Ribulose-1,5-Bisphosphate Carboxylase/Oxygenase (Rubisco) Content in Leaves, Assimilatory Charge and Mesophyll Conductance

Abstract

All factors, which finally influence the photosynthetic rate, can do this by influencing the activity of Rubisco (EC 4.1.1.39) and its substrate, CO₂ and RuBP concentrations. The CO₂ concentration is usually well below the Km(CO₂) of the enzyme and it is the initial slope of the kinetic curve V_M/K_M(CO₂), termed carboxylation conductance, which becomes important. The reciprocal of the conductance, carboxylation resistance must be smaller than other resistances in series, the gas phase resistance (stomatal and boundary layer) and liquid phase diffusion resistance (r_md). The carboxylation conductance in intact leaves in vivo may be found as the initial slope μ of the A vs C_c graph (A net CO₂ uptake rate, C_c, CO₂ concentration at the reaction sites) at low G values. If C_c cannot be calculated because the mesophyll diffusion resistance r_md is unknown, the closest approximation is A vs. C_w or A vs C_i plot (C_w, dissolved CO₂ concentration in cell walls, C_i, intercellular gas space CO₂ concentration). True parameters of the carboxylase can be found only from experiments carried out in nonphotorespiratory conditions (1–2% O₂), otherwise the competing oxygenase reaction consumes a part of RuBP and partially inhibits the carboxylase activity. In the present work we investigate the relationship between Rubisco content and leaf mesophyll conductance in plants grown under different light intensities and allowing leaves to adapt to changed light intensity.