Nutrient uptake and assimilation under varying day and night root zone temperatures in lowland rice

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 (NH4+, NO3-, PO43-, 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 NH4+ and NO3- depended on RZT, whereas K⁺ uptake was higher during the day independent of RZT. Under low VPD, PO43- uptake rate did not differ between day and night, however, under high VPD, the uptake of PO43- 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 NH4+ and NO3- than for PO43- 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.