The effect of different spectral light quality on the photoinhibition of Photosystem I in intact leaves.

Light energy causes damage to Photosystem I (PSI) and Photosystem II (PSII). The majority of the previous photoinhibition studies have been conducted with PSII, which shows much larger photoinhibition than PSI; therefore, relatively little is known about the mechanism of PSI photoinhibition so far. A previous report showed that the photoinhibition action spectrum measured with PSI activity of isolated thylakoid is similar to the absorption spectrum of chlorophyll. However, it is known that the extent of PSI photoinhibition is much smaller in vivo compared to in vitro. It is also possible that the different extent of PSII photoinhibition, caused by different spectral light qualities, can affect the photoinhibition of PSI in vivo because PSI receives electrons from PSII. In the present research, to study the effect of light quality and the effect of the extent of PSII photoinhibition on the PSI photoinhibition in vivo, intact leaves were photoinhibited under four different light qualities. The rate coefficient of PSI photoinhibition was significantly higher in blue and red light compared to white light. The rate of PSI photoinhibition at the same photon-exposure was the largest in blue and red light and followed by white and green light. These results support the notion that light absorption by chlorophyll is responsible for the PSI photoinhibition, even in intact leaves. The variation among light colors in the relationships between the extent of photoinhibition of PSII and that of PSI indicate that PSI and PSII are independently photoinhibited with different mechanisms in the early stage of in vivo photoinhibition.