Impairment of Photosynthesis by Chilling-Temperatures in Tomato

Chilling of attached tomato leaves (cv. Rutgers) in the dark for 16 hours at 1 C decreased both photosynthesis and transpiration. To separate the effects of chilling on stomatal CO(2) conductance from more direct effects of chilling on the chloroplasts' activities, measurements of photosynthesis and transpiration were made at atmospheric and saturating CO(2) levels. At atmospheric CO(2), the inhibition of photosynthesis was approximately 60%, of which about 35% was attributable to the impairment of chloroplast function and about 25% was attributable to decreased stomatal conductance. However, the affinity of the photosynthetic apparatus for CO(2) was not changed by chilling, since the dependence of the relative rate of photosynthesis on the intercellular CO(2) concentration was unaltered. The apparent quantum requirement for CO(2) reduction also was identical in chilled and unchilled plants. This observation contradicts the widely held notion that the chilling-induced inhibition of photosynthesis is caused by an impairment of the water oxidation mechanism. The impairment of chloroplast activity was not a consequence of an unfavorable water status within the leaf, since chilling caused only a small drop (1 bar) in water potential. A small loss of chlorophyll resulted as a secondary effect of chilling, but this loss of chlorophyll was eliminated as a cause of the inhibition of photosynthesis.No recovery of chloroplast activity occurred during the subsequent light period after chilling. The recovery seemed to be inhibited by light or to require both a light period and a dark period or to occur after a considerable lag period. After a period of both light and dark, restoration of stomatal conductance occurred more slowly than did the recovery of chloroplast activity.