Barbados to its development as an im- Grapefruit: History, Use, and Breeding portant agricultural commodity. We review the progress and fluctuations Eliezer S. Louzada1 and Chandrika Ramadugu2 in production and market aspects. Be- cause grapefruit is valued as a highly nutritious fruit, we will discuss the sig- ADDITIONAL INDEX WORDS. biotechnology, drug interaction, mutation breeding, nificant health benefits associated with natural mutation, protoplast, somatic hybrids it. Interaction of grapefruit with cer- SUMMARY. Grapefruit [Citrus 3aurantium (synonym C. 3paradisi)] is an tain commonly used drugs has been important citrus commodity that originated in Barbados in the 17th century. reported; we examine the mode of in- Citrus Grapefruit is the youngest member of the genus . Most commercially teractions and address this concern. important grapefruit cultivars arose through natural and induced mutations, not The first commercial cultivation traditional breeding, of the white-fleshed and seedy Duncan grapefruit. Now, cultivars with a range of flesh colors exist; the pigmentation is correlated with of grapefruit occurred less than 200 lycopene content. A bud sport mutant of grapefruit discovered in Texas has a years ago. The original grapefruit tree deep golden-colored flesh, significantly different from the typical reddish produced very seedy fruit with white pigmentation. In this review, we discuss grapefruit’s journey from its origin in flesh; however, today, several different Barbados and its global establishment including production, marketing, drug cultivars of nearly seedless fruit with a interactions, cultivar development, genetic diversity, and commercially significant range of flesh coloration are available. cultivars. We review the advances in grapefruit cultivar development, the efforts to- ward breeding novel, potentially de- sirable cultivars through conventional  (Citrus maxima), describing it as a rapefruit (Citrus aurantium) breeding, induction of mutations “smaller shaddock.” Macfadyen (1830) belongs to the family Ruta- through irradiations, and other bio- classified the “forbidden fruit” reported Gceae, subfamily Aurantioideae. technological approaches that may be by both Browne and Hughes as Citrus Grapefruit is unique among citrus useful for cultivar improvement. Fi- Âparadisi also known as “barbados (Citrus sp.) fruit due to its distinctive nally, we describe the most important fl fi grapefruit.” Bowman and Gmitter avor, health bene ts, and recent Ca- commercial grapefruit cultivars. ribbean islands’ origin less than 300 (1990) and Gmitter (1995) further con- years ago (Scora, 1975). Most cultivat- firmed the association of the now ex- Commercial importance and tinct “forbidden fruit” with ‘Shaddock’ ed citrus originated in Southeast Asia market history before 2000 BCE. Citron (Citrus med- pummelo and the present-day grape- ica), considered a progenitor species of fruit. Based on chemotaxonomy (Scora THE UNITED STATES LEADS THE et al., 1982) and molecular data (Wu GRAPEFRUIT MARKET. Grapefruit was cultivated citrus (Raveh et al., 2020), fi dates back to 4000 BCE (Webber, et al., 2018), it is presumed that an acci- con ned to the Caribbean islands for 1967). dental hybridization produced the inter- several hundred years. Zoller (1918) fi fl The origin of grapefruit in the speci c hybrid of a pummelo and sweet reported grapefruit ourishing in  Caribbean islands is shrouded in mys- orange [C. aurantium var. sinensis Mexico for decades before Count  fi tery and confusion (Kumamoto et al., (synonym C. sinensis)], now classi ed Odette Philippe brought grapefruit 1987). Hughes (1750) reported a as grapefruit. Earlier, grapefruit was con- seeds to Florida in 1823, raised trees, tree bearing the “forbidden fruit” in sidered an apomictically stabilized hy- and distributed seeds freely (Robin- Barbados, but today’s grapefruit is brid species (Kumamoto et al., 1987). son, 1952). Regardless of the source not a direct descendent of this tree. However, taxonomists now charac- of early grapefruit germplasm, it was Browne (1756) also reported a tree terize grapefruit as a set of cultivars in the United States that grapefruit fi fi bearing the “forbidden fruit” and as- rather than a species; it is classi ed rst gained commercial importance.  sociated it with ‘Shaddock’ pummelo as C. aurantium (M. Schori, per- Initially there was confusion about sonal communication). grapefruit’s nomenclature. Commer- ’ Received for publication 29 June. 2020. Accepted for This review discusses grapefruit s cially it was known as grapefruit publication 12 Mar. 2021. evolution from its identification in (Humes, 1909), and “pomelo” was a Published online 5 May 2021. 1Texas A&M University Kingsville, Citrus Center, 312 N. International Blvd, Weslaco, TX 78599 Units 2Department of Botany and Plant Sciences, Univer- To convert U.S. to SI, To convert SI to U.S., sity of California Riverside, 900 University Avenue, multiply by U.S. unit SI unit multiply by Riverside, CA 92521 0.4047 acre(s) ha 2.4711 We thank Melanie Schori of the U.S. Department 4.1868 calorie(s) kJ 0.2388 of Agriculture, Agricultural Research Service, for 3.7854 gal L 0.2642 help with the recent taxonomic names of citrus 2.54 inch(es) cm 0.3937 cultivars. 0.4536 lb kg 2.2046 E.S.L. is the corresponding author. E-mail: Eliezer. 28.3495 oz g 0.0353 À [email protected]. 28,349,523 oz μg 3.5274  10 8 À This is an open access article distributed under the 28,350 oz mg 3.5274  10 5 À CC BY-NC-ND license (https://creativecommons. 1 ppm μg·mL 1 1 org/licenses/by-nc-nd/4.0/). 0.9072 ton(s) Mg 1.1023 https://doi.org/10.21273/HORTTECH04679-20 ( F À 32) Ä 1.8 F C(C  1.8) 1 32
June 2021 31(3) 243 synonym later discarded in favor of attributed to the processing industry 1922–23 season. Most of the decrease “grapefruit” (Merrill and Lee, 1924; that commercialized canned juice, juice in U.S. grapefruit-bearing area oc- Zoller, 1918). The first commercial concentrate, marmalade, wine additives, curred in Florida; production in the shipments of grapefruit packed in bar- and candied peel. From 1932 to 1937, 1999–2000 season was 2,270,000 rels occurred from 1880 and 1885 grapefruit processing in Florida’s50can- tons, harvested from 114,100 acres from Florida to New York and Phila- neries increased from 80,000 to and declined to 180,400 tons from delphia. By 1889, grapefruit, though 420,000 tons of grapefruit annually. By 24,700 acres in the 2018–19 season. unfamiliar to most consumers, consti- 1936, Texas had 30 canneries (Heid, Acreage in Florida in 2020 reduced to tuted 6% to 7% of Florida’scitrusex- 1937) that processed 260,000 tons of 21,700 acres (USDA, 2020a). port (DePass, 1890). The great freeze grapefruit (about 157 million cans of HLB was first reported from Texas of 1895 drastically reduced all citrus juice) in the 1939–40 season, generating in 2012 (Kunta et al., 2012), but no sig- production in Florida (Humes, 1909), $7 million (Watson, 1940a, 1940b). By nificant reduction in fruit production has including grapefruit. By the 1909–10 1946, the U.S. production increased to been reported after 8 years of disease es- season, the United States produced 2,521,200 tons, of which Florida con- tablishment. In the1999–2000 season, 44,760 tons of grapefruit with 98% tributed 1,280,000 tons and Texas grapefruit production in Texas was contribution from Florida [U.S. De- 920,000 tons (TexSun News, 1948). 