Drought stress delays photosynthetic induction and accelerates
photoinhibition of photosystem I under fluctuating light
Fluctuating light (FL) and drought stress usually occur concomitantly.
However, whether drought stress affects photosynthetic performance under
FL remains unknown. Here, we measured gas exchange, chlorophyll
fluorescence, and P700 redox state under FL in drought-stressed tomato
(Solanum lycopersicum) seedlings. Drought stress significantly affected
stomatal opening and mesophyll conductance after transition from low to
high light and thus delayed photosynthetic induction under FL.
Therefore, drought stress exacerbated the loss of carbon gain under FL.
Furthermore, restriction of CO2 fixation under drought stress aggravated
the over-reduction of photosystem I (PSI) upon transition from low to
high light. The resulting stronger FL-induced PSI photoinhibition
significantly supressed linear electron flow and PSI photoprotection.
These results indicated that drought stress not only affected gas
exchange under FL but also accelerated FL-induced photoinhibition of
PSI. Furthermore, drought stress enhanced relative cyclic electron flow
in FL, which partially compensated for restricted CO2 fixation and thus
favored PSI photoprotection under FL. Therefore, drought stress has
large effects on photosynthetic dark and light reactions under FL.