Asymmetric changes of day and night temperature have already been observed because of Climate Change. However, knowledge on environmental conditions either during day or night serving as trigger for growth processes is scarce. In this study, one rice (Oryza sativa) variety (IR64) was examined to assess the impact of varying temperatures and relative air humidities during day and night periods on biomass, leaf area, and dry matter partitioning between organs. Three different day and night temperature (30/20 °C, 25/25 °C, 20/30 °C) and relative air humidity (40/90%, 65/65%, 90/40%) regimes were established. The effect of relative air humidity on both plant dry matter and leaf area was larger than the effect of temperature, in particular low humidity had a strong negative impact during the night. With high day temperature, the shoot mass fraction increased, whereas the root mass fraction decreased. Specific leaf area increased at high night temperatures and led, along with the high leaf mass fraction at high night humidities, to higher growth rates. The results emphasize the importance of considering relative air humidity when focusing on plant responses to temperature, and strongly suggest that under asymmetric day and night temperature increases in the future, biomass partitioning rather than biomass itself will be affected.

Nitrogen (N) is one of the main nutrients that drive rice grain yield and is intensely managed especially in lowlands under irrigated conditions. A set of experiments was conducted in mid‐ and high‐altitude sites in Rwanda to investigate the response of five genotypes under different sowing dates and different N management. Genotype grain yields were higher and more stable at mid‐altitude across sowing dates. N rates strongly affected grain yield at mid‐altitude (p < .0001), but not at high altitude. Postponing basal N had positive effects on yield and yield components in both sites, with more pronounced effects at high altitude. Increasing N rate beyond 120 kg/ha led to a decrease in percentage of panicles per tiller and spikelet fertility and a decrease in grain yield due to excessive tillers at both high altitude and mid‐altitude. Thus, basal N application should be recommended at high altitude and the increase in N rate up to 120 kg/ha at mid‐altitude. A strict observation of recommended planting date should be followed at high altitude, and the use of cold‐tolerant genotypes is encouraged.

Background: In flooded rice fields, root zone temperatures (RZT) are buffered by the ponded water layer. With global warming, a higher frequency of hot days and hot nights, and the introduction of water-saving irrigation technologies, RZT are likely to vary more widely, particularly between night and day. Aim: It is not known how this will affect nutrient uptake of rice, particularly if the climate-driven transpirational demand increases simultaneously, since nutrient uptake at least partly depends on water uptake. Methods: We investigated the effects of day and night RZT on water and nutrient uptake and nitrogen (N) metabolism under low and high vapor pressure deficit (VPD). Plants of two rice varieties (IR64 and NU838) were grown hydroponically at three root temperature levels (19, 24, and 29°C). For a period of seven days, fresh weight of the plants, nutrient contents of the nutrient solution (${\mathrm{NH}}_4^{+}$, ${\mathrm{NO}}_3^{-}$, ${\mathrm{PO}}_4^{3-}$, K⁺), and water uptake were measured both at the end of the light period and at the end of the dark period. Nitrate reductase (NR), glutamine synthetase (GS), and amino acid (AA) concentrations in the youngest fully developed leaves were examined on the last day and night of the experiment. Results: The share of day and night uptake of ${\mathrm{NH}}_4^{+}$ and ${\mathrm{NO}}_3^{-}$ depended on RZT, whereas K⁺ uptake was higher during the day independent of RZT. Under low VPD, ${\mathrm{PO}}_4^{3-}$ uptake rate did not differ between day and night, however, under high VPD, the uptake of ${\mathrm{PO}}_4^{3-}$ varied between varieties and RZTs. Water uptake of the plants was strongly influenced by VPD, but not by RZT. In contrast, nutrient uptake was hardly influenced by VPD and did not correlate with water uptake, but linearly increased with RZT with an optimum temperature for nutrient uptake above 29°C. This increase was larger for ${\mathrm{NH}}_4^{+}$ and ${\mathrm{NO}}_3^{-}$ than for ${\mathrm{PO}}_4^{3-}$ and K⁺ shifting the nutrient requirements of rice. While the increase of nutrient uptake per °C did not differ between varieties under low VPD, IR64 showed a greater increase in nutrient uptake to RZT at day-time, whereas NU838 showed a greater increase at night-time under high VPD. The activities of NR and GS seemed to respond to the total daily N uptake rather than to different uptake rates during day or night, while AA concentration was strongly correlated to N uptake during the day. Conclusions: With an optimum RZT for nutrient uptake above 29°C, rice plants could benefit from temperature increase caused by either different water management strategies or climate change if fertilizer management was adapted to the new, shifted requirements.