Ability of ‘Valencia’ Sweet Orange to Cold-acclimate on Cold-sensitive Citron Rootstock

Greenhouse-grown l-year-old sweet orange trees [Citrus sinensis (L.) Osbeck cv. Valencia] on cold-hardy trifoliate orange [Poncirus trifoliata (L.) Raf.] and cold-sensitive citron (C. medica L.) rootstocks were exposed to cold-acclimation conditions and freeze-tested at -6.7C for 4 hours in a temperature-programed walk-in freezer room. Nonhardened trees generally did not survive the freeze, whereas coldhardened trees survived with no wood kill on either rootstock. Essentially, all leaves died or abscised during the subsequent 5 weeks in the greenhouse. Freeze survival did not separate rootstocks nor did supercooling in separate trials where Yalencia’ wood reached –8.8C before apparent nucleation. Increases in concentration of carbohydrates and proline and decreases in water content in Yalencia’ leaves during cold hardening were generally associated with increased freeze tolerance. Other tests, that matched 9-month-old seedlings of citron with trifoliate orange rootstock, showed clear differences in the superior cold acclimation of trifoliate orange over citron, which, however, performed better than expected. Table 1. Concentration of various leaf components of ten 1-year-old ‘Valencia’ scions on either trifoliate orange or ‘Etrog’ citron rootstocks before and after cold-hardening treatments, plus subsequent freeze damage. The ability of citrus trees to survive freezes is often associated with the cold hardiness trait of the rootstock (Hearn et al., 1963; Paton et al., 1982; Yelenosky, 1985). Presumably, the less cold hardy the rootstock (tested as seedling trees), the more freeze damage to the scion. There are exceptions (Yelenosky and Hearn, 1967), and differences in freeze damage may not clearly express differences in cold hardiness of the rootstock in citrus field plantings (Gardner and Horanic, 1963; Rouse et al., 1990; Wheaton et al., 1986). Such observations suggest that much more study is needed to assess cause and effect in frozen citrus orchards. Most of the freeze injury observations on citrus scions/rootstocks have been incorporated into data bases that provide useful information to the industry and citrus repositories for comprehensive assessment of germplasm performance and genetic attributes. Such assessments are invaluable in addressing consumer demands, managing quality of existing inventories, developing economic strategies, and providing guidelines for plant genome initiatives. The objective of this study was to determine whether cold acclimation of young trees Received for publication 14 Feb. 1992. Accepted for publication 1 July 1992. Mention of a trademark, 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 charges. Under postal regulations, this paper therefore must be hereby marked advertisement solely to indicate this fact. HORTSCIENCE, VOL. 27(11), NOVEMBER 19 could cause scion cold hardiness to prevail over extreme differences in rootstock cold hardiness. The results of this study add to the data base of the major U.S. juice sweet orange cultivar Valencia to cold, harden on an extremely freeze-sensitive rootstock, ‘Etrog’ citron. This scion/rootstock combination is not commercially grown in the United States and has never been freezetested. Single ‘Valencia’ trees were propagated from state-registered budwood on very coldsensitive citron and very cold-hardy trifoliate orange rootstock seedlings of open-pollinated seed from single source trees; they were grown in 15-liter plastic pots containing ProMix [shredded sphagnum peat with equal parts of vermiculite and perlite and macroand Greenhouse under natural daylight. 21C, 12-h day (≈460 μmol·m ·s photosynthetic humidity for 2 weeks immediately followed by 2 w Mean ± SD (n = 3). 92 micronutrient elements added (Premier Brands, New Rochelle, N.Y.)]. Similarly, seedlings of citron and trifoliate orange were grown for rootstock seedling freeze trials. Single-stem trees/seedlings were maintained in a 50% shaded greenhouse under naturalday conditions. The greenhouse air ranged from 33C during the day to 16C at night. Maximum photosynthetic photon flux (PPF) was 1000 μmol·m ·s at the top of the trees, and relative humidity ranged from a low of 34% during the day to 99% at night. Trees were watered every 2 days and fertilized monthly with a solution of 12N-2.6P-5K that contained micronutrients. Plants tested were 96 to 130 cm tall with 0.8. to 2.1-cm trunk diameters, 10 cm above the bud union for grafted trees, and 52 to 88 cm tall with 0.5to 0.8-cm midstem diameters for rootstock seedlings. A total of 160 seedlings and 80 budded trees for each rootstock was freezetested. Test trials included uniform-appearing, greenhouse-grown plants that were cold acclimated in controlled-environment rooms described by Yelenosky (1979) and nonacclimated controls left in the greenhouse. Programed tempertures, relative humidity, and light conditions during cold acclimation are footnoted in Tables 1 and 2. Concentrations of carbohydrates, amino acids, water status, and osmotic potential of expressed sap in leaves were determined in duplicates from prefreeze composite leaf samples of three leaves per plant using methods and procedures described by Yelenosky and Guy (1977, 1982). Respective freeze tests were done in a separate room, adjacent to cold-hardening rooms, using standard operational procedures and lethal freeze limits (Yelenosky, 1976, 1991). There were no attempts to nucleate the trees with cold water or any other agent during critical freeze tests. Variability in supercooling was determined in separate tests on 12 grafted trees replicated on three consecutive days. Also, bud unions ≈7 cm above soil level were unprotected. Results of freeze tests in tree damage clearly separated cold-hardened from nonhardened trees on both rootstocks (Table 1). However, photon flux), 10C night, and 50% ± 5% relative eeks of 15/4C.