Leaf micromorphology and sugar may contribute to differences in drought tolerance for two apple cultivars

Apple trees (Malus domestica L.) are often exposed to severe water stress in the summertime. We determined how levels of nonstructural carbohydrates (NC) changed in the leaves of two cultivars - drought-tolerant 'QinGuan' and drought-sensitive 'NaganoFuji' - when plants were subjected for 10 d to a sudden water deficit. Photosynthetic performance was investigated in terms of gas exchange and antioxidant enzyme activity. Two separate irrigation scenarios were tested with one-year-old plants grafted onto Malus hupehensis (Pamp.) Rehd. rootstock: 1) water applied to 75% field capacity (FC) (Control, CK) and 2) irrigation withheld to achieve <75% FC ranging from 30 to 75% (Treatment, T). At Days 0, 1, 2, 4, 6, and 10 of the experiment, we recorded net CO2 assimilation, stomatal conductance, leaf transpiration rates, and intercellular CO2 concentrations. All of those parameters showed decreases in plants from both stress scenarios, although those declines were not as dramatic in 'QinGuan'. The photosynthetic rate reduced primarily because of stomatal closure. In both cultivars, water stress induced the accumulation of NC, especially sorbitol, suggesting that this polyol has a role in osmoregulation. The rise in sorbitol levels was much steeper in 'QinGuan'. Likewise, the interaction between superoxide dismutase and peroxidase activities varied between cultivars. In 'QinGuan', the malondialdehyde concentration was much lower and more closely correlated with a high Suc/NC ratio when compared with 'NaganoFuji'. Scanning electron microscopy revealed a drought-resisting apparent structure in 'QinGuan'. Overall, our results from both external and internal examinations demonstrated that 'QinGuan' is more drought-tolerant than 'NaganoFuji'.