CO2 EXCHANGE, PRIMARY PHOTOCHEMICAL-REACTIONS AND ENZYMES OF PHOTOSYNTHETIC CARBON-REDUCTION CYCLE IN BRASSICA PODS DURING WATER-STRESS ANDRECOVERY

Under a moderate water stress (pod water potential, Psi(W), - 1.5 MPa) , induced by stopping irrigation for 3 d, the net photosynthetic rate (P-N) decreased to 50 %, transpiration rate (E) to 85 %, stomatal cond uctance to 65 % and chlorophyll (Chl) content to 82 %, while the activ ities of photosystems (PS) and of some enzymes of the photosynthetic c arbon reduction cycle (NAD- and NADP-glyceraldehyde-3-P dehydrogenases , aldolase) were almost without changes. The exceptions were ribulose- 5-P kinase and 3-phosphoglycerate kinase, the activities of which were reduced to 55 and 79 %, respectively. Under a severe water stress (5 d without irrigation, Psi(W)-2. 1 MPa) all the above characteristics w ere strongly reduced (P-N to 0.5 %, E to 57 %, PS1 to 62 %, PS2 to 37 %, enzyme activities to 48-68 %), but after rehydration the initial ac tivities were restored. The reduction of P-N in pods at a moderate wat er stress is probably related to the decline in activities of 3-phosph oglycerate kinase and ribulose-5-P kinase, while under a severe stress , when P-N is lowered almost to zero, the decline in g(s) and electron transport activities is very important.