Photosynthetic and light-enhanced dark fixation of 14CO2 from the ambient atmosphere and 14C-bicarbonate infiltrated through vascular bundles in maize leaves1

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Abstract
The capacity of light-enhanced dark fixation of 14CO2 from the ambient atmosphere decayed following time-course characteristics of a first-order reaction (half-life, 1–2 min). The level of phosphoenolpyruvate in maize leaves under CO2-free air did not decrease in the dark subsequent to preillumination. These results indicate that phosphoenolpyruvate carboxylase is activated in light and quickly inactivated in the following darkness. Removal of oxygen from the atmosphere did not exert any effect on the products of light-enhanced dark fixation of 14CO2 provided from the atmosphere, the major labeled compounds being malate and aspartate. This confirms that the transfer of carboxyl carbon of C4-acids to form 3-phosphoglycerate is light-dependent. When NaH14CO3 solution was vacuum-infiltrated through vascular tissues of maize leaves, the main initial photosynthetic 14CO2 fixation products were phosphate esters. This indicates that by this technique, 14CO2 could be directly provided to the bundle sheath cells, and was fixed via the reductive pentose phosphate cycle. On the other hand, the main initial 14CO2-fixation products were malate and aspartate even when 14CO2 was provided through vascular tissues in the dark immediately following preillumination. The possible regulatory mechanisms underlying the above findings are discussed.

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