Ecological transcriptomics reveals stress response pathways of a ground-herb species in a waterlogging gradient of Amazonian riparian forests.

Environmental stress is a fundamental facet of life and a significant driver of natural selection in the wild. Gene expression diversity may facilitate adaptation to environmental changes, without necessary genetic change, but its role in adaptive divergence remains largely understudied in Neotropical systems. In Amazonian riparian forests, species distribution is predominantly influenced by species' waterlogging tolerance. The flooding gradient delineates distinct wetland forest types, shaping habitats and species characteristics. Here we investigated the molecular basis of environmental stress response in a tropical ground-herb species (Ischnosiphon puberulus) to environmental variation in Amazonian riparian forests. We compared environmental variables and gene expression profiles from individuals collected in two forest types: Igapó and Terra firme in the Amazonian riparian forests. Predictable seasonal flooding poses a significant challenge in Igapó compared to Terra firme environments, with the former presenting higher water column height and longer flooding duration. Our findings suggest that contrasting environmental conditions related to flooding regimes are important drivers of population genetic differentiation and differential gene expression in I. puberulus. Enriched gene ontology terms highlight associations with environmental stresses, such as defence response, water transport, phosphorylation, root development, response to auxin, salicylic acid and oxidative stress. By uncovering key environmental stress response pathways conserved across populations, I. puberulus offers novel genetic insights into the molecular basis of plant reactions to environmental constraints found in flooded areas of this highly biodiverse neotropical ecosystem.

Rootstocks affect the vulnerability to embolism and pit membrane thickness in Citrus scions.

Embolism resistance of xylem tissue varies among species and is an important trait related to drought resistance, with anatomical attributes like pit membrane thickness playing an important role in avoiding embolism spread. Grafted Citrus trees are commonly grown in orchards, with the rootstock being able to affect the drought resistance of the whole plant. Here, we evaluated how rootstocks affect the vulnerability to embolism resistance of the scion using several rootstock/scion combinations. Scions of 'Tahiti' acid lime, 'Hamlin', 'Pera' and 'Valencia' oranges grafted on a 'Rangpur' lime rootstock exhibit similar vulnerability to embolism. In field-grown trees, measurements of leaf water potential did not suggest significant embolism formation during the dry season, while stomata of Citrus trees presented an isohydric response to declining water availability. When 'Valencia' orange scions were grafted on 'Rangpur' lime, 'IAC 1710' citrandarin, 'Sunki Tropical' mandarin or 'Swingle' citrumelo rootstocks, variation in intervessel pit membrane thickness of the scion was found. The 'Rangpur' lime rootstock, which is known for its drought resistance, induced thicker pit membranes in the scion, resulting in higher embolism resistance than the other rootstocks. Similarly, the rootstock 'IAC 1710' citrandarin generated increased embolism resistance of the scion, which is highly relevant for citriculture.

Salt tolerance in a neotropical orchid in the absence of local adaptation to salt spray.

PREMISE: Salt tolerance has rarely been investigated regionally in the neotropics and even more rarely in Orchidaceae, one of the largest families. Therefore, investigating local adaptation to salt spray and its physiological basis in Epidendrum fulgens, a neotropical orchid species, brings important new insights. METHODS: We assessed the degree of salt tolerance in E. fulgens by testing whether coastal populations are more tolerant to salt, which could point to local adaptation. To understand the physiological basis of such salt tolerance, we exposed wild-collected individuals to salt spray for 60 days, then measured leaf expansion, osmotic potential, sodium leaf concentration, chlorophyll leaf index, chlorophyll fluorescence, relative growth rate, and pressure-volume curves. RESULTS: There is no local adaptation to salt spray since both inland and coastal plants have a high tolerance to salt stress. This tolerance is explained by the ability to tolerate high concentrations of salt in leaf tissues, which is related to the high succulence displayed by this species. CONCLUSIONS: We showed an unprecedented salt tolerance level for an orchid species, highlighting our limited knowledge of that trait beyond the traditional studied groups. Another interesting finding is that salt tolerance in E. fulgens is linked to succulence, is widespread, and is not the result of local adaptation. We suggest that E. fulgens and its allied species could be an interesting group to explore the evolution of important traits related to tolerance to salt stress, like succulence.

