Nitrogen and water supply modulate the effect of elevated temperature on wheat yield

Elevated temperature, water deficit, low nitrogen availability and their interactions, constrain wheat yield in Mediterranean-type environments. Our working hypothesis is that, owing to the non-linearity of yield response to water and nitrogen and the non-linearity of biological processes in response to temperature, the outcome of the interactions between temperature, nitrogen and water is range-dependent. To generate a broad range of conditions in testing this hypothesis, we established two experiments. Experiment 1 combined factorially four sowing times, six cultivars and four nitrogen rates in four locations-seasons. Experiment 2 combined factorially two cultivars, two nitrogen rates, and two thermal regimes to further untangle interactions. Thermal regimes were unheated controls, and crops heated with passive open-top heating chambers increasing temperature by 0.5 degrees C during grain set or grain filling. Across the 384 combinations of treatments in Experiment 1, yield ranged between 0.12 t ha(-1) and 5.96 t ha(-1). The duration of the critical period (300 degrees C d before anthesis to 100 degrees C d after anthesis) decreased with mean temperature at an average rate of 5 d degrees C-1 and accounted for 75% of the variation in yield. We used quantile regression to calculate a temperature-limited yield potential Y-T and derived a linear function between Y-T and mean temperature in the critical period, returning a slope of -0.53 t ha(-1) degrees C-1. Yield gap, i.e., the difference between Y-T and actual yield, was larger in nitrogen and water-deficient crops. Yield-temperature relationships crossed over in response to nitrogen fertilisation. Fertilised crops (100-200 kg N ha(-1)) outyielded their unfertilised counterparts when mean temperature during the critical period was below 13 degrees C, and unfertilised controls were superior above this threshold. Locally calibrated thresholds can be used as a rule-of-thumb adding a further dimension to the management of combined stresses and risk of wheat in Mediterranean-type environments. In Experiment 2, yield responded to the interaction between temperature and nitrogen, whereby elevated temperature during grain set reduced yield by 17% in unfertilised crops with no effect on crops with 100 kg N ha(-1). Elevated temperature during grain fill reduced yield by 14% with no effect of nitrogen or nitrogen x temperature interaction on grain yield.

Automated summary

In Mediterranean-type environments, factors such as elevated temperature, water deficit, and low nitrogen availability, as well as their interactions, constrain wheat yield. Through two experiments, interactions between temperature, nitrogen, and water were found to have a significant impact on yield in complex ways, with different factors affecting yield outcomes.