SOURCE-SINK RELATIONSHIPS IN WHEAT LEAVES INFECTED WITH POWDERY MILDEW .2. CHANGES IN THE REGULATION OF THE CALVIN CYCLE

Alterations in the regulation oi the Calvin cycle were studied in whea t leaves infected with powdery mildew (Erysiphe graminis f. sp. tritic i) during disease development. During infection the triazole fungicide hexaconazole (trade name ANVIL, Zeneca Agrochemicals) was applied to leaves to determine if removal of the fungal sink reversed any pathoge n-induced alterations in host physiology. In mildewed and fungicide tr eated leaves the rate of photosynthesis pet unit leaf area declined bu t was largely unchanged per unit chlorophyll. In addition, the proport ion of excitation energy used photochemically (q(P)) decreased, the pr oportion dissipated non-photochemically (q(N)) increased and a loss of chlorophyll Tvas observed in these leaves. By five days after inocula tion triose-phosphates and fructose-1,6-bisphosphate had significantly increased while 3-phosphoglyceric acid, ribulose-1,5-bisphosphate and the maximum extractable activity oi. ribulose-1,5-bisphosphate carbox ylase/oxygenase, NADP-glyceraldehyde-3-phosphate dehydrogenase and the stromal fructose-1,6-bisphosphatase had decreased in mildewed leaves. In fungicide treated leaves the amount of specific metabolites and th e activity of enzymes was generally intermediate between those in a mi ldewed, and a healthy leaf These data indicate a down-regulation of th e Calvin cycle as a result of a decreased amount of Calvin cycle enzym es. It is suggested this caused an inhibition of the photosynthetic el ectron transfer chain and ultimately, the rate of photosynthesis in bo th mildewed and fungicide treated leaves. (C) Academic Press Limited