Physiological Functions of PsbS-dependent and PsbS-independent NPQ under Naturally Fluctuating Light Conditions

The PsbS protein plays an important role in dissipating excess light energy as heat in photosystem II (PSII). However, the physiological importance of PsbS under naturally fluctuating light has not been quantitatively estimated. Here we investigated energy allocation in PSII in PsbSsuppressed rice transformants ( psbS) under both naturally fluctuating and constant light conditions. Under constant light, PsbS was essential for inducing the rapid formation of light-inducible thermal dissipation ( NPQ), which consequently suppressed the rapid formation of basal intrinsic decay ( f,D), while the quantum yield of electron transport ( II) did not change. In the steady state phase, the difference between the wild type (WT) and psbS was minimized. Under regularly fluctuating light, the reduced PsbS resulted in higher II upon the transition from high light to low light and in lower II upon the transition from low light to high light, indicating that II was, to some extent, controlled by PsbS. Under naturally fluctuating light in a greenhouse, rapid changes in II were compensated by NPQ in the WT, but by f,D in psbS. As a consequence, a significantly lower NPQ integrated NPQ over a whole day) and higher f,D were found in psbS. Furthermore, thermal dissipation associated with photoinhibtion was enhanced in psbS. These results suggest that PsbS plays an important role in photoprotective process at the induction phase of photosynthesis as well as under field conditions. The physiological relevance of PsbS as a photoprotection mechanism and the identities of NPQ and f,D are discussed.