Grass and herb photosynthesis and productivity in a resource-limited Eucalyptus woodland under elevated atmospheric CO2

It has been suggested that plant species from the warmer ecosystems will show different and potentially larger photosynthesis and productivity responses to elevated CO2 (eCO2, ambient + 150 ppm) compared to those from the cold temperate ecosystems, on the basis of higher average annual temperature and greater water deficits in the former ecosystems. Based on these expectations, it has further been predicted that the warm water-limited ecosystems may have a greater potential to sequester the extra C that has been assimilated under eCO2. However, empirical evidences testing these expectations are scarce. The overall aim of this thesis was to investigate the effects of eCO2 on photosynthesis and productivity responses of the evergreen C3 herbaceous species from the understory of a periodically water-limited warm-temperate Eucalyptus woodland. In a three-year field study conducted at the Eucalyptus free-air CO2 enrichment experiment (EucFACE), I investigated how eCO2-induced enhancement of photosynthetic rates (Anet) in herbaceous species varied with seasonal water availability. During the second and third year of CO2 fertilisation at EucFACE, I measured the seasonal photosynthetic acclimation responses to eCO2 in two dominant species- a C3 forb and a C3 grass, and measured responses of peak above-ground biomass to eCO2 for total forbs and grasses. In a glasshouse experiment, I tested whether the species or functional groups growing under similar water inputs and nutrient availability differed in their photosynthetic or biomass allocation and growth responses to eCO2 for two C3 forbs and two C3 grasses. also evidence of photosynthetic acclimation under eCO2 in the dominant C3 herbaceous species, especially during the peak growing season of spring. Also, there was no proportional stimulation of peak above-ground biomass in the understory grasses and forbs, which may have been a result of lack of a ‘water-savings effect’ of eCO2 and/or higher soil nutrient limitation. C3 grasses and C3 forbs differed in their photosynthetic and biomass allocation responses to eCO2. Differences in leaf N content, N allocation and changes in above-ground biomass allocation likely affected the CO2 responsiveness in these functional groups. In particular, there was an ability to maintain greater leaf area index, N allocation to photosynthesis and avoid down-regulation under eCO2 by the grasses but not by the forbs. Findings from the current study suggest that interactions between seasonal water-availability eCO2 will be critical in determining relative Anet enhancement response in herbaceous species from a water-limited ecosystem. However, the enhancement response may not be mediated via a ‘water-savings effect’ of eCO2, which contrasts with the earlier findings from cold temperate ecosystems. Furthermore, evidence of photosynthetic capacity down-regulation in the dominant species and lack of relative increase in biomass under eCO2, suggest a limited capacity of the understory herbaceous species from a grassy woodland to respond to eCO2 and ultimately act as an aboveground C sink in future

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

Carbon-phosphorus cycle models overestimate CO2 enrichment response in a mature Eucalyptus forest.

The importance of phosphorus (P) in regulating ecosystem responses to climate change has fostered P-cycle implementation in land surface models, but their CO2 effects predictions have not been evaluated against measurements. Here, we perform a data-driven model evaluation where simulations of eight widely used P-enabled models were confronted with observations from a long-term free-air CO2 enrichment experiment in a mature, P-limited Eucalyptus forest. We show that most models predicted the correct sign and magnitude of the CO2 effect on ecosystem carbon (C) sequestration, but they generally overestimated the effects on plant C uptake and growth. We identify leaf-to-canopy scaling of photosynthesis, plant tissue stoichiometry, plant belowground C allocation, and the subsequent consequences for plant-microbial interaction as key areas in which models of ecosystem C-P interaction can be improved. Together, this data-model intercomparison reveals data-driven insights into the performance and functionality of P-enabled models and adds to the existing evidence that the global CO2-driven carbon sink is overestimated by models

Physiological and molecular insights into rice-arbuscular mycorrhizal interactions under arsenic stress

