Drought Tolerance of 'Fuji' Apple Trees Grafted onto G, CG, or M Series Rootstocks: Growth and Physiology

In this study, we assessed the drought tolerance of 1-year-old apple (Malus domestica Borkh.) trees exposed to extreme water stress, focusing on growth and physiological responses. After grafting 'Fuji' variety onto M26 and M9 rootstocks or Cornell-Geneva rootstocks (G11, G202, G214, G935, CG4814, and CG5087), the grafted plants were planted in 17-L pots and managed in a greenhouse. After irrigation was stopped, soil water potential dropped to about -700 hPa in 12 days. The leaf water potential and tree vertical growth rate of drought-stressed trees decreased dramatically, and the decreased growth rate of the G202, G935, and M26 trees, which generally produced a large leaf area, was considerable. Trunk cross-sectional area and leaf area of all trees under drought stress were reduced by almost 50% or more than 60%, respectively. Drought-stressed trees were subjected to control-level irrigation during the recovery of about 11 days, but their leaf water potential still did not fully recover to the control level. Photosynthesis-related parameters also showed a substantial decrease as the soil water potential changed, but the difference among trees was not noticeable. Fine root (<2 mm diameter) dry weight showed the greatest decrease in the CG5087 tree, whereas the G935 tree was insensitive. The root:shoot biomass ratio of the CG5087 and M26 trees was 0.24 and 0.23, respectively, and 0.38 for the G214 and CG4814 trees. The CG5087 tree was the most sensitive to drought stress, whereas drought tolerance was higher in the G202, G214, and G935 trees than in the M26 tree, which had a comparable drought tolerance to the G11, CG4814, and M9 trees.