Most of the native forage species, dominant in upland grassland, were able to survive and recover from extreme drought, but with various time lags. Overall the results suggest that the wide range of interspecific functional strategies for coping with drought may enhance the resilience of upland grassland plant communities under extreme drought events.

The study of carbohydrate metabolism in perennial ryegrass (Lolium perenne L. cv. Bravo) during the first 48 h of regrowth showed that fructans from elongating leaf bases were hydrolysed first whereas fructans in mature leaf sheaths were degraded only after a lag of 1.5 h. In elongating leaf bases, the decline in fructan content occurred not only in the differentiation zone (30-60 mm from the leaf base), but also in the growth zone. Unlike other soluble carbohydrates, the net deposition rate of fructose remained positive and even rose during the first day following defoliation. The activity of fructan exohydrolase (FEH; EC 3.2.1.80) was maximal in the differentiation zone before defoliation and increased in all segments, but peaked in the growth zone after defoliation. These data strongly indicate that fructans stored in the leaf growth zone were hydrolysed and recycled in that zone to sustain the refoliation immediately after defoliation. Despite the depletion of carbohydrates, leaves of defoliated plants elongated at a significantly higher rate than those of undefoliated plants, during the first 10 h of regrowth. This can be partly attributed to the transient increase in water and nitrate deposition rate. The results are discussed in relation to defoliation tolerance.

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The aim of this study was to evaluate the putative role of the sucrosyl-galactosides, loliose [␣-d-Gal (1,3) ␣-d-Glc (1,2) ␤-d-Fru] and raffinose 6) ␣-d-Glc (1,2) ␤-d-Fru], in drought tolerance of perennial ryegrass and to compare it with that of fructans. To that end, the loliose biosynthetic pathway was first established and shown to operate by a UDP-Gal: sucrose (Suc) 3-galactosyltransferase, tentatively termed loliose synthase. Drought stress increased neither the concentrations of loliose and raffinose nor the activities of loliose synthase and raffinose synthase (EC 2.4.1.82). Moreover, the concentrations of the raffinose precursors, myoinositol and galactinol, as well as the gene expressions of myoinositol 1-phosphate synthase (EC 5.5.1.4) and galactinol synthase (EC 2.4.1.123) were either decreased or unaffected by drought stress. Taken together, these data are not in favor of an obvious role of sucrosyl-galactosides in drought tolerance of perennial ryegrass at the vegetative stage. By contrast, drought stress caused fructans to accumulate in leaf tissues, mainly in leaf sheaths and elongating leaf bases. This increase was mainly due to the accumulation of long-chain fructans (degree of polymerization Ͼ 8) and was not accompanied by a Suc increase. Interestingly, Suc but not fructan concentrations greatly increased in drought-stressed roots. Putative roles of fructans and sucrosyl-galactosides are discussed in relation to the acquisition of stress tolerance.One of the strategies employed by plants to survive drought stress includes the synthesis of protective compounds, which may act by stabilizing membranes and proteins or mediating osmotic adjustment (Bohnert et al., 1995; Hare et al., 1998; Hoekstra et al., 2001). Included among these protective compounds are the water-soluble carbohydrates (WSCs), Glc, Suc, raffinose, myoinositol, and fructans.Raffinose family oligosaccharides (RFOs) such as raffinose and stachyose accumulate during seed development and are thought to play a role in the desiccation tolerance of seeds (Blackman et al., 1992; Brenac et al., 1997). Raffinose also accumulates in vegetative tissues under drought stress (Taji et al., 2002). RFO biosynthesis requires the presence of galactinol, which is formed by galactinol synthase (GolS; EC 2.4.1.123) from UDP-Gal and myoinositol. Galactinol is the galactosyl donor for the biosynthesis of raffinose from Suc by raffinose synthase (RafS; EC 2.4.1.82). Because galactinol has not been assigned any function in plants other than acting as galactosyl donor for RFOs synthesis, GolS potentially catalyzes a metabolic key step for RFO synthesis. In a recent study, two drought-responsive GolS genes were identified among seven in Arabidopsis (Taji et al., 2002). Overexpression of one of them caused an increase in endogenous galactinol and raffinose as well as an improvement in drought tolerance.In addition to GolS, myoinositol 1-phosphate synthase (INPS; EC 5.5.1.4) is another enzyme that may control the levels of galactinol and raffinose. It repr...