Physiological responses of Catharanthus roseus (periwinkle) to ash yellowsphytoplasmal infection

Photosynthetic responses of periwinkle (Catharanthus roseus) were compared during disease development after infection by two strains of ash yellows (A shY) phytoplasmas that differed in aggressiveness. Healthy plants were inoculated by grafting with diseased scions. Mature lea ves formed before grafting of diseased scion, and leaves formed during dise ase development were monitored for physiological changes. Leaves formed before inoculation remained visually and physiologically asym ptomatic until normal senescence. However, leaves formed after inoculation had reduced photosynthesis, due to reductions in both the maximum carboxyla tion rate (VCmax) and noncyclic electron transport (J(max)). In these leave s, photoinhibition resulted from photoprotective processes rather than phot ooxidative damage to PSII reaction centres. Stomatal conductance (G(5)) was reduced, but this did not increase the stomatal limitation to photosynthes is. Changes in G(5) were not correlated with changes in leaf ABA concentrat ion. Known differences in phytoplasmal aggressiveness were correlated with degre es of inhibition of photosynthesis and occurence of photoinhibition. AshY p hytoplasmas appear to cause metabolic perturbations associated with localiz ed accumulation or multiplication of the pathogen in symptomatic leaves.