237,000 tons from 20,000 acres; in the partment of Agriculture (USDA), United States’ grapefruit acreage 2018–19 season, the yield was 244,000 1945]. increased from 28,800 acres in 1919 to tons from 16,000 acres. The reduction By 1915, Florida’sproduction 198,500 acres in 1947 as new produc- of acreage in Texas occurred because of area consisted of 16,000 acres of bear- tion areas were developed (Fig. 1). freezes,othercitrusdiseasescausedby ing and 45,000 acres of nonbearing, Freezes in 1951, 1962, 1985, and soil-borne pathogens such as Phytoph- 1- to 5-year-old grapefruit trees 1989 intermittently reduced both bear- thora nicotianae, and urbanization. (Vaile, 1915). During the same peri- ing area and production. A look at vari- In California, HLB was first re- od, the California industry had 600 ation in the planting area when freezes ported from a citrus plant in a residen- acres of bearing and 1100 acres of occurred shows that the reduction in tial area in 2012 (Civerolo, 2015); the nonbearing trees. The first commer- acreage was significant in Texas [data disease was restricted to backyard citrus cial grapefruit orchards were planted shown for 10-year blocks (Table 1)]. for 8 years. In 2020, a CLas-positive in Texas in 1915 and produced 120 The other states did not have such asian citrus psyllid was reported from a tons of fruit in the 1919–20 season drastic effects; in some regions, the commercial grove in southern Califor- (Vaile, 1915). planting areas increased after the nia (Citrus Industry News, 2020), in- Grapefruit production increased freeze. In Feb. 2021, grapefruit groves dicating disease spread. substantially from 1919 to 1944, in Texas were hit with a severe freeze, California’s total grapefruit pro- mainly in Florida and Texas. In Flori- with some groves showing a tempera- duction was 241,000 tons from da, in the 1919–20 season, the pro- ture of À7.2 C for several hours; dam- 16,600 acres in the 1999–2000 sea- duction was 236,000 tons; and by ages are still being evaluated (E.S. son and reduced to 128,000 tons 1943–44, it surged to 1,240,000 Louzada, unpublished data). During from 9000 acres in 2018–19 and to tons. In Texas, the production in- the years with most severe freezes, the 87,000 tons in the 2019–20 season. creased from 120 tons in the production values were brought to When an exotic disease like HLB es- 1919–20 season to over 708,400 tons zero or to less than 10,000 tons in tablishes, there is sustained yield de- in the 1943–44 season. Arizona pro- Texas. In 2000, the devastating bacte- cline, and the affected groves may duction increased from 870 tons in rial disease citrus canker (caused by never recover. Unless a permanent so- the 1919–20 season to 122,400 tons Xanthomonas citri ssp. citri)wasrein- lution is found for HLB, it will be in 1943–44; California production troduced to Florida, precipitating an challenging to restore citrus yields to surged from 570 to 99,000 tons for eradication program that lasted until the levels reported before disease es- the same period (USDA, 1945). The 2006. About 1.56 million commercial tablishment. It is also important to escalation of grapefruit production trees and 600,000 dooryard trees were note that hurricanes, like freezes, had between 1910 and 1940 in the Unit- removed (Graham et al., 2004; Zansler a temporary, seasonal effect on the ed States was significant; over 3000% et al., 2005). In 2005, the bacterial Florida grapefruit production. Al- compared with the 300% increase in disease citrus greening or huanglongb- though yield was reduced by hurri- sweet orange production (McLeaish, ing (HLB), associated with Candida- canes, the bearing area was 1939). tus Liberibacter asiaticus (CLas) and unaffected. In Florida, the hurricanes The Texas grapefruit industry was vectored by asian citrus psyllid (Dia- exacerbated the spread of citrus can- confined to the southernmost part of phorina citri),wasreportedinFlorida ker and indirectly affected citrus yield. the state, where it flourished. By 1937, (Halbert, 2005). The effect of HLB in GRAPEFRUITMARKETINOTHER Texas’s 6300 producers accounted for Florida has been significant, and the COUNTRIES. While the grapefruit in- 25% of the grapefruit growers producing disease has destroyed many citrus dustry was booming in the United 24% of the nation’sfruit,secondonlyto groves. The two major diseases in Flo- States, it was developing slowly in Florida’s 64%. Per capita consumption rida, citrus canker and HLB, along other citrus-growing regions of the of grapefruit in the United States was with the seasonal hurricanes, have re- world. In the Caribbean islands 5.7 fruit between 1923–27, increasing sulted in a drastic reduction of U.S. where grapefruit originated, the in- to 7.5 fruit between 1931–35 (Goenig, grapefruit-bearing areas to about dustry was not successful due to vari- 1937). The increased popularity was 46,400 acres, equivalent to the ous factors. In 1929, C. Powel
244 June 2021 31(3) 200 180 )
s 160 e r
c 140 A
0 120 0
0 100 1 ( 80 a e
r 60 A 40 20 0
Crop Season Freeze 1934Freeze 1951 Freeze 1962 Freeze 1985 Freeze 1989 Citrus Canker Eradication HLB detected in Florida
Fig. 1. Evolution of grapefruit bearing area in the United States from 1919 to 2020 (Freie, 1987; Shuler and Scarborough, 1963; U.S. Department of Agriculture, 1950, 1960, 1973, 1981, 2000, 2017b, 2020c); HLB = citrus huanglongbing, 1 acre = 0.4047 ha. summarized the reasons to the West contributed 26,000 tons (USDA, medium-sized fruit (University of Indies Committee in London, as fol- 2020b). Grapefruit is now produced in Michigan, 2016). Consumption of lows: the Jamaican and Bahamian almost all citrus-producing countries half a grapefruit supplies complex nu- grapefruit sectors were small, with with 6.7 million tons produced in the trients essential to a healthy diet; untrained peasants managing from 2019–20 season (USDA, 2020b). In some consider it an antiobesity ingre- 2- to 5-acre groves that produced about a decade (2004–13), the global dient. Public awareness of these health modest quantity of poor-quality fruit. grapefruit production increased 45%, benefits has increased fruit and juice In Grenada, the prime agricultural generating market revenue of $6.5 bil- consumption, and demand in many land was devoted to the cultivation of lion in 2015 (IndexBox Marketing As- countries. nutmeg (Myristica fragrans)andco- sociation, 