Shade compromises the photosynthetic efficiency of NADP-ME less than that of PEP-CK and NAD-ME C4 grasses

The high energy cost and apparently low plasticity of C4 photosynthesis compared with C3 photosynthesis may limit the productivity and distribution of C4 plants in low light (LL) environments. C4 photosynthesis evolved numerous times, but it remains unclear how different biochemical subtypes perform under LL. We grew eight C4 grasses belonging to three biochemical subtypes [NADP-malic enzyme (NADP-ME), NAD-malic enzyme (NAD-ME), and phosphoenolpyruvate carboxykinase (PEP-CK)] under shade (16% sunlight) or control (full sunlight) conditions and measured their photosynthetic characteristics at both low and high light. We show for the first time that LL (during measurement or growth) compromised the CO2-concentrating mechanism (CCM) to a greater extent in NAD-ME than in PEP-CK or NADP-ME C4 grasses by virtue of a greater increase in carbon isotope discrimination (∆P) and bundle sheath CO2 leakiness (ϕ), and a greater reduction in photosynthetic quantum yield (Φmax). These responses were partly explained by changes in the ratios of phosphoenolpyruvate carboxylase (PEPC)/initial Rubisco activity and dark respiration/photosynthesis (Rd/A). Shade induced a greater photosynthetic acclimation in NAD-ME than in NADP-ME and PEP-CK species due to a greater Rubisco deactivation. Shade also reduced plant dry mass to a greater extent in NAD-ME and PEP-CK relative to NADP-ME grasses. In conclusion, LL compromised the co-ordination of the C4 and C3 cycles and, hence, the efficiency of the CCM to a greater extent in NAD-ME than in PEP-CK species, while CCM efficiency was less impacted by LL in NADP-ME species. Consequently, NADP-ME species are more efficient at LL, which could explain their agronomic and ecological dominance relative to other C4 grasses.This research was funded by the Australian Research Council: DP120101603 (OG and SMW), DE130101760 (RES), and CE140100015 (OG and SMW)

C4 photosynthesis: 50 years of discovery and innovation

It is now over half a century since the biochemical characterization of the C4 photosynthetic pathway, and this special issue highlights the sheer breadth of current knowledge. New genomic and transcriptomic information shows that multi-level regulation of gene expression is required for the pathway to function, yet we know it to be one of the most dynamic examples of convergent evolution. Now, a focus on the molecular transition from C3– C4 intermediates, together with improved mathematical models, experimental tools and transformation systems, holds great promise for improving C4 photosynthesis in crops

Cyclic electron flow and light partitioning between the two photosystems in leaves of plants with different functional types

Cyclic electron flow (CEF) around photosystem I (PSI) is essential for generating additional ATP and enhancing efficient photosynthesis. Accurate estimation of CEF requires knowledge of the fractions of absorbed light by PSI (fI) and PSII (fII), which are only known for a few model species such as spinach. No measures of fI are available for C4 grasses under different irradiances. We developed a new method to estimate (1) fII in vivo by concurrently measuring linear electron flux through both photosystems (LEFO2) in leaf using membrane inlet mass spectrometry (MIMS) and total electron flux through PSII (ETR2) using chlorophyll fluorescence by a Dual-PAM at low light and (2) CEF as ETR1—LEFO2. For a C3 grass, fI was 0.5 and 0.4 under control (high light) and shade conditions, respectively. C4 species belonging to NADP-ME and NAD-ME subtypes had fI of 0.6 and PCK subtype had 0.5 under control. All shade-grown C4 species had fI of 0.6 except for NADP-ME grass which had 0.7. It was also observed that fI ranged between 0.3 and 0.5 for gymnosperm, liverwort and fern species. CEF increased with irradiance and was induced at lower irradiances in C4 grasses and fern relative to other species. CEF was greater in shade-grown plants relative to control plants except for C4 NADP-ME species. Our study reveals a range of CEF and fI values in different plant functional groups. This variation must be taken into account for improved photosynthetic calculations and modelling

Cyclic electron flow and light partitioning between the two photosystems in leaves of plants with different functional types

