Is there scope for improving balance between RuBP-regeneration and carboxylation capacities in wheat at elevated CO2?

High Photosynthetic Rates in a Solanum pennellii Chromosome 2 QTL Is Explained by Biochemical and Photochemical Changes

Enhanced photosynthesis is strictly associated with to productivity and it can be accomplished by genetic approaches through identification of genetic variation. By using a Solanum pennellii introgression lines (ILs) population, it was previously verified that, under normal (CO2), IL 2-5 and 2-6 display increased photosynthetic rates by up to 20% in comparison with their parental background (M82). However, the physiological mechanisms involved in the enhanced CO2 assimilation exhibited by these lines remained unknown, precluding their use for further biotechnological applications. Thereby, here we attempted to uncover the physiological factors involved in the upregulation of photosynthesis in ILs 2-5 and 2-6 under normal (CO2) as well as under elevated (CO2). The results provide evidence for increased biochemical capacity (higher maximum carboxylation velocity and maximum electron transport rate) in plants from IL 2-5 and 2-6, whereas the diffusive components (stomatal and mesophyll conductances) were unaltered in these ILs in comparison to M82. Our analyses revealed that the higher photosynthetic rate observed in these ILs was associated with higher levels of starch as well as total protein levels, specially increased RuBisCO content. Further analyses performed in plants under high (CO2) confirmed that biochemical properties are involved in genetic variation on chromosome 2 related to enhanced photosynthesis.

The use and misuse of V(c,max) in Earth System Models

Earth System Models (ESMs) aim to project global change. Central to this aim is the need to accurately model global carbon fluxes. Photosynthetic carbon dioxide assimilation by the terrestrial biosphere is the largest of these fluxes, and in many ESMs is represented by the Farquhar, von Caemmerer and Berry (FvCB) model of photosynthesis. The maximum rate of carboxylation by the enzyme Rubisco, commonly termed V c,max, is a key parameter in the FvCB model. This study investigated the derivation of the values of V c,max used to represent different plant functional types (PFTs) in ESMs. Four methods for estimating V c,max were identified; (1) an empirical or (2) mechanistic relationship was used to relate V c,max to leaf N content, (3) V c,max was estimated using an approach based on the optimization of photosynthesis and respiration or (4) calibration of a user-defined V c,max to obtain a target model output. Despite representing the same PFTs, the land model components of ESMs were parameterized with a wide range of values for V c,max (-46 to +77% of the PFT mean). In many cases, parameterization was based on limited data sets and poorly defined coefficients that were used to adjust model parameters and set PFT-specific values for V c,max. Examination of the models that linked leaf N mechanistically to V c,max identified potential changes to fixed parameters that collectively would decrease V c,max by 31% in C3 plants and 11% in C4 plants. Plant trait data bases are now available that offer an excellent opportunity for models to update PFT-specific parameters used to estimate V c,max. However, data for parameterizing some PFTs, particularly those in the Tropics and the Arctic are either highly variable or largely absent.

Food security: increasing yield and improving resource use efficiency

Food production and security will be a major issue for supplying an increasing world population. The problem will almost certainly be exacerbated by climate change. There is a projected need to double food production by 2050. In recent times, the trend has been for incremental modest yield increases for most crops. There is an urgent need to develop integrated and sustainable approaches that will significantly increase both production per unit land area and the resource use efficiency of crops. This review considers some key processes involved in plant growth and development with some examples of ways in which molecular technology, plant breeding and genetics may increase the yield and resource use efficiency of wheat. The successful application of biotechnology to breeding is essential to provide the major increases in production required. However, each crop and each specific agricultural situation presents specific requirements and targets for optimisation. Some increases in production will come about as new varieties are developed which are able to produce satisfactory crops on marginal land presently not considered appropriate for arable crops. Other new varieties will be developed to increase both yield and resource use efficiency on the best land.

Acclimation to future atmospheric CO2 levels increases photochemical efficiency and mitigates photochemistry inhibition by warm temperatures in wheat under field chambers

