Links of water uptake strategies with physiological characteristics are constrained by climatic conditions for mature apple trees in the loess deposits

Abstract

Exploring the interaction between plant growth and its water uptake patterns is important for the sustainable use of soil water resources in water-limited regions. However, little is known about how climate conditions affect water uptake patterns of apple trees and its link with physiological characteristics (stomatal conductance, leaf water potential, transpiration and assimilation rate). Here, two apple orchards with different tree ages-16 years (A16) and 22 years (A22)-were selected for sampling. We initially collected soil samples down to depths of 20 m, followed by monthly sampling down to 8 m during 2022 (dry year) and 2023 (wet year). Annual branches of apple trees were collected simultaneously with soil samples. We measured water stable isotopes in soil and xylem water monthly, along with physiological characteristics of apple trees, to investigate the relationship between water uptake strategies and physiological characteristics. We found that precipitation indirectly affected water use patterns by influencing shallow soil water. Apple trees exhibited anisohydric regulation, with leaf water potential decreasing as shallow soil water declined and air temperature increased. The stomatal conductance was sensitive to air temperature regardless of shallow soil water. This mechanism helped to maintain high stomatal conductance, thereby buffering the decline in assimilation rates by relying on deep soil water (∼67 %) when the shallow soil water became depleted in 2022, compared to ∼39 % in 2023. Notably, soil water below 6 m was crucial for apple trees to sustain the high assimilation rate (>12 μmol m⁻² s⁻¹), and A22 consumed more soil water below 6 m to sustain high assimilation rates. The results elucidate that depending on deep soil water to sustain stomatal conductance in dry years led to a decrease in soil water deeper than 6 m with stand ages, indicating that apple trees may not be ideal choice for afforestation in the loess deposits.