HORTSCIENCE 27(11):1201-1203. 1992. micronutrient elements added (Premier Brands, New Rochelle, N.Y.)]. Similarly, seedlings of citron and trifoliate orange were Ability of ‘Valencia’ Sweet Orange to grown for rootstock seedling freeze trials. Single-stem trees/seedlings were maintained Cold-acclimate on Cold-sensitive in a 50% shaded greenhouse under natural- day conditions. The greenhouse air ranged Citron Rootstock from 33C during the day to 16C at night. Maximum photosynthetic photon flux (PPF) G. Yelenosky and J.C.V. Vu was 1000 µmol·m-2·s-1 at the top of the trees, U.S. Department of Agriculture, Agricultural Research Service, and relative humidity ranged from a low of 34% during the day to 99% at night. Trees Horticultural Research Laboratory, 2120 Camden Road, Orlando, were watered every 2 days and fertilized FL 32803 monthly with a solution of 12N-2.6P-5K that Additional index words. Citrus, Poncirus, acclimation, freeze survival, carbohydrates, contained micronutrients. Plants tested were 96 to 130 cm tall with 0.8. to 2.1-cm trunk proline, supercooling diameters, 10 cm above the bud union for Abstract. Greenhouse-grown l-year-old sweet orange trees [Citrus sinensis (L.) Os- grafted trees, and 52 to 88 cm tall with 0.5- beck cv. Valencia] on cold-hardy trifoliate orange [Poncirus trifoliata (L.) Raf.] and to 0.8-cm midstem diameters for rootstock cold-sensitive citron (C. medica L.) rootstocks were exposed to cold-acclimation con- seedlings. A total of 160 seedlings and 80 ditions and freeze-tested at -6.7C for 4 hours in a temperature-programed walk-in budded trees for each rootstock was freeze- freezer room. Nonhardened trees generally did not survive the freeze, whereas cold- tested. hardened trees survived with no wood kill on either rootstock. Essentially, all leaves Test trials included uniform-appearing, died or abscised during the subsequent 5 weeks in the greenhouse. Freeze survival did greenhouse-grown plants that were cold ac- not separate rootstocks nor did supercooling in separate trials where Yalencia’ wood climated in controlled-environment rooms reached –8.8C before apparent nucleation. Increases in concentration of carbohy- described by Yelenosky (1979) and nonac- drates and proline and decreases in water content in Yalencia’ leaves during cold climated controls left in the greenhouse. Pro- hardening were generally associated with increased freeze tolerance. Other tests, that gramed tempertures, relative humidity, and matched 9-month-old seedlings of citron with trifoliate orange rootstock, showed clear light conditions during cold acclimation are differences in the superior cold acclimation of trifoliate orange over citron, which, footnoted in Tables 1 and 2. Concentrations however, performed better than expected. of carbohydrates, amino acids, water status, and osmotic potential of expressed sap in The ability of citrus trees to survive freezes could cause scion cold hardiness to prevail leaves were determined in duplicates from is often associated with the cold hardiness over extreme differences in rootstock cold prefreeze composite leaf samples of three trait of the rootstock (Hearn et al., 1963; hardiness. The results of this study add to leaves per plant using methods and proce- Paton et al., 1982; Yelenosky, 1985). Pre- the data base of the major U.S. juice sweet dures described by Yelenosky and Guy (1977, sumably, the less cold hardy the rootstock orange cultivar Valencia to cold, harden on 1982). Respective freeze tests were done in (tested as seedling trees), the more freeze an extremely freeze-sensitive rootstock, a separate room, adjacent to cold-hardening damage to the scion. There are exceptions ‘Etrog’ citron. This scion/rootstock combi- rooms, using standard operational proce- (Yelenosky and Hearn, 1967), and differ- nation is not commercially grown in the dures and lethal freeze limits (Yelenosky, ences in freeze damage may not clearly ex- United States and has never been freeze- 1976, 1991). There were no attempts to nu- press differences in cold hardiness of the tested. cleate the trees with cold water or any other rootstock in citrus field plantings (Gardner Single ‘Valencia’ trees were propagated agent during critical freeze tests. Variability and Horanic, 1963; Rouse et al., 1990; from state-registered budwood on very cold- in supercooling was determined in separate Wheaton et al., 1986). Such observations sensitive citron and very cold-hardy trifoliate tests on 12 grafted trees replicated on three suggest that much more study is needed to orange rootstock seedlings of open-polli- consecutive days. Also, bud unions »7 cm assess cause and effect in frozen citrus or- nated seed from single source trees; they were above soil level were unprotected. chards. Most of the freeze injury observa- grown in 15-liter plastic pots containing Pro- Results of freeze tests in tree damage clearly tions on citrus scions/rootstocks have been Mix [shredded sphagnum peat with equal parts separated cold-hardened from nonhardened incorporated into data bases that provide useful of vermiculite and perlite and macro- and trees on both rootstocks (Table 1). However, information to the industry and citrus repo- sitories for comprehensive assessment of Table 1. Concentration of various leaf components of ten 1-year-old ‘Valencia’ scions on either germplasm performance and genetic attri- trifoliate orange or ‘Etrog’ citron rootstocks before and after cold-hardening treatments, plus subse- butes. Such assessments are invaluable in quent freeze damage. addressing consumer demands, managing quality of existing inventories, developing economic strategies, and providing guide- lines for plant genome initiatives. The objective of this study was to deter- mine whether cold acclimation of young trees

