Effects and physiological mechanism of planting densities on photosynthesis characteristics of flag leaf and grain yield in wheat hybrid C6--38/Py85--1

Using wheat hybrid C6-38/Py85-1 and its parents (female C6-38 and male Py85-1) as materials, the effects and physiological mechanism of different planting densities on photosynthesis of flag leaf and grain yield were analyzed. At different density levels including low (L, 150 plants/m~(2)), middle (M, 300 plants/m~(2)) and high (H, 450 plants/m~(2)) densities, the net photosynthetic rate (P_(n)), chlorophyll concentration (Chl), soluble protein contents (Pro), stomatal gas conductance (g_(s)) and mesophyll gas conductance (g_(m)) in flag leaves were different and decreased with the leaf growth progress and density increase. Meanwhile, above parameters of flag leaves had much more sensitive response to densities in wheat hybrid than in its parents. The heterosis from mid-parent(H_(m)) was gradually increased with the leaf growth progress and density decrease. There existed a tightly relationship between the total CO_(2) assimilation amount during the flag leaf growth period (LSC) and the 1000-grain weight as well as their H_(m), which were decreased with the density increasing. The H_(m) of LSC in wheat hybrid at low and middle density were caused by the comprehensive improvement in average P_(n), LA and photosynthetic active function duration (RSP and PAD), while the H_(m) at high density came only from the improvement in LA. The grain yield under middle density was the highest, but there was no significant difference between those of middle and low densities. The H_(m) of grain yield in wheat hybrid was the highest under low density (41.7%), coming from the improvement of all three yield components, then under middle density (18.4%) and the lowest under high density (3.8%), in the case of middle and high densities, the H_(m) came mainly from the increase of kernel numbers per spike. This study indicated that it is a feasible strategy to develop and promote the heterosis of hybrid on a single plant basis in the production. Suitable decrease of the planting density in hybrid (150--225 plants/m~(2)) will be benefit for the construction of population structure, which has important function to fully increase the photosynthesis and dry matter accumulation in single plant, and finally the H_(m) of grain yield in wheat hybrid will be enhanced to a much higher level.