Abstract
The water-water cycle (WWC) functions as a starter of photosynthesis during the induction from darkness to light. This cycle leads to the rapid formation of NPQ just after illumination. This response may be essential for not only protection of PS II just after a sudden illumination but also for activation of Calvin cycle enzymes including Rubisco activase. The rbcS antisense rice plants with decreased Rubisco have low capacity for carbon assimilation without any decrease in electron transport capacity. The electron flux of WWC is low in these plants. Nevertheless, they have high capacity for the formation of NPQ, and can grow well high light conditions. High NPQ in the antisense plants is also mainly generated by the WWC, but is maintained by the cyclic electron flow around PS I. Thus, the cyclic electron flow may function as the long-term regulation of the PS II quantum yield and NPQ.