Angiosperms follow a convex trade-off to optimize hydraulic safety and efficiency.

Intervessel pits are considered to function as valves that avoid embolism spreading and optimize efficient transport of xylem sap across neighbouring vessels. Hydraulic transport between vessels would therefore follow a safety-efficiency trade-off, which is directly related to the total intervessel pit area (Ap ), inversely related to the pit membrane thickness (TPM ) and driven by a pressure difference. To test this hypothesis, we modelled the relative transport rate of gas (ka ) and water (Q) at the intervessel pit level for 23 angiosperm species and correlated these parameters with the water potential at which 50% of embolism occurs (Ψ50 ). We also measured ka for 10 species using pneumatic measurements. The pressure difference across adjacent vessels and estimated values of ka and Q were related to Ψ50 , following a convex safety-efficiency trade-off based on modelled and experimental data. Minor changes in TPM and Ap exponentially affected the pressure difference and flow, respectively. Our results provide clear evidence that a xylem safety-efficiency trade-off is not linear, but convex due to flow across intervessel pit membranes, which represent mesoporous media within microporous conduits. Moreover, the convex nature of long-distance xylem transport may contribute to an adjustable fluid balance of plants, depending on environmental conditions.

Nitric oxide-releasing nanomaterials: from basic research to potential biotechnological applications in agriculture.

Nitric oxide (NO) is a multifunctional gaseous signal that modulates the growth, development and stress tolerance of higher plants. NO donors have been used to boost plant endogenous NO levels and to activate NO-related responses, but this strategy is often hindered by the relative instability of donors. Alternatively, nanoscience offers a new, promising way to enhance NO delivery to plants, as NO-releasing nanomaterials (e.g. S-nitrosothiol-containing chitosan nanoparticles) have many beneficial physicochemical and biochemical properties compared to non-encapsulated NO donors. Nano NO donors are effective in increasing tissue NO levels and enhancing NO effects both in animal and human systems. The authors believe, and would like to emphasize, that new trends and technologies are essential for advancing plant NO research and nanotechnology may represent a breakthrough in traditional agriculture and environmental science. Herein, we aim to draw the attention of the scientific community to the potential of NO-releasing nanomaterials in both basic and applied plant research as alternatives to conventional NO donors, providing a brief overview of the current knowledge and identifying future research directions. We also express our opinion about the challenges for the application of nano NO donors, such as the environmental footprint and stakeholder's acceptance of these materials.

Estimating the light conversion efficiency by sugarcane: the segmented approach.

The classical method to estimate the light conversion efficiency (εc) gives a single value for the whole crop cycle (εco) but does not reveal any variation along the growing season. We proposed the segmented approach to uncover such variations along sugarcane (Saccharum sp. hybrid) growth cycle. Our analyses revealed that longer sampling intervals could overestimate εco and that the segmented light conversion efficiency (εcs) varied between 0.09 and 5.39 g MJ-1 during the crop cycle. The εcs would provide insights on how the environment affects εc and how to increase biomass production through crop management practices.

Role of bundle sheath conductance in sustaining photosynthesis competence in sugarcane plants under nitrogen deficiency.