2017; USDA, 2017a). FLAVONOIDS. Flavonoids are low- coa (Theobroma cacao). The 200 to molecular-weight phenolic com- 300 acres of land designated for General benefits and nutritional pounds with known antioxidant activ- grapefruit cultivation in Saint Lucia value of grapefruit ity (Codoner-Franch~ and Valls-Belles, was poorly managed. The 400 acres 2010; Liu, 2004). Flavonoids consist Citrus fruit is nutrient-dense with of grapefruit planted in Trinidad was of two benzene rings and an oxygenat- vitamins, folate, phytochemicals like not adequate to establish a thriving in- fl ed heterocycle pyrone ring. Six dietary dustry (Powell, 1929). avonoids, carotenoids, coumarins, flavonoids are recognized: flavones, fla- By 1929, the world grapefruit pro- terpenes, and limonoids (Ramper- vonols, flavanones, flavanols (cate- duction reached 392,000 tons. The saud, 2007; Zhang, 2007). Grapefruit chins), anthocyanins, and isoflavonoids United States and Puerto Rico contrib- is recognized as a functional food, val- (Zhang, 2007). Citrus has four types ued in certain cultures as promoting fl fl uted 372,000 tons, Cuba 8000 tons, Ja- fl of the avonoids: avanones (about maica 4000 tons, South Africa 4000 health due to the avonoid com- 98%), and smaller amounts of flavones, tons, and other countries 4000 tons pounds present in the fresh fruit and flavonols, and anthocyanins (Benaven- (Tutt, 1929). Palestine developed a the juice (Kiani and Imam, 2007; te-Garcia et al., 1997). Naringin, the strong grapefruit industry, with a dra- Tundis et al., 2014; Yu et al., 2005; major flavanone in grapefruit (together matic production increase from 4240 Zou et al., 2016). Table 2 shows nu- with hesperidin and rutin) alter lipid tons in 1930 to 80,000 tons in 1940, tritional information from one-half of and glucose metabolism and improve becoming a vital grapefruit exporter pri- a medium-sized grapefruit (123 g). hyperlipidemia and hyperglycemia in marily to European countries (Watson, Recognized as a low-energy-dense animal models with type 2 diabetes 1940a, 1940b). The global grapefruit food (in calories per gram), it has (Da Silva et al., 2001; Jung et al., market expanded steadily from 1950 to omega-3 and omega-6 fatty acids, 18 2006). Flavonoids are also reported to 2020, with China producing 4.9 million of the 20 amino acids, is rich in glu- have anti-ischemic, antioxidant, vasorelax- tons in 2019–20, more than 10 times tamic and aspartic acids, arginine, pro- ant, and antithrombotic properties (Mink the 485,000 tons produced in the Unit- line, and phenylalanine. It is abundant et al., 2007; Pan et al., 2010). The anti- ed States. However, the Chinese pro- in vitamins, with over 1400 interna- oxidant and free-radical scavenging na- duction also includes pummelo. South tional units (IU) of vitamin A, high in ture of flavonoids mitigate vitamin C Africa is currently in third place with alpha and beta-carotene, lycopene, vi- oxidation (Mink et al., 2007). Naringin 387,000 tons and Mexico in fourth tamin C, in addition to multiples of decreases the risk of atherosclerosis, re- with 350,000 tons. Turkey produced bioactive compounds. Grapefruit is duces radiation-induced chromosome 270,000 tons, Israel 155,000 tons, Eu- moderately rich in potassium, with damage, and stimulates DNA repair in ropean Union 96,000 tons, and others 166 mg per half-portion of a cancer cells (Li et al., 2013; Zhang,
June 2021 31(3) 245 Table 1. Effect of major freezes on grapefruit acreage in the United States. the incidence of myocardial infarctions Florida Texas California Arizona (Kohlmeier et al., 1997). Lycopene treatment of ovariectomized rats sup- Season Area (1000 acres)z pressed bone turnover and restored 1945–46 85.2 78.6 14.5 12.9 bone strength (Ardawi et al., 2016). In 1946–47 85.7 80.0 14.1 12.9 epileptic mice (Mus musculus), lycopene 1947–48 87.6 80.5 13.3 12.2 reversed oxidative damage, demonstrat- 1948–40 89.5 82.0 11.6 10.3 ing neuroprotective potential (Bharad- 1949–50 93.0 56.0 10.7 9.6 waj and Kumar, 2016). In studying 1950–51 94.4 56.0 10.2 9.4 Alzheimer’s disease using cultured hu- y x 1951–52 98.6 17.9 9.7 9.4 man cell and nematode system (Caeno- 1952–53 102.0 20.9 9.4 9.4 rhabditis elegans), lycopene delayed – 1953 54 105.0 22.0 9.1 7.9 paralysis, protected against hydrogen – 1954 55 111.0 24.1 8.2 7.9 peroxide-induced oxidative stress, and decreased pathogenesis (Chen et al., – 1961 62 94.0 45.3 9.5 5.6 2015). Lycopene has been demon- – y x 1962 63 88.0 35.0 10.9 6.0 strated to ameliorate neuropathic pain – 1963 64 83.0 36.0 11.9 6.0 (Zhang et al., 2016), and it is an effec- – 1964 65 84.0 36.0 12.3 6.1 tive prophylactic for cataracts in rat 1965–66 85.8 37.0 12.6 6.7 – model systems (Gupta et al., 2003). 1966 67 87.0 39.0 12.8 6.9 Lycopene dietary supplements have 1967–68 87.5 41.0 12.8 7.6 – been reported as decreasing oxidative 1968 69 90.3 45.0 12.7 4.9 stress and improving bone metabo- 1969–70 98.7 40.0 12.8 6.1 – lism. Analysis of data from human 1970 71 107.2 37.0 12.1 6.3 clinical trials indicated that while lyco- pene supplements are effective in cer- 1983–84 119.6 43.3 21.9 6.8 tain situations, consumption of whole 1984–85y 115.5 19.1w 21.1 7.1 foods containing lycopene significantly 1985–86 105.1 13.5x 20.9 5.7 promotes cardiovascular health (Burton- 1986–87 106.0 15.2 20.8 5.9 Freeman and Sesso, 2014; Caseiro et al., 1987–88 106.0 16.0 20.7 6.0 1988–89 106.9 16.9 19.9 6.5 2020). y URANOCOUMARINS Furanocou- 1989–90 103.0 18.7 19.2 6.4 F . 1990–91 104.2 4.5w 18.3 6.2 marins are secondary metabolites that 1991–92 104.7 7.7x 18.5 5.9 defend plants against insects and 1992–93 111.9 10.1 17.8 5.9 pathogens. They are polyphenolic com- pounds consisting of a furan ring fused z1 acre = 0.4047 ha. yFreezes. with coumarin. These compounds are xProduction below 10,000 tons (9071.8 Mg). associated with antioxidative, anti-in- wZero production. flammatory, and bone health-promot- ing properties in human subjects. In 2007). Naringenin, the aglycone form fleshed parent cultivar (Xu et al., western societies, about 73% of the of naringin, is demonstrated to be 2006); ‘Ruby Red’ has about 1745 mg dietary furanocoumarins are obtained among the more active naringins (Kia- of lycopene in a half fruit (Table 2). through grapefruit consumption (Hung ni and Imam, 2007). The health benefits of lycopene- et al., 2017). The major furanocoumar- CAROTENOIDS. The primary caro- rich foods are numerous. Lycopene ins present in grapefruit are bergamottin, tenoids in grapefruit are lycopene and from grapefruit and tomato (Solanum epoxybergamottin, and 6’,7’-dihydrox- b-carotene. Lycopene is a fat-soluble lycopersicum) inhibited an aggressive ybergamottin (DHB). The furanocou- tetraterpene, a powerful antioxidant form of prostate cancer in rats (Rattus marin content in grapefruit juice can known to prevent heart disease, can- rattus) (Giovannucci, 2005; Gunase- vary