Individual and interactive effects of drought and heat on leaf physiology of seedlings in an economically important crop.

Heat waves in combination with drought are predicted to occur more frequently with climate warming, yet their interactive effects on crop carbon and water balance are still poorly understood. Hence, research on the capacity of crops to withstand and recover from the combined stress is urgently needed. This study investigated the effects of drought and heat wave on a crop species as well as the recovery from the combined stress. Seedlings were grown in growth chambers under two soil water conditions (i.e. well watered and drought stress) at ambient temperature (26°C) for 10 days. Afterwards, half of the seedlings were exposed to a 7-day 42°C heat wave. All the drought-stressed seedlings were then rehydrated upon relief of the heat wave. Leaf gas exchange, the maximum carboxylation capacity (V ), plant growth, relative chlorophyll content and leaf water potential were examined during the experimental period. The heat wave reduced leaf gas exchange rates, V and relative chlorophyll content, while it had no impacts on leaf water potential. In contrast, drought stress led to greater reductions in leaf gas exchange rates, growth and water potential than heat wave alone. Seedlings underwent a greater degree of stress in the combination of drought and heat wave than under the single drought treatment. The recovery of leaf gas exchange from drought stress lagged behind the water potential recovery and was delayed by heat wave. Our results show that drought stress had a predominant role in determining plant physiological responses and the negative impacts of drought stress were exacerbated by heat wave. The greater stress in the combination of drought and heat wave translated into the slower recovery of leaf gas exchange. Therefore, drought combined with heat wave may induce greater risks on crops under future climates.