Drought induces stomata closure and lowers the CO₂ concentration in the mesophyll, limiting CO₂ assimilation and favouring photorespiration. The photosynthetic apparatus is protected under drought conditions by a number of downregulation mechanisms like photosynthetic control, state transitions and activation of cyclic electron transport leading to the generation of a high proton gradient across the thylakoid membrane. In addition, photorespiration may exert a feedback control on photosystem II via the binding of a photorespiratory metabolite at the non-heme iron at the acceptor side of photosystem II (PSII) [1,2].

In state transitions, a part of the light harvesting complex at PSII (state 1) becomes phosphorylated and migrates to PSI (state 2) to balance the excitation between the two photosystems, We have observed that Arabidopsis mutants unable to perform state transitions from state 1 to state 2 are more resistant to drought [3]. We determined chlorophyll fluorescence parameters, stomata opening, relative leaf water content, ROS generation and root growth. Root growth is stimulated in these mutants. Differences between wild type and state transition mutants in respect to growth of the primary root are abolished in the presence of the PSII inhibitor DCMU and the cytochrome b6f inhibitor DBMIB, with DCMU inhibiting and DBMIB stimulating primary root elongation and side root formation. We propose that a signalling molecule deriving from the redox state of the plastoquinone pool and singlet oxygen controls primary root elongation [3].