Quantifying source-sink relationships of rice under high night-time temperature combined with two nitrogen levels

High night-time temperature (HNT) disturbs processes of both assimilate production (source) and assimilate accumulation (sink), and as a result substantially reduces yields of cereal crops. There have been reports that increasing nitrogen application can alleviate the negative impact of high-temperature stress on yield in rice (Oryza sativa L.). However, little is known about the interactive effect of HNT and nitrogen (N) supply on rice grain yield and its underlying source-sink relationships. We conducted two field experiments at the International Rice Research Institute in both the dry (DS) and wet (WS) season of 2012, in which three cultivars with contrasting responses to HNT were grown under two levels of night-time temperature and two levels of N application. HNT significantly decreased grain yield of cv. Gharib at both N levels and in both seasons, while grain yield of cv. PSBRc4 was significantly reduced by HNT at the higher N level only. Among the yield components, grain weight was consistently reduced by HNT in all three cultivars across two seasons while spikeletsm-2 and seed-set were affected by HNT during DS and WS, respectively. In most cases, higher N application reduced grain yield and its components. Thus, in our study, increasing the total N fertilizer did not alleviate the adverse effects of HNT on rice yield. Using a novel modelling approach that quantifies source-sink relationships during grain filling, we found that increased nitrogen did not alleviate the negative impact of HNT on source-sink interactions during grain growth across cultivars and seasons. Nevertheless, the model showed that there were significant differences among cultivars in grain filling duration, grain filling rate and total sink size, modulated by their source-sink relationship in response to HNT, suggesting that breeding programs should select for sink-related traits to improve rice tolerance to HNT