Combined application of biochar and partial root-zone drying irrigation improves water relations and water use efficiency of cotton plants under salt stress

Reduced irrigation in combination with biochar application can improve stomatal anatomy, water relations and intrinsic water use efficiency (WUEᵢ), thus having a positive effect on the alleviation of salinity and drought stresses. A split-root pot experiment was executed to explore the effects of two biochar applications (WSP: wheat straw biochar; SWP: soft wood biochar) in combination with three irrigation strategies (FI: full irrigation; DI: deficit irrigation, PRD: partial root-zone drying irrigation) on stomatal anatomy, water relations and WUEᵢ of cotton plants under normal soil (S0, EC=0.36 dS m⁻¹) and saline soil (S1, EC=16.55 dS m⁻¹). The results revealed that both salinity and drought stresses negatively affected plant water relations and reduced stomatal conductance, carbon isotope discrimination (Δ¹³C), stomatal size (SS) and hydraulic conductance (Kₗ) while increased leaf abscisic acid (ABA) concentration ([ABA]ₗₑₐf). However, biochar amendment under salt stress significantly decreased [ABA]ₗₑₐf while improved leaf water relations, increased Kₗ, stomatal density (SD), SS, Δ¹³C and maximum stomatal conductance (gₛₘₐₓ). Meanwhile, compared with FI and DI, PRD plants had greater SD, gₛₘₐₓ, and WUEᵢ but lowered stomatal conductance (gₛ). Among all treatments, the combined application of WSP and PRD significantly increased SD and gₛₘₐₓ, improved leaf water relations, Kₗ and [ABA]ₗₑₐf and WUEᵢ. It is concluded that the altered stomatal features caused by the biochar and irrigation treatments are associated with changes in [ABA]ₗₑₐf and Kₗ, and have a major role in affecting plant hydraulic integrity and water use efficiency of cotton exposed to salinity stress.