high-throughput-analysis-non-photochemical-quenching-crops-using

High-Throughput Analysis of Non-Photochemical Quenching in Crops Using Pulse Amplitude Modulated Chlorophyll Fluorometry

Photosynthesis is not optimized in modern crop varieties, and therefore provides an opportunity for improvement. Speeding up the relaxation of ...

University of Illinois at Urbana-Champaign

University of Cambridge

Arundo donax has attracted interest as a potential bioenergy crop due to a high apparent productivity. It uses C3 photosynthesis yet appears competitive with C4 grass biomass feedstock's and grows in warm conditions where C4 species might be expected to be that productive. Despite this there has been no systematic study of leaf photosynthetic properties. This study determines photosynthetic and photorespiratory parameters for leaves in a natural stand of A. donax growing in southern Portugal. We hypothesise that A. donax has a high photosynthetic potential in high and low light, stomatal limitation to be small and intrinsic water use efficiency unusually low. High photosynthetic rates in A. donax resulted from a high capacity for both maximum Rubisco (Vc,max 117 μmol CO2 m(-2) s(-1)) and ribulose-1:5-bisphosphate limited carboxylation rate (Jmax 213 μmol CO2 m(-2) s(-1)) under light-saturated conditions. Maximum quantum yield for light-limited CO2 assimilation was also high relative to other C3 species. Photorespiratory losses were similar to other C3 species under the conditions of measurement (25%), while stomatal limitation was high (0.25) resulting in a high intrinsic water use efficiency. Overall the photosynthetic capacity of A. donax is high compared to other C3 species, and comparable to C4 bioenergy grasses. 

High C3 photosynthetic capacity and high intrinsic water use efficiency underlies the high productivity of the bioenergy grass Arundo donax.

Prediction of stomatal conductance is a key element to relate and scale up leaf-level gas exchange processes to canopy, ecosystem and land surface models. The empirical models that are typically employed for this purpose are simple and elegant formulations which relate stomatal conductance on a leaf area basis to the net rate of CO assimilation, humidity and CO concentration. Although light intensity is not directly modelled as a stomatal opening cue, it is well-known that stomata respond strongly to light. One response mode depends specifically on the blue-light part of the light spectrum, whereas the quantitative or 'red' light response is less spectrally defined and relies more on the quantity of incident light. Here, we present a modification of an empirical stomatal conductance model which explicitly accounts for the stomatal red-light response, based on a mesophyll-derived signal putatively initiated by the chloroplastic plastoquinone redox state. The modified model showed similar prediction accuracy compared to models using a relationship between stomatal conductance and net assimilation rate. However, fitted parameter values with the modified model varied much less across different measurement conditions, lessening the need for frequent re-parameterization to different conditions required of the current model. We also present a simple and easy to parameterize extension to the widely used Farquhar-Von Caemmerer-Berry photosynthesis model to facilitate coupling with the modified stomatal conductance model, which should enable use of the new stomatal conductance model to simulate ecosystem water vapour exchange in terrestrial biosphere models.

Predicting light-induced stomatal movements based on the redox state of plastoquinone: theory and validation.

Improving photosynthetic efficiency is widely regarded as a major route to achieving much-needed yield gains in crop plants. In plants with C3 photosynthesis, increasing the diffusion conductance for CO2 transfer from substomatal cavity to chloroplast stroma (gm) could help to improve the efficiencies of CO2 assimilation and photosynthetic water use in parallel. The diffusion pathway from substomatal cavity to chloroplast traverses cell wall, plasma membrane, cytosol, chloroplast envelope membranes, and chloroplast stroma. Specific membrane intrinsic proteins of the aquaporin family can facilitate CO2 diffusion across membranes. Some of these aquaporins, such as PIP1;2 in Arabidopsis thaliana, have been suggested to exert control over gm and the magnitude of the CO2 assimilation flux, but the evidence for a direct physiological role of aquaporins in determining gm is limited. Here, we estimated gm with four different methods under a range of light intensities and CO2 concentrations in two previously characterized pip1;2 knock-out lines as well as pip1;3 and pip2;6 knock-out lines, which have not been previously evaluated for a role in gm. This study presents the most in-depth analysis of gm in Arabidopsis aquaporin knock-out mutants to date. Surprisingly, all methods failed to show any significant differences between the pip1;2, pip1;3, or pip2;6 mutants and the Col-0 control.

Photosynthetic efficiency and mesophyll conductance are unaffected in Arabidopsis thaliana aquaporin knock-out lines.

Photosynthetic induction describes the transient increase in leaf CO uptake with an increase in light. During induction, efficiency is lower than at steady state. Under field conditions of fluctuating light, this lower efficiency during induction may cost > 20% of potential crop assimilation. Accelerating induction would boost photosynthetic and resource-use efficiencies. Variation between rice accessions and potential for accelerating induction was analysed by gas exchange. Induction during shade to sun transitions of 14 accessions representing five subpopulations from the 3000 Rice Genome Project Panel (3K RGP) was analysed. Differences of 109% occurred in the CO fixed during the first 300 s of induction, 117% in the half-time to completion of induction, and 65% in intrinsic water-use efficiency during induction, between the highest and lowest performing accessions. Induction in three accessions with contrasting responses (AUS 278, NCS 771 A and IR64-21) was compared for a range of [CO ] to analyse limitations. This showed in vivo capacity for carboxylation at Rubisco (V ), and not stomata, as the primary limitation to induction, with significant differences between accessions. Variation in nonsteady-state efficiency greatly exceeded that at steady state, suggesting a new and more promising opportunity for selection of greater crop photosynthetic efficiency in this key food crop.

