Photosynthetic parameters of leaves of wild type and Cyt b₆/f deficient transgenic tobacco studied by CO₂ uptake and transmittance at 800 nm

Parallel measurements of CO₂ assimilation and 800 nm transmission were carried out on intact leaves of wild type and cytochrome b₆/f deficient transgenic tobacco grown at different light intensities and temperatures, with the aim to diagnose rate-limiting processes in photosynthesis and investigate their adaptations to growth conditions. Maximum CO₂- and light-saturated photosynthetic rate, mesophyll conductance, assimilatory charge and specific carboxylation efficiency were determined from CO₂ fixation measurements and postillumination P700 rereduction time constant was measured from the transient of the 800 nm signal. Results show that growth conditions continue to modulate the expression of genes in transgenic plants, interfering with the antisense modulation, but under all environmental conditions the antisense treatment to decrease Cyt b₆/f complexes ensured that the control of electron/proton transport rate by proton backpressure on the PSI donor side was stronger than the control by electron backpressure on the PSI acceptor side. Coordinated control of gene expression and enzyme activation ensures that different parts of the photosynthetic machinery - components of the electron transport chain, ribulose-1,5-bisphosphate carboxylase/oxygenase, enzymes of the sucrose and starch synthesis chains - are synthesized more or less proportionally under different environmental conditions and in case of mild genetic interference.