The symbiotic associations between plants, microbes and fungi are examples of living in harmony. The intimate association between the arbuscular mycorrhizal fungi (AMF) and their host plants benefits the latter in nutrient (viz., phosphate, nitrogen etc.) acquisition in exchange of carbohydrates. Arsenic (As) accumulation in rice grains has become a serious issue in some parts of world having high As levels in soil and groundwater. To this end, experiments have demonstrated ameliorative potential of AMF colonization on As stress in rice. AMF colonization not only influences As concentrations in grains but also the speciation of As and reduces the ratios of inorganic/organic As concentrations. Positive influences of AMF colonization have also been linked to alteration in transport of As and phosphate, photosynthetic reactions and improved growth. A role of 14-3-3 proteins in AMF colonization under As stress is also suggested in recent studies. Importantly, grain yield has been found to increase in presence of AMF colonization. In this review, we discuss the molecular intricacies of rice-AMF in the context of As stress

Arsenic stress affects the expression profile of genes of 14-3-3 proteins in the shoot of mycorrhiza colonized rice

The intimate association between the arbuscular mycorrhizal fungi and host plants helps the latter in phosphate acquisition in exchange of carbohydrates and in enhanced stress tolerance. Similarly, the ubiquitous 14-3-3 protein family is known to be a major regulator of plant metabolism and stress responses. However, the involvement of mycorrhiza and plant 14-3-3 proteins interaction in plant response to environmental stimuli, such as arsenic (As) stress, is yet unknown. In this study, we analysed the impact of the As stress on the expression profile of 14-3-3 genes in the shoot of mycorrhiza colonized rice (Oryza sativa) plants. Ten day old rice seedlings were kept for 45 days for mycorrhizal colonisation (10 g inoculum per 120 g soilrite) and were then subjected to 12.5 µM arsenate [As(V)] exposure for 1 and 3 days, in hydroponics. Arsenate stress resulted in significant change in expression of 14-3-3 protein genes in non-colonized and mycorrhiza colonized rice plants which indicated As mediated effects on 14-3-3 proteins as well as interactive impact of mycorrhiza colonization. Indeed, mycorrhiza colonization itself induced up-regulation of all 14-3-3 genes in the absence of As stress. The results thus indicate that 14-3-3 proteins might be involved in As stress signalling and the mycorrhiza induced As stress response of the rice plants

No CO2 fertilization effect on plant growth despite enhanced rhizosphere enzyme activity in a low phosphorus soil

Aims: Accurate predictions of plant responses to elevated CO2 (eCO2) levels require a better understanding of uptake and allocation of resources that affect growth. While it is well-established that plants can increase C transfer belowground to increase access to nutrients in N-limited environments, there is less studies investigating the role of these adaptations in P-limited systems. In this study, we aimed to characterise plant nutrient use and acquisition strategies under future atmospheric [CO2] scenarios, under P-limited conditions and determine how different levels of water availability regulate these strategies. Methods: We exposed four grass species commonly found in the understory of P-limited Australian Eucalyptus woodlands to eCO2 over a period of 14 weeks, under two contrasting water levels. We assessed a suite of root morphological and chemical traits and the rhizosphere activity of extracellular enzymes related to C, N and P cycles, along with changes in plant allocation patterns and use efficiency of these nutrients. Results: Elevated CO2 effects on root functional attributes were species-specific, but clear trends towards increased phosphatase activity were observed across species, leading to a lower C:P ratio. In contrast, water supply affected root morphological attributes, but interactions between CO2 and water levels on functional traits were minor. Greater water availability also stimulated microbial activity in the rhizosphere, but without observable changes in the relative demand for N and P relative to C. Conclusions: Despite changes in rhizosphere processes, eCO2 did not lead to increased plant biomass regardless of water supply, suggesting primary nutrient limitation and a lack of positive rhizosphere feedbacks to plant growth

Biochemical and Structural Diversification of C₄ Photosynthesis in Tribe Zoysieae (Poaceae)

