Rice night-time thirst: Genotype nutrient needs reflected in nocturnal transpiration

To enhance rice grain protein content, understanding strategies to improve nitrogen uptake is crucial. While the impact of transpiration on nitrogen flux is known in trees, its role in rice is unclear due to inconsistent results. Our study used a phenomics facility for real-time transpiration measurements during the entire crop growth period. We hypothesized that genotypes respond differently to transpiration regulation based on nitrogen needs. This study investigates the morphological responses and grain protein content (GPC) of two genotypes of rice, GEN-RIC_784 and GEN-RIC_384, under varying light and nitrogen conditions. GEN-RIC_784 exhibited lower reductions in biomass and total leaf area under limiting nitrogen and light compared to GEN-RIC_384. Both genotypes showed comparable reduction in biomass and leaf area when low nitrogen was combined with low light (LN + LL) condition. GEN-RIC_784 flowered early under low light, while GEN-RIC_384 did so only in LN + LL conditions. GEN-RIC_384 experienced significant yield reductions under all treatments except LN + LL, while maintaining high GPC compared to control. In contrast, GEN-RIC_784 showed a >50% reduction in GPC under low nitrogen conditions. Cumulative water transpired decreased notably only under LN + LL for both genotypes. GEN-RIC_384 had higher daytime transpiration declines across treatments and increased nighttime transpiration in CN + LL and LN + AL treatments. Daytime transpiration rates per leaf area were higher across treatments compared to controls. Water use efficiency decreased in both genotypes, most prominently under LN + LL. Across growth stages, transpiration trends varied, with notable increases under LN + AL and LN + LL. GEN-RIC_784 showed higher transpiration during vegetative stages, while GEN-RIC_384 showed higher nocturnal transpiration under CN + LL. Nitrogen supplementation affected shoot growth and chlorophyll content, particularly in GEN-RIC_384, with notable reductions when nitrogen was withheld at night. The study underscores the complex genotype-light-nitrogen interactions in rice, offering insights for enhancing rice productivity and grain quality under diverse environmental conditions.