Abstract
Summary Photosynthetic responses of periwinkle (Catharanthus roseus) were compared during disease development after infection by two strains of ash yellows (AshY) phytoplasmas that differed in aggressiveness. Healthy plants were inoculated by grafting with diseased scions. Mature leaves formed before grafting of diseased scion, and leaves formed during disease development were monitored for physiological changes. Leaves formed before inoculation remained visually and physiologically asymptomatic until normal senescence. However, leaves formed after inoculation had reduced photosynthesis, due to reductions in both the maximum carboxylation rate (Vcmax) and noncyclic electron transport (Jmax). In these leaves, photoinhibition resulted from photoprotective processes rather than photooxidative damage to PSII reaction centres. Stomatal conductance (Gs) was reduced, but this did not increase the stomatal limitation to photosynthesis. Changes in Gs were not correlated with changes in leaf ABA concentration. Known differences in phytoplasmal aggressiveness were correlated with degrees of inhibition of photosynthesis and occurence of photoinhibition. AshY phytoplasmas appear to cause metabolic perturbations associated with localized accumulation or multiplication of the pathogen in symptomatic leaves. AbbreviationsA, net photosynthesis rate; Aa, net photosynthesis rate at ambient [CO2]; Amax, net photosynthesis rate at saturated [CO2]; AshY, ash yellows; Ci, intercellular [CO2]; Fo, Fm, Fv, minimal, maximum and variable fluorescence, respectively; F′m, Fs, maximal and steady state fluorescence in light‐adapted state; Fv/Fm, intrinsic efficiency of PSII; Gs, stomatal conductance; Jmax, maximum noncyclic electron transport; Ls, stomatal limitation; ΦPSII, quantum yield of PSII; qNP, nonphotochemical quenching coefficient; RuBP, ribulose‐1,5‐biophosphate; Vcmax, maximum rate of carboxylation of Rubisco; vpd, leaf‐to‐air vapor pressure deficit; WAG, week after grafting.
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