Atmospheric CO2 (ca) has increased since the last glacial period, increasing photosynthetic water use efficiency and improving plant productivity. Evolution of C4 photosynthesis at low ca led to decreased stomatal conductance (gs), which provided an advantage over C3 plants that may be reduced by rising ca. Using controlled environments, we determined how increasing ca affects C4 water use relative to C3 plants. Leaf gas exchange and mass per area (LMA) were measured for four C3 and four C4 annual, crop-related grasses at glacial (200 µmol mol-1), ambient (400 µmol mol-1), and super-ambient (640 µmol mol-1) ca. C4 plants had lower gs, which resulted in a water use efficiency advantage at all ca and was broadly consistent with slower stomatal responses to shade, indicating less pressure on leaf water status. At glacial ca, net CO2 assimilation and LMA were lower for C3 than for C4 leaves, and C3 and C4 grasses decreased leaf hydraulic conductance (Kleaf) similarly, but only C4 leaves decreased osmotic potential at turgor loss. Greater carbon availability in C4 leaves at glacial ca generated a different hydraulic adjustment relative to C3 plants. At current and future ca, C4 grasses have advantages over C3 grasses due to lower gs, lower stomatal sensitivity, and higher absolute water use efficiency.
The rise in atmospheric [CO2] is associated with increasing air temperature. However, studies on plant responses to interactive effects of [CO2] and temperature are limited, particularly for leaf structural attributes. In this study, Eucalyptus saligna plants were grown in sun-lit glasshouses differing in [CO2] (290, 400, and 650 µmol mol–1) and temperature (26 °C and 30 °C). Leaf anatomy and chloroplast parameters were assessed with three-dimensional confocal microscopy, and the interactive effects of [CO2] and temperature were quantified. The relative influence of leaf structural attributes and chemical properties on the variation of leaf mass per area (LMA) and photosynthesis within these climate regimes was also determined. Leaf thickness and mesophyll size increased in higher [CO2] but decreased at the warmer temperature; no treatment interaction was observed. In pre-industrial [CO2], warming reduced chloroplast diameter without altering chloroplast number per cell, but the opposite pattern (reduced chloroplast number per cell and unchanged chloroplast diameter) was observed in both current and projected [CO2]. The variation of LMA was primarily explained by total non-structural carbohydrate (TNC) concentration rather than leaf thickness. Leaf photosynthetic capacity (light- and [CO2]-saturated rate at 28 °C) and light-saturated photosynthesis (under growth [CO2] and temperature) were primarily determined by leaf nitrogen contents, while secondarily affected by chloroplast gas exchange surface area and chloroplast number per cell, respectively. In conclusion, leaf structural attributes are less important than TNC and nitrogen in affecting LMA and photosynthesis responses to the studied climate regimes, indicating that leaf structural attributes have limited capacity to adjust these functional traits in a changing climate.
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.
We examined the hypothesis that root and shoot factors influence growth responses to elevated CO2 of the C4 grass Panicum coloratum var. makarikiense cv. Bambatsi (NAD-ME malic enzyme subtype) when well watered and droughted. Plants were grown at CO2 partial pressures (pCO2) of 36 (ambient) and 100 Pa (elevated) in pot ed soil in growth chambers for 3 weeks with adequate water (day 0) before being subjected to 15 d of drought. At day 15, enhancement of shoot growth by elevated pCO2 was 70% under drought, and 44% when well watered. During the drought period, leaf CO2 assimilation rates (A) and stomatal conductance (g) (measured at 36 Pa CO2) declined after day 2, but the decline was faster at 36 Pa CO2, and by day 9, A was negligible and intercellular pCO2 had sharply increased compared with 100 Pa CO2. Changes in carbon metabolism and water relations occurred during drought and elevated CO2 generally delayed these changes. Leaf growth rates were higher at elevated CO2 at day 0 and during drought. Importantly, the decline in soil water content was slower at elevated pCO2 due to lower transpiration rates. This explained the slower decline in A, gand shoot water relations at elevated CO2 and indicates that root factors were responsible for their decline. In contrast, leaf growth rates were higher at elevated CO2, irrespective of soil water content. We conclude that both soil and leaf factors contribute to the greater growth response of P. coloratum to high CO2 under drought, and that reduced transpiration rates explains their enhanced growth.
Abstract Environmental change requires more crop production per water use to meet the rising global food demands. However, improving crop intrinsic water use efficiency (iWUE) usually comes at the expense of carbon assimilation. Sorghum is a key crop in many vulnerable agricultural systems with higher tolerance to water stress (WS) than most widely planted crops. To investigate physiological controls on iWUE and its inheritance in sorghum, we screened 89 genotypes selected based on inherited haplotypes from an elite line or five exotics lines, containing a mix of geographical origins and dry versus milder climates, which included different aquaporin (AQP) alleles. We found significant variation among key highly heritable gas exchange and hydraulic traits, with some being significantly affected by variation in haplotypes among parental lines. Plants with a higher proportion of the non-stomatal component of iWUE still maintained iWUE under WS by maintaining photosynthetic capacity, independently of reduction in leaf hydraulic conductance. Haplotypes associated with two AQPs (SbPIP1.1 and SbTIP3.2) influenced iWUE and related traits. These findings expand the range of traits that bridge the trade-off between iWUE and productivity in C4 crops, and provide possible genetic regions that can be targeted for breeding.
