Effects of changing assimilate supply on starch synthesis in maize kernels under high temperature stress

High temperature stress (HT) significantly reduces maize yield by reducing the starch accumulation in kernels. However, the effect of HT on starch synthesis is controversial, whether by reducing the supply of assimilates or directly affecting the starch synthesis in the kernels. To clarify the potential mechanisms, a heat sensitive maize hybrid Xianyu 335 (XY) was subjected to 30/20°C (maximum/ minimum temperature, control) and 40/30°C for 7 consecutive days at the seed setting stage. Synchronous pollination (SP), apical pollination (AP) and shading treatments were adopted to change the inherent source-sink ratio in maize plant. Results showed that the weight of apical kernels reduced by 11.9% under 40°C in SP treatment. The 13C contents, starch accumulation and activity of cell wall invertase (CWIN) also decreased by 15.9, 36.7, and 16.4% respectively under HT. In shading treatment, 40/30°C caused even higher reduction in 13C contents, starch accumulation and CWIN activity because of the decreased assimilates supply. On the contrary, in AP treatment, starch contents and CWIN were increased 22.0 and 18.5% under 40/30°C, which caused similar weight and 13C contents with the kernels in SP and shading regardless of the temperature treatments. Consistent with the apical kernels in AP treatment, HT treatment did not have negative effects on the middle kernels both in the SP and shading treatments, which the kernel weight and starch content were not decreased under HT. All the kernels were exposed to the same environment with HT or control, but their responses to HT were different. The originally disturbed starch synthesis in the apical kernels under HT could be salvaged by increasing the carbon supply through AP treatment. The different performance between the middle kernels, apical kernels in AP and the apical kernels in SP or shading treatments meant that reduced carbon supply was a critical mechanism for the inhibited starch accumulation. Our findings provided a theoretical basis for further improving the understanding of kernel abortion under HT.