The role of bundle sheath conductance (gbs) in sustaining sugarcane photosynthesis under nitrogen deficiency was investigated. Sugarcane was grown under different levels of nitrogen supply and gbs was estimated using simultaneous measurements of leaf gas exchange and chlorophyll fluorescence at 21% or 2% [O2] and varying air [CO2] and light intensity. Maximum rates of PEPC carboxylation, Rubisco carboxylation, and ATP production increased with an increase in leaf nitrogen concentration (LNC) from 1 to 3 g m-2. Low nitrogen supply reduced Rubisco and PEPC abundancies, the quantum efficiency of CO2 assimilation and gbs. Because of reduced gbs, low photosynthetic rates were not associated with increased leakiness under nitrogen deficiency. In fact, low nitrogen supply increased bundle sheath cell wall thickness, probably accounting for low gbs and increased estimates of [CO2] at Rubisco sites. Effects of nitrogen on expression of ShPIP2;1 and ShPIP1;2 aquaporins did not explain changes in gbs. Our data revealed that reduced Rubisco carboxylation was the main factor causing low sugarcane photosynthesis at low nitrogen supply, in contrast to the previous report on the importance of an impaired CO2 concentration mechanism under N deficiency. Our findings suggest higher investment of nitrogen into Rubisco protein would favour photosynthesis and plant performance under low nitrogen availability.

The Pneumatron: An automated pneumatic apparatus for estimating xylem vulnerability to embolism at high temporal resolution.

Xylem vulnerability to embolism represents an important trait to determine species distribution patterns and drought resistance. However, estimating embolism resistance frequently requires time-consuming and ambiguous hydraulic lab measurements. Based on a recently developed pneumatic method, we present and test the "Pneumatron", a device that generates high time-resolution and fully automated vulnerability curves. Embolism resistance is estimated by applying a partial vacuum to extract air from an excised xylem sample, while monitoring the pressure change over time. Although the amount of gas extracted is strongly correlated with the percentage loss of xylem conductivity, validation of the Pneumatron was performed by comparison with the optical method for Eucalyptus camaldulensis leaves. The Pneumatron improved the precision of the pneumatic method considerably, facilitating the detection of small differences in the (percentage of air discharged [PAD] < 0.47%). Hence, the Pneumatron can directly measure the 50% PAD without any fitting of vulnerability curves. PAD and embolism frequency based on the optical method were strongly correlated (r2 = 0.93) for E. camaldulensis. By providing an open source platform, the Pneumatron represents an easy, low-cost, and powerful tool for field measurements, which can significantly improve our understanding of plant-water relations and the mechanisms behind embolism.

Encapsulation of S-nitrosoglutathione into chitosan nanoparticles improves drought tolerance of sugarcane plants.

The entrapment of NO donors in nanomaterials has emerged as a strategy to protect these molecules from rapid degradation, allowing a more controlled release of NO and prolonging its effect. On the other hand, we have found beneficial effects of S-nitrosoglutathione (GSNO) - a NO donor - supplying to sugarcane plants under water deficit. Here, we hypothesized that GSNO encapsulated into nanoparticles would be more effective in attenuating the effects of water deficit on sugarcane plants as compared to the supplying of GSNO in its free form. The synthesis and characterization of chitosan nanoparticles containing GSNO were also reported. Sugarcane plants were grown in nutrient solution, and then subjected to the following treatments: control (well-hydrated); water deficit (WD); WD + GSNO sprayed in its free form (WDG) or encapsulated (WDG-NP). In general, both GSNO forms attenuated the effects of water deficit on sugarcane plants. However, the encapsulation of this donor into chitosan nanoparticles caused higher photosynthetic rates under water deficit, as compared to plants supplied with free GSNO. The root/shoot ratio was also increased when encapsulated GSNO was supplied, indicating that delayed release of NO improves drought tolerance of sugarcane plants. Our results provide experimental evidence that nanotechnology can be used for enhancing NO-induced benefits for plants under stressful conditions, alleviating the negative impact of water deficit on plant metabolism and increasing biomass allocation to root system.

S-nitrosoglutathione spraying improves stomatal conductance, Rubisco activity and antioxidant defense in both leaves and roots of sugarcane plants under water deficit.

