Photosynthesis and chloroplast functioning within individual pustules of Uromyces muscari on bluebell leaves

Abstract

Photosynthesis and chloroplast functioning is examined within pustules of Uromyces muscari on bluebell leaves at different stages of development from presporulation to green island formation. Green island tissue is shown to be photosynthetically active by ¹⁴CO₂ fixation, OZ evolution and in vivo chlorophyll fluorescence kinetics. Chloroplast measurements indicate that, whilst there is little reduction in chloroplast number per unit area in pustules of U. muscari, the chloroplast volume, the chlorophyll concentration and the ratio of chlorophyll a : b decline, suggesting that chlorophyll is lost from individual chloroplasts. Oxygen evolution, both per unit area and per unit chlorophyll, is reduced in pustules in comparison to green areas of an infected leaf and to uninfected leaves. Two major parameters of in vivo chlorophyll fluorescence kinetics (Fvar and F.) are progressively reduced in pustules. This suggests that non-cyclic electron transport, and general chloroplast integrity, are impaired as the disease develops. These results are discussed in the context of current theories concerning the mechanisms by which parasites induce a block in the electron transport chain, and a new model is proposed, based on a change in the chlorophyll content of the photosystem reaction centres.