A consistent CO₂ assimilation rate and an enhanced root development drives the tolerance mechanism in Ziziphus jujuba under soil water deficit

Shifting toward stress-resistant planting material has become a pre-requisite for fruit tree culture under arid to the semiarid environment. No information currently exists on the physiological basis of water stress tolerance in Ziziphus jujuba at the saplings stage, an important fruit tree of semiarid regions of Central Punjab. Therefore, an experiment was conducted under controlled conditions where plants were subjected to three watering regimes: control irrigation, a medium, and a high-water deficit treatment. During the experiment, growth, biomass production and allocation in different plant organs (leaves, stem, and root) were measured. The physiological response of Z. jujuba under water deficit was determined through a gas exchange system and CO₂ assimilation rate, stomatal conductance, and water use efficiency were evidenced. Biomass production/allocation in leaves and stem decreased significantly with an increase in soil water deficit. Root biomass production remained unchanged (p = 0.907), but allocation increased significantly under high soil water deficit (p < 0.001). Net CO₂ assimilation rate (A) remained similar at 6.54 ± 0.32 μmol m⁻² s⁻¹ but, stomatal conductance (gs) decreased significantly from 94.0 to 69.2 mmol m⁻² s⁻¹ under high water deficit treatment. Water use efficiency (ratio between Net CO₂ assimilation rate and transpiration rate) also increased significantly (p = 0.012) under high water deficit treatment. Ziziphus jujuba showed a medium degree of plasticity under water deficit conditions, which was mainly driven by a sustained CO₂ assimilation rate and more biomass allocation for root development.