Résumé
The ability to survive severe summer water stress determines the persistence, and therefore autumn yield, of temperate grasses in the Mediterranean environment. In the island of Corsica, sward survival and changes in carbohydrate content were investigated over 2 years, in two contrasting populations of cocksfoot (Dactylis glomerata L.): KM2, an early-flowering population of Mediterranean origin and Lutetia, a late-flowering cultivar bred in North West France. In Expt 1 (1987/88) three spring defoliation intensities were imposed. In Expt 2 (1989/90) three levels of water deficit were imposed under a rain shelter. In spring, dry matter (DM) yields and water-soluble carbohydrate (WSC) concentrations in leaf bases were similar (20-35 % of DM) for both populations. From June to September, WSC reserves tended to decline in Lutetia, whereas accumulation of WSC was maintained until August (45 % of DM) in KM2. Fructans formed the major component, and therefore followed the same seasonal pattern, as total WSC. Contents of trisaccharide and sucrose increased in late summer as drought progressed. Contents of glucose and fructose were lower but increased in August or in September according to treatment. In both field experiments during summer, KM2 had 50 % more live tillers/ml than Lutetia, possibly because it remained in positive carbon balance for a longer period. WSC levels in summer and regrowth in autumn were reduced both by intense defoliation in spring and by severe water stress in summer. On average, autumn yields of KM2 were five times greater than those of Lutetia and the recovery yields were closely correlated with fructan contents of leaf bases in summer (r = 0.80; P < 0.001). The role of fructans and low DP-sugars as solutes in osmotic adjustment and as substrates for regrowth are discussed, and the differences between populations are analysed.