USE OF POTASSIUM NITRATE ON FLOWERING

Mike A. Nagaol and Melvin S. Nishina2 lDepartment of Horticulture and 2Cooperative Extension Service, Hawaii County College of Tropical Agriculture and Human Resources University of Hawaii at Manoa

The mango ( indica L.), a member of Elisa and Davenport 1992). It was concluded that the Anarcardiaceae, is a popular fruit of the water stress was not responsible for flower tropics and occupies a position in the tropics induction, but could enhance the response to cool similar to that of apples in temperate regions. temperatures. Similar conclusions have also been Originating in the Indo-Burma region, the mango obtained by workers in Australia (Whiley 1992). has since spread to nearly all tropical areas of the Reliable flowering is necessary to obtain world. On the Indian subcontinent, it has been consistent mango production in the tropics. In under cultivation for at least 4,000 years. Hawaii, the Pacific, and the Caribbean, where land In the tropics, are usually grown at is a limiting factor, mango is a potentially elevations between sea level and 1,200 meters, but important fruit crop if increased production, does best below 610 meters in climates with a reliable bearing, and off-season bearing can be pronounced dry season. In Hawaii mango will achieved. In Hawaii, biennial bearing is common; grow best at elevations from sea level to 457 a heavy crop one year may prevent further fruiting meters (1,500 ft) (Chia et a!. 1988). Mangos grow for two years or longer. In the Hawaiian Islands, best at temperatures between 27 - 24°C and are leeward sections, which are drier during the winter susceptible to frost damage. Annual rainfall in months, are considered ideal for mango growing areas ranges from 25 to 250 cm. In the production (Yee 1979). wet-humid tropics, persistent rains and high humidity during flowering can cause a reduction in pollination and fruit set. Regulating Mango Flowering Flowers are borne on inflorescences (panicles) Tropical climates are conducive to year-round which are usually terminal, but panicles may also vegetative growth of perennial tropical fruit crops, arise from axillary buds. Flowers are either male but flowering and fruit set are usually seasonal. or hermaphroditic and may number from 300 to Flowering from one season to the next is 3,000 on each panicle, depending on cultivar. The unreliable, because the environmental signals for percentage of hermaphroditic flowers varies with flower initiation are often inconsistent, subtle, or cultivar and upon early or late emergence of the poorly defined. An alternative to dependance panicles (Chanda and Pal 1986). upon environmental signals for flower initiation is Flowering period in mango is mainly related to the development of management strategies that weather patterns and to some degree to cultivar can substitute for these signals. differences under the same climatic conditions In Hawaii, one method to extend the (Singh 1960, 1977; Whiley 1985). Flowering in availability of fruit within or slightly beyond the Hawaii usually begins in January, with fruit ripening period of May-June to September is by maturing from May-June through September growing different cultivars. There are usually some (Hamilton et al. 1992; Yee 1979). In the milder seedling and off-season fruits available at other southern and western regions of India, flowering times. Figure 1 illustrates the bearing pattern for begins in November or December; however, in important mango cultivars in Hawaii. Bearing northern areas where harsher climates prevail, patterns in Hawaii show that 'Harders' produces flowering is more precise and occurs later, in late-season fruits in the fall; '' and 'Rapoza' February or March (Chanda and Pal 1986). are also late season cultivars that mature from Flowering in the U.S. occurs between January and August through October (Hamilton et a!. 1992). In March and in the Philippines from December to Hawaii and other tropical areas with high rainfall, January. flowering, fruit set, and fruit quality are often Recent studies in Florida suggest that low diminished by pathogens favored by wet temperature is the environmental factor with the conditions that occur during or soon after the greatest influence on flower induction (Nunez- flowering season.

