Relationship between leaf development and primary photosynthetic products in the C4 plant Portulaca oleracea L.

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Abstract
The photosynthetic products of Portulaca oleracea differ greatly depending on leaf age and length of exposure to 14CO2. Mature leaves of P. oleracea fix 14CO2 primarily into organic and amino acids during a 10-s exposure period. Less than 2% of the 14CO2 fixed appears in phosphorylated compounds. In contrast, incorporation into amino acids can account for over 60% of the total 14CO2 fixed by young leaves in an equal time period, and incorporation into alanine alone can account for up to one half of this amount. Senescent leaves display a quantitative shift of primary products toward phosphorylated compounds with a concomitant reduction of the label residing in malate and asparate. About 8 times more phosphoglyceric acid is produced in senescent leaves than in mature leaves. The aspartate/ malate ratio is not constant and depends on the length of time the leaves are exposed to 14CO2 and the age of the leaves under study. It appears as if the stage of leaf development is one of the most important factors determining the operation of a particular enzyme system in C4 plants.