*4.3. The Mehler Peroxidase Reaction (Water–Water Cycle)*

The term water–water cycle was coined by Asada (1999) [33] to indicate a process wherein electrons from LEF are extracted from water at the oxygen evolving complex of PSII, through the intermediate electron transfer chain, to PSI and to O2, reforming H2O. In plants, most of the O2 reduction occurs the by the Mehler peroxidase reaction (sometimes referred to as the water–water cycle (WWC)), electrons are transferred from low-potential donors (most probably F(X/A/B) within PSI to molecular oxygen, bypassing terminal NADP<sup>+</sup> reduction, and producing superoxide. The resulting reactive oxygen species are rapidly detoxified by the activities of superoxide dismutase and the plastid ascorbate peroxidases [33]. This results in H<sup>+</sup> translocation through water oxidation and the Q-cycle, without parallel NADPH production, increasing ATP/NADPH supply. Note, however, that PSI-involvement is a not a strict requirement of a WWC (i.e., the 'traditional' Mehler peroxidase reaction), and the activity of the plastid terminal (plastoquinol) oxidase, also serves to bypass NADPH production, albeit at low capacity [61]. Alternative forms of the WWC are also found in moss, algae and cyanobacteria, where flavodiiron proteins function as NADPH-dependent oxygen reductases [62]. While the WWC may be important under certain stress conditions, current evidence suggests that it does not operate at significant rates under a variety of conditions (i.e., <5% of LEF in tobacco when the C3 was inhibited [63]), and so it will not be further considered in the context of energy balancing [64,65].
