Contrasting anatomical and biochemical controls on mesophyll conductance across plant functional types

Variation in photosynthetic capacity of Salvia przewalskii along elevational gradients on the eastern Qinghai-Tibetan Plateau, China

Elevational variation in plant growing environment drives diversification of photosynthetic capacity, however, the mechanism behind this reaction is poorly understood. We measured leaf gas exchange, chlorophyll fluorescence, anatomical characteristics, and biochemical traits of Salvia przewalskii at elevations ranging from 2400 m to 3400 m above sea level (a.s.l) on the eastern Qinghai-Tibetan Plateau, China. We found that photosynthetic capacity showed an initial increase and then a decrease with rising elevation, and the best state observed at 2800 m a.s.l. Environmental factors indirectly regulated photosynthetic capacity by affecting stomatal conductance (gs), mesophyll conductance (gm), maximum velocity of carboxylation (Vc max), and maximum capacity for photosynthetic electron transport (Jmax). The average temperature (T) and total precipitation (P) during the growing season had the highest contribution to the variation of photosynthetic capacity of S. przewalskii in subalpine areas, which were 25% and 24%, respectively. Photosynthetic capacity was mainly affected by diffusional limitations (71%-89%), and mesophyll limitation (lm) played a leading role. The variation of gm was attributed to the effects of environmental factors on the volume fraction of intercellular air space (fias), the thickness of cell wall (Tcw), the surface of mesophyll cells and chloroplasts exposed to intercellular airspace (Sm, Sc), and plasma membrane intrinsic protein (PIPs, PIP1, PIP2), independent of carbonic anhydrase (CA). Optimization of leaf tissue structure and adaptive physiological responses enabled plants to efficiently cope with variable climate conditions of high-elevation areas, and the while maintaining high levels of carbon assimilation.

The long and tortuous path towards improving photosynthesis by engineering elevated mesophyll conductance

The growing demand for global food production is likely to be a defining issue facing humanity over the next 50 years. To tackle this challenge, there is a desire to bioengineer crops with higher photosynthetic efficiencies, to increase yields. Recently, there has been a growing interest in engineering leaves with higher mesophyll conductance (gm), which would allow CO2 to move more efficiently from the substomatal cavities to the chloroplast stroma. However, if crop yield gains are to be realised through this approach, it is essential that the methodological limitations associated with estimating gm are fully appreciated. In this review, we summarise these limitations, and outline the uncertainties and assumptions that can affect the final estimation of gm. Furthermore, we critically assess the predicted quantitative effect that elevating gm will have on assimilation rates in crop species. We highlight the need for more theoretical modelling to determine whether altering gm is truly a viable route to improve crop performance. Finally, we offer suggestions to guide future research on gm, which will help mitigate the uncertainty inherently associated with estimating this parameter.

Harnessing photosynthetic C18O16O discrimination dynamics under leaf water nonsteady state to estimate mesophyll conductance: a new, regression-based method

Mesophyll conductance (gm) is a crucial plant trait that can significantly limit photosynthesis. Measurement of photosynthetic C18O16O discrimination (Δ18O) has proved to be the only viable means of resolving gm in both C3 and C4 plants. However, the currently available methods to exploit Δ18O for gm estimation are error prone due to their inadequacy in constraining the degree of oxygen isotope exchange (θ) during mesophyll CO2 hydration. Here, we capitalized on experimental manipulation of leaf water isotopic dynamics to establish a novel, nonsteady state, regression-based approach for simultaneous determination of gm and θ from online Δ18O measurements. We demonstrated the methodological and theoretical robustness of this new Δ18O-gm estimation approach and showed through measurements on several C3 and C4 species that this approach can serve as a benchmark method against which to identify previously-unrecognized biases of the existing Δ18O-gm methods. Our results highlight the unique value of this nonsteady state-based approach for contributing to ongoing efforts toward quantitative understanding of mesophyll conductance for crop yield improvement and carbon cycle modeling.

Photosynthetic plasticity aggravates the susceptibility of magnesium-deficient leaf to high light in rapeseed plants: the importance of Rubisco and mesophyll conductance

Plants grown under low magnesium (Mg) soils are highly susceptible to encountering light intensities that exceed the capacity of photosynthesis (A), leading to a depression of photosynthetic efficiency and eventually to photooxidation (i.e., leaf chlorosis). Yet, it remains unclear which processes play a key role in limiting the photosynthetic energy utilization of Mg-deficient leaves, and whether the plasticity of A in acclimation to irradiance could have cross-talk with Mg, hence accelerating or mitigating the photodamage. We investigated the light acclimation responses of rapeseed (Brassica napus) grown under low- and adequate-Mg conditions. Magnesium deficiency considerably decreased rapeseed growth and leaf A, to a greater extent under high than under low light, which is associated with higher level of superoxide anion radical and more severe leaf chlorosis. This difference was mainly attributable to a greater depression in dark reaction under high light, with a higher Rubisco fallover and a more limited mesophyll conductance to CO2 (gm ). Plants grown under high irradiance enhanced the content and activity of Rubisco and gm to optimally utilize more light energy absorbed. However, Mg deficiency could not fulfill the need to activate the higher level of Rubisco and Rubisco activase in leaves of high-light-grown plants, leading to lower Rubisco activation and carboxylation rate. Additionally, Mg-deficient leaves under high light invested more carbon per leaf area to construct a compact leaf structure with smaller intercellular airspaces, lower surface area of chloroplast exposed to intercellular airspaces, and CO2 diffusion conductance through cytosol. These caused a more severe decrease in within-leaf CO2 diffusion rate and substrate availability. Taken together, plant plasticity helps to improve photosynthetic energy utilization under high light but aggravates the photooxidative damage once the Mg nutrition becomes insufficient.