Water deficit is a major environmental constraint on crop productivity and performance and nitric oxide (NO) is an important signaling molecule associated with many biochemical and physiological processes in plants under stressful conditions. This study aims to test the hypothesis that leaf spraying of S-nitrosoglutathione (GSNO), an NO donor, improves the antioxidant defense in both roots and leaves of sugarcane plants under water deficit, with positive consequences for photosynthesis. In addition, the roles of key photosynthetic enzymes ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and phosphoenolpyruvate carboxylase (PEPC) in maintaining CO2 assimilation of GSNO-sprayed plants under water deficit were evaluated. Sugarcane plants were sprayed with water or GSNO 100 µM and subjected to water deficit, by adding polyethylene glycol (PEG-8000) to the nutrient solution. Sugarcane plants supplied with GSNO presented increases in the activity of antioxidant enzymes such as superoxide dismutase in leaves and catalase in roots, indicating higher antioxidant capacity under water deficit. Such adjustments induced by GSNO were sufficient to prevent oxidative damage in both organs and were associated with better leaf water status. As a consequence, GSNO spraying alleviated the negative impact of water deficit on stomatal conductance and photosynthetic rates, with plants also showing increases in Rubisco activity under water deficit.

Exogenous nitric oxide improves sugarcane growth and photosynthesis under water deficit.

MAIN CONCLUSION: Nitric oxide (NO)-mediated redox signaling plays a role in alleviating the negative impact of water stress in sugarcane plants by improving root growth and photosynthesis. Drought is an environmental limitation affecting sugarcane growth and yield. The redox-active molecule nitric oxide (NO) is known to modulate plant responses to stressful conditions. NO may react with glutathione (GSH) to form S-nitrosoglutathione (GSNO), which is considered the main reservoir of NO in cells. Here, we investigate the role of NO in alleviating the effects of water deficit on growth and photosynthesis of sugarcane plants. Well-hydrated plants were compared to plants under drought and sprayed with mock (water) or GSNO at concentrations ranging from 10 to 1000 µM. Leaf GSNO sprayed plants showed significant improvement of relative water content and leaf and root dry matter under drought compared to mock-sprayed plants. Additionally, plants sprayed with GSNO (≥ 100 µM) showed higher leaf gas exchange and photochemical activity as compared to mock-sprayed plants under water deficit and after rehydration. Surprisingly, a raise in the total S-nitrosothiols content was observed in leaves sprayed with GSH or GSNO, suggesting a long-term role of NO-mediated responses to water deficit. Experiments with leaf discs fumigated with NO gas also suggested a role of NO in drought tolerance of sugarcane plants. Overall, our data indicate that the NO-mediated redox signaling plays a role in alleviating the negative effects of water stress in sugarcane plants by protecting the photosynthetic apparatus and improving shoot and root growth.

Plant pneumatics: stem air flow is related to embolism - new perspectives on methods in plant hydraulics.

Wood contains a large amount of air, even in functional xylem. Air embolisms in the xylem affect water transport and can determine plant growth and survival. Embolisms are usually estimated with laborious hydraulic methods, which can be prone to several artefacts. Here, we describe a new method for estimating embolisms that is based on air flow measurements of entire branches. To calculate the amount of air flowing out of the branch, a vacuum was applied to the cut bases of branches under different water potentials. We first investigated the source of air by determining whether it came from inside or outside the branch. Second, we compared embolism curves according to air flow or hydraulic measurements in 15 vessel- and tracheid-bearing species to test the hypothesis that the air flow is related to embolism. Air flow came almost exclusively from air inside the branch during the 2.5-min measurements and was strongly related to embolism. We propose a new embolism measurement method that is simple, effective, rapid and inexpensive, and that allows several measurements on the same branch, thus opening up new possibilities for studying plant hydraulics.

Xylem sap residue in cut-open conduits can affect gas discharge in pneumatic experiments.

