Photosynthetic response of pepper plants to wilt induced by Verticillium dahliae and soil water deficit.

Greenhouse experiments were conducted to compare stress effects caused by Verticillium dahliae and drought on gas exchange, chlorophyll (Chl) fluorescence and photosynthetic pigments of pepper plants. Three treatments were compared: Verticillium inoculated plants (+V), non-inoculated well-watered plants (-V) and non-inoculated plants subjected to progressive drought (D). Gas exchange, fluorescence and photosynthetic pigments were measured and represented along a gradient of relative water content (RWC) and stomatal conductance (g(s)). Net photosynthesis (A) and electron transport rate (ETR) decreased, as RWC and g(s) declined, similarly in both +V and D plants. However, dark respiration (R(D)) and photorespiration (R(L)) tended to increase in inoculated plants compared to those subjected to soil drought, as g(s) decreased. Photoinhibitory damage was not observed in infected or in droughted plants. Soil drought decreased intrinsic PSII efficiency (Phi(exc.)), which seemed to result in part from enhanced xanthophyll cycle- and/or lutein-related thermal energy dissipation. Nevertheless, the fact that 1-Phi(exc.) increased in D only at high values of the de-epoxidation state of the xanthophyll cycle (DPS) suggests that DeltapH could be the major factor controlling thermal energy dissipation in this treatment. By contrast, antheraxanthin, zeaxanthin and lutein, as well as Phi(exc.), were not markedly affected in +V. Water stress appeared to be the main limitation to photosynthesis in Verticillium infected plants, probably through stomatal closure, together with impaired mesophyll conductance (g(m)). However, our results indicate differential effects of V. dahliae on dark respiration, photorespiration, g(m) and on the capability of thermal energy dissipation under low g(s) values.