Changes in CO 2 and water vapour exchange rates in leaves of Quercus robur infected by Microsphaera alphitoides (powdery mildew)

Abstract

Infra-red gas analysis techniques were used to follow CO 2 exchange in leaves of Quercus robur L. seedlings infected with Microsphaera alphitoides Griffon and Maublanc. Infected leaves showed an initial increase in net photosynthesis, followed by a rapid decline, to levels significantly lower than those of control leaves on the second day after inoculation. Photorespiration rates in infected leaves were reduced throughout the 7 day experimental period. Dark respiration increased to a peak 5 days after inoculation which was followed by a period of decline. Overall respiration (photorespiration and dark respiration) was found to decrease from the time of inoculation. The decrease in photorespiration was responsible for the transient increase in net photosynthesis that occurred soon after infection. Transpiration rates in infected material increased within 2 to 3 days after infection and the difference between healthy and infected tissue was greatest in the dark. No effect of mildew on stomatal movements was observed until the sixth day, when there was a reduction in the aperture of stomata in the light. The greater part of the increased transpiration in infected material took place via the mycelium. The relative turgidity and water potential of infected leaves were reduced from the fourth day after inoculation although the decreases were probably not responsible for the reduction in photosynthetic rate. Some solute loss from infected tissue was indicated on the sixth day. It was concluded that the reduction in the net photosynthetic rate of diseased leaves was due to biochemical changes other than those directly related to a change in the water status of the tissue.