The relationship between the cold tolerance of tomato and cucumber organs and photosynthesis

Tomato (Lycopersicon esculentum L.) and cucumber (Cucumis sativus L.) were grown in a climatic chamber at an average daily temperature of about 20 deg rees C to the stage of the fifth (cucumber) and eighth (tomato) true leaves and then exposed to -2 degrees C. The carbon dioxide exchange of the middl e leaves was studied as related to the cold tolerance of the whole plants a nd separate organs. The temperature at which 50% of plants survived was -0. 3 and 0.7 degrees C for tomato and cucumber plants, respectively. The index of the injury of separate organs was calculated from the characteristics o f the electric conductivity of water extracts obtained from plant tissues. In the temperature range from 2 to -2 degrees C, the values of the injury i ndex of tomato and cucumber leaves were higher than those of the stems and roots. At -1 degrees C, the injury index of the cucumber stem was higher th an that of tomato. At temperatures from 2 to -2 degrees C, the injury index of cucumber leaves was, on average, by 15% higher than that of tomato leav es. In tomato leaves, the rate of apparent CO2 assimilation measured at 20 degrees C was about fourfold higher than in cucumber leaves (during the per iod preceding the measurements, the night temperature was 10-14 degrees C). Al a night temperature of 10-14 and 3-7 degrees C preceding the measuremen ts, the respective ratio of the true photosynthesis to dark respiration in cucumber leaves was two- and fivefold lower than in tomato leaves. The remo val of tomato fruits at the end of the growing period resulted in a decreas e in the plant injury index, which was most clearly expressed in the upper leaves. The results obtained are discussed in relation to the source-sink m echanisms of plant adaptation to cold and frost.