Rootstocks with Different Vigor Influenced Scion–Water Relations and Stress Responses in Ambrosia(TM) Apple Trees (Malus Domestica var. Ambrosia)

In recent years, awareness has been raised around the benefits of diversifying rootstocks, in order to enhance tree health and sustain apple fruit production under the influence of climate change. However, performances of many rootstocks under stresses remain unclear. This study aimed to set the first step towards a much-needed comprehensive evaluation on water relationships and stress responses of scion–rootstock systems for the popular apple cultivar Ambrosia(TM) (Malus domestica var. Ambrosia), because its production and horticultural knowledge had been largely limited to the Malling 9 rootstock (M.9). Five rootstocks were evaluated in a greenhouse water deficit experiment and at the onset of heat stress in a field trial in Summerland, British Columbia, Canada. Multiple stress indicators of leaves and fruits were analyzed to elucidate water use strategies and drought resistance mechanisms. The rootstocks led to differences in scion vigor, and stomatal and photosynthetic characteristics. The largest semi-dwarfing Geneva 202 (G.202) demonstrated more water use and higher stress susceptibility. Large dwarfing Geneva 935 (G.935) and Malling 26 (M.26) showed more stringent stomatal control and reduced water use under stresses, typical of a drought-avoidance strategy. The smallest large dwarfing M.9NIC29(®) and the small dwarfing Budagovsky 9 (B.9) led to smaller and denser stomata. B.9 demonstrated the most stable water status and drought tolerance. The study suggested that scion stress responses were influenced by rootstock vigor and tree water use strategies. It implied the necessity of vigor-specific irrigation management for alleviating stresses and achieving production goals of different rootstocks.