Photoperiod control of the initial phase of frost hardiness development in Pinus radiata

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Abstract
The development of frost hardiness in Pinus radiata was investigated to establish whether there is quantitative relationship between photoperiod and the hardening process. Three controlled environment experiments were carried out. In the first, seedlings were exposed to a photoperiod that shortened from 13 h at a rate of 3 min d−1 to 9.5 h. At intervals, the photoperiod was either held constant or lengthened. In the second experiment, seedlings were exposed to one of five constant photoperiods between 9 and 12 h for up to 90 d. In the third, seedlings were exposed to photoperiods shortening at rates of 1 or 5 min d−1. Frost hardiness was also measured during the natural photoperiod‐controlled stage of hardening in outdoor‐grown seedlings. Frost hardiness developed at a constant rate in response to a shortening photoperiod once it had declined to about 12 h. This rate was consistent with the hardening rate that occurred in outdoor‐grown seedlings. Hardening stopped when the photoperiod became constant, indicating a tight coupling between changes in photoperiod and hardiness development. When the photoperiod was held constant, the extent of frost hardiness was directly dependent on the photoperiod but the rate of hardening was apparently independent of the length of photoperiod. However, the rate of hardening was dependent on the rate at which the photoperiod shortened, increasing linearly with increases in the rate of change in photoperiod between 0 and 3 min d−1. These results suggest shortening photoperiods control the first stage of the hardening process by regulating the rate of hardening. Frost hardening was inherently unstable once the maximum hardiness was reached since spontaneous dehardening occurred in spite of the controlled conditions. Dehardening also occurred when the photoperiod was lengthened suggesting that the cue for dehardening to commence was the shift from shortening to lengthening photoperiods.