The influence of chilling on photosynthesis and activities of some enzymes of sucrose metabolism in Lycopersicon esculentum Mill.

The effects of chilling stress on leaf photosynthesis and sucrose metabolism were investigated ill tomato plants (Lycopersicon esculentum Mill. cultivar Marmande). Twenty-one-day-old seedlings wore grown in a growth chamber at 25/23 degrees C (day/night) (control) and at 10/8 degrees C (day/night) (chilled) for 7 days. The most evident effect of chilling was the marked reduction of plant growth and of CO2 assimilation as measured after 7 days, the Latter being associated with a decrease in stomatal closure and an increase in Ci. The inhibition in photosynthetic rate was also related to an impairment of photochemistry of photosystem II (PSII), as seen from the slight, but significant change in the ratio of F-V/F-m. The capacity of chilled leaves to maintain higher qp values with respect to the controls suggests that some protection mechanism prevented excess reduction of PSII accepters. The results of the determination of starch and soluble sugar content could show that chilling impaired sucrose translocation. The activity of leaf invertase increased significantly in chilled plants, while that of other sucrose-metabolizing enzymes was not affected by growing temperature. Furthermore, the increase in invertase (neutral and acid) activity, which is typical of senescent tissue characterized by reduced growth, seems to confirm that tomato is a plant which is not a plant genetically adapted to low temperatures.