Low-temperature tolerance of photosynthesis and carbohydrate metabolic patterns account for the difference in tomato and cucumber cold tolerance

Tomato (Lycopersicon esculentum L., cv. Sibirskii skorospelyi) and cucumber (Cucumis sativus L., cv. Konkurent) plants were grown in soil culture at 18 degrees C, 5 klx, and a 16-h photoperiod to the stage of 3 to 5 leaves. The plants were exposed to 6 degrees C for 5 days at the same illuminance and photoperiod. Cold tolerance, CO2 assimilation rate, water and sugar contents in leaves, and ultrastructure of leaf parenchyma cells and chloroplasts were studied. Most of the cucumber plants did not survive chilling, while the tomato plants remained undamaged. In cucumber plants, photosynthesis was irreversibly inhibited by the end of the chilling period, while in tomato plants, photosynthetic activity persisted and subsequently recovered at 18 degrees C. In both species, chilling decreased leaf water content and the number of thylakoids per granum, whereas sugar content increased. The cucumber contained five times more starch grains per chloroplast section than the tomato. The area occupied by starch grains on the chloroplast section increased by five times in cucumber leaves after exposure to cold, while in tomato leaves, this area remained almost unchanged. The cells of both species had no lipid globules in cytoplasm or plastoglobules in the chloroplasts. It was suggested that the differences in cold tolerance of tomato and cucumber plants were caused by contrasting sensitivities of photosynthesis and carbohydrate metabolism to low temperatures.