Osmotic Components and Properties Associated With Genotypic Differences in Osmoregulation in Wheat

Abstract

Solutes contributing to genotypic differences in osmoregulation in water-stressed wheat plants were investigated using samples of sap from expanded flag leaves of breeding lines with high and low osmoregulation. The sap samples were collected from leaves using a hand press after freezing and thawing. The leaves were sampled during a period of increasing stress created by withholding water from breeding lines growing in potting mix in the glasshouse. Measurements of potassium, free amino acids, proline, glucose, sucrose and fructose, as well as conductivity and refractive index, were made on the sap. The osmotic potential of the same sap samples was determined with a thermocouple psychrometer both before and after these measurements were made. Separate determinations of osmotic potential were also made on sections of frozen and thawed tissue from the same leaves used for sap samples, but without expressing the sap. The level of osmoregulation was expressed as the solute accumulation between full and zero turgor. The lines showed either very low (0.18 MPa) or very high (1.35 MPa) levels of solute accumulation. These levels were similar to a single gene effect observed in other lines. High osmoregulation was largely (78%) due to potassium accumulation, with amino acids the only other important contributor (22%). No sugar accumulation was found. The osmotic potential due to the solutes potassium, amino acids and sugars accounted for, on average, 90% of the sap osmotic potential measured with a thermocouple psychrometer. The major components were potassium (42-53%) and amino acids (35-43%). In high lines, increases in the proportion of potassium with increased stress were associated with changes in the relationship between sap osmotic potential and either tissue osmotic potential or that derived from the component solutes. The various methods of measuring osmotic potential and effects on the calculation of solute accumulation are discussed.