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• 'I" from "on"-year shoots, suggesting that failure to flower was associated with higher GA levels in shoot tips (Pal and Ram 1978). Analysis of the Jan Feb Mar Apr May Jun Jul Aug Sap Oct Nov Dec Jan xylem sap at various stages during shoot Cultivar Bearing Period development have shown that low gibberellin 0'///?>2 Ah Ping June - July Flowering June - July content was associated with periods during flower Harders June - September bud formation (Chen 1987). Application of ~ Manzanillo June - July Bearing MomiK June - July paclobutrazol, an inhibitor of GA synthesis, caused Excel July Gouveia July - August precocious flowering in trees and promoted Pope July - October White Pirie July flowering in bearing trees (Kulkarni 1988). This Keitt August - October Rapoza August - October finding supports the hypothesis that endogenous GA may play a regulatory role in mango by inhibiting flowering. The first studies to demonstrate that Figure 1. Flowering and bearing pattern of mango potassium nitrate could induce flowering of mango cultivars in Hawaii. trees were from the Philippines (Barba 1974, Bondad and Linsangan 1979; Bueno and Valmayor 1974). Flowering was evident within Success has been achieved in stimulating seven days after treatment and was effective on mango flowering with chemical treatments. In the shoots that were between 4.5 and 8.5 months old Philippines, smudging has been used to obtain when treated. Bondad and Linsangan (1979) earlier and increased flowering of '' and reported that concentrations of potassium nitrate '' mango (Dutcher 1972; Gonzales 1923; between 1 and 8 percent stimulated flowering of Madamba 1978). Ethylene has been identified as seedling 'Carabao' and 'Pahutan' trees and 'Pico' the active agent responsible for flowering during trees within one week after sprays were applied. smudging (Dutcher 1972). Smudging is done The treatment was effective for stimulating continuously for several days and is stopped if flowering of trees that had remained vegetative flower buds do not appear within two weeks. The well beyond normal bearing ages, for advancing process may be repeated 1-2 months later, but the flowering and fruiting periods, and for results are uncertain. The ethylene-generating breaking the biennial bearing habits of trees. agent, ethephon, applied at 125 - 200 ppm, Potassium nitrate is currently recommended in the induced flowering of 'Carabao' mango in the Philippines for inducing uniform flowering and for Philippines within six weeks after treatment the production of off-season fruits in the 'Pico' (Dutcher 1972). Flower induction also occurred at and 'Carabao' cultivars (Madamba 1978). In India, concentrations between 500 and 1,000 ppm; workers have reported variable results with however, defoliation was also experienced at the potassium nitrate (Pal et. aI1979). Areas that have higher concentrations (Bondad 1976). Ethephon reported success with potassium nitrate include has also been successful in India for increasing Trinidad with '' (James et al. 1992), flowering of ' and 'Deshehari' during "off' the Coast with '' and 'ZiU' (Goguey years (Chacko et al. 1972, 1974; Chanda and Pal 1992) and Mexico with 'Manila' and '' 1986) and for inducing earlier production in (Nunez-Elisea 1985; 1986). juvenile plants (Chacko et al. 1974). In 10-year-old In Mexico, studies by Nunez-Elisea (1986) 'Haden', 500-1,000 ppm applied one month before have shown that 'Haden' shoots should be six the normal flowering date increased flowering by months of age or older. In the case of 'Manila', 40 - 55 percent (Nunez-Elisea et al. 1980). These shoots could be as young as 3-4 months of age and results are contrary to those obtained by Pal et al. be responsive. Leaves should be dark green with a (1979), who found ethephon ineffective after five mature, "woody" texture and well developed consecutive years of treatment, and by Sen et al., terminal buds. Upon treatment with a 4 percent who reported an increase in flowering during "on" potassium nitrate solution, slight leaf wilting can years but failed to stimulate flowering during "off' be observed within two days, and at 10 days buds years. Applications of giberellic acid (GA) to begin to swell. A second application is made at 15- mango trees have been inhibitory to flowering 20 days after the first application if the response is (Bakr et al. 1981; Tomer 1984). Higher GA levels poor. Application should be made prior to have been extracted from "off'-year shoots than emergence of the flowers, because flowers are

