Time Trends for Change in Osmotic Adjustment and Water Relations of Leaves of Cenchrus ciliaris During and After Water Stress

Abstract

Buffel grass was subjected to a soil drying cycle for 5 weeks in a semi-arid environment. As water stress developed, the leaf water relations characteristics of these plants (Dry treatment) were compared with those of irrigated plants (Wet treatment). Leaf water potential (Ψ) of the Dry treatment measured at 1400 h decreased to a minimum of -6.9 MPa.

The stressed leaves adjusted osmotically, with the osmotic potential at full turgor (Ψ_π¹⁰⁰) decreasing (becoming more negative) linearly with time (0.017 MPa day^-1) and with decreasing water potential measured at 1400 h (0.11 MPa per 1 MPa decrease in Ψ). Maximum osmotic adjustment (Ψ_π¹⁰⁰ Wet -Ψ_π¹⁰⁰ Dry) was 0.66 MPa, and this change together with lower cell wall elasticity decreased by 1.03 MPa the water potential (Ψ⁰) at which the stressed leaves lost turgor. Differences between the stress- acclimated Dry leaves and the Wet leaves in bound water, turgid weight:dry weight ratio and the relative water content at which they reached zero turgor were small and inconsistent. At 18 days after rewatering, the Ψ_π¹⁰⁰ value of acclimated leaves was still 0.18 MPa lower than that of the control leaves.

The substantial shift in Ψ⁰ gained the stress-acclimated leaves only one extra day before they lost turgor at 1400 h, and only 2.5 extra days before being permanently wilted. This small gain in time and the rapid cessation of leaf growth even before positive turgor was completely lost suggests that osmotic adjustment may not contribute greatly to continued leaf growth in water-stressed plants of buffel grass.