Cyclic electron flow (CEF) around photosystem I (PSI) is essential for generating additional ATP and enhancing efficient photosynthesis. Accurate estimation of CEF requires knowledge of the fractions of absorbed light by PSI (fI) and PSII (fII), which are only known for a few model species such as spinach. No measures of fI are available for C4 grasses under different irradiances. We developed a new method to estimate (1) fII in vivo by concurrently measuring linear electron flux through both photosystems (LEFO2) in leaf using membrane inlet mass spectrometry (MIMS) and total electron flux through PSII (ETR2) using chlorophyll fluorescence by a Dual-PAM at low light and (2) CEF as ETR1—LEFO2. For a C3 grass, fI was 0.5 and 0.4 under control (high light) and shade conditions, respectively. C4 species belonging to NADP-ME and NAD-ME subtypes had fI of 0.6 and PCK subtype had 0.5 under control. All shade-grown C4 species had fI of 0.6 except for NADP-ME grass which had 0.7. It was also observed that fI ranged between 0.3 and 0.5 for gymnosperm, liverwort and fern species. CEF increased with irradiance and was induced at lower irradiances in C4 grasses and fern relative to other species. CEF was greater in shade-grown plants relative to control plants except for C4 NADP-ME species. Our study reveals a range of CEF and fI values in different plant functional groups. This variation must be taken into account for improved photosynthetic calculations and modelling.JVS gratefully acknowledges the award of a Higher Degree Research Scholarship funded through the Centre of Excellence for Translational Photosynthesis and Western Sydney Universit

Water availability affects seasonal CO₂-induced photosynthetic enhancement in herbaceous species in a periodically dry woodland

Elevated atmospheric CO2 (eCO2) is expected to reduce the impacts of drought and increase photosynthetic rates via two key mechanisms: first, through decreased stomatal conductance (gs) and increased soil water content (VSWC) and second, through increased leaf internal CO2 (Ci) and decreased stomatal limitations (Slim>). It is unclear if such findings from temperate grassland studies similarly pertain to warmer ecosystems with periodic water deficits. We tested these mechanisms in three important C3 herbaceous species in a periodically dry Eucalyptus woodland and investigated how eCO2-induced photosynthetic enhancement varied with seasonal water availability, over a 3 year period. Leaf photosynthesis increased by 10%–50% with a 150 μmol mol-1 increase in atmospheric CO2 across seasons. This eCO2-induced increase in photosynthesis was a function of seasonal water availability, given by recent precipitation and mean daily VSWC. The highest photosynthetic enhancement by eCO2 (>30%) was observed during the most water-limited period, for example, with VSWC 2 there was neither a significant decrease in gs in the three herbaceous species, nor increases in VSWC, indicating no “water-savings effect” of eCO2. Periods of low VSWC showed lower gs (less than ≈ 0.12 mol m-2 s-1), higher relative Slim (>30%) and decreased Ci under the ambient CO2 concentration (aCO2), with leaf photosynthesis strongly carboxylation-limited. The alleviation of Slim by eCO2 was facilitated by increasing Ci, thus yielding a larger photosynthetic enhancement during dry periods. We demonstrated that water availability, but not eCO2, controls gs and hence the magnitude of photosynthetic enhancement in the understory herbaceous plants. Thus, eCO2 has the potential to alter vegetation functioning in a periodically dry woodland understory through changes in stomatal limitation to photosynthesis, not by the “water-savings effect” usually invoked in grasslands

Precise phenotyping for improved crop quality and management in protected cropping : a review

Protected cropping produces more food per land area than field-grown crops. Protected cropping includes low-tech polytunnels utilizing protective coverings, medium-tech facilities with some environmental control, and high-tech facilities such as fully automated glasshouses and indoor vertical farms. High crop productivity and quality are maintained by using environmental control systems and advanced precision phenotyping sensor technologies that were first developed for broadacre agricultural and can now be utilized for protected-cropping applications. This paper reviews the state of the global protected-cropping industry and current precision phenotyping methodology and technology that is used or can be used to advance crop productivity and quality in a protected growth environment. This review assesses various sensor technologies that can monitor and maintain microclimate parameters, as well as be used to assess plant productivity and produce quality. The adoption of precision phenotyping technologies is required for sustaining future food security and enhancing nutritional quality

Current technologies and target crops : a review on Australian protected cropping

Protected cropping offers a way to bolster food production in the face of climate change and deliver healthy food sustainably with fewer resources. However, to make this way of farming economically viable, we need to consider the status of protected cropping in the context of available technologies and corresponding target horticultural crops. This review outlines existing opportunities and challenges that must be addressed by ongoing research and innovation in this exciting but complex field in Australia. Indoor farm facilities are broadly categorised into the following three levels of technological advancement: low-, medium- and high-tech with corresponding challenges that require innovative solutions. Furthermore, limitations on indoor plant growth and protected cropping systems (e.g., high energy costs) have restricted the use of indoor agriculture to relatively few, high value crops. Hence, we need to develop new crop cultivars suitable for indoor agriculture that may differ from those required for open field production. In addition, protected cropping requires high start-up costs, expensive skilled labour, high energy consumption, and significant pest and disease management and quality control. Overall, protected cropping offers promising solutions for food security, while reducing the carbon footprint of food production. However, for indoor cropping production to have a substantial positive impact on global food security and nutritional security, the economical production of diverse crops will be essential

