Stomatal Resistance to Low Leaf Water Potential at Different Growth Stages Affects Plant Biomass in Glycine max L.

Two cultivars of Glycine max (L) Merril. (TGX536-02D and TGX 923-2E designated cv.A and cv.B respectively), were subjected to water stress for a period of 7 days at the vegetative (T2), flowering/fruiting (T3) and seed development (T4) stages of growth, with a control (T1). Stress was considered moderate at T2 stage (ψ_w : -1.53 to -1.57 MPa) and severe at T3and T4 stages (ψ_w : -2.23 to -2.67 MPa). Leaf water potential (ψ_w) and relative water content (RWC) were significantly reduced (p<0.01) by the stress treatment at all stages of growth and a strong positive correlation (r = 0.79 to 0.95) was found between them. Cv.A showed greater sensitivity to water stress by having lower RWC and leaf ψ_w values than cv.B with similar treatments. Cv.B had higher abaxial resistance (Rs) than cv.A at the T3 and T4 stages and higher adaxial Rs at the T2 and T3 stages of growth. The lower Rs of cv.A at the T3 and T4 stages resulted in lower tolerance to water stress than cv.B as shown by the significant reduction (p<0.05) in the biomass of T3 and T4 plants. Furthermore, T3 and T4 plants of cv.B showed leaf area adjustments. Hence, cv.B is better adapted to stress at the reproductive stage than cv.A.