Nitrogen fertilizer application does not always improve available carbohydrate per spikelet but decreases chalkiness under high temperature in rice (Oryza sativa L.) grains

Abstract

The objective of this study was to understand the relationship between N fertilizer application and available carbohydrates per spikelet (ACPS) or amyloplast molecular chaperones (cpHSP70–2) on rice grain chalkiness. In two consecutive years (2020 and 2021), five rice cultivars with different photoperiod sensitivities, ACPS, and activity in cpHSP70–2 were grown in Kyoto, Japan under varied N fertilizer and temperature applications. The cultivars comprised near-isogenic lines (NILs) of Chinese japonica Liaojing 5 (LG5) with or without an erect panicle trait (EP or NEP) and Japanese japonica cultivars, Nipponbare, Kinmaze, and flo11–2 mutant with reduced activity in cpHSP70–2. The results showed that flo11–2 had the highest chalky ratio. The chalky ratio increased with the average of daily maximum temperature (Tmax) for 20 d after flowering and decreased with N fertilization in all cultivars. The chalky ratio was marginally higher in LG5-EP, possibly because ACPS was lower than in LG5-NEP. However, the grain projection area (grain area measured from a scanned grain image), which is a proxy of ACPS, correlated with the relative chalky ratio for the LG5-NEP and Japanese japonica cultivars at high temperatures, negatively and positively, respectively. The genotypically different responses of grain projection area to N fertilization, along with a negative correlation between ACPS and N dosage in NILs of LG5 reported in Idowu et al. (2022) suggest that ACPS is inconsequential for chalkiness. Meanwhile, N fertilization increased the cpHSP70–2 accumulation per grain, suggesting the involvement of cpHSP70–2 in lowering of chalkiness under N fertilization. • Chalky ratio (CR) of rice increased with the average of daily maximum temperature. • CR was the highest in the flo11-2 mutant and decreased by nitrogen (N) application. • Response of grain projection area to N dosages was different between genotypes. • Available carbohydrate per spikelet (source/sink) was inconsequential for chalkiness. • High N increased plastid molecular chaperone cpHSP70, which might reduce chalkiness.