Neoformation and summer arrest are common sources of tree plasticity in response to water stress of apple cultivars.

Background and Aims Depending on the species, water stress affects different growth and developmental processes, mainly due to changes in hydraulic properties and hormonal signalling. This study compared the impact of water stress on tree development and organ growth in three apple cultivars. Methods Trees were differentially irrigated to induce water stress or to provide well-watered conditions in their second and third years of growth. Effects of water stress were evaluated at tree scale by shoot number and proportions of the different types of shoots, and at shoot scale by metamer appearance rate, growth duration and arrest time, as well as organ size. Key Results Water stress promoted early growth cessation, prolonged summer arrests and decreased growth resumptions, thus modifying within-tree shoot demography in favour of short shoots. Growth cessations occurred in mild water stress conditions before any difference in stem water potential appeared. No major impact was observed on organ size. Consistently with tree ontogeny, the number of shoots that resumed growth after summer arrest decreased with years, but more in water-stressed than well-watered conditions. Conclusions Even though the impact of water stress differed slightly among cultivars, the reduction in neoformation and increase in summer arrest played a common role in apple tree morphological responses and led to stress avoidance by early reduction of tree leaf area. [ABSTRACT FROM AUTHOR]