Adjustment of leaf photosynthesis to shade in a natural canopy: rate parameters

The present study was performed to investigate the adjustment of the rate parameters of the light and dark reactions of photosynthesis to the natural growth light in leaves of an overstorey species, Betula pendula Roth, a subcanopy species, Tilia cordata P. Mill., and a herb, Solidago virgaurea L., growing in a natural plant community in Jarvselja, Estonia. Shoots were collected from the site and individual leaves were measured in a laboratory applying a standardized routine of kinetic gas exchange, Chl fluorescence and 820 nm transmittance measurements. These measurements enabled the calculations of the quantum yield of photosynthesis and rate constants of excitation capture by photochemical and non-photochemical quenchers, rate constant for P700(+) reduction via the cytochrome b(6)f complex with and without photosynthetic control, actual maximum and potential (uncoupled) electron transport rate, stomatal and mesophyll resistances for CO2 transport, K-m(CO2) and V-m of ribulose-bisphosphate carboxylase-oxygenase (Rubisco) in vivo. In parallel, N, Chl and Rubisco contents were measured from the same leaves. No adjustment toward higher quantum yield in shade compared with sun leaves was observed, although relatively more N was partitioned to the light-harvesting machinery in shade leaves (H. Eichelmann et al., 2004). The electron transport rate through the Cyt b(6)f complex was strongly down-regulated under saturating light compared with darkness, and this was observed under atmospheric, as well as saturating CO2 concentration. In vivo V-m measurements of Rubisco were lower than corresponding reported measurements in vitro, and the k(cat) per reaction site varied widely between leaves and growth sites. The correlation between Rubisco V-m and the photosystem I density was stronger than between V-m and the density of Rubisco active sites. The results showed that the capacity of the photosynthetic machinery decreases in shade-adjusted leaves, but it still remains in excess of the actual photosynthetic rate. The photosynthetic control systems that are targeted to adjust the photosynthetic rate to meet the plant's needs and to balance the partial reactions of photosynthesis, down-regulate partial processes of photosynthesis: excess harvested light is quenched non-photochemically; excess electron transport capacity of Cyt b(6)f is down-regulated by DeltapH-dependent photosynthetic control; Rubisco is synthesized in excess, and the number of activated Rubisco molecules is controlled by photosystem I-related processes. Consequently, the nitrogen contained in the components of the photosynthetic machinery is not used at full efficiency. The strong correlation between leaf nitrogen and photosynthetic performance is not due to the nitrogen requirements of the photosynthetic apparatus, but because a certain amount of energy must be captured through photosynthesis to maintain this nitrogen within a leaf.