Trunk radial growth, water and carbon relations of mature apple trees on two size-controlling rootstocks during severe summer drought

The use of size-controlling rootstocks is central to modern high-density fruit production systems. While biological mechanisms responsible for vigor control are not fully understood, differences in water relations and carbohydrate storage ability have been suggested as two potential factors. To better understand the processes that control growth vigor, we analyzed the trunk radial variation at seasonal and diurnal timescales and measured the midday leaf water potential (Psi(MD)), leaf gas exchange and concentrations of non-structural carbohydrates (NSC) in apple trees of variety 'Jonagold' grafted on two rootstocks of contrasting growth vigor (dwarfing J-TE-G vs invigorating J-TE-H). The measurements were conducted during an exceptionally hot and dry summer. We found that smaller annual trunk radial increments in dwarfed trees were primarily due to an earlier cessation of trunk secondary growth. The interdiurnal trunk circumference changes (Delta C) were slightly lower in dwarfed trees, and these trees also had fewer days with positive Delta C values, particularly during the driest summer months. The trunks of dwarfed trees shrank gradually during the drought, showed less pronounced diurnal variation of trunk circumference and the maximum trunk daily shrinkage was only weakly responsive to the vapor pressure deficit. These results indicated that lower turgidity in the cambial region may have limited the trunk radial expansion in dwarfed trees during the hot and dry days. Dwarfed trees also maintained lower Psi(MD) and leaf gas exchange rates during the summer drought. These parameters decreased in parallel for both rootstock combinations, suggesting their similar drought sensitivity. Similar concentrations and seasonal dynamics of NSC in both rootstock combinations, together with their similar spring growth rates, suggest that NSC reserves were not directly limiting for growth. Our results support the prominent role of water relations in rootstock-induced size-controlling mechanisms and highlight the complexity of this topic.

Automated summary

The use of size-controlling rootstocks is essential for high-density fruit production systems. Water relations and carbohydrate storage ability may play important roles in controlling growth vigor. Our study on apple trees grafted on two rootstocks with different growth vigor revealed that dwarfed trees had smaller trunk radial increments due to an earlier cessation of trunk secondary growth. Dwarfed trees also showed less interdiurnal trunk circumference changes and fewer days with positive changes during dry summer months. These trees exhibited lower turgidity in the cambial region, resulting in limited trunk radial expansion. Additionally, dwarfed trees maintained lower leaf water potential and leaf gas exchange rates during the summer drought. The concentrations and seasonal dynamics of non-structural carbohydrates were similar in both rootstock combinations, indicating that NSC reserves were not directly limiting growth. Overall, our results highlight the importance of water relations in rootstock-induced size control mechanisms and emphasize the complexity of this topic.