based on the flesh coloration (red, cer, and macular degeneration. The kera et al., 2007). In pancreatic cancer pink, or white), the fruit’s maturity level, yellow, orange, and red pigmentation patients, cancerous lesions decreased fruit part, juice processing technique, of grapefruit is dependent on the ratios when lycopene was incorporated into and storage conditions. The health ben- of lycopene, b-carotene, trace amounts the diet (Kucuk et al., 2001). Lyco- efits of furanocoumarins are well docu- of phytofluene, zeaxanthine, b-cryp- pene reduced liver cancer in rats mented in the literature (Chudnovskiy toxanthine, and zeta carotene (Curl (Agca et al., 2012) and oral and breast et al., 2014; Madrigal-Bujaidar et al., and Bailey, 2006; Zhang et al., 2016). cancer in humans (Gupta et al., 2015; 2013; Mahgoub, 2002; Wang and Re- White-fleshed grapefruit cultivars also Sato et al., 2002). The antiplatelet uss, 2018). Bergamottin inhibits cancer have trace amounts of lycopene (Cam- and antiapoptotic effects of lycopene cell growth and suppresses fibrosarcoma eron et al., 1964) and a significant reduce arterial stiffness (Mozos et al., metastasis in humans (Ko et al., 2018). b-carotene content. The red Marsh 2018), making it an effective antiathero- Furanocoumarins are also known to in- grapefruit mutants have 790-fold high- sclerotic, antioxidant, anti-inflammatory, fluence bone health. Bergapten has anti- er carotenoid content than the white- and antihypertensive agent able to lower osteoporosis activity demonstrated by
246 June 2021 31(3) in vitro and in vivo experiments (Hung intestines. Commonly used medica- has a list of common drugs that are et al., 2017). tions that show altered activity when known to cause furanocoumarin-in- consumed with grapefruit juice include duced pharmacokinetic changes. Drug interactions HMG-CoA reductase inhibitors pre- ALLEVIATING THE PROBLEM OF Grapefruit juice is known to in- scribed for lowering cholesterol (atorva- DRUG INTERACTION ASSOCIATED WITH teract with several commonly adminis- statin, simvastatin, lovastatin), calcium GRAPEFRUIT CONSUMPTION. Abetter tered prescription medications. The channel blockers prescribed for blood understanding of the interactions be- first drug interaction was reported in pressure regulation (felodipine, nifedi- tween grapefruit furanocoumarins and 1989; Bailey et al. observed that the pine), certain antihistamines (fexofena- oral drugs in the human alimentary ca- concentrations of felodipine, a calci- dine), immunosuppressants (cyclosporine, nal is needed. Speculations based on um channel blocker, were elevated in tacrolimus), pain medications (metha- in vitro experiments and studies con- apatient’s blood plasma after grape- done), and some psychiatric drugs ducted with model animals are not al- fruit consumption (Bailey et al., (buspirone, triazolam, diazepam, mid- ways clinically relevant. Alternative 1989). According to current reports, azolam, sertraline). Grapefruit and its medications that are unaffected by pharmacological activity of 85 pre- processed products are contraindi- grapefruit juice consumption are valid scription drugs is affected if grapefruit cated when these drugs are adminis- options. Activity of certain antilipemic is consumed with an oral medication tered (Hanley et al., 2011). statins such as fluvastatin, pravastatin, with low-to-medium bioavailability Overdoses of antilipemic drugs and rosuvastatin are unaffected by si- (Bailey et al., 2013). The drug interac- such as statins can be toxic at excessive multaneous consumption of grapefruit tions are caused by the disruption of systemic concentrations and potential- juice. Fluvastatin and rosuvastatin are the enteric cytochrome P4503A4 en- ly lead to rhabdomyolysis; skeletal metabolized in the liver by a different zyme (CYP3A4) activity. Under nor- muscle damage caused by excessive enzyme, CYP2C9, that does not inter- mal conditions, the drugs are myoglobin release into the blood re- act with furanocoumarins; pravastatin is metabolized in the small intestine by sults in renal failure (Lilja et al., metabolized in the liver, and hence the CYP3A4 before the system absorbs 1998b). Cyclosporine and tacrolimus bioavailability is unaffected by con- them. The furanocoumarins berga- are administered for preventing organ sumption of grapefruit (Owira and Oje- mottin and DHB inhibit CYP3A4 ac- rejection after liver transplants; fura- wole, 2010). Because of the significant tivity in the gastrointestinal tract, nocoumarin interactions can cause health benefits of grapefruit consump- resulting in reduced intestinal metabo- drug overdose leading to nephrotoxi- tion, it is unreasonable to completely lism of the drugs (Owira and Ojewole, city (Ducharme et al., 1995). In- avoid grapefruit products (Mertens-Tal- 2010). Because the drug’s pre-systemic creased concentrations of several cott et al., 2006). Recently generated degradation is decreased, bioavailabili- anticancer agents in the system can re- grapefruit hybrids with low concentra- ty and absorption are increased, result- sult in a situation called “torsade de tions of furanocoumarins, such as ‘914’, ing in drug overdoses. pointes.” In this condition, the pa- ‘Aliza’,and‘Cookie’, may be used for SPECIFIC INTERACTIONS. Several tient may develop abnormal heart patients desiring to obtain nutritional types of drugs are affected by the fura- rhythm leading to sudden cardiac benefits from grapefruit in their diet (Fi- nocoumarin-induced process in the death (Libersa et al., 2000). Table 3 del et al., 2016; Gmitter, 2014).
Table 2. Nutritional information of 1/2 grapefruit [2–3/4 inches (7.0 cm) diameter, 123 g (4.34 oz), with pink or red flesh] (Nutrition Data, 2020). General informationz Amino acidsz Vitaminsz Calories 51.7 Tryptophan 9.8 mg Vitamin A 1415 IU Energy density 0.39 kcal/g Threonine 16.0 mg Retinol activity equivalent 71.3 mg Carbohydrates 13.1 g Isoleucine 9.8 mg Alpha carotene 3.7 mg Dietary fiber 2.0 g Leucine 18.4 mg Beta carotene 844 mg Starch 0.0 g Lysine 23.4 mg Beta cryptoxanthin 7.4 mg Sugar 8.5 g Methionine 8.6 mg Lycopene 1745 mg Total fat 0.2 g Cystine 9.8 mg Lutein 1 zeaxanthin 6.2 mg Total omega-3 fatty acids 9.8 g Phenylalanine 56.6 mg Vitamin C 38.4 mg Total omega-6 fatty acids 35.7 g Tyrosine 9.8 mg Vitamin E (alpha tocopherol) 0.2 mg Protein 0.9 g Valine 18.4 mg Thiamin 0.1 mg Mineralsz Arginine 107 mg Niacin 0.3 mg Calcium 27.1 mg Histidine 9.8 mg Vitamin B6 0.1 mg Iron 0.1 mg Alanine 29.5 mg Folate 16.0 mg Magnesium 11.1 mg Aspartic acid 170 mg Food folate 16.0 mg Phosphorus 22.1 mg Glutamic acid 242 mg Pantothenic acid 0.3 mg Potassium 166.0 mg Glycine 18.4 mg Dietary folate equivalents 16.0 mg Zinc 0.1 mg Proline 77.5 mg Choline 9.5 mg Selenium 0.1 mg Serine 34.4 mg Betaine 0.1 mg – – z1 calorie = 4.1868 kJ, 1 g = 0.0353 oz, 1 mg = 3.5274 Â 10 5 oz, 1 mg = 3.5274 Â 10 8 oz, IU = international units.
