Specific phytohormones levels in leaves and spikes of wheat explains the effects of elevated CO2 on drought stress at the flowering stage

This study aims to understand the combined impact of elevated CO₂ and drought stress at flowering stage to explain the adaptation of bread wheat to future climate change scenarios. Four wheat genotypes with 24 replications of each were grown in two different greenhouses, maintaining 400 (ambient) and 800 (elevated) ppm levels of CO₂. Irrigation was withheld at flowering to impose drought to 10 replications while 10 were allowed to grow normally. Daily water consumption was recorded until the pot-water of drought plants reached 10 % of the well-watered ones. This study was aided by the measurement of ecophysiology, phytohormones, and yield-related traits. In comparison to normal CO₂, plants consumed the pot water quickly under elevated CO₂. Further, the threshold value of the fraction of transpirable soil water, at which the relative transpiration is diverging from 1 was different at the two levels of CO₂, and among genotypes. Drought significantly reduced plant water relations, gas exchange parameters, grain yield, and yield-related traits but enhanced osmotic adjustment, kernel abortion, and most of the phytohormones in leaves and spikes. Elevated CO₂ though increased gas exchange parameters significantly under well-watered conditions but these parameters were significantly reduced under combined effect with drought and resultantly, lower yield-related traits were recorded. Moreover, we also identified a strong positive association between leaf trans-zeatin and a strong negative association of leaf and spike ABA and ACC with grain yield indicating that maintenance of a higher level of leaf trans-zeatin or lower levels of ABA and ACC can help plants to adapt better to the combination of elevated CO₂ and drought.