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 (, , , 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 and depended on RZT, whereas K+ uptake was higher during the day independent of RZT. Under low VPD, uptake rate did not differ between day and night, however, under high VPD, the uptake of 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 and than for 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.

Implementation of water‐saving irrigation practices in lowland rice results in increased availability of nitrate (NO3−) in the soil and favours germination of upland weeds. Since plant species show a specific preference for either ammonium (NH4+) or NO3− as nitrogen (N) source, changes in both soil NO3− concentration and weed flora may affect the competition between rice and weeds. Further, the transpirational demand of the atmosphere might affect growth and competitiveness of lowland (wetland) and upland (dryland) weeds differently due to their adaptation to different ecological environments. Therefore, the study aimed to evaluate the effects of N source on growth, N uptake and competition between rice and common upland and lowland weeds under high and low vapour pressure deficit (VPD). Two rice (Oryza sativa) varieties (NU838 and KD18) differing in growth characteristics and two weed species (Echinochloa crus‐galli and Solanum nigrum) differing in their natural habitat were selected and grown hydroponically as monoculture or mixed culture at low or high VPD. N was supplied as 75%/25% or 25%/75% NH4+/NO3−. N uptake rates were measured in the first week, whereas dry matter (DM), N concentration in the plant, total N uptake and the activities of nitrate reductase and glutamine synthetase in the fresh leaves were determined two weeks after the onset of treatments. Independent of N source, both rice varieties and E. crus‐galli took up a larger share of NH4+, whereas S. nigrum took up a larger share of NO3−. N uptake of rice and E. crus‐galli was hardly affected by N source, whereas high NO3− led to significantly higher N uptake rates and total N uptake of S. nigrum. NU838 showed a higher competitiveness against weeds than KD18. In competition, high NO3− decreased the competitiveness of E. crus‐galli against NU838 but increased the competitiveness of S. nigrum against NU838. High VPD did not affect DM but increased N uptake of S. nigrum, leading to increased competitiveness of the weed at high transpirational demand. Competitiveness for N uptake appears to be an important trait as the relative N concentration in mixed plant communities was correlated with the activity of N‐assimilating enzymes and leaf growth, with a stronger response in rice than in weeds. Our results support the hypothesis that increased availability of NO3− in aerobic rice soils may be advantageous for the competitiveness of upland weeds, especially at high VPD, whereas it may be disadvantageous for common lowland weeds.

Background In anaerobic lowland fields, ammonium (NH4+) is the dominant form of nitrogen (N) taken up by rice plants, however, with the large expansion of water‐saving irrigation practices, nitrification is favored during drained periods, leading to an increased availability of nitrate (NO3−). Aim Since the uptake and assimilation of the two N‐sources differ in their demand of photosynthates, leaf gas exchange may be subject to adjustments in response to N‐sources, particularly at high evaporative demand, when stomatal conductance (gs) is very sensitive. Methods Three experiments were carried out to study leaf gas exchange of various lowland rice varieties in response to N‐source at low and high vapor pressure deficit (VPD). In the first experiment, seedlings of 12 rice varieties were grown at high VPD for 3 weeks. From this, four rice varieties differing in gs and CO2 assimilation rate (A) were selected and grown for 2 weeks at low VPD, and after that, they were shifted to high VPD for 1 week, whereas in the third experiment, the same varieties were grown separately at low and high VPD conditions for 2 weeks. In all three experiments, plants were grown hydroponically in nutrient solution with N‐sources as sole NH4+ or NO3−. Results At high VPD, NO3− nutrition led to a higher gs and A in four out of 12 varieties (IR64, BT7, NU838, and Nipponbare) relative to NH4+ nutrition, while no effect was observed at low VPD or after a short‐term exposure to high VPD. Further, varieties with a high intrinsic water‐use efficiency (WUEi; IR64 and BT7) showed the strongest response to N‐source. Higher gs was partially supported by increased root/shoot ratio, but could not be fully explained by the measured parameters. However, higher A in NO3−‐fed plants did not always result in increased plant dry matter, which is probably related to the higher energy demand for NO3− assimilation. Our results suggest that at high VPD, NO3− nutrition can improve leaf gas exchange in varieties having a high WUEi, provided a sufficient water supply. Conclusion Therefore, intensified nitrification under water‐saving irrigation measures may improve leaf gas exchange and the growth of rice plants under high transpirational demand. However, choice of variety seems crucial since large varietal differences were observed in response to N‐source. Further, breeding strategies for genotypes adapted to aerobic soil conditions should consider responses to NO3−, potentially using gas exchange measurements as a screening tool.