June 2021 31(3) 247 Processing to reduce the active cited as a reason for disliking multiple embryos are clones of the ingredient involved in drug interac- grapefruit (Obenland et al., 2018). mother plant. When grapefruit is used tions of grapefruit offers potential so- The flavonoid naringin [4',5,7-tri- as the seed parent in breeding, the zy- lution (Girennavar et al., 2008). hydroxyflavanone-7-b-L-rhamno- gotic hybrid embryos resulting from Furanocoumarin concentration in ex- glucoside-(1,2)-a-D-glucopyrano- sexual reproduction generally do not tracted grapefruit juice is affected by side] and the limonoid limonin develop due to fierce competition the processing conditions. Use of MA (7,16-dioxo-7,16-dideoxylimon- from the numerous, robust nucellar packaging (that uses materials and for- diol) produce the bitterness. If the embryos. It is challenging to create a mats with an appropriate level of gas naringin concentration is higher sufficiently large hybrid population –1 barrier) reduces the concentration of than 300–400 mg·mL ,grapefruit for the selection of superior cultivars furanocoumarins (Hung et al., 2017). juice is considered bitter (Soares by embryo rescue methods because Pasteurization of juice at 60 C for 60 and Hotchkiss, 1998). Grapefruit the weaker zygotic embryos are elimi- min reduced levels of bergamottin and sweetness, measured by the hedon- nated due to lack of nourishment. Be- DHB (Cancalon et al., 2011). Grape- ic score (indicating acceptable sen- cause citrus is a perennial crop with a fruit juice treated with adsorption res- sory quality) increases with the long juvenility period, generating im- ins to remove furanocoumarins did not sugar content. In a study of four proved cultivars through traditional ’ increase the drug felodipine ssystemic grapefruit cultivars, Obenland et al. breeding is time-consuming. Field tri- bioavailability in human volunteers (2018) noted that the bitterness per- als for new cultivar evaluations require (Paine et al., 2006). Treating grape- ceived by a trained sensory panel corre- considerable land and resources. Nu- fruit juice with heat or ultraviolet light lated negatively with the hedonic score cellar embryony and economic con- fi signi cantly reduced undesirable phar- (Obenland et al., 2018). Bitterness varies siderations are the predominant macokinetic interactions (Uesawa and with cultivar type, maturity, and juicing factors limiting the availability of new Mohri, 2006a, 2006b). methods. Juice extracted from the whole grapefruit hybrids. Grapefruit consumption is a con- fruit will have a higher flavonoid concen- fl fi Nucellar embryony played sig- cern in vulnerable elderly patients on tration because avonones are signi - nificant role in the serendipitous or- multiple medications (Bressier and cantly higher in the segment membrane igin of grapefruit in Barbados. The Bahl, 2003). In patients with liver cir- and peel (Rouseff et al., 1987). Grape- English settlers on Barbados Island, rhosis, CYP3A4 activity is crucial (Kia- fruit juice can be “debittered” through not familiar with the asexual propa- ni and Imam, 2007). Rhabdomyolosis adsorption and enzymatic hydrolysis by gation commonly practiced for cit- from an overdose of cholesterol-re- naringinase (Chien et al., 2001). rus, cultivated ‘Shaddock’ pummelo ducing statins occurs in about four from seed. The self-incompatible per 100,000 people per year (Lee Grapefruit breeding ‘Shaddock’ presumably outcrossed et al., 2016). Drug interaction studies Grapefruit cultivars have a narrow with a sweet orange resulting in a using model organisms are often genetic base; hybridization to increase hybrid with apomictic traits inher- based on administering high volumes genetic diversity is desirable. Com- ited from the sweet orange pollen of grapefruit juice or related products mercially significant grapefruit market [i.e., 1–1.5 L juice per day, 1.5 kg cultivars did not originate through parent. Repeated propagation from the seeds that generated “true-to- marmalade per week (Bailey et al., conventional breeding. Many of these ” 2013)]. Such studies often misrepre- can be traced back to a series of natu- type progeny probably led to the sent the dietary intake of patients ral budsports or induced mutations of multiplication of grapefruit trees under normal conditions. Lee et al. the original white grapefruit that pro- that later stabilized as an apomictic (2016) discuss the low risk of duced cultivars with flesh colors rang- species (Kumamoto et al., 1987). rhabdomyolysis associated with statins SEXUAL BREEDING. Tangelos [C. ing from white to pinkish, peach, and   consumed with grapefruit. The phyto- red (Fig. 2). A deep-golden–fleshed aurantium (synonym C. tangelo)] chemicals in grapefruit confer cardio- grapefruit (Fig. 3) discovered in Texas is are the oldest known grapefruit hy- vascular benefits that often outweigh presumed to be a somatic mutant of a brids that resulted from a planned the associated risks caused by drug in- white grapefruit resembling ‘Duncan’ breeding program. These mandarin- teractions. Selecting alternate drugs (E.S. Louzada, unpublished data). A like hybrids were generated from a ‘ ’ may allow incorporating moderate grapefruit family tree, depicting cultivars cross of the Dancy mandarin (Citrus amounts of grapefruit in the diet. It is that originated by spontaneous bud reticulata)seedparentwiththe prudent to consider the net health sports, induced mutations, and by sexual ‘Duncan’ grapefruit pollen parent. Three benefits before contraindicating crosses (leading to grapefruit-like culti- well-known tangelos are ‘Orlando’, grapefruit consumption (Lee et al., vars),isshowninFig.4. ‘Minneola’,and‘Seminole’ (Hodgson, 2016; Paine et al., 2006). LIMITATIONS TO SEXUAL BREED- 1967). Recently, a grapefruit-like hybrid ‘ ’ IMPROVING COMMERCIAL ACCEPT- ING. Like most citrus, grapefruit has a named Dany was produced by a cross ABILITY OF GRAPEFRUIT. In addition high degree of sporophytic apomixis of ‘Michal’ mandarin with ‘Duncan’ to reducing the negative interac- leading to polyembryony; multiple ge- grapefruit (Fidel et al., 2016). A cross of tions of grapefruit with certain pre- netically identical embryos develop ‘Pearl’ tangelo with a ‘Marsh’ or scription drugs, reducing bitterness from a single fertilized egg cell by nat- ‘Duncan’ grapefruit produced a hybrid would enhance consumer approval ural embryogenesis of cells from the with a low amount of volatiles and a of grapefruit. While bitterness is an walls of the embryo sac (Kotlunow milder juice than ‘Marsh’ (Shaw et al., accepted characteristic, it is often et al., 1996) (Fig. 5). These vigorous 2001).
248 June 2021 31(3) Table 3. Medications known to interact with furanocoumarins in grapefruit products. Class of drug Interacting drugz Manufacturer Adverse effects References Statins Atorvastatin (Lipitor) Viatris, Canonsburg, PA Rhabdomyolosis Dreier and Endres, 2004 Simvastatin (Zocor) Merck Sharp & Dohme, Lilja et al., 1998b Northumberland, UK Lovastatin (Mevacor) Merck Sharp & Dohme Calcium channel blockers Felodipine (Plendil) AstraZeneca, Sodert€ €alje, Torsade de pointes Dresser et al., 2000 Sweden Amiodarone (Pacerone) Upsher-Smith, Maple Agosti et al., 2012 Grove, MN Beta blockers Carvedilol (Coreg) GlaxoSmithKline, Atrioventricular disorder Bailey and Dresser, Research Triangle 2012 Park, NC Psychiatric drugs Buspirone (BuSpar) Bristol-Myers Squibb Dizziness, sedation Lilja et al., 1998a Co., Princeton, NJ Diazepam (Valium) Roche Laboratories, Ozdemir et al., 1998 Little Falls, NJ Midazolam (Versed) Roche Laboratories Trazodone (Desyrel) Pragma Pharmaceuticals, Locust Valley, NY Triazolam (Halcion) Pfizer, New York, NY Anticancer agents Nilotinib (Tasigna) Novartis, East Hanover, Torsade de pointes Yin et al. 2010 NJ Crizotinib (Xalkori) Pfizer Myelotoxicity Rothenstein and Letarte, 2014 Everolimus (Afinitor) Novartis Grabowsky, 2013 Hormone drugs Estrogen Risk of breast cancer Monroe et al., 2013 Immunosuppressants Cyclosporine (Neoral) Novartis Nephrotoxicity Yee et al., 1995 Tacrolimus (Prograf) Astellas Pharma, Peynaud et al., 2007 Northbrook, IL Antihistamines Carbamazepine (Tegretol) Novartis Nausea, muscle Garg et al., 1998 weakness Astemizole Bio-Techne, Torsade de pointes Hermans et al., 2003 Minneapolis, MN zRepresentative drug with trade name in parentheses.
