Photosynthesis in green-islands on powdery mildewinfected barley leaves

Abstract

Net photosynthetic oxygen evolution and dark respiration were measured in green-islands and surrounding senescing tissue from detached barley leaves infected with the powdery mildew fungus, Erysiphe gramims f.sp. hordei Marchai. Green-islands exhibited a reduction in net photosynthesis, expressed either on a unit area basis (14%) or a chlorophyll basis (32%). The apparent quantum yield of photosynthesis, which is thought to be a sensitive indicator of damage to the electron transport system, was reduced by 47% in green-island tissue. Concentrations of total chlorophyll, carotenoids and soluble protein, and rates of dark respiration, were not significantly altered in green-islands, when compared to controls. However, the chlorophyll a:b ratio was reduced substantially in green-island tissue. In senescing regions, both total chlorophyll and carotenoids were greatly reduced compared to controls. Following the initial procedure to establish steady state oxygen evolution, senescing tissue from infected leaves was unable to photosynthesize. Instead, oxygen was consumed at all photon flux densities examined. The possible mechanisms underlying the changes in photosynthesis in discrete regions of mildewed barley leaves are discussed.