SMITH: BORON & ARSENIC ON GRAPEFRUIT 99 to be established for other, varieties that have Marsh grapefruit. Proc. Fla. State Hort. Soc. £6:62-65. 4. 1954. The effect of lead arsenate sprays on delayed maturity because of excess acidity. quality and maturity of Ruby Red grapefruit. Proc. Fla. State Hort. Soc. 67:38-42. 5. Metcalf, J. F., and H. M. Vines. 1965. Effects of ar Literature Cited senate on oxidative phosphorylation in grapefruit mitochon dria. Proc. Assoc. South. Agr. Workers, Inc. 62:242-243. 1. Deszyck, E. J. 1954. Effect of variable rates of lead 6. Miller, R. L. 1933. The effect of lead arsenate insecti arsenate on maturity and quality of Ruby Red grapefruit. cides on citrus fruit. Proc. Fla. State Hort. Soc. 46:57-63. Fla. Agr. Exp. Sta. Ann. Rpt. 156-157. 7. Reitz, H. J. 1949. Arsenic sprays on grapefruit in re 2. 1956. Leaf and fruit drop as related to bo lation to the new citrus code. Proc. Fla. State Hort. Soc. rax and leaf arsenate sprays on mature Marsh grapefruit 62:49-55. trees. Fla. Agr. Exp. Sta. Ann. Rpt. 161-162. 8. Smith, P. F. 1955. Relation of boron level to produc 3. , and J. W. Sites. 1953. The effect of borax tion and fruit quality of grapefruit and oranges. Proc. Fla. and lead arsenate sprays on the total acid and maturity of State Hort. Soc. 68:54-59. TOLERANCE OF TRIFOLIATE ORANGE SELECTIONS AND HYBRIDS TO FREEZES AND FLOODING George Yelenosky mature trees in the field; but the relative ratings of the cultivars tend to be similar between field U.S. Horticultural Research Laboratory and controlled-environment tests, even with un Agricultural Research Service budded young trees (14). In this study, the objec U.S. Department of Agriculture tives were to test the reaction of citrus-breeding Orlando selections to severe freezing and flooding and to R. T. Brown identify potential citrus-breeding material with cold-hardiness and flooding tolerance. Plaquemines Parish Experiment Station Fort Sulphur, Louisiana Materials and Methods and C. J. Hearn Selections of trifoliate orange '(Poncirus try foliata [L.] Raf.), trifoliate orange hybrids, and U.S. Horticultural Research Laboratory mandarins (Citrus reticulata Blanco) were used Agricultural Research Service in this study. Hybrids included citranges (C. U.S. Department of Agriculture sinensis [L.] Osbeck X P. trifoliata), citrumelo Orlando (C. paradisi Mac. X P. trifolata), citradia CPB 50097 (C.aurantium L. X P. trifoliata), citran- Abstract. Freezes and floods in Louisiana, darin (P. trifoliata X C. reticulata), citrangedin freezes in Georgia, and controlled-environment (citrange X Calamondin [C. reticulata var. aus- studies in Florida indicate seven selections of tsra X Fortunella sp.], and citrangor ([C. sinen trifoliate orange (Poncirus trifoliate [L.] Raf.), sis X P. trifoliata] X C. sinensis). Plants were a citradia (C. aurantium L. X P. trifoliata), and from open-pollinated seed, germinated in a glass a citrumelo (C. paradisi Macf. X P. trifoliata) are house. One- to 1 1/2-year-old plants were trans sources of relatively high tolerance of freezes and planted to the field and used in controlled-environ flooding. The highest yield of fruit/tree after ad ment tests. verse conditions was not always directly associ Field plantings were near Port Sulphur, ated with the tolerance of freezes and flooding. Louisiana, and Byron, Georgia. These sites were selected because they may have moderate to The effects of adverse environment on citrus severe freezes and/or floods. Rootstocks were trees sometimes can be lessened by appropriate tested as scion-rootstock combinations in Louis rootstocks (1, 3, 5, 8). In continuing studies of iana, primarily as unbudded plants in Georgia, cultivar evaluations, we tested citrus rootstock and as unbudded plants in controlled-environ selections as budded and unbudded trees in the ment tests in Florida. In Louisiana, budded tops field and as unbudded plants in controlled en were 'Owari' satsuma mandarin (C. reticulata) vironments. Results of controlled-environment and Washington' navel orange (C, sinensis,) tests with young plants do not apply equally to Buds were from validated sources. Test trees 100 FLORIDA STATE HORTICULTURAL SOCIETY, 1973 were randomized in blocks replicated 6 to 8 times, the group was selected for cold-hardiness condi with three single-tree replications per scion-root- tioning, and the remaining half of the group was stock combination. In Georgia, unbudded root- retained as glasshouse (unconditioned) controls. stocks of 65 to 200 plants each were transplanted Conditioning was either slight or moderate 2 ft apart in rows 20 ft apart. in controlled environment rooms 9 ft X 9 ft X 6 ft. Temperatures in the field were recorded with Slight hardening off, such as might occur hygrothermographs in standard weather shelters. naturally in central Florida, consisted of 