Mechanistic Basis for High Night Temperature Induced Carbon Imbalance and Yield Loss in Winter Wheat.

Greater increase in minimum night temperature than maximum day temperatures is recorded at the farm, regional and at global scale. A stronger correlation of minimum night temperature with historical grain yields necessitates characterization of mechanisms underlying high night temperature (HNT) induced yield losses in cereals. Based on preliminary results obtained from a controlled environment study using six diverse winter wheat genotypes, two genotypes (Tascosa and TX86A5606) contrasting for HNT response were selected for detailed physiological and biochemical analysis under optimum (15^oC) and high (23^oC) night temperature exposure. Night respiration and photosynthesis was recorded every other day starting from heading to maturity both under optimum and high night temperature. Plant tissue was sampled every other day starting from heading until 12 days after heading (DAH) and at maturity to quantify temporal changes in ATP and carbon partitioning in different plant parts. HNT induced a significant increase in night respiration compared to optimum night temperature (ONT) at 4 and 6 DAH in flag leaves of TX86A5606 (sensitive), with longer duration of exposure leading to acclimation with leaf carbon loss through respiration. However, HNT stress exposure at the whole plant level recorded >60% reduction in spike weight and grain yield of TX86A5606 compared to ONT. Additionally, grain starch concentration in TX86A5606 was significantly reduced under HNT over ONT, while grain protein concentration was unaffected by temperature treatments. Contrastingly, no such significant changes in night respiration, grain starch concentration and yield were noticed in Tascosa (tolerant) under HNT over ONT. To connect the spatial i.e. leaf to whole plant HNT exposure induced carbon loss through respiration, ATP and carbon partitioning (soluble and insoluble carbohydrates) in different plant parts is being assessed. These physiological and biochemical responses will be discussed in relation to high night temperature induced carbon imbalance leading to biomass and yield losses.