Study on Photosynthesis of Leaves at Different Positions of Cucumber in Solar Greenhouse

The Photosynthetic rate (Pn) and carboxylation efficiency (CE) of leaves at middle position of cucumber in solar greenhouse were the highest, secondly were the leaves at upper and middle to lower positions, and the leaves at lower position were the lowest. The saturation light of the leaves at middle position were the highest either, but the light compensation point of the leaves at upper, middle and lower positions were debasing as the leaf position descent. In the growing period, the Pn of all leaves were increasing as PFD increased. The diurnal variation of Pn in leaves at different positions were one peak pattern curve, and the peak appeared at 12 o'clock. With the increase of the plant density, the PFD of different layer leaves were decreased, especially the leaves at middle to lower and lower positions. Effect of plant density on the Pn of upper position leaves was not markedly, but on the light compensation point was obviously. The Pn, saturation light of lower position leaves were decreasing as the plant density increased, but the effect of plant density on the light compensation point was not markedly. Leaves of cucumber had certain capacity of adaptation and adjustment to light. The weaker the PFD, the higher the apparent quantum yield (AQY). So the AQY increased as the leaf position descended.