The two most prominent grape- taking medications that interact hybrids have only traces of furanocou- fruit hybrid cultivars to date are Or- with furanocoumarins (Nature Bio- marins (Fidel et al., 2016), and, there- oblanco (Soost and Cameron, technology, 2013). Tetraploid cit- fore, are expected to have less 1980) and Melogold (Soost and rusplantshavebeenusefulto interaction with prescription drugs. Cameron, 1986). Both cultivars are produce seedless triploid offspring Except for ‘Melogold’ and triploid and seedless, resulting from by interploid crosses. ‘Oroblanco’, the grapefruit hybrids a cross between a diploid (2n) low- Toolapong et al. (1996) pro- developed are not yet commercially acid pummelo with a tetraploid (4n) duced 51 triploid grapefruit hybrids available. The ‘New Zealand’ grape- white Marsh grapefruit. A red by 2n  2n cross of ‘Banpeiyu’ pum- fruit, assumed to be a pummelo hy- ‘Oroblanco’-like grapefruit, named melo with ‘Ruby Red’ grapefruit. brid, is commercially important in ‘Einat’, was recently reported from a Crosses of sweet orange or mandarin New Zealand (Hodgson, 1967). Fig- cross of a tetraploid ‘Hudson’ with pummelo generate novel citrus ure 4 shows a detailed grapefruit fami- grapefruit with a diploid low-acid types with grapefruit characteristics. A ly tree. pummelo (Fidel et al., 2016). An- grapefruit-like hybrid was produced MUTATION BREEDING. Mutation other red-fleshed grapefruit, also by crossing the seed parent ‘Nakon’ has been a significant source of genet- produced by a cross of a diploid pummelo with the ‘Shamouti’ sweet ic variation in all citrus species. Natu- low-acid pummelo with a tetraploid orange (Shaw et al., 2001). When ral or induced mutations have ‘Ruby Red’ grapefruit, was patented mandarin cultivars Orah and Frua produced most of the commercial cul- under the name ‘914’ (Gmitter, were crossed with Chandler and Sia- tivars of sweet orange or grapefruit. 2014). The hybrid cultivar 914 has mese Sweet pummelo, respectively, The mandarin group is more amena- an extremely low furanocoumarin two grapefruit-like hybrids (Aliza and ble to conventional breeding because content, making it safe for people Cookie) were produced. These two many cultivars are monoembryonic.
June 2021 31(3) 249 desirable traits: seedlessness and in- new cultivar was named Rio Red, cur- tense flesh pigmentation. Most culti- rently the most important commercial vars have a yellow peel with a red grapefruit in Texas (Hensz, 1985). blush; shaded fruit develops a more Recently another budsport mutation homogeneous red peel color without of ‘Rio Red’, with a homogeneous blush (Lado et al., 2015). Cold deep red peel and flesh with reduced weather enhances a red peel color bitterness, was discovered and patent- (E.S. Louzada, unpublished data). An ed as TR-1 (Louzada and Del Rio, exception to the red flesh color was 2020). A seedless version of the observed in Texas where a deep gold- deep-golden–fleshed grapefruit was en-fleshed seedy grapefruit (Fig. 3) produced by irradiation with gam- was found. It is presumed to have ma rays at 50 Gy (E.S. Louzada, un- originated from a ‘Duncan’-like white published data) (Fig. 3). In South grapefruit because reversion to a see- Africa, the ‘RedHeart’ grapefruit, for- dy, white grapefruit was observed in merly called ‘Flamingo’, was devel- Fig. 2. Slices of grapefruit showing fl some trees (E.S. Louzada, unpub- oped through induced mutations of esh pigmentation ranging from white ‘ ’ ‘ ’ to various shades of red and a new lished data). Duncan grapefruit was the Henderson grapefruit, and it is cultivar of grapefruit with golden- initially an essential cultivar in Florida, reported to have a lower naringin con- colored flesh is shown. but it was later replaced by the nearly tent (Citrogold, 2020) (Fig. 4). seedless Marsh grapefruit and then by SECTORED GRAPEFRUIT CHIMERA. Ruby Red. Two new cultivars, Sham- A chimera is composed of cells from bar and Thompson, originated from more than one genetically distinct or- Marsh grapefruit as budsports. ganism. The first report of a chimera ‘Thompson’ itself has branched into was described in the 1600s as a shoot several commercially important pig- emerging from a graft junction of a mented cultivars by naturally occur- bitter orange (Citrus Âaurantium) ring budsports and induced mutations and a citron (Frank and Chitwood, (Fig. 4). 2016). The occurrence of sectorial INDUCED MUTATIONS. The first chimeras has been studied in several documented induced mutation in cultivars of sweet orange, grapefruit, plants was reported in 1927. The tangelo, and tangor (C. reticulata  same year, low-alkaloid forms of sweet C. sinensis). Chimera incidence ranges lupine (Lupinus albus) variants result- from 0.009% to 0.271% in citrus. Culti- ing from induced mutations were se- vation of progeny seedlings from fully lected, domesticated, and developed developed or under-developed seeds into a crop species. Since 1964, with derived from the chimeric fruit may be fl FAO/International Atomic Energy valuable for selecting cultivars with im- Fig. 3. Grapefruit with golden esh fl color, presumably a natural mutant Agency support, more than 3200 mu- proved fruit esh and peel color, matu- from a seedy, white grapefruit tant cultivars derived from over 210 rity, reduced pest, and disease resistance resembling ‘Duncan’. plant species have been released (Shu (Bowman et al., 1991). E.S. Louzada et al., 2011). (unpublished data) recovered grapefruit Technologies inducing mutations plants producing fruit with improved Mutations have also resulted in the have successfully produced improved peel and flesh color from a grapefruit development of several patented commercial grapefruit cultivars with sectorial chimera (Fig. 6) now being cultivars. The advantage of clonally red flesh. ‘Star Ruby’, produced from evaluated for horticultural performance. propagated cultivated citrus is the ‘Hudson’, was the first successful cul- maintenance of desirable mutations. tivar produced by irradiation with Role of biotechnology Horticultural traits of mutant culti- thermal neutrons (Hensz, 1971). In- SOMATIC HYBRIDIZATION AND CY- vars are often lost in the zygotic terestingly, in Texas where ‘Star BRIDIZATION. Biotechnology has seedling progeny resulting from sex- Ruby’ was developed, it did not gain been instrumental in developing ual reproduction. Figure 4 shows commercial importance. ‘Star Ruby’ is novel cultivars of crop plants. In details of grapefruit types produced a highly valued commodity in South citrus, protoplast fusion was used by natural seedling variation, bud- Africa, Australia, Israel, Turkey, Spain, successfully to develop new tetra- sport mutation, and parentage of and Cyprus (Da Grac¸a et al., 2004). ploid breeding lines for rootstock specific cultivars. Hensz irradiated ‘Ruby Red’ grape- development and for breeding SPONTANEOUS BUDSPORT MUTA- fruit with thermal neutrons, produc- seedless triploid forms using a tet- TION. The seedy, white-fleshed ing fruit with about three times the raploidparentallineininterploid ‘Duncan’ grapefruit is presumed to be red color intensity of the original crosses (Grosser et al., 1996, 1998; the original source of the several seed- ‘Ruby Red’ and thus designated this Guo et al., 2000; Ohgawara et al., ed and seedless cultivars with various as ‘A&I-1-48’. A budsport mutant of 1989). Cybrids are produced by peels and flesh colors. Grapefruit bud- ‘A&I-1-48’ produced fruit with flesh fusing only the cytoplasm of the sport mutants were selected for two five times as red as ‘Ruby Red’.The two selected cultivars; potentially,
250 June 2021 31(3) Fig. 4. Origin of major grapefruit (GF) cultivars. (A) Cultivars developed by sexual hybridization. Parentage of selected hybrids is shown. (B) Cultivars resulting from spontaneous or induced mutations. Major events involved in GF origin are shown in black rectangles. Names of budsport cultivars are shown without borders. Dotted rectangles represent cultivars arising through irradiations. Seedling-derived cultivars are shown in boxes with dashes. Year and place of cultivar development are indicated: Florida (FL), Texas (TX), California (CA), South Africa (SA), and Argentina (ARG). Modified from Da Grac¸a et al. (2004).
