Comparison of Osmotic Adjustment Responses to Water and Temperature Stresses in Spring Wheat and Sudangrass

Abstract

This study explores the mechanisms of osmotic adjustment by comparing the growth of spring wheat and sudangrass, which exhibit different degrees of osmotic adjustment, under soil water and temperature stresses. Leaf water potential (ψ 1), osmotic potential (π), and rate of leaf area growth of spring wheat and sudangrass seedlings were measured at combinations of five soil water potentials, from -0·03 to -0·25 MPa, and six root temperatures, from 14 to 36°C. Spring wheat exhibit little osmotic adjustment. The leaf osmotic potential was not affected by either soil water or root temperature stress. Osmotic potential of sudangrass decreased in parallel with the decreasing leaf water potential as a result of osmotic adjustment. As soil water potential decreased from -0·03 to -0·25 MPa, the rates of growth and photosynthesis of spring wheat both decreased by about 30%. For sudangrass with the same range of soil water potential, the photosynthesis rate decreased by only 10% while the leaf area growth rate decreased by 49%. We introduce a dimensionless index (R) to quantify the degree to which environmental stresses alter the balance between production of photosynthates and their use for growth. The index, R, is equal to 1 when stress reduces growth and photosynthesis by the same degree, i.e. the balance between production and consumption of photosynthate is not disturbed. R is smaller than 1 when growth is reduced more than photosynthesis. R was equal to 1 for spring wheat where there was no osmotic adjustment. For sudangrass, R decreased from 1 to 0·25 as osmotic potential decreased from -1·10 to -1·63 MPa. These findings lead to the hypothesis that osmotic adjustment could result from an imbalance between production, consumption and translocation of photosynthates under stressed conditions.