Carboxylation capacity is the main limitation of carbon assimilation in High Arctic shrubs.

Increases in shrub height, biomass and canopy cover are key whole-plant features of warming-induced vegetation change in tundra. We investigated leaf functional traits underlying photosynthetic capacity of Arctic shrub species, particularly its main limiting processes such as mesophyll conductance. In this nutrient-limited ecosystem, we expect leaf nitrogen concentration to be the main limiting factor for photosynthesis. We measured the net photosynthetic rate at saturated light (Asat) in three Salix species throughout a glacial valley in High-Arctic tundra and used a causal approach to test relationships between leaf stomatal and mesophyll conductances (gsc, gm), carboxylation capacity (Vcmax), nitrogen and phosphorus concentration (Narea, Parea) and leaf mass ratio (LMA). Arctic Salix species showed no difference in Asat compared to a global data set, while being characterized by higher Narea, Parea and LMA. Vcmax, gsc and gm independently increased Asat, with Vcmax as its main limitation. We highlighted a nitrogen-influenced pathway for increasing photosynthesis in the two prostrate mesic habitat species. In contrast, the erect wetland habitat Salix richardsonii mainly increased Asat with increasing gsc. Overall, our study revealed high photosynthetic capacities of Arctic Salix species but contrasting regulatory pathways that may influence shrub ability to respond to environmental changes in High Arctic tundra.

When and where soil is important to modify the carbon and water economy of leaves.

Photosynthetic 'least-cost' theory posits that the optimal trait combination for a given environment is that where the summed costs of photosynthetic water and nutrient acquisition/use are minimised. The effects of soil water and nutrient availability on photosynthesis should be stronger as climate-related costs for both resources increase. Two independent datasets of photosynthetic traits, Globamax (1509 species, 288 sites) and Glob13C (3645 species, 594 sites), were used to quantify biophysical and biochemical limitations of photosynthesis and the key variable Ci /Ca (CO2 drawdown during photosynthesis). Climate and soil variables were associated with both datasets. The biochemical photosynthetic capacity was higher on alkaline soils. This effect was strongest at more arid sites, where water unit-costs are presumably higher. Higher values of soil silt and depth increased Ci /Ca , likely by providing greater H2 O supply, alleviating biophysical photosynthetic limitation when soil water is scarce. Climate is important in controlling the optimal balance of H2 O and N costs for photosynthesis, but soil properties change these costs, both directly and indirectly. In total, soil properties modify the climate-demand driven predictions of Ci /Ca by up to 30% at a global scale.

Persistent negative temperature response of mesophyll conductance in red raspberry (Rubus idaeus L.) leaves under both high and low vapour pressure deficits: a role for abscisic acid?

The temperature dependence of mesophyll conductance (gm ) was measured in well-watered red raspberry (Rubus idaeus L.) plants acclimated to leaf-to-air vapour pressure deficit (VPDL) daytime differentials of contrasting amplitude, keeping a fixed diurnal leaf temperature (Tleaf ) rise from 20 to 35 °C. Contrary to the great majority of gm temperature responses published to date, we found a pronounced reduction of gm with increasing Tleaf irrespective of leaf chamber O2 level and diurnal VPDL regime. Leaf hydraulic conductance was greatly enhanced during the warmer afternoon periods under both low (0.75 to 1.5 kPa) and high (0.75 to 3.5 kPa) diurnal VPDL regimes, unlike stomatal conductance (gs ), which decreased in the afternoon. Consequently, the leaf water status remained largely isohydric throughout the day, and therefore cannot be evoked to explain the diurnal decrease of gm . However, the concerted diurnal reductions of gm and gs were well correlated with increases in leaf abscisic acid (ABA) content, thus suggesting that ABA can induce a significant depression of gm under favourable leaf water status. Our results challenge the view that the temperature dependence of gm can be explained solely from dynamic leaf anatomical adjustments and/or from the known thermodynamic properties of aqueous solutions and lipid membranes.​.

Threshold response of mesophyll CO2 conductance to leaf hydraulics in highly transpiring hybrid poplar clones exposed to soil drying.

