Varietal Differences in Chilling-Induced Depression of Photosynthesis and Leaf Growth in Cucumber Seedlings

Abstract

To estimate the varietal differences in chilling tolerance of cucumber (Cucumis sativus L.), young seedlings belonging to 20 varieties and grown at 25°C, were chilled at 4°C or 7°C for 2 days and changes in the photosynthetic rate and increase of leaf area were investigated.
The photosynthetic rate decreased more rapidly in the seedlings exposed to 4°C than in those exposed to 7°C, and the decline became more severe as the duration of chilling increased. The chilling-induced depression of photosynthesis was less pronounced in the spring varieties (except ‘Sagamihanjiro’, ‘Hokkyoku-2’ and ‘Ougonmegami-2’) than in the summer varieties.
The increase in the apparent leaf area (length×width of leaf) ceased during the chilling treatment, and the normal rate was not restored even after the temperature rose to 20°C. The increase in the apparent leaf area after chilling was more depressed in seedlings exposed to 4°C than in those exposed to 7°C, and was more suppressed in younger seedlings or true leaves. The chilling-induced depression of the increase in the apparent leaf area was less pronounced in the spring varieties than in the summer varieties.
There was a positive correlation (r=0.864, significant at 5% level) between the chilling-induced depression of photosynthesis and of the increase in the apparent leaf area. Both parameters were also positively correlated (r=0.763, significant at 1% level) among various varieties.
These results indicate that the chilling tolerance of cucumber can be estimated by analyzing the variations of photosynthesis and the apparent leaf area of cucumber plants exposed to low temperatures.