The effect of freezing nights on photosynthesis, stomatal conductance, and internal CO₂ concentration in seedlings of Engelmann spruce (Picea engelmannii Parry)

The effect of freezing night temperatures on net photosynthesis, stomatal conductance, and internal CO 2 concentration was investigated in unhardened seedlings of Engelmann spruce. Exposure to – 2.5°C in the dark for 10 h caused a slight and reversible reduction in gas‐exchange parameters on the following days. Substantial and irreversible inhibition of photosynthesis occurred after exposure to ‐4°C or –5°C. Despite a parallel decline in stomatal conductance and net photosynthesis, exposure to a hard freeze caused a decrease in the stomatal limitation to gas exchange. Hard‐freeze conditions (less than – 4°C) also caused a decrease in carboxylation efficiency and apparent quantum yield, indicating a freeze‐induced failure of the dark reactions and electron transport. There was no significant difference in the photosynthetic response to freezing temperatures in different elevational populations of spruce, although acclimatory adjustments were observed. Gas exchange in seedlings grown under cool conditions (14°C day/9°C night) was less affected and recovered more rapidly after exposure to a hard freeze than in seedlings grown under warm conditions (24°C day/19°C night).