Effects of Shading on Photosynthesis, Dry Matter Partitioning and N、P、K Concentrations in Leaves of Tomato Plants at Different Growth Stages
不同生长阶段遮荫对番茄光合作用、干物质分配与叶N、P、K的影响

PAR (photosynthetic active radiation) is an important factor affecting tomato ( Lycopersicon esculentum Mill.) plant growth, development and yield. But high irradiation may not always be beneficial to the plant at all growth stages. The effects of two strengths of shading (40% and 75% increases in shaded area) for 8 days at three growth stages (i.e. early flowering (EF), peak flowering (PF) and late flowering (LF)) on photosynthesis, dry matter accumulation and allocation of dry matter to roots, stems and leaves, mineral nutrient (N, P, and K) in leaves and yield were measured. Different effects caused by shading at different growth stages were found: (1) Shading caused large increases in leaf conductance and intercellular CO 2 concentrations at all three growth stages, significantly reduced the leaf photosynthetic rates of EF stage, but the photosynthetic rates in 40% shaded leaves at PF stage were increased gradually depending on the length of shade time. This trend becomes more obvious and the average photosynthetic rates of 40% shaded leaves in an 8 day shading period at the LF stage was increased by 22%, (2) The 75% shading treatment at the EF and PF stages reduced the dry weights of root and stem tissues while the 40% shading at the LF stage caused a 22% to 43% increase, but shading did not affect leaf dry weight at most growth stages, (3) Shading at the EF and PF stages had no significant effects on total leaf N, P, and K concentrations, but 40% shading at the LF stage significantly increased the leaf N, P, and K concentrations by approximate 20%, 60% and 35% compared to unshaded controls. (4) Only shading at the LF stage significantly enhanced the marketable fruit yield of the 40% shaded tomato plants, but did not affect the fruit yield of shaded plants at the EF stage. These results suggested that the adverse influences caused by high irradiation and high temperatures could be overcome and a good growth, better dry matter accumulation and yield also could be obtained if some moderate shading was applied at the LF stage at the summer midday time, which might be useful in agriculture.