Mesophyll conductance (gm) has been shown to impose significant limitations to net CO2 assimilation (A) in various species during water stress. Net CO2 assimilation is also limited by stomatal conductance to water (gsw), both having been shown to co-vary with leaf hydraulic conductance (Kleaf). Lately, several studies have suggested a close functional link between Kleaf, gsw, and gm. However, such relationships could only be circumstantial since a recent study has shown that the response of gm to drought could merely be an artefactual consequence of a reduced intercellular CO2 mole fraction (Ci). Experiments were conducted on 8-week-old hybrid poplar cuttings to determine the relationship between Kleaf, gsw, and g m in clones of contrasting drought tolerance. It was hypothesized that changes in gsw and Kleaf in response to drought would not impact on gm over most of its range. The results show that Kleaf decreased in concert with g sw as drought proceeded, whereas gm measured at a normalized Ci remained relatively constant up to a g sw threshold of ~0.15 mol m(-2) s(-1). This delayed gm response prevented a substantial decline in A at the early stage of the drought, thereby enhancing water use efficiency. Reducing the stomatal limitation of droughted plants by diminishing the ambient CO2 concentration of the air did not modify gm or Kleaf. The relationship between gas exchange and leaf hydraulics was similar in both drought-tolerant and drought-sensitive clones despite their contrasting vulnerability to stem cavitation and stomatal response to soil drying. The results support the hypothesis of a partial hydraulic isolation of the mesophyll from the main transpiration pathway.

Variable mesophyll conductance revisited: theoretical background and experimental implications.

The CO(2) concentration at the site of carboxylation inside the chloroplast stroma depends not only on the stomatal conductance, but also on the conductance of CO(2) between substomatal cavities and the site of CO(2) fixation. This conductance, commonly termed mesophyll conductance (g(m) ), significantly constrains the rate of photosynthesis. Here we show that estimates of g(m) are influenced by the amount of respiratory and photorespiratory CO(2) from the mitochondria diffusing towards the chloroplasts. This results in an apparent CO(2) and oxygen sensitivity of g(m) that does not imply a change in intrinsic diffusion properties of the mesophyll, but depends on the ratio of mitochondrial CO(2) release to chloroplast CO(2) uptake. We show that this effect (1) can bias the estimation of the CO(2) 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 g(m) . We further show that the amount of photo(respiratory) CO(2) that is refixed by Rubisco can be directly estimated through measurements of g(m) .

Production of Biopharmaceuticals in Nicotiana benthamiana-Axillary Stem Growth as a Key Determinant of Total Protein Yield.

Data are scarce about the influence of basic cultural conditions on growth patterns and overall performance of plants used as heterologous production hosts for protein pharmaceuticals. Higher plants are complex organisms with young, mature, and senescing organs that show distinct metabolic backgrounds and differ in their ability to sustain foreign protein expression and accumulation. Here, we used the transient protein expression host Nicotiana benthamiana as a model to map the accumulation profile of influenza virus hemagglutinin H1, a clinically promising vaccine antigen, at the whole plant scale. Greenhouse-grown plants submitted to different light regimes, submitted to apical bud pruning, or treated with the axillary growth-promoting cytokinin 6-benzylaminopurine were vacuum-infiltrated with agrobacteria harboring a DNA sequence for H1 and allowed to express the viral antigen for 7 days in growth chamber under similar environmental conditions. Our data highlight the importance of young leaves on H1 yield per plant, unlike older leaves which account for a significant part of the plant biomass but contribute little to total antigen titer. Our data also highlight the key contribution of axillary stem leaves, which contribute more than 50% of total yield under certain conditions despite representing only one-third of the total biomass. These findings underline the relevance of both considering main stem leaves and axillary stem leaves while modeling heterologous protein production in N. benthamiana. They also demonstrate the potential of exogenously applied growth-promoting hormones to modulate host plant architecture for improvement of protein yields.

Greater efficiency of water use in poplar clones having a delayed response of mesophyll conductance to drought.

Improvement of water use efficiency is a key objective to improve the sustainability of cultivated plants, especially fast growing species with high water consumption like poplar. It is well known that water use efficiency (WUE) varies considerably among poplar genotypes, and it was recently suggested that the use of the mesophyll-to-stomatal conductance ratio (gm/gs) would be an appropriate trait to improve WUE. The responses of 7-week-old cuttings of four hybrid poplar clones and one native Balsam poplar (Populus balsamifera L.) to a water stress-recovery cycle were examined to evaluate the relation between the gm/gs ratio and transpiration efficiency (TE), a leaf-level component of WUE. A contrasting gs response to water stress was observed among the five clones, from stomatal closure early on during soil drying up to limited closure in Balsam poplar. However in the hybrids, the decline in gm was consistently delayed by a few days compared with gs. Moreover, in the most water use-efficient hybrids, the recovery following rehydration occurred faster for gm than for gs. Thus, the delay in the response of gm to drought and its faster recovery upon rewatering increased the gm/gs of the hybrids and this ratio scaled positively with TE. Our results support the use of the gm/gs ratio to select genotypes with improved WUE, and the notion that breeding strategies focusing mainly on stomatal responses to soil drying should also look for a strong curvilinearity between net carbon assimilation rate and gs, the indication of a significant increase in gm/gs in the earlier stages of stomatal closure.