Virtually all current estimates of the maximum carboxylation rate ( V cmax ) of ribulose-1,5-bisphosphate carboxylase/ oxygenase (Rubisco) and the maximum electron transport rate ( J max ) for C 3 species implicitly assume an infinite CO 2 transfer conductance ( g i ). And yet, most measurements in perennial plant species or in ageing or stressed leaves show that g i imposes a significant limitation on photosynthesis. Abbreviations : A c , RuBP-saturated CO 2 assimilation rate; A j , RuBP-limited CO 2 assimilation rate; A n , net CO 2 assimilation rate; C , gas phase CO 2 concentration in an in vitro assay media; C c , chloroplastic CO 2 concentration; C i , intercellular CO 2 concentration; C i *, intercellular CO 2 photocompensation point; G *, chloroplastic CO 2 photocompensation point; G , CO 2 compensation point; g i , CO 2 transfer conductance; g s , stomatal conductance; I , incident irradiance; J , CO 2 -saturated electron transport rate; J max , maximum, light-and CO 2 -saturated electron transport rate; a , quantum efficiency (number of electrons transferred per incident photon); q , curvature factor of the nonrectangular hyperbola describing the light response of J ; K c and K o , Michaelis-Menten constants for RuBP carboxylation and oxygenation, respectively; K m (CO 2 ) i , apparent Michaelis-Menten constant for CO 2 evaluated at C i ; O , O 2 concentration; R d , mitochondrial respiration in the light; RuBP, ribulose-1,5-bisphosphate; Rubisco, ribulose-1,5-bisphosphate carboxylase/oxygenase; S c/o , Rubisco specificity factor; V cmax , maximal carboxylation rate; W c , RuBP-saturated carboxylation rate.

The CO2 concentration at the site of carboxylation inside the chloroplast stroma depends not only on the stomatal conductance, but also on the conductance of CO2 between substomatal cavities and the site of CO2 fixation. This conductance, commonly termed mesophyll conductance (gm), significantly constrains the rate of photosynthesis. Here we show that estimates of gm are influenced by the amount of respiratory and photorespiratory CO2 from the mitochondria diffusing towards the chloroplasts. This results in an apparent CO2 and oxygen sensitivity of gm that does not imply a change in intrinsic diffusion properties of the mesophyll, but depends on the ratio of mitochondrial CO2 release to chloroplast CO2 uptake. We show that this effect (1) can bias the estimation of the CO2 photocompensation point and non-photorespiratory respiration in the light; (2) can affect the estimates of ribulose 1·5-bisphosphate carboxylase/oxygenase (Rubisco) kinetic constants in vivo; and (3) results in an apparent obligatory correlation between stomatal conductance and gm. We further show that the amount of photo(respiratory) CO2 that is refixed by Rubisco can be directly estimated through measurements of gm.

The effect of nitrogen stress on needle δ 13 C, water-use efficiency (WUE) and biomass production in irrigated and dry land white spruce (Picea glauca (Moench) Voss) seedlings was investigated. Sixteen hundred seedlings, representing 10 controlled crosses, were planted in the field in individual buried sand-filled cylinders. Two nitrogen treatments were imposed, nitrogen stressed and fertilized. The ranking of δ 13 C of the crosses was maintained across all combinations of water and nitrogen treatments and there was not a significant genetic versus environmental interaction. The positive relationships between needle δ 13 C, WUE and dry matter production demonstrate that it should be possible to use δ 13 C as a surrogate for WUE, and to select for increased WUE without compromising yield, even in nitrogen deficient environments. Nitrogen stressed seedlings had the lowest needle δ 13 C in both irrigated and dry land conditions. There was a positive correlation between needle nitrogen content and δ 13 C that was likely associated with increased photosynthetic capacity. There was some indication that decreased nitrogen supply led to increased stomatal conductance and hence lower WUE. There was a negative correlation between intrinsic water use efficiency and photosynthetic nitrogen use efficiency (NUE). This suggests that white spruce seedlings have the ability to maximize NUE when water becomes limited. There was significant genetic variation in NUE that was maintained across treatments. Our results suggest that in white spruce, there is no detectable effect of anaplerotic carbon fixation and that it is more appropriate to use a value of 29‰ ('Rubisco only') for the net discrimination against 13 C during CO 2 fixation. This leads to excellent correspondence between values of C i /C a derived from gas exchange measurements or from δ 13 C.Key-words: Picea glauca, carbon isotopic composition, carbon isotope discrimination, dry matter production, leaf nitrogen content, nitrogen use efficiency, water use efficiency.Abbreviations: A, assimilation rate; a, fractionation against 13 C for CO 2 diffusion through air; b, net fractionation against

A novel A -C i curve (net CO 2 assimilation rate of a leaf -A n -as a function of its intercellular CO 2 concentration -C i ) analysis method (Plant, Cell & Environment 27, 137-153, 2004) was used to estimate the CO 2 transfer conductance ( g i ) and the maximal carboxylation ( V cmax ) and electron transport ( J max ) potentials of ageing, non-senescing Pseudotsuga menziesii leaves in relation to their nitrogen (N) content and protein and pigment composition. Both g i and the stomatal conductance ( g sc ) of leaves were closely coupled to V cmax , J max and A n with all variables decreasing with increasing leaf age. Consequently, both C i and C c (chloroplastic CO 2 concentration) remained largely conserved through successive growing seasons. The N content of leaves, as well as the amount of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and other sodium dodecyl sulfate-soluble proteins, increased during the first three growing seasons, then stabilized or decreased only slightly afterwards. Thus, the age-related photosynthetic nitrogen use efficiency (PNUE) decline of leaves was not a consequence of decreased allocation of N towards Rubisco and other proteins involved in bioenergetics and light harvesting. Rather, loss of photosynthetic capacity was the result of the decreased activation state of Rubisco and proportional down-regulation of electron transport towards the photosynthetic carbon reduction (PCR) and photorespiratory (PCO) cycles in response to a reduction of CO 2 supply to the chloroplasts' stroma. This study emphasizes the regulatory potential and homeostaticity of C c -rather than photosynthetic metabolites or C i -in relation to the commonly observed correlation between photosynthesis and g sc .Key-words : A -C i curves; CO 2 transfer conductance; leaf ageing; photosynthesis model Pseudotsuga menziesii ; Rubisco activity.Abbreviations : A c , RuBP-saturated CO 2 assimilation rate; A j , RuBP-limited CO 2 assimilation rate; A n , net CO 2 assimilation rate; α , leaf absorptance; δ 13 C, stable carbon isotope composition; ∆ , carbon isotope discrimination; C c and C i , chloroplastic and intercellular CO 2 concentration, respectively; ELISA, enzyme-linked immunosorbtion assay; Γ *, chloroplastic CO 2 photocompensation point; Γ , CO 2 compensation point; g i , CO 2 transfer conductance; g sc , stomatal conductance to CO 2 ; I , incident irradiance; J , photochemical electron transport rate; J max , maximal photochemical electron transport rate; Θ , curvature factor of the non-rectangular hyperbola describing the light response of J ; Φ , quantum yield of photochemical electron flow; K c and K o , Michaelis-Menten constants for RuBP carboxylation and oxygenation, respectively; LMA, leaf dry mass allocated per unit area; N, nitrogen; PCR, photosynthetic carbon reduction; PCO, photosynthetic carbon oxidation; PNUE, photosynthetic nitrogen use efficiency; PPFD, photosynthetic photon flux density; R d , mitochondrial respiration in the light; RuBP, ribulose-1,5-bisphosphate; Rubisco, r...