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Shaoli Lu and Mark Rieger

One-year-old kiwifruit [Actinidia deliciosa (A. Chev.) C.F. Liang et R. Ferguson var. deliciosa] vines were grown under 8- and 16-hour photoperiods to study the influence of photoperiod on cold acclimation and determine the potential level of hardiness that young vines attain. Vines were acclimated by reducing growth chamber temperature at 2-week intervals, beginning at 31/20C (16 hours/8 hours) and ending with 15/5C after 8 weeks. Vines receiving an 8-hour photoperiod were more cold hardy than vines receiving a 16-hour photoperiod after 4 weeks of acclimation as determined by electrolyte leakage from stem tissues. Moreover, vines receiving an 8-hour photoperiod survived freezing at – 9C at the end of the 8-week acclimation period, whereas those receiving a 16-hour photoperiod were killed at – 6C. Vine survival and electrolyte leakage of sterns were highly correlated (r = – 0.79 to – 0.90).

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Shaoli Lu, Mark Rieger, and Michael J. Duemmel

Ovary temperatures of upward and downward facing flowers of `Junegold' Peach (Prunus persica (L.) Batsch) were measured on 5 nights in March 1991 to determine whether differential survival of ovaries following frost was related to flower orientation. Flowering twigs were removed from mature trees and positioned horizontally ≈ 1.5 m above ground level prior to occurrence of low temperatures (0-5C). Thermocouples were inserted through the hypanthium to contact ovaries of 10 upward and 10 downward facing flowers, and temperature and meteorological data were logged every five minutes. Under clear, calm conditions, temperature of upward facing flowers averaged 0.33C lower than that of downward facing flowers during the coldest period of the night, with maximal differences of 0.77C. Under cloudy, calm conditions, temperature differences between upward and downward facing flowers were less frequently observed and lower in magnitude (0.08 - 0.15C). Under windy conditions (>2.5 m/s), no temperature difference between upward and downward facing flowers occurred, despite strongly negative net radiation. Based on known values of ovary cold tolerance, it is concluded that differences in survival of Up to 38% could occur due to flower orientation when air temperature reaches critical values.