Osmotic Adjustment in Lolium perenne; Its Heritability and the Nature of Solute Accumulation

Plants derived from Lolium perenne L. cv. ‘Melle’ were selected on the basis of extreme high or low lamina solute potential (ψs) and pair-crossed to produce divergent G2 lines. The high and low lines had mean lamina ψs values significantly different from each other and from ‘Melle’, and showed an enhanced range of phenotypic expression of ψs during drought. Both mature lamina ψs and meristem ψs values of droughted plants were highly heritable. Extreme G2 genotypes were selected and clonally replicated for further study. During drought mean ψ2 values fell from −1·21 to −1·80 MPa. Fructans of large molecular weight, and total free amino acids, especially proline, all accumulated when estimated on a dry-matter (DM) or plant-water (PW) basis. Oligosaccharide content was largely unchanged. Minerals declined when estimated on a DM basis but accumulated on a PW basis because hydration (g water in turgid tissue per g DM) declined more rapidly. In comparison with genotypes having high constitutive ψs, low-ψs genotypes (a) were larger, had faster leaf extension rate, had fewer tillers, and were proportionally more affected by drought, (b) showed greater osmotic adjustment, (c) contained and accumulated much more fructan (but not oligosaccharides), and amino acids, especially proline, and (d) accumulated more mineral ions on a PW basis, but less on a DM basis. The relatively high repeatabilities for organic solutes in particular show that further divergent selection for individual solutes would not be difficult. Solutes accumulated, probably because they were not consumed in growth. There was no evidence of ‘competition’ between growth and osmotic adjustment for metabolites, or that plants which accumulated more solutes were better able to recover when water deficits were relieved.