Relationship between leaf development, carboxylase enzyme activities and photorespiration in the C4-plant Portulaca oleracea L.

Abstract

Ribulose diphosphate (RuDP) and (PEP) phosphoenolpyruvate carboxylase enzyme activities were studied in young, mature, and senescent Portulaca oleracea leaves. While the absolute amount of both the C₃ (RuDP) and C₄ (PEP) carboxylase is less in senescent leaves than in mature leaves, RuDP carboxylase activity is reduced to a lesser degree. In senescent leaves, PEP carboxylase activity equals 10% of that in mature tissue, but RuDP carboxylase is 27% of that in mature leaves. The same ontogenetic series was also used to determine photorespiration rates and responses to several gas treatments. Young and mature leaves were unaffected by changes in the light regime or oxygen concentrations, and exhibited typical C₄-plant light/dark ¹⁴CO₂ evolution ratios. Senescent leaves, on the other hand, have photorespiration ratios similar to C₃-plants. In addition, senescent leaves were affected by minus CO₂, 100% O₂ and N₂ in a manner expected of C₃-plants, but not C₄-plants. These results are discussed in terms of a relative increase in activity of the C₃ cycle in later developmental stages in this plant.