A study was conducted over 2 years to determine whether growth under elevated CO(2) (700 μmol mol(-1) ) and temperature (ambient + 4 °C) conditions modifies photochemical efficiency or only the use of electron transport products in spring wheat grown in field chambers. Elevated atmospheric CO(2) concentrations increased crop dry matter at maturity by 12-17%, while above-ambient temperatures did not significantly affect dry matter yield. In measurements with ambient CO(2) at ear emergence and after anthesis, growth at elevated CO(2) concentrations decreased flag leaf light-saturated carbon assimilation. The quantum yield of electron transport (Φ(PSII) ) measured at ambient CO(2) and higher irradiances increased at ear emergence and decreased after anthesis in plants grown at elevated CO(2) . At higher light intensities, but not in low light, photochemical quenching (qP) decreased after growth in elevated CO(2) conditions. Growth under CO(2) enrichment increased dark- (Fv:Fm) and light-adapted (Fv':Fm') photochemical efficiencies, and decreased the chlorophyll a:b ratio, suggesting an increase in light-harvesting complexes relative to PSII reaction centres. A relatively higher decrease in carbon assimilation than the decrease in Φ(PSII) pointed to a sink other than CO(2) assimilation for electron transport products at defined growth stages. With higher light intensities, warmer temperatures increased Φ(PSII) and Fv':Fm' at ear emergence and decreased Φ(PSII) after anthesis; in ambient-but not elevated-CO(2) , warmer temperatures also decreased qP after anthesis. CO(2) fixation increased or did not change with temperature, depending on the growth stage and year. We conclude that elevated CO(2) decreases the carbon assimilation capacity, but increases photochemistry and resource allocation to light harvesting, and that elevated levels of CO(2) can mitigate photochemistry inhibition as a result of warm temperatures.

Photosynthetic Acclimation in Rice Leaves to Free-air CO2 Enrichment Related to Both Ribulose-1,5-bisphosphate Carboxylation Limitation and Ribulose-1,5-bisphosphate Regeneration Limitation

Net photosynthetic rates (Pns) in leaves were compared between rice plants grown in ambient air control and free-air CO2 enrichment (FACE, about 200 micromol mol(-1) above ambient) treatment rings. When measured at the same CO2 concentration, the Pn of FACE leaves decreased significantly, indicating that photosynthetic acclimation to high CO2 occurs. Although stomatal conductance (Gs) in FACE leaves was markedly decreased, intercellular CO2 concentrations (Ci) were almost the same in FACE and ambient leaves, indicating that the photosynthetic acclimation is not caused by the decreased Gs. Furthermore, carboxylation efficiency and maximal Pn, both light and CO2-saturated Pn, were decreased in FACE leaves, as shown by the Pn-Ci curves. In addition, the soluble protein, Rubisco (ribulose-1,5-bisphosphate caboxylase/oxygenase), and its activase contents as well as the sucrose-phosphate synthase activity decreased significantly, while some soluble sugar, inorganic phosphate, chlorophyll and light-harvesting complex II (LHC II) contents increased in FACE leaves. It appears that the photosynthetic acclimation in rice leaves is related to both ribulose-1,5-bisphosphate (RuBP) carboxylation limitation and RuBP regeneration limitation.

Reading a CO2 signal from fossil stomata

The inverse relationship between atmospheric CO₂ and the stomatal index (proportion of epidermal cells that are stomata) of vascular land plant leaves has led to the use of fossil plant cuticles for determining ancient levels of CO₂ . In contemporary plants the stomatal index repeatedly shows a lower sensitivity atmospheric CO₂ levels above 340 ppm in the short term. These observations demonstrate that the phenotypic response is nonlinear and may place constraints on estimating higher-than-present palaeo-CO₂ levels in this way. We review a range of evidence to investigate the nature of this nonlinearity. Our new data, from fossil Ginkgo cuticles, suggest that the genotypic response of fossil Ginkgo closely tracks the phenotypic response seen in CO₂ enrichment experiments. Reconstructed atmospheric CO₂ values from fossil Ginkgo cuticles compare well with the stomatal ratio method of obtaining a quantitative CO₂ signal from extinct fossil plants, and independent geochemical modelling studies of the long-term carbon cycle. Although there is self-consistency between palaeobiological and geochemical CO₂ estimates, it should be recognized that the nonlinear response is a limitation of the stomatal approach to estimating high palaeo-CO₂ levels.

Effects of low atmospheric CO(2) on plants: more than a thing of the past

In recent geological time, atmospheric CO(2) concentrations were 25-50% below the current level. Photosynthetic productivity of C(3) plants is substantially reduced at these low CO(2) levels, particularly at higher temperatures and during stress. Acclimation of photosynthesis to reduced CO(2) levels might compensate for some of this inhibition, but plants have a limited capacity to modulate Rubisco and other photosynthetic proteins following CO(2) reduction. Because of this, low CO(2) probably acted as a significant evolutionary agent, selecting plants adapted to CO(2) deficiency. Adaptations to low CO(2) might still exist in plants and might constrain responses to a rising CO(2) concentration.