resistance by fusion of the meso- diploid grapefruit (Kunitake et al., phyll protoplast from three different 2002). grapefruit cultivars with the protoplast GENETIC ENGINEERING. Success in from embryogenic suspension cells of plant genetic engineering depends on the citrus canker-resistant ‘Meiwa’ a successful Agrobacterium tumefaciens kumquat (Fortunella Âcrassifolia). Sat- infection for genetic transformation, pute et al. (2015) reported somatic and the ability to efficiently regenerate hybridization of ‘Dancy’ mandarin plants through tissue culture. Genetic suspension cells with haploid ‘Ruby transformation has been accomplished Red’ grapefruit and obtained one in most of the citrus species, including triploid hybrid and several cybrids grapefruit (Moreira-Dias et al., 2000; resembling grapefruit. The fusion Niedz et al., 2015; Pena~ et al., 2004). of ‘Dancy’ mandarin with ‘Duncan’ Grapefruit epicotyl explant is the tis- Fig. 5. Germinated seed of ‘Rio Red’ grapefruit generated grapefruit cy- sue of choice for biolistic, direct intro- grapefruit demonstrating the presence brids with fruit that maintain high duction, and A. tumefaciens-mediated of various embryos in a single seed concentrations of soluble solids transformation (Wu et al., 2016). A (polyembryony). during the summer season; an im- comprehensive study of the experi- provement over normal ‘Duncan’ mental parameters that influence the that loses fruit quality and has un- regeneration efficiency of citrus root- unique hybrids with horticulturally timely seed germination (vivipary). stocks and scions is available (Niedz interesting combinations can be Somatic triploid grapefruit hybrids et al., 2015). developed. Omar et al. (2017) re- with normal fruit morphology were Disease resistance, seedlessness, ported the production of grapefruit produced by electrofusion of the and flesh pigmentation are probably cybrids with putative citrus canker diploid ‘Shogun’ mandarin with a the three most important horticultural
June 2021 31(3) 251 suppressors that affect host resistance sweet orange; recent genome se- and facilitate viral colonization. Calvo quence analyses have confirmed this (2013) targeted these suppressors with assumption (Wu et al., 2018). Chlo- small interfering RNA and obtained roplast genome analysis, a powerful transgenic plants resistant to the virus; tool to study the female parent’scon- however, the resistance was compro- tribution, indicates that pummelo is mised when CTV strains with divergent the seed parent and sweet orange is suppressors were used to challenge the the pollen parent of grapefruit. Based Fig. 6. A grapefruit sectored chimera transgenic plants. Several groups are on molecular evidence, grapefruit is found in Texas showing changes in now conducting research to produce assumed to be the result of a back- rind and flesh coloration. transgenic citrus resistant to HLB and cross of sweet orange with pummelo citrus canker, two of the most economi- (Nicolosi et al., 2000). Phylogenomic cally significant citrus diseases. Grosser karyotypes studies clustered grapefruit target traits for improvement of all cit- et al. (2011) produced several transgen- in an intermediary position between rus cultivars. Grapefruit is susceptible to ic plants of sweet orange, grapefruit, pummelo and sweet orange. It was es- several diseases, including quick decline lime (Citrus Âaurantiifolia), and timated that about 63% of the grape- caused by Citrus tristeza virus (CTV), ‘Carrizo’ citrange [(C. Âaurantium) Â fruit genome was contributed by the citrus canker, and HLB. In a controlled C. trifoliata] expressing antimicrobial pummelo parent (Oueslati et al., greenhouse study of representative citrus peptides. Selected plants showed signifi- 2017). From the initial point of hy- cultivars exposed to psyllid vectors carry- cantly higher resistance to citrus canker bridization to now, the commercially ing the HLB pathogen, grapefruit culti- compared with untransformed controls significant grapefruit cultivars have vars were among the most susceptible in detached leaf assays. E.S. Louzada maintained their homogeneous genetic (McCollum et al., 2016). As discussed (unpublished data) produced transgenic identity because of apomixis. Grape- earlier, in the United States, the Florida ‘Valencia’ and ‘Hamlin’ sweet oranges fruit has a very narrow genetic base be- grapefruit industry recently declined due and ‘Ruby Red’ grapefruit expressing a cause all the important commercial to citrus canker and HLB (USDA, gene responsible for enhanced calcium cultivars originated from spontaneous 2019). The development of resistance to signals; the transgenic plants showed budsports or induced mutations. Cor- HLB, citrus canker, and other dis- strong resistance to root rot caused by azza-Nunes et al. (2002) used ran- eases is probably one of the most P. nicotianae and citrus canker. Evalua- dom amplified polymorphic DNA desired outcomes of a grapefruit ge- tion for HLB resistance is in progress. and simple sequence repeat markers netic improvement program. Even In studies conducted to understand cy- to study the genetic variability of though commercial transgenic cit- tokinin’s role in conferring resistance to grapefruit cultivars. Marsh Seedless, rus cultivars are not yet developed, pathogens, Pasquali et al. (2009) trans- Duncan, Thompson Pink, Foster, several successful genetic transfor- formed grapefruit with an isopentenyl- and Red Blush cultivars were 100% mation reports of grapefruit are transferase (ipt) gene known to increase like each other in the genomic re- available. Luth and Moore con- cytokinin levels. The expression of ipt gionsincludedintheanalysis.The ducted the first transformation of was driven by a floral promoter. The only grapefruit cultivars that showed grapefruit using epicotyl segments transformed plants had 30% higher polymorphism at the nucleotide level (Luth and Moore, 1999; Moore chlorophyll content in the fully expand- were accessions known to be hybrids et al., 2000). Yang et al. (2000) ed uppermost leaves compared with with grapefruit parentage, such as Tri- generated transgenic ‘Rio Red’ control plants. A 9- to 32-fold increase umph (C. aurantium), Royal grape- grapefruit plants expressing a CTV in endogenous concentration of trans- fruit, and others. Sharma et al. (2015) coat protein gene and an antiviral zeatin riboside (the most active form of studied the genetic diversity using mi- agglutinin gene from the snowdrop naturally occurring cytokinin) was ob- crosatellite markers and reported that plant (Galanthus nivalis). Febres served. Significantly higher expression ‘Star Ruby’, ‘Foster’,and‘Duncan’ et al. (2003) transformed ‘Duncan’ of specific pathogenesis-related genes were 100% similar