Ameliorated Effect of Shading Treatment in Strong Light Period on Senescence and Photosynthesis in Cucumber 'Jinchun No. 5' Seedlings

To gain a physiological understanding of the effects of light intensity on the cucumber (Cucumis sativus L.), we subjected 'Jinchun No. 5' seedlings to shading treatment in three plots with light transmission of 100% (unshaded), 80%, and 60% in strong light periods for 45 days, and examined active oxygen species, senescence indicators and photosynthetic properties on the 15th, 30th, and 45th days of shading treatment respectively. The amounts of active oxygen species superoxide anions and hydrogen peroxide in leaves fell with decreasing light intensity on each of the 15th, 30th, and 45th days of shading treatment respectively. The amounts of chlorophyll and protein also decreased and that of malondialdehyde increased - all indicators of senescence - with decreasing light intensity on each of the 15th, 30th, and 45th days of shading treatment respectively. The photosynthetic property stomatal conductance in the 80% and 60% transmission plots exceeded that in the 100% transmission plot on the 15th and 30th days of shading treatment. No significant difference was seen in sub-stomatal CO2 concentration between all three light transmission plots. Both transpiration and net photosynthesis rates rose with decreasing light intensity. These results suggest that the shading treatment during strong light periods may suppress the generation and accumulation of active oxygen species, inhibit senescence and also ameliorate photosynthesis in cucumber 'Jinchun No. 5' seedlings.