Drought is a lesser evil than cold for photosynthesis and assimilation metabolism of maize

Cold is one of the abiotic environmental factors that severely affect plant metabolism. It causes changes in the fluidity of biological membranes, induces oxidative and osmotic stress, photoinhibition, reduces photosynthetic rates and slows down numerous metabolic enzyme activities. Some effects of cold overlap with drought response processes. Drought itself has a dualistic effect on plants. Severe drought stress is detrimental, while a mild preceding drought may prepare the otherwise sensitive maize to cope with cold. This study focused on the effects of mild drought on photosynthesis, sugar and amino acid metabolism at low temperature. Maize (Zea mays L.) plants were drought-hardened (15±5% relative soil moisture) between stages V4 and V6 at 25°C for 10 days and then subjected to cold treatmet at 10°C for 2 weeks at two light levels (PPFD=450 and 150 µmol m^-2 s^-1). The control population was watered daily (35±5% soil moisture), and in completely dried pots, 5±3% soil moisture was measured. The present results confirmed that photosynthetic performance and glutamine biosynthesis were positively affected by drought under both light intensities. Plants exposed to combined cold and moderate drought stress had a higher dry weight than those exposed to cold stress alone. The metabolism of several sugars, organic acids and amino acids was maintained by mild drought acclimation. The cold protective DHN2-like dehydrin gene was induced by moderate drought treatment, which may have contributed to the drought-induced higher tolerance to low temperature. However, cold stress markers were lower in low light than in normal growth light under drought acclimation, suggesting that drought-induced stress defense may require a certain level of light.