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The daily course of photosynthesis at low temperatures in 2 coniferous species, Pinus ponderosa Laws., and Pseudotsuga menziesii (Mirb.) Franco, were studied using controlled environment facilities. After having been grown at a 23 degrees day, and 19 degrees night for a year, seedlings were acclimatized for 4 months to either a 3 degrees , 7 degrees or 11 degrees day all under 1200 ft-c of light and followed by a 16-hour night at 3 degrees . Measurement of photosynthesis at 1200 ft-c revealed 3 separate responses. First, the rapidity at which the plants attained their maximum photosynthesis when the lights were turned on depended upon the species, the current temperature, and the previous temperature condition to which the plants had become acclimatized. The warmer the day temperature the sooner the daily maximum was reached. Second, fluctuations in the rate of photosynthesis during the day varied with the species and the day temperature. Photosynthesis in both fir and pine kept at an 11 degrees day and pines kept at a 7 degrees day attained a daily peak rate followed by a decline. This decline occurred even though temperature and light were kept constant, the CO(2) level was returned to 320 ppm from 290 ppm, and the plants were kept well watered. At a 3 degrees day neither species showed this decline. Third, a plant transferred to another temperature acquired a new stable daily photosynthetic pattern. The number of days required for stabilization depended upon the previous temperature history of the plant. The adjustment rate was faster when the temperature was raised than when it was lowered.

Kinetics of the Daily Rate of Photosynthesis at Low Temperatures for two Conifers