Total soil water content accounts for augmented ABA leaf concentration and stomatal regulation of split-rooted apple trees during heterogeneous soil drying

摘要

A split-rooted containerized system was developed by approach grafting two, 1-year-old apple (Malus×domestica Borkh. cv ‘Gala’) trees to investigate the effect of soil moisture heterogeneity and total soil moisture content (θ_v) on tree water relations, gas exchange, and leaf abscisic acid (ABA) concentration [ABA_leaf]. Four irrigation treatments comprising a 2×2 factorial experiment of irrigation volume and placement were imposed over a 30-day period: control (C) [100% of crop evapotranspiration (ET_c)] applied to both containers; PRD100 (100% ET_c) applied to one container only; and two treatments receiving 50% ET_c applied to either one (PRD50) or both containers (DI50). Irrigation between PRD (partial rootzone drying) root compartments was alternated when θ_v reached ~35% of field capacity. Maximum daily sap flow of the irrigated roots of PRD100 exceeded that of C roots throughout the experimental period. Pre-dawn water potential (Ψ_pd) was similar between C and PRD100; however, daily water use and mid-day gas exchange of PRD100 was 30% lower. Slightly higher [ABA_leaf] was observed in PRD100, but the effect was not significant and could not explain the observed reductions in leaf gas exchange. Both 50% ET_c treatments had similar, but lower θ_v, Ψ_pd, and gas exchange, and higher [ABA_leaf] than C and PRD100. Regardless of treatment, the container having the lower θ_v of a split-rooted system correlated poorly with [ABA_leaf], but when θ_v of both containers or θ_v of the container possessing the higher soil moisture was used, the relationship markedly improved. These results imply that apple canopy gas exchange and [ABA_leaf] are responsive to the total soil water environment.