Photosynthetic activity of stems in two Clusia species

Abstract

Key message: In stems ofClusia,CO2concentrated in the xylem sap in CAM trees can be fixed by PEPC and Rubisco, while in C3trees only Rubisco is engaged. Abstract: The photosynthetic characteristics of 7–8-year-old stems of two tropical trees representing the Clusiaceae family were compared: Clusia multiflora Kunth. described as an obligate C3 and Clusia rosea Jacq. as an obligate CAM plant. Photosynthetic gas exchange, xylem CO2 concentration, chlorophyll distribution, 13C discrimination, daily malate and citrate fluctuations and the abundance of Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase) and PEPC (phosphoenolpyruvate carboxylase) proteins were measured in leaves and stems. In stems of both species a low CO2 efflux (in the range of 0.05–0.1 µmol m−2 s−1) was observed as a result of extremely low cork conductance for water vapor (0.15–0.2 mmol m−2 s−1). This led to the CO2 concentration in xylem sap reaching 5.2 (CO2*) mmol l−1. The substantial amount of chlorophyll in the outer part of the bark and light-induced decrease of CO2 concentration within the xylem can be explained by photosynthetic activity in this tissue. Moreover, Western blotting analyses proved the presence of Rubisco in the stems of both Clusia species; however, PEPC was only found in C. rosea. Additionally, daily fluctuations in the concentration of citrate and malate (higher than in leaves) and significant enrichment in 13C in C. rosea stems were observed. These facts allow us to conclude that the examined stems of C. rosea and C.multiflora represent specific types of photosynthetic metabolism.