Comparison of drought resistance of rootstocks 'M9-T337' and 'M26' grafted with 'Huashuo' apple

Drought stress is one of the main limiting factors in apple (Malus domestica Borkh.) cultivation. Rootstock plays an important role in the drought tolerance of apple plants. ‘M.9-T337’ is a novel apple rootstock that was recently introduced and widely cultivated in China. In this study, we selected the new, widely promoted Chinese apple variety ‘Huashuo’ as the scion and grafted it onto ‘M.9-T337’ (HM9). Another combination, ‘Huashuo’/‘M.26’/Malus robusta Rehd. (HM26), served as the experimental control to analyse drought resistance in the two hybrids. We believe that this empirical approach is more representative than merely studying rootstock seedlings. After sustained drought stress for over 1 month, the leaf relative water content had decreased in both types of plants, but to a lesser extent in HM26 than in HM9. The SPAD values increased in both plants, but without significant difference. Drought stress reduced photosynthetic activity in both plants, and the net photosynthetic rate was higher in HM26 than in HM9. The observed changes in the chlorophyll fluorescence parameters indicated that drought had damaged the PSII activity centres of both plants, photosynthetic electron transfer was inhibited, and excessive excitation energy accumulated. However, compared to HM26, HM9 displayed lower maximal PSII quantum photochemical efficiency and potential PSII activity. Moreover, HM9 showed lower antioxidant enzyme activity than HM26 under drought stress. A membership function analysis confirmed that ‘M.9-T337’ was less drought resistant than ‘M.26’. Nevertheless, ‘M.9-T337’ could still recover after prolonged drought stress, indicating it also had good drought resistance.