LONG-TERM WATER-STRESS INACTIVATES RUBISCO IN SUBTERRANEAN CLOVER

In long-term field experiments, during consecutive years, microswards of subterranean clover were irrigated to minimise water deficits or su bjected to progressively increasing drought over 30 days. Both leaf wa ter potential and relative water content steadily decreased during the experiments. Plants affected by drought grew more slowly and photosyn thesis was decreased. Photosynthetic rate (A) and Rubisco were analyse d in relation to midday water potentials and relative water contents. The difference in A between droughted and irrigated plants increased p rogressively, in part as a result of decreased stomatal conductance an d CO2 concentration within leaf (Ci). However, A-Ci curves suggest tha t the photosynthetic capacity in plants experiencing long-term stress was reduced by 50% when compared with irrigated plants. Drought decrea sed both the initial and the total Rubisco activity per unit area in a similar way but did not reduce the amount of Rubisco protein per unit leaf area. Thus, the specific activity of Rubisco, rather than its ac tivation state, decreased suggesting that under water stress the activ e sites were blocked by inhibitors.