, Volume 178, Issue 2, pp 258-266

Some relationships between contents of photosynthetic intermediates and the rate of photosynthetic carbon assimilation in leaves of Zea mays L.

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The relationship between the gas-exchange characteristics of attached leaves of Zea mays L. and the contents of photosynthetic intermediates was examined at different intercellular partial pressure of CO2 and at different irradiances at a constant intercellular partial pressure of CO2. (i) The behaviour of the pools of the C4-cycle intermediates, phosphoenolpyruvate and pyruvate, provides evidence for light regulation of their consumption. However, light regulation of phosphoenolpyruvate carboxylase does not influence the assimilation rate at limiting intercellular partial pressures of CO2. (ii) A close correlation between the pools of phosphoenolpyruvate and glycerate-3-phosphate exists under many different flux conditions, consistent with the notion that the pools of C4 and C3 cycles are connected via the interconversion of glycerate-3-phosphate and phosphoenolpyruvate. (iii) The ratio of triose-phosphate to glycerate-3-phosphate is used as an indicator of the availability of ATP and NADPH. Changes of this ratio with CO2 and with irradiance are compared with results obtained in C3 leaves and indicate that the mechanism of regulation of carbon assimilation by light in leaves of C4 plants may differ from that in C3 plants. (iv) The behaviour of the ribulose-1,5-bisphosphate pool with CO2 and irradiance is contrasted with the behaviour of these pools measured in leaves of C3 plants.