Effect of shading on photosynthesis and growth in two Phaseolus aureus varieties

In order to determine the suitability of Phaseolus aureus as interplanting crops, field experiments and pot trials were conducted under full natural light, and 30 and 72% shading to test the shade tolerance of two Mung bean varieties, including Small Robinia-flower Round-leaf (SRRPA) and Big Lace-leaf Mung bean(BLPA) in 2015. The photosynthetic parameters, chlorophyll content, and RuBPCase activity in leaves of the Mung beans were measured during the first bloom stage. In addition, the response of growth and nutritional quality to weak light was studied. The results showed that, the net photosynthetic rate (Pn), stomatal conductance (Gs), transpiration rate (Tr), water use efficiency (WUE) and RuBPCase activity of BLPA significantly decreased by 24%, 18%, 12%, 13%, and 21%, respectively, than that under full light, whereas these parameters showed no significant change at 30% shading in SRRPA in response to shading. Photosynthetic parameters, including Pn, Gs, Tr, and WUE, in the two Mung bean varieties were significantly decreased by more than 11% under 72% shading. Vegetative growth of BLPA was significantly inhibited, but that of SRRPA revealed no significant change at 30% shading. In addition, vegetative growth of the Mung beans was significantly inhibited at 72% shading, and dry matter yield decreased by more than 34%, compared to those under full light. Shading treatments significantly reduced seed yield by over 37% compared to that under full light. These results indicated that SRRPA had the ability of resisting shade, and could tolerate 30% shading, which was suitable for intercropping.