DROUGHT EFFECTS ON GROWTH AND CARBON PARTITIONING IN A TALL FESCUE SWARD GROWN AT DIFFERENT RATES OF NITROGEN-FERTILIZATION

The effects of drought on plant water and nitrogen status, shoot growt h and carbon partitioning between root and shoot, were studied during two summer regrowth cycles of a tall fescue sward grown at different r ates of application of nitrogen fertilizer. Nitrogen fertilizer was th e main factor influencing the relative partitioning of the newly assim ilated C-14. Drought did not prevent the expression of the effect of n itrogen fertilization on carbon partitioning. The highest carbon parti tioning coefficients to roots (15-30 per cent) were measured in swards having the lowest nitrogen status. With the high nitrogen fertilizati on rate, water shortage increased the carbon partitioning to roots fro m 6 to 13 per cent (means for both regrowths, before rewatering). This increase mainly occurred in the surface horizon of soil (0-20 cm). Ho wever, an estimate in absolute terms showed that root growth was not i ncreased by drought. Water shortage impaired the sward nitrogen status . It is therefore proposed that at least part of the change in the car bon allocation pattern induced by drought was mediated via an effect o n nitrogen nutrition. The importance of the drought-induced modificati on of carbon partitioning is discussed.