In order to determine the adaptability of legumes as the interplanting crops in fruit yards, field and pot experimental treatments with full natural light and weak light (48% of full natural light) regimes were conducted in 2014 to test the shade tolerance and physiological mechanisms of four legume species.

The leaf photosynthetic characteristic parameters, photosynthetic pigments contents and the activities of ribulose-1,5-bisphosphate carboxylase (RuBPCase) were measured during the first bloom stage. The responses of growth to weak light were likewise studied.

The results showed that the maximum net photosynthetic rate, light saturation point, light compensation point, apparent quantum yield and dark respiration rate of the four legume species changed into those of shade-tolerant plants under the weak light stress. The weak light reduced the net photosynthetic rate, stomatal conductance, transpiration rate, water use efficiency and RuBPCase activity of the legumes. In contrast, the weak light increased intercellular CO₂ concentration significantly. Contents of chlorophyll (Chl) a and Chl b in leaves of Vigna cylindrica (VCS) and Vigna radiata (VR) increased significantly, while Chl a/b in the leaves decreased dramatically after shading. Other two species changed photosynthetic pigments contents after shading. The weak light changed the growth of the four legume species, such as reducing stem diameter and branching quantity, reducing root nodule and root-shoot ratio, decreasing dry matter yield and dry matter accumulation efficiency, debasing leaf area and leaf thickness, reducing leaf quantity and leaf area index. Crotalaria assamica (CA) can not bloom under the weak light stress. Flowers were not strong enough to seed for Senna occidentalis (SO) under the weak light. The weak light also changed reproductive growth of VCS and VR significantly, such as debasing flowers quantity and flowering time, as well as decreasing seed yield. In conclusion, according to the responses of photosynthesis and growth to weak light regime in the legumes, we concluded that the shade tolerance ranking of four legume species from high to low is as follows: VCS, VR, SO and CA. VCS and VR are suitable for orchard interplanting. However, SO and CA are not suitable for orchard interplanting.