Abstract
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 AmbrosiaTM (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.
Highlights
In apple horticulture, the use of rootstock is an effective approach to manage tree growth and yield, to improve resource use efficiency, and to confer scion with desirable traits of resilience against a variety of abiotic and biotic stressors [1]
Rootstocks with different vigor and water transport capacity are assumed to alter the water demand, water use strategies and drought resistance of the scion, it remains a debate whether the performance of smaller rootstocks with lower water demand would necessarily exceed that of larger rootstocks under water-limited conditions, and whether the scions with different growing habits would inversely impact the performance of specific rootstocks
Rehydration led to a further decrease in photosynthetic rates (Pn) in M.9 and G.935, but not in M.26, B.9 or G.202
Summary
The use of rootstock is an effective approach to manage tree growth and yield, to improve resource use efficiency, and to confer scion with desirable traits of resilience against a variety of abiotic and biotic stressors [1]. Water transport in the scion–rootstock system may be further restricted by xylem dysfunction and increased resistivity of the graft union [5]. Such limitations constrain scion hydraulics and tree growth, by the well-known mechanism of altering the root-to-shoot hydraulic signals. Other mechanisms, such as interfered systemic chemical communications via endogenous growth regulators, nutrients, RNAs and proteins, are less understood [6,7,8]. Rootstocks with different vigor and water transport capacity are assumed to alter the water demand, water use strategies and drought resistance of the scion, it remains a debate whether the performance of smaller rootstocks with lower water demand would necessarily exceed that of larger rootstocks under water-limited conditions, and whether the scions with different growing habits would inversely impact the performance of specific rootstocks
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