Investigating Factors That Affect the Rate of Photosynthesis
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In the presence of adequate amounts of light, higher CO2 concentrations support higher photosynthetic rates. The reverse is also true: low CO2 concentrations can limit the amount of photosynthesis in C3 plants. Carbon dioxide is a trace gas in the atmosphere, presently accounting for about 0.039%, or 390 parts per million (ppm), of air. Currently the CO2 concentration of the atmosphere is increasing by about 1 to 3 ppm each year. By 2100 the atmospheric CO2 concentration could reach 600 to 750 ppm unless fossil fuel emission are controlled. Carbon dioxide and methane, play a role similar to that of the glass roof in a greenhouse. The increased CO2 concentration and temperature associated with the greenhouse effect can influence photosynthesis. At current atmospheric CO2 concentrations, photosynthesis in C3 plants is CO2 limited, but this situation could change as atmospheric CO2 concentrations continue to rise. Under laboratory conditions, most C3 plants grow 30 to 60% faster when CO2 concentration is doubled (to 600-750 ppm), and the growth rate becomes limited by the nutrient available to the plant.
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When exposed to excess light, leaves must dissipate the surplus absorbed light energy so that it does not harm the photosynthetic apparatus. Heat production and the xanthophylls cycle appears to be important avenues for dissipation of excess light energy. The xanthophylls cycle comprises the three carotenoids violaxanthin, antheraxanthin, and zeaxanthin. Experiments have shown that zeaxanthin is the most effective of the three xanthophylls in heat dissipation. The zeaxanthin content increases at high irradiances and decreases at low irradiances. In leaves growing under full sunlight, zeaxanthin and antheraxanthin can make up 60% of the total xanthophyll cycle pool at maximal irradiance levels attained at midday (). Contrary to the diurnal cycling of this pool observed in summer, zeaxanthin levels remain high all day during the winter. Presumably this mechanism maximizes dissipation of light energy, thereby protecting the leaves against photooxidation during winter.