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PGR5 has been reported as an essential factor for the activity of the ferredoxin-dependent cyclic electron transport around photosystem I. To elucidate the role of PGR5 on C3 photosynthesis, we characterize photosynthetic electron transport rate (ETR), CO2 assimilation and growth in the Arabidopsis thaliana pgr5 mutant in various CO2 regimes. In low light grown pgr5, CO2 assimilation rate and ETR under air conditions were similar to the wild-type at low irradiance, but decreased at saturating irradiance. Under low light conditions in air, pgr5 showed same growth as the wild-type but a significant growth reduction compared to the wild-type above 150 μmol photons m−2 s−1. This growth impairment was largely suppressed under high CO2 concentration. Based on the intercellular CO2 concentration dependency of CO2 assimilation, ETR and P700 oxidation measurements, we conclude that reduction of photosynthesis and growth result from ATP deficiency and inactivation of photosystem I.

Photosynthesis can be heterogeneously distributed over a leaf, especially during periods of environmental stress. Heterogeneity has usually been explained by patchy stomatal closure [1,2] but local metabolic limitations might also be involved, e. g. during induction by light [3]. Discrimination between stomatal and metabolic limitations of photosynthesis has been usually done by measuring photosynthetic capacity under nonlimiting CO2 availability [2, 4]. We investigated the potential contribution of stomatal conductance and metabolism in determining heterogeneity of photosynthesis by exposing the leaf to short pulses of high CO2 concentration while imaging the leaf chlorophyll fluorescence. Relationship between diffusive and metabolism dependent limitations was characterised for detached leaves of Rosa rubiginosa after ABA-feeding.

CO2 diffusion into the leaf to the chloroplast not only depends on stomatal conductance but also on the conductance along the path CO2 follows into the leaf mesophyll from the substomatal cavities to the catalytic sites of Rubisco. Recent methodological advances showed that a significant drop in the CO2 partial pressure may occur along this path [1–5]. Such a drop reveals a low conductance to CO2 diffusion in the mesophyll. A lower conductance in the mesophyll has been shown in leaves of woody plants than in herbaceous plants [4–7].

Mutations that depress total chlorophyll content such as found in barley(l), maize(2), pea(3), soybean(4) and tobacco(5) generally result in an increased chlorophyll a:b ratio. Mutants of pea (3) and tobacco (5) have a decreased capacity for CO2 assimilation, and the change in the chl a:b ratio in a mutant of maize (2) reflects a reduction in LHC2. Recently a single, temperature-sensitive, recessive nuclear gene mutation of cowpea (Vigna unguiculata (L.) Walp) has been found which results in a reduced leaf chlorophyll content (6) but which exhibits similar vigour and productivity in the field compared to the parent (A.E. Hall personal communication). This study examines the changes induced by this single gene mutation in the LHC2 and PS2 contents of thylakoids and assesses their consequences for photosynthetic electron transport and carbon fixation.

Recently the development of techniques capable of measuring in vivo simultaneously the photochemical (qQ) and non-photochemical (qNP) components of chlorophyll fluorescence quenching, absorbance changes at 820 nm and photosynthetic gas exchange of leaves have allowed quantitative estimation of the quantum efficiencies of photosystems 2 (ϕPS2) and 1 (ϕPSl)and non-cyclic electron transport in leaves (1–6). These studies have provided evidence that non-photochemical processes associated with qNP could explain the observed reduction of quantum yield of open PS2 reaction centres as light intensity is increased (1–3,5,6).

The capacity to grow in a range of light conditions, which may vary in both spectral composition and intensity as a result of shading by vegetative structures, is a major determinant of plant distribution in the natural environment. The lower, shaded leaves of many crop species are also subject to shading for a large part of their development. Changes in the spectral composition and intensity of the incident light are known to alter the stoichiometries of the pigment-protein complexes in the thylakoid membranes of higher plants, and these alterations are presumed to confer an optimising benefit between the extremes of adaptation to different light environments to which any one species is able to adapt. In this study consideration of changes in light intensity and light quality is integrated using a growth system which truly mimics the natural environment. Using Silene dioica, a species with a high degree of plasticity for adaptation to sun and shade light environments, sun and shade adapted plants are compared in two respects.

Recently developed techniques that allow the estimation of the quantum efficiencies of photosystems I and II in vivo (1, 2, 3) have allowed us to adopt a quantitative approach to the analysis of the relationship between both photosystems and carbon assimilation (3, 4, 5). In this report we will describe the key findings of this and other related work which relates to the co-ordination of the quantum efficiencies of both photosystems, and the co-ordination of the thylakoids and the Calvin cycle activity.

It has been proposed that drought by restricting carbon assimilation by CO2 depletion (stomatal closure) or by biochemical alterations in the chloroplast could predispose the photosynthetic apparatus to photoinhibition. However experimental data have shown contradictory results depending upon the considered species : in Nerium oleander (1) and in low light adapted willow (2), they indicated a susceptibility to photoinhibition during drought while those made in sunflower (3) and in cotton (4) demonstrated that water stress does not modify photosynthetic apparatus susceptibility to photoinhibition. This discrepancy questions the underlying causes of this susceptibility and suggests that the water stress induced slowdown of carbon metabolism is not the main factor that predisposes the plants to photoinhibition during drought.