Carbon assimilation by tree stems: potential involvement of phosphoenolpyruvate carboxylase

In woody species, the photosynthesis of stems, especially young branches, occurs by refixing part of the internal respiratory CO2. The present study aims to improve the physiological characterization of stem photosynthesis by examining enzymatic characteristics. During an entire growing season, three enzymatic activities that are linked to C3 and C4 metabolism were investigated in relation to the CO2 efflux and chlorophyll content of current year stems of European beech and were compared to the corresponding characteristics of leaves. High activities of phosphoenolpyruvate carboxylase (PEPC) and NADP malic enzyme were detected in stems (up to 13 times and 30 times higher in stems than in leaves, respectively), whereas Rubisco activity remained low in comparison with leaves. Stem maximal Rubisco and PEPC activities occurred at the beginning of the season when the total chlorophyll content and the CO2 assimilation rate were also maximal. Stems were characterized by a PEPC:Rubisco ratio that was equal to 2.5 [an intermediate value between that of C3-plants (about 0.1) and that of C4-plants (about 10)], whereas it was equal to 0.1 in leaves. Eight other tree species were also measured and the PEPC:Rubisco ratio was, on average, equal to 3.6. The potential role of PEPC in stem carbon assimilation is discussed in relation to its known involvement in the anaplerotic function of C3 plants and in the carbon metabolism of the C4 pathway.