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PS II is one of the most sensitive component of the photosyntehtic apparatus to different environmental stresses. Heat stress causes the hyperfluidization of the thilakoid membranes, the deactivation of PS II including the dissociation of LHC from the core complexes and inactivation of oxygen evolving system(1). The heat sensitivity of plants can be caracterized by the thermostability of PS II measured by temperature-dependent changes of initial fluorescence (Fo) of dark adapted leaves (2). In parallel with the heating, the initial level of fluorescence yield increases intensively above a critical temperaure (Tc). The Fo vs T curve and the critical temperaure depend on the composition and physical state of the thylakoid membranes (3). It seems that the thermostability of the photosynthetic apparatus of preilluminated plants is higher than of the untreated samples, which manifests in the upshift of Tc measured on dark adapted leaves (4).

The excess of the absorbed energy must be sufficiently dissipated to maintain the integrity of the photosynthetic apparatus. One of these protective energy dissipating mechanisms the xanthophyll epoxidation cycle working on the lumenal side of grana thylakoids recently has been thoroughly investigated in many species and under a wide range of environmental and physiological conditions [1,2,3,4].

Atrazine-resistant (AR) weed biotypes have a Ser264-Gly mutation on the D1 protein, causing a decreased binding of atrazine and QB, and some structural/functional changes in PSII complexes (1). The photosynthetic function of AR plants has been reported to be more sensitive to a high light intensity (2–5). Adverse effects of photoinhibition (PI) on the photosynthetic apparatus occur when the capacity of photoprotection (damage and repair) of the plant is exceeded. It has been suggested that a higher susceptibility to the PI of AR biotypes may be caused by an altered DI protein turnover (2) and/or by the limited conversion of xanthophyll cycle components (4,5). A well-studied photoprotective mechanism in plants is associated with the xanthophyll cycle activity and non-photochemical quenching. The increase in the amount of zeaxanthin seems to correlate with an increase in non-photochemical fluorescence quenching of chlorophyll, and is suggested to be a mechanism for the protection of PSII against PI via non-radiative energy dissipation (6).