genetically. grapefruit with genomic segments was also recorded. from CTV to obtain transgenic Genetic resources: Most plants with resistance. Among sev- Novel, powerful molecular techni- eral transgenic plants transformed ques are currently available for the ge- important cultivars netic engineering of plants. As these with the coat protein gene from dif- ‘DUNCAN’. This cultivar is histori- ferent CTV strains, one plant was techniques are established and per- cally significant because most grape- resistant, six were partially resistant, fected for citrus, and more high-quality fruit cultivars in the market today are and 27 were susceptible to the virus complete genome sequences become derived from Duncan (Fig. 7A). Al- (Febres et al., 2007). C¸evic et al. available, there will be new opportuni- though named in 1892, the mother (2012) reported the production of ties for successful manipulation of the tree was planted around 1830 (Hodg- transgenic ‘Duncan’ grapefruit ex- grapefruit genome. son, 1967). ‘Duncan’ is currently not fi pressing the wild-type and mutant Germplasm biodiversity and commercially signi cant because the RNA-dependent RNA polymerase fruit is very seedy; it was replaced by genes of CTV; the disease resistance molecular markers the more desirable seedless cultivar status of the transgenic plants has For over two centuries, grapefruit Marsh (Fig. 7B) and then by the pig- not been reported. CTV has several was presumed to be an interspecific mented mutants. As of 2000, Florida intra and intercellular silencing hybrid between a pummelo and a produced about 368 tons of ‘Duncan’
252 June 2021 31(3) Fig. 7. Most important grapefruit cultivars derived from the white seeded Duncan, first grapefruit planted in the United States, to the dark-red Rio Red grapefruit. (A) ‘Duncan’, (B) ‘White Marsh’, (C) ‘Ruby Red’, (D) ‘Rio Red’, and ‘ ’ (E) Star Ruby . Fig. 8. Grapefruit cultivar TR-1, a natural mutant of Rio Red, showing grapefruit per year designated solely important in Texas; it is also planted in external and internal coloration. for juice processing. Arizona, California, and Mexico. Small ‘MARSH’. Also known as ‘Marsh plantings are reported in Argentina, Is- Seedless’ and ‘White Marsh’,thiscul- rael, Turkey, South Africa, and Spain tivar was discovered in 1850 (Shamel (Da Grac¸a et al., 2004). et al., 1918), but it gained promi- ‘STAR RUBY’. It originated nence after 1886. It is seedless or through irradiation of ‘Hudson’ grape- nearly seedless, with internal colora- fruit of seeds with thermal neutrons. tion ranging from pale- to light-yel- This cultivar was released in Texas dur- low at maturity (Fig. 7B) (Hodgson, ing 1970. The fruit peel is yellow with 1967). ‘Marsh’ is still considered im- a slight tendency to red coloration and portant in most grapefruit-producing a clear red blush. The flesh color is countries; horticulturally superior cul- deep red, with more than three times fl ‘ ’ tivars with pigmented esh have now the pigmentation of Ruby Red ‘ ’ fl Fig. 9. A 5-year-old tree of TR-1 replaced the white- eshed types (Hensz, 1971) (Fig. 7E). Problems grapefruit showing fruit production (Saunt, 2000). with alternate bearing, sunscald, mi- inside the canopy. The rootstock was ‘RUBY RED’. Abudsportmuta- cronutrient deficiencies, and herbicides ‘C-22’ (Poncirus trifoliata 3 Citrus tion from ‘Thompson’ was discovered damage reduced cultivar Star Ruby’s sunki). in Texas in 1929 and patented in value in Texas. ‘Star Ruby’ is currently 1934. It became a significant com- commercially important in South Afri- mercial cultivar worldwide wherever ca, Israel, Australia, Cyprus, and Tur- because of its nutritional value and grapefruit was cultivated (Saunt, key (Da Grac¸a et al., 2004). the numerous medicinal properties at- 2000). ‘Ruby Red’ is the premier cul- ‘FLAME’. This cultivar is a seed- tributed to phytochemicals. Although tivar in Florida, with about 25,700 ling tree derived from Henderson grapefruit is less than 300 years in age, acres of bearing trees as of 2016 grapefruit, found in Texas, and re- an extensive range of cultivars with (USDA, 2017a). The fruit has very leased in 1987. Fruit is round with a different peel and flesh colors are now few seeds; the flesh has more pigmen- good appearance, the fruit rind is thin available; most of these originated tation than ‘Thompson’, and the peel with a red-bronze blush, and the flesh through natural or induced muta- has a reddish blush. The internal fruit coloration is red, similar to ‘Rio Red’. tions. Molecular marker studies with quality is like ‘Marsh’ except for pig- ‘Flame’ became important in Argenti- grapefruit cultivars indicate a high de- mentation pattern (Saunt, 2000) (Fig. na and Florida. gree of genetic similarity despite the 7C). ‘Redblush’ grapefruit also origi- ‘TR-1’. A new spontaneous mu- morphological and horticultural dif- nated as a natural mutant from tant derived from ‘Rio Red’ grapefruit ferences discernible among cultivars. ‘Thompson’ in 1929, but it did not was discovered in Texas and patented Because the somatic mutations cause attain the popularity of ‘Ruby Red’. in 2020 (Louzada and Del Rio, observable diversity in genetically sim- ‘RIO RED’. Originating as a bud- 2020). The rind and flesh are very ilar cultivars, grapefruit is considered a sport mutant from ‘A&I-1-48’,itisa red, and the flavor is mild with re- monocrop. The use of grapefruit as a grapefruit cultivar produced in Texas duced bitterness. The trees of ‘TR-1’ seed parent in conventional breeding by irradiation of Ruby Red budwood tend to produce numerous fruit clus- is challenging because of its complex with thermal neutrons. ‘Rio Red’ (Fig. ters inside the canopy (Figs. 8 and 9). biology; the polyembryony trait limits 7D) has a yellow peel with various de- Release for commercial planting is ex- the production of the hybrid progeny grees of reddish color developing pected by 2022. required to select superior cultivars. when fruit are shaded inside the cano- Biotechnology has been instrumental py. The flesh is deep red but lighter Conclusions and prospects in producing grapefruit cybrids and than ‘Star Ruby’,withflesh color fad- Grapefruit is the youngest mem- tetraploid somatic hybrids that are ing later in the season. The fruit tends ber of the citrus species; it has an in- useful for producing commercially to sheep-nose, developing a depressed triguing, almost romantic history, its valuable triploid cultivars by interploid button resulting in a snout-like appear- unique origin shrouded in mystery. It crosses. Because genetic transforma- ance (Patil, 2001). This cultivar is is considered a superior citrus fruit tion techniques are well developed for
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