Growth, carbohydrate reserves and drought survival strategies of contrasting Dactylis glomerata populations in a Mediterranean environment.
The survival of cocksfoot (D. glomerata) swards and their changes in carbohydrate content were investigated on loamy clay soil in Corsica, France. Five contrasting populations of D. glomerata, cv. KM2, Amoucha, Dora, Roskilde and Lutetia from the south of France, Algeria, continental Italy, Denmark and north west France, resp., were studied. In the summer of 1992, swards were either well-irrigated or subjected to 76 days of drought under a rain-out shelter. Under drought conditions, there was no measureable summer yield of any cocksfoot population. There were large differences in recovery growth in autumn, which was highly correlated (P<0.001) with tiller density (r=0.81), water-soluble carbohydrate (WSC) content (r=0.82) and fructan content in leaf bases (r=0.85) recorded in August. Under irrigation conditions, no real summer dormancy was observed for any population. Summer yield varied from 13 to 244 g dry matter (DM) per m2 and was highly correlated (P<0.001) with glucose and fructose contents in leaf bases in September (r=0.65 and 0.74, resp.). It was suggested that, amongst the five cocksfoot populations examined, there were two strategies for surviving summer drought. The first is summer semi-dormancy, which was observed in the populations from Algeria and the south of France, which are adapted to summer drought and exhibited seasonal growth patterns relatively insensitive to irrigation (summer yields <110 g DM m-2). High tiller density was associated with accumulation in leaf bases of high levels of WSC (>40% of DM) and fructans (>30% of DM) in the summer, followed by rapid recovery of leaf extension after irrigation in early September. The autumn yield of water-stressed plants was only 16-24% lower than their combined summer and autumn yields when irrigated in summer. The 2nd strategy, summer activity was observed in populations from Denmark, northwest France and continental Italy, which were unadapted to long periods of drought and responded to irrigation (summer yields >200 g DM m-2). Under long and intense drought, their tiller densities reduced significantly, and the carbohydrate reserves in leaf bases were continuously utilized and not restored (WSC amounts were 15-25% of DM in September). This was associated with high tiller mortality, low growth resumption in autumn and, therefore, a lack of persistence.