Photosynthetic, photochemical and osmotic regulation changes in tobacco resistant and susceptible to Alternaria alternata

Alternaria alternata is a necrotrophic fungus that may cause severe losses. To further elucidate the resistance mechanisms, we investigated the impacts of increasing biotic stress, or severity levels (5%, 25% and 50%), due to A. alternata infection on two tobacco cultivars with varying resistance levels. Under severe stress, the photosynthetic pigment, net photosynthetic rate (Pn), stomatal conductance (Gs) and stomatal limitation value (Ls) all decreased in both cultivars, whereas the intercellular CO 2 concentration (Ci) exhibited an upward trend. Moreover, A. alternata caused adverse effects on PSII electron transport rate and chlorophyll a fluorescence parameters, including the maximal quantum yield of fluorescence ( F v / Fm ), PSII potential efficiency ( F v / F 0 ), PSII actual quantum yield ( Φ PSII ), photochemical quenching ( q P ) and non-photochemical quenching (NPQ), and ultimately decreased PSII photochemical activity. In addition, the contents of MDA, proline and total soluble sugar increased under stress. The adverse effect of A. alternata on the resistant cultivar was less harmful than the susceptible cultivar. We conclude that the reduction of photosynthesis is a result of non-stomatal limitations. However, the resistant cultivar has better self-protection of its photosynthetic system, improved light energy utilization ability and greater accumulation of osmotic regulation substances for mitigating the damage on cellular membranes. These attributes may account for the resistance to A. alternata .