Original Paper Abstract: The global modelling of photosynthesis is based on exact knowledge of the leaf photosynthetic machinery. The ca- pacities of partial reactions of leaf photosynthesis develop at dif- ferent rates, but it is not clear how the development of photore- actions and the Calvin cycle are co-ordinated. We investigated the development of foliar photosynthesis in the temperate de- ciduous tree Betula pendula Roth. using a unique integrated op- tical/gas exchange methodology that allows simultaneous esti- mation of photosystem I and II (PS I and PS II) densities per leaf area, interphotosystem electron transport activities, and ribu- lose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) kinetic properties. We combined these measurements with in vitro de- terminations of Rubisco, soluble protein and chlorophyll con- tents. We observed a strong increase in leaf photosynthetic ca- pacity in developing leaves per leaf area, as well as per dry mass, that was paralleled by accumulation of leaf Rubisco. Enhanced mesophyll conductance was the outcome of increased carboxy- lation capacity and increased CO 2 diffusion conductance. How- ever, Rubisco was only partly activated in the leaves, according to in vivo measurements of Rubisco kinetics. The amount of ac- tive Rubisco increased in proportion with development of PS I, probably through a direct link between Rubisco activase and PS I electron transport. Since the kinetics for post-illumination P700 re-reduction did not change, the synthesis of cytochrome b 6 f complex was also proportional to PS I. The synthesis of PS II began later and continued for several days after reaching the full PS I activity, but leaf chlorophyll was shared equally between the photosystems. Due to this, the antenna of PS II was very large and not optimally organized, leading to greater losses of excita- tion and lower quantum yields in young leaves. We conclude that co-ordinated development of leaf photosynthesis is regu- lated at the level of PS I with subordinated changes in PS II con- tent and Rubisco activation. Key words: Photosystems, quantum yield, Rubisco activation, mesophyll diffusion resistance. Abbreviations: AC: assimilatory charge (post-illumination CO 2 fixation) Cyt b 6 f : cytochrome b 6 f complex LHCII and LHCI: total antenna size of PS II and PS I M A : dry matter mass per leaf area PI: plastochone index PQ: plastoquinone PS II and PS I: photosystem II and photosystem I P700: donor pigment of PS I RuBP: ribulose 1,5-bisphosphate Rubisco: ribulose 1,5-bisphosphate carboxylase oxygenase Introduction The adjustment of the biosphere to global change can be pre- dicted by mathematical modelling of canopy photosynthesis. The parametrization of these models relies upon the availabil- ity of experimental data obtained in situ, at the natural growth site, and systematically covering the lifespan of leaves. The general limitation of experiments on leaf development has been that the leaf photosynthetic apparatus was only partly characterized, or was assessed by combining not fully compa- rable in vivo and in vitro techniques. Thus, the conclusions on controlling factors of leaf photosynthesis have had to rely on a synthesis of many independent studies (S Ï estµk et al., 1985c). To overcome this limitation, we developed a gas exchange/op- tical routine that provides a unique opportunity to assess the in vivo activity of many partial reactions of photosynthesis (Laisk et al., 2002). In this work, we apply this routine to in- vestigate the development of the photosynthetic machinery in expanding birch leaves. Leaf photosynthetic potentials increase after bud burst, reach a stable value in mature leaves, and decrease with leaf age. Thus, canopy photosynthesis is dependent on leaf development and senescence during significant periods of the growing season (Wilson et al., 2000; Wilson et al., 2001), particularly in spe- cies continuously forming leaves. Although the developmental responses importantly modify the canopy photosynthetic pro- ductivity, it is still not clear whether the leaf ontogenetic de- velopment represents an increase in bulk leaf photosynthetic capacity, with all components of photosynthetic machinery Development of Leaf Photosynthetic Parameters in Betula pendula Roth Leaves: Correlations with Photosystem I Density H. Eichelmann, V. Oja, B. Rasulov, E. Padu, I. Bichele, H. Pettai, Ü. Niinemets, and A. Laisk Department of Plant Physiology, Institute of Molecular and Cell Biology, University of Tartu, Tartu, Estonia Received: November 7, 2003; Accepted: February 4, 2004 Plant Biology 6 (2004): 307±318  Georg Thieme Verlag KG Stuttgart ´ New York ISSN 1435-8603 ´ DOI 10.1055/s-2004-820874 307