Flavescence dorée phytoplasma deregulates stomatal control of photosynthesis in Vitis vinifera.

Flavescence dorée (FD) is among major grapevine diseases causing high management costs; curative methods against FD are unavailable. In FD infected plants, a decrease in photosynthesis is usually recorded, but stomatal control of leaf gas exchange during FD infection and recovery is unknown. During one year when grapevines experienced a water stress and another with no drought we measured the seasonal time course of gas exchange rates in two cultivars (‘Barbera’ and ‘Nebbiolo’) different in the response to FD infection and recovery, as assessed by symptom observation and phytoplasma detection through PCR analysis. Chlorophyll fluorescence was also evaluated at maximum symptom severity in ‘Barbera’, the cultivar showing the most severe stress response to FD infection, causing the highest damage in vineyards of north-western Italy. In FD infected plants, net photosynthesis and transpiration gradually decreased during the season more during the un-droughted year than upon drought. During recovery, healthy (PCR negative) plants infected two years before, but not those still infected the year before, regained the gas exchange performances measured pre-infection. The relationships between stomatal conductance and the residual leaf intercellular CO2 concentration (ci) discriminated healthy versus FD infected and recovered plants; at the same ci, FD infected leaves had higher non-photochemical quenching than healthy ones. We conclude that metabolic, not stomatal, leaf gas exchange limitation in FD infected and recovered grapevines is at the basis of plant response to FD disease. In addition, we suggest that such response is limited upon water stress, by showing that water stress superimposes on FD infection in terms of stomatal and metabolic non-stomatal limitations to carbon assimilation.