Considerable attention has been paid to address methodological concerns related to measurements of embolism in conduits of angiosperm xylem. A fast, easy, and cheap method is based on gas extraction measurements from dehydrating samples to obtain pneumatic vulnerability curves (VCs). Here, we tested the assumption that cutting open conduits leads to gas-filled lumina when these are cut in air at fairly high water potentials, which is required to detect embolism in intact conduits. We performed VCs with the Pneumatron for 12 angiosperm species, and extracted sap from cut-open vessels in branches of nine species under early stages of branch dehydration. The optical method was applied to Citrus plants as an alternative reference method to estimate embolism resistance. We found an increase in gas discharge during early stages of dehydration, which affected the pneumatic VCs for most of the species studied. Xylem sap residue was not absorbed immediately by surrounding tissue in cut-open conduits in six of the nine species, but gradually disappeared over time during progressive dehydration. The amount of gas discharged increased until all residual sap was absorbed, and was not related to embolism. We conclude that residual xylem sap in cut-open conduits affects early stages of pneumatic VCs, and represents a novel artefact that can easily be corrected for. Yet, it remains unclear why exactly the air-water meniscus in cut-open conduits did not fully withdraw to the conduit end wall in most species. By analysing the slope of VCs over time, we could improve estimations of embolism resistance, as evidenced by a strong agreement between the pneumatic and the optical methods. Since residual sap in cut-open conduits of some species could slightly underestimate embolism resistance, we propose to apply a correction for this artefact based on the high time resolution measurements taken with a Pneumatron.

A unit pipe pneumatic model to simulate gas kinetics during measurements of embolism in excised angiosperm xylem.

The pneumatic method has been introduced to quantify embolism resistance in plant xylem of various organs by applying a partial vacuum to cut-open xylem. Despite the similarity in vulnerability curves between the pneumatic and other methods, a modeling approach is needed to investigate if changes in xylem embolism during dehydration can be accurately quantified based on gas diffusion kinetics. Therefore, a unit pipe pneumatic (UPPn) model was developed to estimate gas extraction from intact conduits, which were axially interconnected by inter-conduit pit membranes to cut-open conduits. The physical laws used included Fick's law for diffusion, Henry's law for gas concentration partitioning between liquid and gas phases at equilibrium and the ideal gas law. The UPPn model showed that 91% of the extracted gas came from the first five series of embolized, intact conduits and only 9% from the aqueous phase after 15 s of simulation. Considering alternative gas sources, embolism resistance measured with a pneumatron device was systematically overestimated by 2-17%, which corresponded to a typical measuring error of 0.11 MPa for P50 (the water potential equivalent to 50% of the maximum amount of gas extracted). It is concluded that pneumatic vulnerability curves directly measure embolism of intact conduits due to the fast movement of gas across interconduit pit membranes, while gas extraction from sap and diffusion across hydrated cell walls is about 100 times slower. We expect that the UPPn model will also contribute to the understanding of embolism propagation based on temporal gas dynamics.

Impacts of ground-level ozone on sugarcane production.

Sugarcane is a vital commodity crop often grown in (sub)tropical regions which have been experiencing a recent deterioration in air quality. Unlike for other commodity crops, the risk of air pollution, specifically ozone (O3), to this C4 crop has not yet been quantified. Yet, recent work has highlighted both the potential risks of O3 to C4 bioenergy crops, and the emergence of O3 exposure across the tropics as a vital factor determining global food security. Given the large extent, and planned expansion of sugarcane production in places like Brazil to meet global demand for biofuels, there is a pressing need to characterize the risk of O3 to the industry. In this study, we sought to a) derive sugarcane O3 dose-response functions across a range of realistic O3 exposure and b) model the implications of this across a globally important production area. We found a significant impact of O3 on biomass allocation (especially to leaves) and production across a range of sugarcane genotypes, including two commercially relevant varieties (e.g. CTC4, Q240). Using these data, we calculated dose-response functions for sugarcane and combined them with hourly O3 exposure across south-central Brazil derived from the UK Earth System Model (UKESM1) to simulate the current regional impact of O3 on sugarcane production using a dynamic global vegetation model (JULES vn 5.6). We found that between 5.6 % and 18.3 % of total crop productivity is likely lost across the region due to the direct impacts of current O3 exposure. However, impacts depended critically on the substantial differences in O3 susceptibility observed among sugarcane genotypes and how these were implemented in the model. Our work highlights not only the urgent need to fully elucidate the impacts of O3 in this important bioenergetic crop, but the potential implications air quality may have upon tropical food production more generally.