Received for publication 14 Feb. 1992. Accepted for publication 1 July 1992. Mention of a trade- mark, warranty, proprietary product, or vendor does not constitute a guarantee by the U.S. Department of Agriculture and does not imply its approval to the exclusion of other products or vendors that may also be suitable. The cost of publishing this paper was defrayed in part by the payment of page zGreenhouse under natural daylight. charges. Under postal regulations, this paper y21C, 12-h day (»460 µmol·m-2·s-1 photosynthetic photon flux), 10C night, and 50% ± 5% relative therefore must be hereby marked advertisement humidity for 2 weeks immediately followed by 2 weeks of 15/4C. x solely to indicate this fact. Mean ± SD (n = 3).

HORTSCIENCE, VOL. 27(11), NOVEMBER 1992 1201 Table 2. Concentration of various components in 9-month-old cold-hardy trifoliate orange and cold-sensitive citron seedlings and percentage of 10 seedlings killed at each of four freezing conditions.

zTemperature decreased 1.1C/h, after 2 h equilibration at 4C, to respective minima and durations without light; thaw rates 1.1C/h to 4C. y Means ± SD (n = 3) for tissue analyses, and means ± SE (n = 10) for freeze kill. x21C, 12-h day (460 µmol·m-2·s-1 photosynthetic photon flux), 10C night and 50% ± 5% relative humidity. w15C 12-h day (460 µmol·m-2·s-1 photosynthetic photon flux), 4C night and 50% ± 5% relative humidity. differences in scion freeze damage of cold- in water content is needed to survive - 6.7C seedling trials, which indicated that citron acclimated trees did not separate cold-hardy for 4 h is not known. In contrast to no wood has considerable cold-acclimation potential trifoliate orange from cold-sensitive citron kill of cold-hardened ‘Valencia’ trees on ci- (Table 2). rootstock. ‘Valencia’ scion essentially was tron rootstock (Table 1), citron seedlings Cold hardiness ratings of rootstocks (tested killed without cold acclimation but survived showed considerable damage after a similar as seedlings) may not necessarily be defini- with no apparent wood damage after cold cold-hardening treatment and subsequent tive markers for tree cold hardiness (Young, acclimation on either trifoliate orange or freeze test (Table 2). Our data clearly indi- 1969), and a wide range of cold hardiness ‘Etrog’ citron. Apparent increases in solute cated that citron seedlings are more vulner- ratings of rootstocks may be applicable to concentrations and decreased water content able to freeze damage than are trifoliate orange management goals (Wheaton et al., 1990). that were found in leaves of cold-hardened seedlings. Citron’s apparent intolerance to Cold-hardening conditions used in this study trees compare favorably with known cold- freezing temperatures as a seedling was not may have been near optimal and, therefore, acclimation events in citrus (Yelenosky and expressed in budded trees such as ‘Valencia’ obscured differences in survival that might Guy, 1982). The differences found in solute orange on ‘Etrog’ citron rootstock in this result during less cold hardening on such a concentrations and water content between study. The general pattern of increased sol- wide range of rootstock cold hardiness. rootstocks are difficult to interpret because utes and decreased hydration during cold ac- Seemingly, rootstock differences in cold of no significant differences in freeze sur- climation was as evident in seedlings as in hardiness are best expressed in scions during vival of trees after cold acclimation. Higher budded trees. However, it is not known less-than-optimal cold hardening, and dif- solute concentrations and lower water con- whether the magnitude of such changes could ferences would become less noticeable with tent in leaves of cold-acclimated ‘Valencia’ account for the differences in freeze damage. increases in cold-hardening conditions just on trifoliate orange rootstock suggest a greater There was no indication in this study that before major freezes. freeze tolerance than trees on citron root- supercooling was a significant factor. All trees stock that survived equally as well. In this started to freeze, regardless of rootstock and Literature Cited instance, a temperature lower than -6.7C cold acclimation, from - 6.3 to - 7.3C mean Gardner, F.E. and G. Horanic. 1963. Cold tol- might have been needed to separate root- temperatures ±0.4C SE. The range of su- erance and vigor of young citrus trees on var- stocks that were not expected to have equal percooling for individual trees was from - 4.8 ious rootstocks. Proc. Fla. Hort. Soc. 76:105- scion survival after cold acclimation and to -8.8C, not uncommon for young citrus 110. subsequent freeze test. Leaf tissue analyses trees under controlled temperatures (Yele- Hearn, C.J., W.C. Cooper, R.O. Register, and apparently are not totally adequate to express nosky, 1991). However, nothing is known R. Young. 1963. Influence of variety and root- rootstock influence on scion freeze toler- about supercooling of mature citrus trees un- stock upon freeze injury to citrus trees in the ance, especially with young trees, which do der natural conditions. 1962 freeze. Proc. Fla. State Hort. Soc. 76:75- The ability of ‘Valencia’ scion to survive 81. not acclimate to the same level as mature Paton, D.M., G.I. Moss, and A.S. Carter. 1982. trees; thus, the results of this study may not -6.7C for 4 h equally well on rootstocks Frost damage in citrus. Austral. Citrus News apply to mature trees (Young et al., 1960). that represent extremes in citrus cold hardi- July, p. 2. Very little is known about rootstock influ- ness demonstrates that the effect of rootstock Rouse, R.E., E.D. Holcomb, Jr., D.P.H. Tucker, ence in citrus cold hardening other than an on scion cold hardiness is not as straightfor- and C.O. Youtsey. 1990. Freeze damage sus- apparent significant association with degree ward as is often assumed. There are in- tained by 27 citrus cultivars on 21 rootstocks in of damage in freeze situations (Rouse et al., stances where rootstocks were separated the budwood foundation grove, Immokalee. Proc. 1990; Yelenosky et al., 1981), and some as- statistically based on damage when young Fla. State Hort. Soc. 103:64-67. sociation with citrus “dormancy” (Young, trees were exposed to a freeze (Yelenosky, Wheaton, T.A., W.S. Castle, J.D. Whitney, 1970). We do not know the exact relation- 1976), and this raises the level of unknowns D.P.H. Tucker. and R.P. Muraro. 1990. A high density citrus Planting. Proc. Fla. State Hort. ship of solute concentration and water con- in dealing with rootstock influence in citrus Soc. 103:55-59. tent with freeze survival, although there are freeze survival. For example, the survival of Wheaton, T.A., J.D. Whitney, W.S. Castle, and significant correlations in field studies (Ye- the bud union on the assumed extremely cold- D.P.H. Tucker. 1986. Tree spacing and root- lenosky and Guy, 1982). How much of an sensitive-rated citron rootstock at - 6.7C for stock affect growth, yield, fruit quality, and increase in solute concentration and decrease 4 h was unexpected but was borne out in the freeze damage of young ‘Hamlin’ and ‘Valen-