- 62- Table 1. Percent terminals flowering after foliar Table 2. Percent terminals flowering after treatments of seedling mango trees at Kalapana, potassium nitrate foliar treatments of seedling Hawaii. Treatment date: Feb. 6, 1986. mango trees at Kalapana, Hawaii. Treatment date: May 29, 1986. Weeks after treatment 123 4 5 Weeks after treatment 234 5 6 Control 0.4 1.9 7.7 12.4 17.2 Potassium nitrate Control 0 0 0 0 0 2% 0 0.8 39.2 58.0 66.2 Potassium nitrate 4% 1.2 1.6 51.0 76.3 83.7 2% 1.3 4.4 11.1 12.1 13.1* Ethephon 4% 1.0 4.0 10.0 11.9 15.9* 1000 ppm 0 0.6 0.6 0.6 0.6 *Vegetative flushes also stimulated by potassium nitrate. usually damaged by the potassium nitrate sprays. Off-season flowering was also stimulated when Harvesting occurs at about five months after application was made to seedling trees in May treatment. Advancing the flowering season in after the flowering season was completed (Table Mexico has enabled growers to get fruits into the 2). Nearly 16 percent of the terminals treated with market at an earlier date, extend the harvest 4 percent potassium nitrate flowered by six weeks season, and harvest crops during the drier periods. after treatment. Our results also showed that Work in Mexico showed that mango flowering terminals that flowered were associated with could also be stimulated with ammonium nitrate specific trees; some trees in the test exhibited no sprays (Macias-Gonzales et a1. 1992; Nuiiez-Elisea response, while others produced vegetative 1988, Nuiiez-Elisea and Caldeira 1992). terminals after treatment. Concentrations of 2 percent ammonium nitrate These results suggest that potassium nitrate were sufficient to promote early flowering in did not induce flowering, but probably stimulated 'Haden', 'Tommy Atkins', 'Kent', 'Diplomatico' growth of terminal buds. Flowering was and 'Manila'. The similar results between determined by the condition of the terminal bud ammonium and potassium nitrate indicate that the or the environmental conditions at the time nitrate ion is the active portion of the molecule. potassium nitrate application was made. Our Experiments in Hawaii by the authors showed results with seedling trees also showed that that 2 and 4 percent potassium nitrate sprays genotypic differences among trees exist with applied to mature seedling trees early in the regard to flowering responses to potassium nitrate. flowering season (February, 1986) stimulated Some trees were highly responsive to the flowering (Table 1). A single application treatment and flowered, while others produced stimulated flowering within three weeks after vegetative shoots instead of panicles. treatment, and maximum response was observed Potassium nitrate application to mature at about four weeks. A stimulation in flowering 'Haden' trees in Pahala and Waimanalo also was not observed on terminals treated with 1,000 showed that flowering was stimulated in October ppm ethephon. and November (Table 3 and 4). Stimulation of

Table 3. Percent flowering of 'Haden' mango Table 4. Mango cultivar flowering response to terminals at three weeks after treatment with 4.0 potassium nitrate treatment. November 16, 1992; percent potassium nitrate spray at Waimanalo, Pahala, Hawaii. Hawaii. Treatment Date: Oct. 31, 19li6 Haden +++ Control o Fairchild + * Potassium nitrate 17.1 * Keitt

*Significant at P = 0.05. *Mixed panicles produced.

