On the cover: Leaves are green. This apparently implies that leaves do not use green light efficiently in photosynthesis. Then, why are leaves green? Terashima et al. (pp. 684-697) readdress this enigmatic question. Employing a newly developed 'differential quantum yield method', they quantitatively evaluated the efficiency of green or red monochromatic light in white light. The rate of leaf photosynthesis attained by a given white light was measured first and subsequently a weak monochromatic light was superimposed on the white light. The ratio of the increment in the rate of photosynthesis to the photon flux density of the monochromatic light expresses the in situ quantum yield of this monochromatic light in the white light. The results with sunflower leaves clearly showed that, in weak white light, red light was more efficient, reflecting a greater absoptance (percentage light absorption) of red light than green light. However, in medium or strong white light, green light drove leaf photosynthesis more efficiently than red light. In strong white light, the quantum yield of photosynthesis is lower in the chloroplasts located near the top surface than in those located deeper in the leaf. Thus, green light that penetrated further deeper in the mesophyll more effectively increased the rate of leaf photosynthesis. Two tri-furcated optical fibers, for white light and monochromatic light, respectively, were used to irradiate the leaf uniformly in an assimilation cuvette of a gas exchange system. Photo by Shu Ma.
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