70 °F Water above ground level was considered as (21.1°C), 12-hr days and 50°F (10°C) nights flooding. In Louisiana, tree tolerance of adverse for 2 consecutive weeks. Moderate hardening off, conditions was rated by tree survival counts 1 to similar to southern California conditions, con 6 months after freezing or flooding. Also, annual sisted of slight hardening followed by 2 consecu crop yields of surviving trees were determined tive weeks of 60°F (15.6°C) days and 40°F for 5 consecutive years immediately after adverse (4.4°C) nights. Light, both incandescent and weather conditions. In Georgia, survival counts fluorescent, was about 2,000 ft-c at the top of were made annually in April and October for 3 the plants. Temperatures were controllable ± 1°F years after unbudded rootstocks were planted. (0.6°C), relative humidity (RH) was maintained Some of the rootstocks included in field trials at 60%, and air movement averaged about 3/4 were also included in controlled-environment mph. tests. Controlled freeze tests included glasshouse Controlled-environment tests involved plants (nonconditioned) and conditioned plants simul potted in 3-liter cans filled with a mixture of 1 taneously. After treatments, plants were trans part sand, 2 parts vermiculite, 7 parts peat, and ferred to an adjacent controlled freeze room a trace of mixed fertilizers. Tests were made on preset at 35°F (1.7°C) and 50% RH. After 1 hr 1- to 1 1/2-year-old uniform plants in groups at 35° F, the temperature was decreased 2°/hr of 30 to 60 single-plant replicates. One-half of to 20°F. (-6.7°C) for 4 hr and then returned to Table 1» Field survival of unbudded citrus treesy after 3 consecutive winters in Georgia with re spective low temperatures of 16°, 3°, and 18° F -16,1°, and -7.8° C. respectively) Rootstock selections es planted Survival (No.) (%) Trifoliate orange2 2,775 100 Citradia CPB 50097 88 67 Citrumello 4475 80 54 Citrumello 4551 112 27 Yum6 citrange 132 19 (Temple X trifoliata) 48 12 Changsha mandarin 65 5 Shekwasha mandarin 202 1 Cleopatra mandarin 152 0 YTrees were 1 year old at time of first winter# Z75 plants of each of 37 different selectionso YELENOSKY, ET AL: TRIFOLIATE ORANGE HARDINESS 101 35° at 2°/hr. After 3 hr of gradual thawing, plants after three consecutive freezes in a flatwoods were placed in a glasshouse. Four weeks later, planting of different scion-rootstock combina foliage kill and stem dieback were rated. tions, more trees on 'Rusk' needed to be replaced than trees on either 'Carrizo' or 'Troyer' (10). Results and Discussion In Louisiana, trees on 'Rusk' were susceptible to freezing and flooding (Tables 2, 3). However, Selections of citrus rootstocks differed in tolerance to test conditions. This was apparent Table 2. Survival of 5-year-old Owari with both unbudded and budded trees exposed to satsuma trees on different rootstocks freezing and flooding in the field and with un 30 days after a 20°F freeze (-6.7°C) budded potted seedlings in controlled conditioning in February, preceded by 14°F (-10°C) and freezing. The selected rootstocks did not show in December, near Port Sulphur« La. marked differences in cold tolerance without hard- Rootstocks Survival ening-off treatments before the controlled freezes. (total of 12 ea) Also, temperatures as low as 3°F (—16.1 °C) did Trifoliate orange selections: 100 not separate selections of P. Trifoliata in Georgia, Rich 22-2 92 where 100% survived with little or no injury English small Towne F 92 (Table 1). The same selections were uninjured by English large 83 a 16°F (-8.9°C) freeze in Texas in 1962 (12). Rubidoux 56-6 83 Trifoliate orange selections, however, can be sep Jacobson 56-5 75 arated on the basis of vigor in Georgia and Swingle 75 Florida (11). Kryder medium 75 Trifoliate orange rootstocks of 'Owari' Christiansen 56-4 67 satsuma trees exposed to 14°F (-10°C) and 20°F Benecke 56-3 58 (-6.7°C) in Louisiana showed differences in tree Yamaguchi 56-7 58 survival (Table 2). Only trees on selection Rich Argentina 58 22-2 had 100% survival 1 month after the freezes. Chambers small 58 This selection is vigorous in Florida (11). P. Chambers large 42 trifoliata selections, of which more than 75% Chambers large 42 survived freezes in Louisiana, included 'English Towne G 25 small', 'English large', 'Towne F', and 'Rubidoux Citranges: 75 56-6\ 'Towne G* and 'Chambers large' were the Uvalde 50 only selections that had more than 50% of the Morton Carrizo 50 trees killed. Troyer 50 Differences in number of trees killed were Rusk 33 apparent among citranges as rootstocks of sat suma trees in Louisiana. Seventy-five percent of Table 3, Percent survival of 3-year- the trees on 'Uvalda' citrange rootstock survived, old Washington navel orange trees as did 50% on 'Carrizo', 'Morton', and 'Troyer', on citrange rootstocks after a hur- and 33% on 'Rusk' (Table 2).