Chlorophyll fluorescence and CO2 assimilation of black spruce seedlings following frost in different temperature and light conditions

Effects of artificial frosts on light-saturated photosynthesis (A(max)) and ground, maximal and variable fluorescence variables (F-o, F-m, and F-v and F-v/F-m) were monitored on 1-year-old foliage of black spruce seedlings (P icea mariana (Mill.) BSP) grown at high (25 degrees C), moderate(15 degrees C) and low (5 degrees C) temperatures and moderate (240 mu mol m(-2) s(-1) ) and low (80 mu mol m(-2) s(-1)) irradiances. Photoinhibition of 1-year-ol d foliage was greater in seedlings grown in moderate light than in seedling s grown in low light. Photoinhibition increased with decreasing growth cham ber temperature at both irradiances. Most changes in F-v/F-m were caused by changes in F-v. Exposure to -4 degrees C decreased both F-v/F-m and A(max) compared with control values. The effect of the -4 degrees C frost treatme nt was greater in seedlings grown in low light than in seedlings grown in m oderate light, probably because seedlings grown in moderate light were alre ady partially photoinhibited before the frost treatment. Following -4 degre es C treatment, neither F-v/F-m nor A(max) recovered in seedlings grown in low light. Light-saturated photosynthesis decreased with decreasing growth chamber temperature. Light-saturated photosynthesis was more sensitive to t he -3 and -4 degrees C frost treatments in seedlings grown at 25 degrees C than in seedlings grown at 15 and 5 degrees C. The A(max) of seedlings grow n at 15 degrees C was sensitive only to the -4 degrees C frost treatment, w hereas A(max) of seedlings grown at 5 degrees C was not sensitive to any of the frost treatments. Recovery of A(max) following frost took longer in se edlings grown at high temperatures than in seedlings grown at low temperatu res. For seedlings grown at the same temperature but under different irradi ances, both A(max) and F-v/F-m reflected damage to the photosynthetic syste m following a moderate frost. However, for seedlings grown at the same irra diance but different temperatures, A(max) provided a more sensitive indicat or of frost damage to the photosynthetic system than F-v/F-m ratio.