Short-term heat stress at booting stage inhibited nitrogen remobilization to grain in rice

Extreme heat-stress events have become more frequent due to climate change often with devastating effects on rice production. Accumulation and translocation of nitrogen (N) in rice organs is an important process that determines rice yield and quality. To assess the effects of short-term heat stress during booting stage on the accumulation and remobilization of N in rice plant organs, a pot-experiment in phytotron chambers was conducted using two cultivars (Nanjing 41 and Wuyunjing 24) at mean temperatures of 27 degrees C, 31 degrees C, 35 degrees C and 39 degrees C for 2, 4 and 6 days at booting stage. The results showed that high temperatures of 35 degrees C and 39 degrees C for 4 and 6 days significantly increased N concentration in leaves, stems, panicles and grains. Severe heat stress strongly reduced N translocation from leaves and stems into grains, resulting in an increased N distribution in stems and leaves. Moreover, N translocation efficiency of the vegetative parts decreased with increasing heat degree-days (HDD), and when HDD > 21.5 degrees C d, N translocation efficiency was negative, indicating re-accumulation of N in vegetative organs under severe heat stress. N concentration in leaves was positively associated with photosynthetic rate. Dry matter partitioning index of leaves and stems was positively correlated with their N concentration, whereas grain dry matter partitioning index was negatively correlated with grain N concentration. Heat stress reduced the ratio of grain number to leaf area, thereby reducing the proportion of sink to source. These results suggested that the low N translocation efficiency under heat stress could be due to a decrease in sink capacity. Our findings demonstrate that projected climate warming is likely to induce a significant reduction in N accumulation in rice grains by inhibiting the translocation of N from vegetative organs to grains.