SOURCE-SINK RELATIONSHIPS IN WHEAT LEAVES INFECTED WITH POWDERY MILDEW .1. ALTERATIONS IN CARBOHYDRATE-METABOLISM

The effects of powdery mildew (Erysiphe graminis f. sp. tritici) on th e carbohydrate metabolism of wheat leaves was studied during infection . During infection the triazole Fungicide hexaconazole (trade name ANV IL, Zeneca Agrochemicals) was applied to leaves to determine if remova l of the fungal sink reversed any pathogen-induced alterations in host physiology. Five days after inoculation there was a large accumulatio n of sucrose, glucose, fructose and hexose-phosphates and an increased partitioning of current photosynthate into starch rather than sucrose in both mildewed leaves and those treated with fungicide 2 and 3 days after inoculation. Within 48 h of inoculation, acid invertase activit y had doubled and this increase was maintained until 7 days after inoc ulation. Application of hexaconazole to a mildewed leaf caused a trans ient decline in invertase activity but over subsequent days activity i ncreased again until it resembled thar observed in a mildewed leaf. He xaconazole severely inhibited fungal development and hence the size of the fungal sink. This suggests that the enhanced invertase activity w as triggered by a signal from the Fungus bur was not directly related to the size of the Fungal sink. These results suggest that (i) the sti mulation of invertase activity in mildewed leaves altered the source-s ink relationship of the leaf leading to an accumulation of soluble car bohydrates and a decrease in sucrose synthesis and (ii) that applicati on of hexaconazole successfully prevented development of the fungus bu r that the metabolic alterations which occurred following the initial infection Mere not reversible within the time scale studied. (C) 1995 Academic Press Limited