Drought-triggered leaf transcriptional responses disclose key molecular pathways underlying leaf water use efficiency in sugarcane (Saccharum spp.).

Drought is a major constraint to sugarcane (Saccharum spp.) production and improving the water use efficiency (WUE) is a critical trait for the sustainability of this bioenergy crop. The molecular mechanism underlying WUE remains underexplored in sugarcane. Here, we investigated the drought-triggered physiological and transcriptional responses of two sugarcane cultivars contrasting for drought tolerance, 'IACSP97-7065' (sensitive) and 'IACSP94-2094' (tolerant). After 21 days without irrigation (DWI), only 'IACSP94-2094' exhibited superior WUE and instantaneous carboxylation efficiency, with the net CO2 assimilation being less impacted when compared with 'IACSP97-7065'. RNA-seq of sugarcane leaves at 21 DWI revealed a total of 1,585 differentially expressed genes (DEGs) for both genotypes, among which 'IACSP94-2094' showed 617 (38.9%) exclusive transcripts (212 up- and 405 down-regulated). Functional enrichment analyses of these unique DEGs revealed several relevant biological processes, such as photosynthesis, transcription factors, signal transduction, solute transport, and redox homeostasis. The better drought-responsiveness of 'IACSP94-2094' suggested signaling cascades that foster transcriptional regulation of genes implicated in the Calvin cycle and transport of water and carbon dioxide, which are expected to support the high WUE and carboxylation efficiency observed for this genotype under water deficit. Moreover, the robust antioxidant system of the drought-tolerant genotype might serve as a molecular shield against the drought-associated overproduction of reactive oxygen species. This study provides relevant data that may be used to develop novel strategies for sugarcane breeding programs and to understand the genetic basis of drought tolerance and WUE improvement of sugarcane.

Water stress signaling and hydraulic traits in three congeneric citrus species under water deficit.

Morpho-physiological strategies to deal with water deficit vary among citrus species and the chemical signaling through ABA and anatomical, hydraulic, and physiological traits were evaluated in saplings of Rangpur lime, Swingle citrumelo and Valencia sweet orange. Trunk and roots of Swingle citrumelo presented lower vessel diameter and higher vessel frequency as compared to the other species. However, relative water content at the turgor loss point (RWCTLP), the osmotic potential at full turgor (Ψ0), the osmotic potential at the turgor loss point (ΨTLP), bulk modulus of elasticity (ε) and the xylem water potential when hydraulic conductivity is reduced by 50% (Ψ50) and 88% (Ψ88) indicated similar hydraulic traits among citrus species, with Rangpur lime showing the highest hydraulic safety margin. Roots of Rangpur lime and Swingle citrumelo were more water conductive than ones of Valencia sweet orange, which was linked to higher stomatal conductance. Chemical signaling through ABA prevented shoot dehydration in Rangpur lime under water deficit, with this species showing a more conservative stomatal behavior, sensing, and responding rapidly to low soil moisture. Taken together, our results suggest that Rangpur lime - the drought tolerant species - has an improved control of leaf water status due to chemical signaling and effective stomatal regulation for reducing water loss as well as decreased root hydraulic conductivity for saving water resources under limiting conditions.

Sink strength of citrus rootstocks under water deficit.

