Energy dissipation in C3 plants under drought.

A general quantification of the relative contribution of different light energy dissipation processes to total dissipation under different drought conditions is lacking. Here we compare six studies, including enough data for such a general quantification, to build up a general pattern of the relative importance of several energy dissipation mechanisms in response to drought in C3 plants. Such a general pattern apparently emerges independently of specific acclimation to drought, but largely dependent on CO2 availability in the chloroplasts, which may be regulated under drought by adjustments in stomatal and mesophyll conductances. Under irrigation and saturating light, more than 50% of absorbed light is thermally dissipated, while photosynthesis dissipates 20-30% and photorespiration 10-20%. Under mild drought, the contribution of photosynthesis decreases, and that of photorespiration increases in a compensatory manner. During moderate to severe drought, the contribution of both photosynthesis and photorespiration decreases, and thermal dissipation increases up to 70-90% of the total light absorbed. The contribution of other processes, like the Mehler reaction, is shown to be very low under both irrigation and drought. Therefore, in C3 plants subjected to different degrees of drought, more than 90% of the total energy absorbed by leaves is dissipated by the sum of thermal dissipation, photorespiration and photosynthesis.