Absorbance spectra of chlorophylls or pigments extracted from green leaves show that green light is absorbed only weakly. Action spectra of photosynthesis for thin algal solutions, transparent thalli of ordinary green algae, and leaves of aquatic angiosperms also show that green light is less effective than red light. As has been pointed out by Nishio (2000), these facts are often confused, and it is frequently argued that green light is inefficient for photosynthesis in green leaves. However, many spectra of absorptance (the value of light absorption) measured with integrating spheres have shown clearly that ordinary, green leaves of land plants absorb a substantial fraction of green light (McCree 1972, Inada 1976, Gates 1980). It is also known that green light, once absorbed by the leaves, drives photosynthesis with high efficiency (Björkmann 1968, Balegh and Biddulph 1970, McCree 1972, Inada 1976). On an absorbed quantum basis, the efficiency or photosynthetic quantum yield of green light is comparable with that of red light, and greater than that of blue light. The difference between the quantum yields of green and blue light is particularly large in woody plants grown outdoors in high light. The question of how much green light is absorbed and used in photosynthesis by the green leaves of land plants has therefore been solved. In this mini-review, however, we aim at further clarifying another important role of green light in photosynthesis, by considering the intra-leaf profiles of light absorption and photosynthetic capacity of chloroplasts.
First, we briefly explain light absorption by the leaf. Secondly, we examine the light environment within the leaf. Thirdly, we compare the vertical, intra-leaf profile of photosynthetic capacity with that of light absorption. We also discuss some serious problems with the use of pulse amplitude modulated (PAM) fluorometry in assessing leaf electron transport rate and photoinhibition. Fourthly, we propose a new method to measure the quantum yield of any monochromatic light in white light, and demonstrate the effectiveness of green light in strong white light. Based on these arguments, we finally revisit the enigmatic question of why leaves are green.