- 63 - flowering during these periods could enable chloroethylphosphonic acid (Ethrel) on producers to obtain fruits five months later mango, I. Flower induction in "off' year in (April), which would be about two months earlier Langra trees. Indian J. Horticulture 29:1-4. than the usual seasonal production in Hawaii. Chacko, E.K, RR Kohli, and G.S. Randhawa. Preliminary tests with other varieties show that 1974. Investigations on the use of 2- 'Fairchild' was not as responsive as 'Haden' to chloroethylphosphonic acid (Ethephon CEPA) applications made in November, while no for the control of bienniel bearing in mango. response was observed with 'Keitt' (Table 4). Scientia Horticulturae 2:389-398. The mode of action for potassium nitrate during Chacko, E.K., RR Kohli, RD. Swamy, and G.S. flower induction is not fully understood; therefore, Randhawa. 1974. Effect of 2- an explanation for the variable results between chloroethyphosphonic acid on flower induction cultivars and application periods remains unclear. in juvenile mango () The influence of endogenous GA levels (possible seedlings. Physiol. Plant. 32: 188-190. flowering inhibitors) and the interaction between Chanda, KL., and RN. Pal. 1986. Mangifera shoot age and environmental conditions on the indica. In AH. Halevy (ed), CRC Handbook response to potassium nitrate are not known. of flowering. CRC Press, Boca Raton, Florida. Critical to obtaining reliable seasonal and off­ p.211-230. seasonal flowering in Hawaii is the identification Chen, W-S. 1987. Endogenous growth substances of varieties that are responsive to potassium in relation to shoot growth and flower bud nitrate, determining the influence of application development of mango. J. American Soc. times and the type of shoots that will respond to Horticultural Sci. 112:260-363. potassium nitrate, and the development of Chia, c.L., RA Hamilton, and D.O. Evans. 1988. management strategies that force development of Mango. Hawaii Inst. Tropical Agriculture and responsive shoots at any period during the year. Human Resources, Commodity Fact Sheet To use chemical treatments effectively to control MAN-3(A). mango fruit production. application should be Dutcher, RD. 1972. Induction of early flowering assessed in relationship to the plant's growth in 'Carabao' mango in the Philippines by phenology. The overall objective of future studies smudging and ethephon application. should be to develop a management system that HortScience 7:343. ensures consistent seasonal yields. A successful Gonzales, L.G. 1923. The smudging of mango management system could stabilize production by trees and its effects. Philippine Agriculture eliminating biennial bearing and by altering the 12:15-27. time of production to off-season periods. Goguey, T. 1992. Results obtained with three different flower inducing chemicals applied to Literature Cited the mangos 'Kent' and ''. IV International Bakr, E.I., KM. Abdalla, M.A Meligi, and AI. Mango Symposium, Miami, Florida. p. 37. Ishmail. 1981. Floral differentiation as affected Hamilton, RA., c.L. Chia, and D.O. Evans. 1992. by growth regulators, ringing, and defoliation. Mango cultivars in Hawaii. Hawaii Inst. Egyptian J. Horticulture 8:161-166. Tropical Agriculture and Human Resources, Barba, RC. 1974. Induction of flowering of mango Information Text Series 042. by chemical spray. Crop Sci. Soc. Philippines James, M., D. Lennox, and L. Roberts-Nkruma. Proc.5:154-160. 1992. The effect of potassium nitrate and Bondad, N.D. 1976. Response of some tropical boron treatments on mango (cv. Tommy and subtropical fruits to pre- and post-harvest Atkins) flowering and fruit retention. IV applications of ethephon. Economic Botany International Mango Symposium, Miami, 30:67-80. Florida. p. 81. Bondad, N.D., and E. Linsangan. 1979. Flowering Kulkarni, v.J. 1988. Chemical control of tree vigor in mango induced with potassium nitrate. and the promotion of flowering and fruiting in HortScience 14:527-528. mango (Mangifera indica L.) using Bueno, P.B., and RV. Valmayor. 1974. Potassium paclobutrazol. J. Horticultural Sci. 63:557-566. nitrate: Key to mango flowering. Agric. Los Macias-Gonzales, J.L., V.M. Medina-Urrutia, and Banos 13:4. R Nuiiez-Elisea. 1992. Promotion of early Chacko, E.K, RR Kohli. and G.S. Randhawa. flowering in mango with ammonium nitrate 1972. Studies on the effect of 2- sprays as influenced by cultivar and shoot age.

- 64- IV International Mango Symposium, Miami, Whiley, A. W. 1985. Mango crop management - a Florida. p. 4l. review. Maroochy Horticultural Research Madamba, J.e. 1978. The Philippines recommends Station, 4th Report, 1984-1985. for mango. Philippine Council for Agriculture Whiley, A.W. 1992. Environmental effects on and Resources Research. phenology and reproductive physiology of Nufiez-Elisea, R 1985. Flowering and fruit set of a mango - A review. IV International Mango monoembryonic and a polyembryonic mango Symposium, Miami, Florida. p. 54. as influenced by potassium nitrate sprays and Yee, W. 1979. The mango in Hawaii. Univ. of shoot decapitation. Proc. Florida State Hawaii, Cooperative Extension Service, Horticultural Soc. 98: 179-183. Circular 388. Nufiez-Elisea, R 1986. Produccion temprara de mango 'Haden' y 'Manila' con aspersiones de nitrato de potasio. Centro de Investigaciones • Agricolas del Pacifico Centro Campo Agricola Q: Will any form of nitrate work? Experimental Tecoman. Foleto para A: We suspect that the nitrate is the active Productores No.8. part. Ammonium nitrate has been used in Mexico NUfiez-Elisea, R 1988. Nitrato de amonio: nueva because it is cheaper than potassium, and the alternativa para adelantar la floracion y concentrations used were the same, about two cosecha del mango. Centro de Investigaciones percent. Agricolas del Pacifico Centro Campo Agricola Dr. Davenport: Roberto Nufiez-Elisea has Experimental Tecoman. Desplegable para looked at many different salts, and it appears that Productores No.4. the nitrate is the active part. You can substitute Nufiez-Elisea, R, A.E. Becerriland, and A. the cation and still get the stimulatory effect as Martinez Garza. 1980. The effect of ethrel on long as the nitrate is present, but without the the flowering of mango cv. Haden. Chapingo nitrate, with potassium chloride for example, there 23/24:43-49. is no effect. Ammonium nitrate is cheaper but Nufiez-Elisea, R, and T. L. Davenport. 1992. you have to use about half the concentration, 2 Influence of water stress on vegetative growth percent, compared to the 4 percent routinely used and flowering of mango trees. IV with potassium nitrate. If more than 2 percent International Mango Symposium, Miami, ammonium nitrate is used, they get leaf bum. Florida. p. 42. _ A: There are some reports that urea has a Nufiez-Elisea, Rand M.L. Caldeira. 1992. Forcing marginal effect, but that may vary with locations. early flowering of 'Haden' mango with NH4S03 sprays. IV International Mango Q: What is the critical factor influencing the Symposium, Miami, Florida. p. 43. plant to change toward flowering or vegetative Pal, RN., KL. Chadha, and M.RK Rao. 1979. growth. Effect of different plant growth regulators and A: We need to speculate about that, but it is other chemicals on flowering behavior of likely temperature. Dr. Davenport's work mango. Paper presented at Mango Workers indicated that water stress is not the factor. The Meeting held at Panji, Goa, May 2-5, 1979. actual temperatures and durations required may Pal, S. and S. Ram. 1978. Endogenous gibberellins be specific to variety. The variation we observed of mango shoot tips and their significance in in response to spraying in a seedling orchard flowering. Scientia Horticulturae 9:369-379. indicates a strong genetic component. Sen, P.K, M. Bandopadhyay, S.S. Roy, and RN. Basu. 1973. Use of ethrel in controlling non­ Q: What has to be affected by the uniform bearing of mango. Indian Agric. temperature, the leaves, the roots, or both? 17:185-288. Dr. Davenport: The leaves. The promoter is Singh, L.B. 1960. The mango; Botany, cultivation in the leaves. and utilization. Leonard Hill Ltd., London. Q: Could you cool them by spraying with cool Singh, L.B. 1977. Ecophysiology of tropical crops. water at night? Academic Press, New York. p. 479 Dr. Davenport: I have not done any Tomer, E. 1984. Inhibition of flowering in mango experiments with that, but it is possible that by gibberellic acid. Scientia Horticulturae misting to bring about evaporative cooling could 24:299-303. have an effect.

