Effects of moisture supply, temperature regime, and light on frost-hardiness changes in 2-year-old Douglas-fir seedlings were studied. Frost-hardiness was measured by an electrolytic method. Eight-hour light (3000ensp;ft-c) periods caused increase in hardiness, compared with 16-hour light periods, when seedlings grew under a 12-hour day/night temperature regime of 13 deg;C/7.5 deg;C. Long days retarded development of hardiness, under otherwise natural conditions in autumn, but did not prevent plants becoming frost-hardy later. Moisture supply had no direct effect on hardiness, although low supply may have reduced response to photoperiod. Development of hardiness was dependent on night temperature when day and night temperatures were in the range 7.5deg;ndash;24 deg;C, and lower night temperatures favored hardening. Loss of hardiness was not influenced by photoperiod.Seedlings did not harden in darkness at 2.5 deg;C over 9 weeks, but hardening occurred if low intensity light (40 or 100ensp;ft-c) was provided. Continuous low-intensity light caused more hardening than 8-hour light periods, and 100ensp;ft-c caused more hardening than 40ensp;ft-c. An exception was that seedlings previously exposed to 16-hour high-intensity photoperiods showed some ability to harden in darkness.Factors important in development of frost-hardiness were day length, night temperature, and light. Only temperature affected loss of hardiness.
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