Plants protect themselves against excessive light by the induction of Delta pH-dependent nonphotochemical quenching (qE) that is associated with de-epoxidation of violaxanthin (V) to zeaxanthin (Z) in thylakoid membranes. In this work, we report that low light (12 micromol photons m(-2) s(-1)) is sufficient for a marked stimulation of the V to Z conversion in shortly preheated wheat leaves (5 min, 40 degrees C), but without a substantial increase in qE. Re-irradiation of these leaves with high light led to a rapid induction of nonphotochemical quenching, implying a potential photoprotective role of low-light-induced Z in preheated leaves. On the contrary to low light conditions, preheated leaves exposed to high light behaved similar to nonheated leaves with respect to the V to Z conversion and qE induction. The obtained results indicate that low-light-induced lumen acidification in preheated leaves is high enough to activate V de-epoxidation, but not sufficiently high to induce the formation of quenching centers.