Carbon allocation between source and sink organs determines plant growth and is influenced by environmental conditions. Under water deficit (WD), plant growth is inhibited before photosynthesis and shoot growth tends to be more sensitive than root growth. However, the modulation of the source-sink relationship by rootstocks remains unsolved in citrus trees under WD. Citrus plants grafted on Rangpur lime are drought tolerant, which may be related to a fine coordination of the source-sink relationship for maintaining root growth. Here, we followed 13C allocation and evaluated physiological responses and growth of Valencia orange trees grafted on three citrus rootstocks (Rangpur lime, Swingle citrumelo and Sunki mandarin) under WD. As compared with plants on Swingle and Sunki rootstocks, ones grafted on Rangpur lime showed higher stomatal sensitivity to the initial variation of water availability and less accumulation of non-structural carbohydrates in roots under WD. High 13C allocation found in Rangpur lime roots indicates this rootstock has high sink demand associated with high root growth under WD. Our data suggest that Rangpur lime rootstock used photoassimilates as sources of energy and carbon skeletons for growing under drought, which is likely related to increases in root respiration. Taken together, our data revealed that carbon supply by leaves and delivery to roots are critical for maintaining root growth and improving drought tolerance, with citrus rootstocks showing differential sink strength under WD.

A User Manual to Measure Gas Diffusion Kinetics in Plants: Pneumatron Construction, Operation, and Data Analysis.

The Pneumatron device measures gas diffusion kinetics in the xylem of plants. The device provides an easy, low-cost, and powerful tool for research on plant water relations and gas exchange. Here, we describe in detail how to construct and operate this device to estimate embolism resistance of angiosperm xylem, and how to analyse pneumatic data. Simple and more elaborated ways of constructing a Pneumatron are shown, either using wires, a breadboard, or a printed circuit board. The instrument is based on an open-source hardware and software system, which allows users to operate it in an automated or semi-automated way. A step-by-step manual and a troubleshooting section are provided. An excel spreadsheet and an R-script are also presented for fast and easy data analysis. This manual aims at helping users to avoid common mistakes, such as unstable measurements of the minimum and maximum amount of gas discharged from xylem tissue, which has major consequences for estimating embolism resistance. Major advantages of the Pneumatron device include its automated and accurate measurements of gas diffusion rates, including highly precise measurements of the gas volume in intact, embolised conduits. It is currently unclear if the method can also be applied to woody monocots, gymnosperm species that possess torus-margo pit membranes, or to herbaceous species.

Elevated CO2 concentration increases photosynthetic sensitivity to nitrogen supply of sorghum in a genotype-dependent manner.

We hypothesized that elevated [CO2] only increases sorghum photosynthesis under low nitrogen availability and evaluated whether cultivars BRS373 (grain), BRS511 (saccharine) and BRS655 (forage) differ in their sensitivity to nitrogen and [CO2]. Plants were grown in growth chambers where air [CO2] was 400 (a[CO2]) or 800 (e[CO2]) µmol CO2 mol-1 and supplied with nutrient solution containing 211 (HN) or 48 (LN) ppm N for 45 days. Photosynthetic traits were measured in fully expanded leaves as well as leaf nitrogen and biomass accumulation. e[CO2] increased the sensitivity of photosynthesis to LN, with all sorghum cultivars having lower maximum Rubisco carboxylation rate, effective quantum efficiency of PSII and stomatal conductance at LN than at HN. As compared to HN, LN caused lower photosynthesis of BRS373 at a[CO2] and lower maximum PEPC carboxylation rate at e[CO2]. Actually, the metabolic limitation of photosynthesis by LN (Lm) was high in BRS373 at a[CO2] and slightly reduced at e[CO2]. On the other hand, Lm was increased in BRS511 and BRS655 at e[CO2]. Based on photosynthesis, the grain cultivar BRS373 was the most sensitive to LN. Although the number of leaves and of tillers and the leaf area were lower at LN than at HN for BRS373 and BRS655 after 45 days of growth, shoot biomass was not significantly affected. We found significant variation in photosynthetic responses to LN and e[CO2] among sorghum cultivars, likely associated with different patterns of nitrogen and carbon partitioning. Such findings must be considered when predicting crop performance in a changing environment.