Linking photosynthesis and leaf N allocation under future elevated CO2 and climate warming in Eucalyptus globulus

Leaf-level photosynthetic processes and their environmental dependencies are critical for estimating CO2 uptake from the atmosphere. These estimates use biochemical-based models of photosynthesis that require accurate Rubisco kinetics. We investigated the effects of canopy position, elevated atmospheric CO2 [eC; ambient CO2 (aC)+240 ppm] and elevated air temperature (eT; ambient temperature (aT)+3 °C) on Rubisco content and activity together with the relationship between leaf N and Vcmax (maximal Rubisco carboxylation rate) of 7 m tall, soil-grown Eucalyptus globulus trees. The kinetics of E. globulus and tobacco Rubisco at 25 °C were similar. In vitro estimates of Vcmax derived from measures of E. globulus Rubisco content and kinetics were consistent, although slightly lower, than the in vivo rates extrapolated from gas exchange. In E. globulus, the fraction of N invested in Rubisco was substantially lower than for crop species and varied with treatments. Photosynthetic acclimation of E. globulus leaves to eC was underpinned by reduced leaf N and Rubisco contents; the opposite occurred in response to eT coinciding with growth resumption in spring. Our findings highlight the adaptive capacity of this key forest species to allocate leaf N flexibly to Rubisco and other photosynthetic proteins across differing canopy positions in response to future, warmer and elevated [CO2] climates.This is a contribution from the Hawkesbury Forest Experiment. We thank Burhan Amiji and Dr Craig Barton for their assistance in undertaking gas exchange. This research was supported by funding from ARC grant DP160102452, the Forest Industries Climate Change Research Fund from the Australian Department of Agriculture, and the Commonwealth Government through the Education Investment Fund

Short-term thermal photosynthetic responses of C4 grasses are independent of the biochemical subtype

C4 photosynthesis evolved independently many times, resulting in multiple biochemical pathways; however, little is known about how these different pathways respond to temperature. We investigated the photosynthetic responses of eight C4 grasses belonging to three biochemical subtypes (NADP-ME, PEP-CK, and NAD-ME) to four leaf temperatures (18, 25, 32, and 40 °C). We also explored whether the biochemical subtype influences the thermal responses of (i) in vitro PEPC (Vpmax) and Rubisco (Vcmax) maximal activities, (ii) initial slope (IS) and CO2-saturated rate (CSR) derived from the A-Ci curves, and (iii) CO2 leakage out of the bundle sheath estimated from carbon isotope discrimination. We focussed on leakiness and the two carboxylases because they determine the coordination of the CO2-concentrating mechanism and are important for parameterizing the semi-mechanistic C4 photosynthesis model. We found that the thermal responses of Vpmax and Vcmax, IS, CSR, and leakiness varied among the C4 species independently of the biochemical subtype. No correlation was observed between Vpmax and IS or between Vcmax and CSR; while the ratios Vpmax/Vcmax and IS/CSR did not correlate with leakiness among the C4 grasses. Determining mesophyll and bundle sheath conductances in diverse C4 grasses is required to further elucidate how C4 photosynthesis responds to temperature.BVS was supported by a postgraduate research award funded by the Australian Research Council and the Hawkesbury Institute for the Environment at Western Sydney University. This research was funded by the following grants from the Australian Research Council: DP120101603 (OG, SMW), DE130101760 (RES), and CE140100015 (OG, SvC, SMW)

Sugar sensing responses to low and high light in leaves of the C4 model grass Setaria viridis

Although sugar regulate photosynthesis, the signalling pathways underlying this process remain elusive, especially for C4 crops. To address this knowledge gap and identify potential candidate genes, we treated Setaria viridis (C4 model) plants acclimated to medium light intensity (ML, 500 µmol m-2 s-1) with low (LL, 50 µmol m-2 s-1) or high (HL, 1000 µmol m-2 s-1) light for 4 days and observed the consequences on carbon metabolism and the transcriptome of source leaves. LL impaired photosynthesis and reduced leaf content of signalling sugars (glucose, sucrose and trehalose-6-phosphate). Contrastingly, HL strongly induced sugar accumulation without repressing photosynthesis. LL more profoundly impacted leaf transcriptome, including photosynthetic genes. LL and HL contrastingly altered the expression of HXK and SnRK1 sugar sensors and trehalose pathway genes. The expression of key target genes of HXK and SnRK1 were affected by LL and sugar depletion, while surprisingly HL and strong sugar accumulation only slightly repressed the SnRK1 signalling pathway. In conclusion, we demonstrate that LL profoundly impacted photosynthesis and the transcriptome of S. viridis source leaves, while HL altered sugar levels more than LL. We also present the first evidence that sugar signalling pathways in C4 source leaves may respond to light intensity and sugar accumulation differently to C3 source leaves