1202 HORTSCIENCE, VOL. 27(11), NOVEMBER 1992 cia’ orange trees. Proc. Fla. State Hort. Soc. in the main stem of ‘Valencia’ orange trees. Yelenosky, G., R. Young, C.J. Hearn, H.C. Bar- 99:29-32. Cryobiology 28:382-390. rett, and D.J. Hutchison. 1981. Cold hardiness Yelenosky, G. 1976. Cold hardening young ‘Val- Yelenosky, G. and C.J. Hearn. 1967. Cold dam- of citrus trees during the 1981 freeze in Florida. encia’ trees on Swingle citrumelo (C.P.B. 4475) age to young mandarin hybrid trees and differ- Proc. Fla. State Hort. Soc. 94:46-51. and other rootstocks. Proc. Fla. State Hort. Soc. ent rootstocks on flatwood soil. Proc. Fla. State Young, R. 1970. Induction of dormancy and cold 89:9-10. Hort. Soc. 80:53-56. hardiness in citrus. HortScience 5:411-413. Yelenosky, G. and C.L. Guy. 1977. Carbohydrate Yelenosky, G. 1979. Accumulation of free proline accumulation in citrus. Bot. Gaz. 138:13-17. Young, R.H. 1969. Cold hardening in citrus seed- in citrus leaves during cold hardening of young Yelenosky, G. and C.L. Guy. 1982. Seasonal var- lings as related to artificial hardening condi- trees in controlled temperature regimes. Plant iations in physiological factors implicated in cold tions. J. Amer. Soc. Hort. Sci. 94:612-614. Physiol. 64:425-427. hardiness of citrus trees, p. 561-573. In: P.H. Young, R.H., A. Peynado, and W.C. Cooper. Yelenosky, G. 1985. Cold hardiness in citrus. Hort. Li and A. Sakai (eds.). Plant cold hardiness and 1960. Effect of rootstock-scion combination and Rev. 7:201-237. freezing stress: Mechanisms and crop implica- dormancy on cold hardiness of citrus. J. Rio Yelenosky, G. 1991. Supercooling and freezing tions. vol. 2. Academic, New York. Grande Valley Hort. Soc. 14:58-65.

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