Variation in photosynthetic induction between rice accessions and its potential for improving productivity.

Improving leaf intrinsic water-use efficiency (), the ratio of photosynthetic CO assimilation to stomatal conductance, could decrease crop freshwater consumption.  has primarily been studied under steady-state light, but light in crop stands rapidly fluctuates. Leaf responses to these fluctuations substantially affect overall plant performance. Notably, photosynthesis responds faster than stomata to decreases in light intensity: this desynchronization results in substantial loss of . Traits that could improve  under fluctuating light, such as faster stomatal movement to better synchronize stomata with photosynthesis, show significant natural diversity in C species. However, C crops have been less closely investigated. Additionally, while modification of photosynthetic or stomatal traits independent of one another will theoretically have a proportionate effect on , in reality these traits are inter-dependent. It is unclear how interactions between photosynthesis and stomata affect natural diversity in , and whether some traits are more tractable drivers to improve . Here, measurements of photosynthesis, stomatal conductance and  under steady-state and fluctuating light, along with stomatal patterning, were obtained in 18 field-grown accessions of the C crop sorghum. These traits showed significant natural diversity but were highly correlated, with important implications for improvement of . Some features, such as gradual responses of photosynthesis to decreases in light, appeared promising for improvement of . Other traits showed tradeoffs that negated benefits to , e.g., accessions with faster stomatal responses to decreases in light, expected to benefit , also displayed more abrupt losses in photosynthesis, resulting in overall lower . Genetic engineering might be needed to break these natural tradeoffs and achieve optimal trait combinations, e.g., leaves with fewer, smaller stomata, more sensitive to changes in photosynthesis. Traits describing  at steady-state, and the change in  following decreases in light, were important contributors to overall  under fluctuating light.

Drivers of Natural Variation in Water-Use Efficiency Under Fluctuating Light Are Promising Targets for Improvement in Sorghum.

Although improving photosynthetic efficiency is widely recognized as an underutilized strategy to increase crop yields, research in this area is strongly biased towards species with C3 photosynthesis relative to C4 species. Here, we outline potential strategies for improving C4 photosynthesis to increase yields in crops by reviewing the major bottlenecks limiting the C4 NADP-malic enzyme pathway under optimal and suboptimal conditions. Recent experimental results demonstrate that steady-state C4 photosynthesis under non-stressed conditions can be enhanced by increasing Rubisco content or electron transport capacity, both of which may also stimulate CO2 assimilation at supraoptimal temperatures. Several additional putative bottlenecks for photosynthetic performance under drought, heat, or chilling stress or during photosynthetic induction await further experimental verification. Based on source-sink interactions in maize, sugarcane, and sorghum, alleviating these photosynthetic bottlenecks during establishment and growth of the harvestable parts are likely to improve yield. The expected benefits are also shown to be augmented by the increasing trend in planting density, which increases the impact of photosynthetic source limitation on crop yields.

Improving C4 photosynthesis to increase productivity under optimal and suboptimal conditions.

Water deficit currently acts as one of the largest limiting factors for agricultural productivity worldwide. Additionally, limitation by water scarcity is projected to continue in the future with the further onset of effects of global climate change. As a result, it is critical to develop or breed for crops that have increased water-use efficiency and that are more capable of coping with water-scarce conditions. However, increased intrinsic water-use efficiency (iWUE) typically comes with a trade-off to CO2 assimilation as all gas exchange is mediated by stomata, through which CO2 enters the leaf while water vapor exits. Previously, promising results were shown about using guard-cell targeted overexpression of hexokinase to increase iWUE without incurring a penalty to photosynthetic rates or biomass production. Here, two homozygous transgenic lines expressing AtHXK1 constitutively (35SHXK2 and 35SHXK5) and a line that had guard-cell targeted overexpression of AtHXK1 (GCHXK2) were evaluated relative to wild type (WT) for traits related to photosynthesis and yield. In this study significantly better iWUE was found in GCHXK2 relative to WT without negatively impacting CO2 assimilation, although results were dependent upon leaf age and proximity of precipitation event to gas exchange measurement.

Article	Year
High C3 photosynthetic capacity and high intrinsic water use efficiency underlies the high productivity of the bioenergy grass Arundo donax. Scientific reports\| PubMed ID: 26860066	2016
Predicting light-induced stomatal movements based on the redox state of plastoquinone: theory and validation. Photosynthesis research\| PubMed ID: 30891661	2019
Photosynthetic efficiency and mesophyll conductance are unaffected in Arabidopsis thaliana aquaporin knock-out lines. Journal of experimental botany\| PubMed ID: 31731291	2020
Variation in photosynthetic induction between rice accessions and its potential for improving productivity. The New phytologist\| PubMed ID: 32124982	2020
Drivers of Natural Variation in Water-Use Efficiency Under Fluctuating Light Are Promising Targets for Improvement in Sorghum. Frontiers in plant science\| PubMed ID: 33597965	2021
Improving C4 photosynthesis to increase productivity under optimal and suboptimal conditions. Journal of experimental botany\| PubMed ID: 34268575	2021
Guard-cell Targeted Overexpression of Arabidopsis Hexokinase 1 May Improve Water Use Efficiency in Field-Grown Tobacco Plants. Journal of experimental botany\| PubMed ID: 35595294	2022
Genetic variation in photosynthesis: many variants make light work. Journal of experimental botany\| PubMed ID: 35606158	2022

Johannes Kromdijk

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