The increasing demand for quality, year-round food production in limited space has led to the widespread adoption of protected cropping. Effectively monitoring and maintaining crops within these facilities requires substantial labour and expertise. Traditional manual monitoring is labour intensive and time consuming. Therefore, non-destructive image-based techniques, particularly those utilising 3D structural data, have gained attention. We developed a stereo vision-based system to estimate the height of vertically supported tall plants in protected facilities, given plant height serves as a vital measure of crop growth. Our system uses a mobile platform with a top-angle view of a stereo vision depth camera for data acquisition and machine learning in its core for data analysis. First, we collected weekly RGB and depth (RGBD) streams from plant gutters in three glasshouse compartments with different light treatments. We used part of the RGB data collected to train and validate a deep learning segmentation model to detect plant tops and bases. Detected tops and bases of an image were then mapped to the generated 3D scene using the depth image of the same frame. Thresholds and 3D clustering are used respectively to remove background and eliminate outliers in top and base detection mapped to 3D space. Finally, the height of each plant was calculated using the cluster centres of the tops and bases of the plants. Manually measured heights of ten selected plants per environment were used to validate the height estimations. Similar growing patterns were observed between imaged and manually measured plant heights, which showed strong correlations of 0.87, 0.96, and 0.79 R2 scores, respectively, under unfiltered ambient light, Smart Glass film, and shifted light. These promising results demonstrate the feasibility of our proposed method for a vertically supported capsicum crop in a commercial-scale protected crop facility.
Abstract Sustaining crop productivity and resilience in water‐limited environments and under rising temperatures are matters of concern worldwide. We investigated the leaf anatomical traits that underpin our recently identified link between leaf width (LW) and intrinsic water use efficiency (iWUE), as traits of interest in plant breeding. Ten sorghum lines with varying LW were grown under three temperatures to expand the range of variation of both LW and gas exchange rates. Leaf gas exchange, surface morphology and cross‐sectional anatomy were measured and analysed using structural equations modelling. Narrower leaves had lower stomatal conductance ( g s ) and higher iWUE across growth temperatures. They also had smaller intercellular airspaces, stomatal size, percentage of open stomatal aperture relative to maximum, hydraulic pathway, mesophyll thickness, and leaf mass per area. Structural modelling revealed a developmental association among leaf anatomical traits that underpinned g s variation in sorghum. Growing temperature and LW both impacted leaf gas exchange rates, but only LW directly impacted leaf anatomy. Wider leaves may be more productive under well‐watered conditions, but consume more water for growth and development, which is detrimental under water stress.
The use of light-blocking film (LBF) is a promising strategy to reduce energy consumption in high-tech glasshouses. However, it also reduces specific light spectra which affect the physiological responses of plants. The LBF reduces 8–25% of canopy-level photosynthetically active radiation (PAR) while targeting a reduction in biologically irrelevant heat-generating light. Here, we investigated the mesophyll and stomatal responses of a Capsicum annum genotype grown in a high-tech glasshouse facility under the LBF. Our results demonstrated that LBF significantly increased the rate of stomatal closure to stimulus (exogenously applied ABA). The capsicum crops grown under LBF also exhibited significantly faster stomatal response to changes in light intensities (e.g. low to high PAR), rather than to light spectral differences (e.g. blue light that induces stomatal opening), which are potentially due to upregulated expression of photoreceptors and light harvesting genes [Phototropin 1 (PHOT1), phytochrome A (PHYA) and Ribulose-1,5-bisphosphate carboxylase/oxygenase small subunit (RBCS)] in guard cells. Moreover, capsicum leaves under LBF also exhibited faster electron physiological responses to light intensity in mesophyll rather than to red light spectrum, which determines electron transfer in mesophyll for photosynthesis. However, leaf mesophyll in LBF showed enhanced K+ and Cl− efflux, Ca2+ influx, and reduced capability in proton pumping than those under control conditions, suggesting impaired mesophyll cell ion homeostasis in LBF. We propose that the LBF significantly affected stomatal responses to the light, which is partially linked with its modified mesophyll ionic status required for optimal photosynthesis in glasshouse capsicum plants.
Future food security is a major concern of the 21st century with the growing global population and climate changes. In addressing these challenges, protected cropping ensures food production year-round and increases crop production per land area by controlling environment conditions. Maintaining the growth and health of crops in these facilities is essential to ensure optimum food production. However, this is a laborious work and is currently done manually. Image-based non-destructive plant phenotyping is an emerging research area that reduces the skilled labour cost while enhancing the monitoring of crop growth, health, and identifying phenotype-genotype relations for plant breeding. With the proliferations of protected infrastructures and targeted plants, different technologies and sensor setups are needed for image-based crop monitoring. Conveyor-type plant-to-sensor systems, bench-top or gantry-based systems are commonly found in research facilities focussing on phenotyping of small, relatively short, or movable model plants. This review examines the literature on crop monitoring and phenotyping platforms in both field and protected facilities and explains different camera technologies and their ability to extract different plant traits. The review highlights the future research directions of image-based monitoring of commercial scale protected crops where crops can be relatively tall or vertically supported under semi controlled environments, which presents new challenges and is rarely covered in the literature.
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