Cold Hardiness Comparison of Young Trees of 'ambersweet' with 'valencia' Orange during Controlled Freezes

Two-year-old 'Ambersweet' ( Citrus reticulata Blanco X [ C poroc/f'siMacf. X C. reticulata]) X midseason orange, C. s/nensfs(L.) Osb. trees on three rootstocks, sour orange (C. aurantium L), Cleopatra mandarin (C. reticulate), and Carrizo citrange (C. sinensisX Poncirus trifoliate* (L) Raf.) grow ing in 15-liter pots were exposed to cold-hardening regimes in controlled-temperature rooms and freeze-tested at -6.7°C and colder for three hours. The ability of 'Ambersweet' to cold harden did not exceed that of control variety trees, 'Valencia' orange ( C. sinensis[L.) Osb.) based on degree of freeze injury, freezing behavior, and biochemical changes in the tissues. Differences in the above factors were significant only between hardened and unhardened trees of both varieties. Results suggest 'Ambersweet' is a moderately cold-hardy type similar to 'Valencia' orange. The 1989 citrus release, 'Ambersweet' orange hybrid, developed in the ARS/USDA breeding program at the A. H. Whitmore Foundation Farm (2), is expected to be planted extensively throughout the Florida citrus industry in the next few years. The performance of 'Ambersweet' trees to date has been encouraging in both productivity and freeze survival. However, years of observations will be needed to make final evaluations on the role and perform ance of 'Ambersweet' under different field conditions and stresses in the industry. Until such time, initial observations on performance of young trees, as new plantings are estab lished, are extremely important to help make decisions in managing 'Ambersweet' in the Industry. This report de scribes the freeze tolerance of young 'Ambersweet' trees during a series of freeze tests for the purpose of expanding freeze survival information critical to site selection for new plantings. Materials and Methods Trees. Test trees were developed on 3 different rootstocks, Carrizo citrange, Cleopatra mandarin, and sour orange, which were used in the original development of 'Ambersweet' trees (2). Single trees propagated from A. H. Whitmore Foundation Farm budwood on rootstock seed lings from open-pollinated seed were grown in 15-liter plastic pots containing Astatula fine sand Florida field soil. Single-stem trees were maintained in a 50% shaded greenhouse under natural-day conditions. Air tempera tures in the greenhouse ranged from 32°C during the day to a 20°C minimum at night. Maximum photosynthetic 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. Proc. Fla. State Hort. Soc. 104: 1991. photon flux density (PPFD) was approximately 1000 junol-s^nr2 at the top of the trees, and relative humidity (RH) ranged from a low of 35% during the day to 98% at night. Trees were watered every 2 days and fertilized monthly with a solution of 12N-2.6P-5K liquid that con tained micronutrients. Test trees ranged in height from 95 to 112 cm, and 0.9 to 1.1 cm in stem diameter, half the length of the scion above the budunion. Cold hardening. Equal number and uniform appearing trees of each scion variety on common rootstocks were ar bitrarily assigned simultaneously to 4 weeks of cold har dening and 4 weeks of nonhardening in separate program med temperature rooms. Cold hardening consisted of 20°C 12-hr days with approximately 450 |xmol-s-um-2 PPFD at the top of the trees and 10°C nights for 2 consecu tive weeks, followed by 2 weeks of 15°C days and 4°C nights. Nonhardening was at similar light conditions with 30°C days and 20°C nights for 4 weeks. Relative humidity fluctuated from 40% to 60% in both rooms during harden ing treatments, and trees were watered daily. Immediately following the 4-week cold hardening, a similar group of trees was used in a 1-week cold hardening scheme where temperature was abruptly lowered 5°C every 24 hr, going from 30°C to 2°C in 6 days. All other conditions remained the same except a greenhouse was the nonhardening treatment in this instance. This treat ment was imposed to estimate the responsiveness of the two varieties to a sharply contrasting temperature regime than the standard 4 weeks known to induce significant cold hardiness in container-grown citrus trees (7). Respective greenhouse trees, in addition to those used above, were used as controls throughout the study. Freeze tests. Trees of each variety on common rootstocks from different cold-hardening treatments were tested at -6.7°C and -8°C for 3 hr, and -10°C for 1 hr. Tests were done in a separate controlled-temperature room with 40% to 60% RH and no light. Trees were temperature equilib rated at 2°C for 3 hr immediately before temperature was lowered at a rate of 5°C per hr to minimum lows and du rations, and thawed at 5°C per hr to original starting tem perature of 2°C. Freeze-tested trees remained at room tem perature (approximately 25°C) for 3 or more hours before being returned to the greenhouse for 5 weeks of observa tions on freeze injury. Tests were done on successive days, at temperatures of-6.7°C, -8°C, and -10°C, respectively. Injury was based on the number of leaves that were killed or abscised from the original number and the length of wood dieback relative to total length of scion. The approx imate temperature of the scion-stem and apparent mo ment that ice started to form in the wood (nucleation tem perature) were based on the release of latent heat of freez ing (exotherm). This was indicated by copper-constantan thermocouples (36 gauge, 0.12 mm diameter) inserted 2 to 3 mm into the central vascular system of a leaf trace at mid-stem level. Thermocouples were connected to an auto mated data collection system accurate to ± 0.1°C at 22°C ambient and a resolution of 0.015° (8).