Production and Action of Active Oxygen Species in Photosynthetic Tissues

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Abstract
Photosynthetic organisms have a high capacity for conversion of light energy to chemical energy under favorable conditions; eight light quanta are required for the reduction of one molecule of carbon dioxide to the level of carbohydrate; this represents 36% efficiency for 680-nm light. The high efficiency is attained only under conditions where the concentration of carbon dioxide, the availability of mineral nutrients, and other environmental conditions are not limiting, but light intensity is limiting photosynthesis. Natural environments are not always adequate to satisfy these conditions for the high photosynthetic yield. The atmospheric concentration of carbon dioxide often limits photosynthesis. Transport of CO2 to the chloroplasts is impeded by water and salt stresses. Furthermore, the combination of environmental conditions is not always favorable to photosynthesis. Rather, an unfavorable combination such as high light intensity and water stress is not unusual. In addition to carbon dioxide, the presence of oxygen, varying temperatures, and the physiological state of the leaf, especially in terms of sink capacity, have large effects on photosynthetic efficiency. As a result, the conversion efficiency of sunlight by plants under natural environments is only 3 to 4% at most, compared with the expected maximum efficiency of 12% for solar energy, even when C4 plants are cultured under optimal conditions.

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