C4 photosynthesis has evolved independently multiple times in grass lineages with nine anatomical and three biochemical subtypes. Chloridoideae represents one of the separate events and contains species of two biochemical subtypes, NAD-ME and PEP-CK. Assessment of C4 photosynthesis diversification is limited by species sampling. In this study, the biochemical subtypes together with anatomical leaf traits were analyzed in 19 species to reveal the evolutionary scenario for diversification of C4 photosynthesis in tribe Zoysieae (Chloridoideae). The effect of habitat on anatomical and biochemical diversification was also evaluated. The results for the 19 species studied indicate that 11 species have only NAD-ME as a decarboxylating enzyme, while eight species belong to the PEP-CK subtype. Leaf anatomy corresponds to the biochemical subtype. Analysis of Zoysieae phylogeny indicates multiple switches between PEP-CK and NAD-ME photosynthetic subtypes, with PEP-CK most likely as the ancestral subtype, and with multiple independent PEP-CK decarboxylase losses and its secondary acquisition. A strong correlation was detected between C4 biochemical subtypes studied and habitat annual precipitation wherein NAD-ME species are confined to drier habitats, while PEP-CK species prefer humid areas. Structural adaptations to arid climate include increases in leaf thickness and interveinal distance. Our analysis suggests that multiple loss of PEP-CK decarboxylase could have been driven by climate aridization followed by continued adaptive changes in leaf anatomy

One hundred important questions facing plant science: an international perspective

Hundred Important Questions Facing Plant Science Research’ project aimed to capture a global snapshot of the current issues and future questions facing plant science. This revisiting builds on the original 2011 paper. Over 600 questions were collected from anyone interested in plants, which were reduced to a final list of 100 by four teams of global panellists. There was remarkable consensus on the most important topics between the global subpanels. We present the top 100 most important questions facing plant science in 2022, ranging from how plants can contribute to tackling climate change, to plant-defence priming and epigenome plasticity. We also provide explanations of why each question is important. We demonstrate how focussing on climate change, community and protecting plant life has become increasingly important for plant science over the past 11 years. This revisiting illustrates the collaborative and international need for long-term funding of plant science research, alongside the broad community-driven efforts to actively ameliorate and halt climate change, while adapting to its consequences

Microbial competition for phosphorus limits the CO 2 response of a mature forest

The capacity for terrestrial ecosystems to sequester additional carbon (C) with rising CO2 concentrations depends on soil nutrient availability1, 2. Previous evidence suggested that mature forests growing on phosphorus (P)-deprived soils had limited capacity to sequester extra biomass under elevated CO2 (refs. 3–6), but uncertainty about ecosystem P cycling and its CO2 response represents a crucial bottleneck for mechanistic prediction of the land C sink under climate change7. Here, by compiling the first comprehensive P budget for a P-limited mature forest exposed to elevated CO2, we show a high likelihood that P captured by soil microorganisms constrains ecosystem P recycling and availability for plant uptake. Trees used P efficiently, but microbial pre-emption of mineralized soil P seemed to limit the capacity of trees for increased P uptake and assimilation under elevated CO2 and, therefore, their capacity to sequester extra C. Plant strategies to stimulate microbial P cycling and plant P uptake, such as increasing rhizosphere C release to soil, will probably be necessary for P-limited forests to increase C capture into new biomass. Our results identify the key mechanisms by which P availability limits CO2 fertilization of tree growth and will guide the development of Earth system models to predict future long-term C storage

Microbial competition for phosphorus limits the CO 2 response of a mature forest

The capacity for terrestrial ecosystems to sequester additional carbon (C) with rising CO2 concentrations depends on soil nutrient availability1, 2. Previous evidence suggested that mature forests growing on phosphorus (P)-deprived soils had limited capacity to sequester extra biomass under elevated CO2 (refs. 3–6), but uncertainty about ecosystem P cycling and its CO2 response represents a crucial bottleneck for mechanistic prediction of the land C sink under climate change7. Here, by compiling the first comprehensive P budget for a P-limited mature forest exposed to elevated CO2, we show a high likelihood that P captured by soil microorganisms constrains ecosystem P recycling and availability for plant uptake. Trees used P efficiently, but microbial pre-emption of mineralized soil P seemed to limit the capacity of trees for increased P uptake and assimilation under elevated CO2 and, therefore, their capacity to sequester extra C. Plant strategies to stimulate microbial P cycling and plant P uptake, such as increasing rhizosphere C release to soil, will probably be necessary for P-limited forests to increase C capture into new biomass. Our results identify the key mechanisms by which P availability limits CO2 fertilization of tree growth and will guide the development of Earth system models to predict future long-term C storage