Effect of Osmotic Stress in Early Stages of Ontogenesis on Root Respiration, Growth, Sugar Content, and Cell Injury in Maize Seedlings Differing in Drought Sensitivity

Cultivars of maize (Zea mays L.) with different sensitivity to drought were exposed to 0.3 mol/L sorbitol (-1.4 MPa water potential) for 24 h. Exposure to water deficiency significantly reduced the growth of both shoots (coleoptile and hypocotyl) and roots. Shoot growth was inhibited more than the growth of roots.Osmotic stress enhanced accumulation of soluble sugars. Electrolyte leakage, a cell injury index, was slightly increased after 0.3 mol/L sorbitol. Respiration was measured in the presence and absence of 2,6-dichloro-phenol indophenol. 2,6-Dichloro-phenol indophenol did not influence respiration rates, because statistically equal results were observed under both conditions. Total respiration (VT) decreased after osmoticum treatment. There were no significant differences in the VT among the cultivars analysed.The decrease in vT was caused by a decline in the activities and capacities of both cytochrome (Vcyt, Vcyt) and alternative pathway (valt, valt) of respiration. A high residual respiration (vres) was observed, up to 27% of total uninhibited respiration. The result of uncoupler use clearly indicated that coupling was maintained after 24 h of osmotic stress. The recovery of the respiration rate was comparable with that of non-stressed control rates. According to these observations, no possible mitochondrial damage is expected. Water deficiency did not induce a stimulation of the alternative oxidase, so we assume that the stimulation of the alternative pathway is not related to drought stress resistance; rather, the function of the alternative pathway is to balance carbon metabolism and electron transport in a response to a changing environment.