- 65 - Q: How low do night temperatures get in the with mangos. But it is not true that fertilizing summertime in Hawaii? mangos with nitrogen will always promote A: Perhaps the low 70s on a clear night. vegetative growth and suppress flowering. Actually, nitrate applied to the ground also can Dr. Davenport: I can tell you that if you have stimulate flowering; if you stimulate the tree to a variety that responds to spraying, you can have a grow at a time when it is capable of flowering, it grove with rows that flower starting in early will flower. In Jamaica, a person who thought it October and with other rows stimulated to flower was the potassium that was stimulating flowering every two weeks afterward, on a schedule. In the was applying potassium chloride to the soil. He following year, you have the potential to move that fertilized his trees at the right time and got early schedule three months forward if you want to, flowering. This is why water stress will appear to because it will take about nine months to go work as well. If you have an extended period of through the reproductive process and get a new water stress with none of the flushing that might flush of leaves. This is being done. You can have occurred during that period, the leaves manage sections of your orchard to flower at any maintain their capability for induction and the time you choose. You must be careful, however, trees will flower when they are watered. about your rainy season, so that your flowers are not getting rained on. You have to be aware of Q: How can growers using paclobutrazol in your seasonal effects and the disease control you Central America export mangos to the U.S., where need to exercise to get quality fruits. the chemical is not approved for mango? Dr. Davenport: Actually, the grower I Q: Is there any regulation affecting our use of mentioned is exporting his mangos to Europe. potassium nitrate sprays? The people at ICI claim that there are no residues A: I spoke with the Hawaii Department of in the fruit, even if the chemical is applied through Agriculture about the legality of spraying the trees. the root system. This claim is consistent with I interpret it as applying foliar fertilizer, and there results from research with deciduous fruits like seems to be no problem with that. apples and pears. Under the conditions that we are proposing to use it, as a foliar spray during Q: Why does nitrate sprayed on the leaves vegetative growth phases, it is even less likely that stimulate flowering while nitrate applied to the it will be found in the fruits. As long as the ground stimulates vegetative growth? inspection agencies find no residue, then there will A: No one knows what the potassium nitrate be no problem exporting the fruits to the u.s. But does. It appears that this only works with mangos. there is always the risk, the possibility, that they Dr. Davenport: It has also been used with will find a residue and refuse a whole shipment. grapes, which is where the idea came from to try it

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