Low internal air space in plants with crassulacean acid metabolism may be an anatomical spandrel

Crassulacean acid metabolism (CAM) is a photosynthetic adaptation found in at least 38 plant families. Typically, the anatomy of CAM plants is characterized by large photosynthetic cells and a low percentage of leaf volume consisting of internal air space (% IAS). It has been suggested that reduced mesophyll conductance (gm) arising from low % IAS benefits CAM plants by preventing the movement of CO2 out of cells and ultimately minimizing leakage of CO2 from leaves into the atmosphere during day-time decarboxylation. Here, we propose that low % IAS does not provide any adaptive benefit to CAM plants, because stomatal closure during phase III of CAM will result in internal concentrations of CO2 becoming saturated, meaning low gm will not have any meaningful impact on the flux of gases within leaves. We suggest that low % IAS is more likely an indirect consequence of maximizing the cellular volume within a leaf, to provide space for the overnight storage of malic acid during the CAM cycle.

Higher global gross primary productivity under future climate with more advanced representations of photosynthesis

Gross primary productivity (GPP) is the key determinant of land carbon uptake, but its representation in terrestrial biosphere models (TBMs) does not reflect our latest physiological understanding. We implemented three empirically well supported but often omitted mechanisms into the TBM CABLE-POP: photosynthetic temperature acclimation, explicit mesophyll conductance, and photosynthetic optimization through redistribution of leaf nitrogen. We used the RCP8.5 climate scenario to conduct factorial model simulations characterizing the individual and combined effects of the three mechanisms on projections of GPP. Simulated global GPP increased more strongly (up to 20% by 2070-2099) in more comprehensive representations of photosynthesis compared to the model lacking the three mechanisms. The experiments revealed non-additive interactions among the mechanisms as combined effects were stronger than the sum of the individual effects. The modeled responses are explained by changes in the photosynthetic sensitivity to temperature and CO2 caused by the added mechanisms. Our results suggest that current TBMs underestimate GPP responses to future CO2 and climate conditions.

Improving photosynthetic efficiency toward food security: Strategies, advances, and perspectives

Photosynthesis in crops and natural vegetation allows light energy to be converted into chemical energy and thus forms the foundation for almost all terrestrial trophic networks on Earth. The efficiency of photosynthetic energy conversion plays a crucial role in determining the portion of incident solar radiation that can be used to generate plant biomass throughout a growth season. Consequently, alongside the factors such as resource availability, crop management, crop selection, maintenance costs, and intrinsic yield potential, photosynthetic energy use efficiency significantly influences crop yield. Photosynthetic efficiency is relevant to sustainability and food security because it affects water use efficiency, nutrient use efficiency, and land use efficiency. This review focuses specifically on the potential for improvements in photosynthetic efficiency to drive a sustainable increase in crop yields. We discuss bypassing photorespiration, enhancing light use efficiency, harnessing natural variation in photosynthetic parameters for breeding purposes, and adopting new-to-nature approaches that show promise for achieving unprecedented gains in photosynthetic efficiency.

Nutrient availability regulates Deschampsia antarctica photosynthetic and stress tolerance performance in Antarctica

Deschampsia antarctica is one of the only two native vascular plants in Antarctica, mostly located in the ice-free areas of the Peninsula's coast and adjacent islands. This region is characterized by a short growing season, frequent extreme climatic events, and soils with reduced nutrient availability. However, it is unknown whether its photosynthetic and stress tolerance mechanisms are affected by the availability of nutrients to deal with this particular environment. We studied the photosynthetic, primary metabolic, and stress tolerance performance of D. antarctica plants growing on three close sites (<500 m) with contrasting soil nutrient conditions. Plants from all sites showed similar photosynthetic rates, but mesophyll conductance and photobiochemistry were more limiting (~25%) in plants growing on low-nutrient availability soils. Additionally, these plants showed higher stress levels and larger investments in photoprotection and carbon pools, most probably driven by the need to stabilize proteins and membranes, and remodel cell walls. In contrast, when nutrients were readily available, plants shifted their carbon investment towards amino acids related to osmoprotection, growth, antioxidants, and polyamines, leading to vigorous plants without appreciable levels of stress. Taken together, these findings demonstrate that D. antarctica displays differential physiological performances to cope with adverse conditions depending on resource availability, allowing it to maximize stress tolerance without jeopardizing photosynthetic capacity.