Plant responses to climate warming : physiological adjustments and implications for plant functioning in a future, warmer world

The Earth’s average temperatures have been rising since the start of the Industrial Revolution, in major part driven by rising concentrations of carbon dioxide. Correlated with the rise of atmospheric CO2 from ~280 ppm (before the start of the Industrial Revolution) to the current 410 ppm, the average temperature has warmed by about 1°C since 1880 (Ciais et al., 2013). However, as CO2 concentrations continue to rise, the Earth will experience further warming, although how much warming will also depend on political will and human capacity to reduce carbon emissions in the near future. As such, rising temperatures will create new climate conditions in many places, affecting species’ functioning and their current geographical distributions

Konvensyen Myprospec tumpu revolusi industri 4.0

Rising atmospheric concentrations of CO 2 (C a) can reduce stomatal conductance and transpiration rate in trees, but the magnitude of this effect varies considerably among experiments. The theory of optimal stomatal behaviour predicts that the ratio of photosynthesis to transpiration (instantaneous transpiration efficiency, ITE) should increase in proportion to C a. We hypothesized that plants regulate stomatal conductance optimally in response to rising C a. We tested this hypothesis with data from young Eucalyptus saligna Sm. trees grown in 12 climate-controlled whole-tree chambers for 2 years at ambient and elevated C a. Elevated C a was ambient + 240 ppm, 60% higher than ambient C a. Leaf-scale gas exchange was measured throughout the second year of the study and leaf-scale ITE increased by 60% under elevated C a, as predicted. Values of leaf-scale ITE depended strongly on vapour pressure deficit (D) in both CO 2 treatments. Whole-canopy CO 2 and H 2O fluxes were also monitored continuously for each chamber throughout the second year. There were small differences in D between C a treatments, which had important effects on values of canopy-scale ITE. However, when C a treatments were compared at the same D, canopy-scale ITE was consistently increased by 60%, again as predicted. Importantly, leaf and canopy-scale ITE were not significantly different, indicating that ITE was not scale-dependent. Observed changes in transpiration rate could be explained on the basis that ITE increased in proportion to C a. The effect of elevated C a on photosynthesis increased with rising D. At high D, C a had a large effect on photosynthesis and a small effect on transpiration rate. At low D, in contrast, there was a small effect of C a on photosynthesis, but a much larger effect on transpiration rate. If shown to be a general response, the proportionality of ITE with C a will allow us to predict the effects of C a on transpiration rate

Canopy position affects photosynthetic adjustments to long-term elevated CO2 concentration (FACE) in a mature Pinus taeda L. forest

Few studies have examined the effects of elevated CO2 concentration ([CO2]) on the physiology of intact forest canopies, despite the need to understand how leaf-level responses can be aggregated to assess effects on whole-canopy functioning. We examined the long-term effects of elevated [CO2] (ambient + 200 ppm CO2) on two age classes of needles in the upper and lower canopy of Pinus taeda L. during the second through sixth year of exposure to elevated [CO2] in free-air (free-air CO2 enrichment (FACE)) in North Carolina, USA. Strong photosynthetic enhancement in response to elevated [CO2] (e.g., +60% across age classes and canopy locations) was observed across the years. This stimulation was 33% greater for current-year needles than for 1-year-old needles in the fifth and sixth years of treatment. Although photosynthetic stimulation in response to elevated [CO2] was maintained through the sixth year of exposure, we found evidence of concurrent down-regulation of Rubisco and electron transport capacity in the upper-canopy sunlit leaves. The lower canopy showed no evidence of down-regulation. The upper canopy down-regulated carboxylation capacity (Vcmax) and electron transport capacity (Jmax) by about 17–20% in 1-year-old needles; however, this response was significant across sampling years only for Jmax in 1-year-old needles (P < 0.02). A reduction in leaf photosynthetic capacity in aging conifer needles at the canopy top could have important consequences for canopy carbon balance and global carbon sinks because 1-year-old sunlit needles contribute a major proportion of the annual carbon balance of these conifers. Our finding of a significant interaction between canopy position and CO2 treatment on the biochemical capacity for CO2 assimilation suggests that it is important to take canopy position and needle aging into account because morphologically and physiologically distinct leaves could respond differently to elevated [CO2]

Probing the inner sanctum of leaf phosphorus : measuring the fractions of leaf P

Background: In spite of the importance of phosphorus (P) to plant physiological function and growth, relatively few studies have quantified foliar P fractions in native plants in natural environments. Understanding how these P fractions vary with P availability, soil type and parent material should provide information on the importance of P storage versus its partitioning to cell ultrastructure versus active biochemical compounds. In the latest study evaluating foliar P fractions, McQuillan et al. (2020), this issue, have enlisted a novel technique to estimate these foliar P fractions for major groups of functional and structural compounds in native species of different taxa across sites west of the Great Dividing Range of Australia. Scope: Combined with recent studies of diverse tropical species, there is a conservative amount of lipid-membrane P and nucleic acid P across a threefold range of leaf P concentrations, from very low leaf P concentration to what could arguably be considered moderately low leaf P concentrations (0.3 to 1.0 mg g−1 leaf P concentration). Conclusions: The findings provide insight into how overall leaf P concentrations are partitioned, including that P investment in structural components of the leaf like membrane phospholipids is remarkably conservatively regulated. Further insights await a quantification of organelle-specific P fractions on well-preserved samples, so importance of the storage versus biochemical functions of orthophosphate can be elucidated. These insights will be important for incorporating functional components of P and P biogeochemistry into models of ecosystem function, for understanding how P may regulate global change responses

Is phosphorus limiting in a mature Eucalyptus woodland? : phosphorus fertilisation stimulates stem growth

Aims: Few direct tests of phosphorus (P) limitation on highly-weathered soils have been conducted, especially in mature, native Eucalyptus stands. We tested whether growth in a mature >80-year old stand of Eucalyptus tereticornis in Cumberland Plain Woodland was limited by P, and whether this P-limitation affected leaf photosynthetic capacity. Methods: P was added to trees at the native woodland site at 50 kg ha-1 year-1 in each of 3 years, and stem and leaf responses were measured. Results: Leaf P concentrations before fertilisation were 2], because photosynthesis in elevated [CO2] may become further constrained by required phosphate pools within the photosynthetic apparatus

Acclimation of light and dark respiration to experimental and seasonal warming are mediated by changes in leaf nitrogen in Eucalyptus globulus

Quantifying the adjustments of leaf respiration in response to seasonal temperature variation and climate warming is crucial because carbon loss from vegetation is a large but uncertain part of the global carbon cycle. We grew fast-growing Eucalyptus globulus Labill. trees exposed to + 3 degrees C warming and elevated CO2 in 10-m tall whole-tree chambers and measured the temperature responses of leaf mitochondrial respiration, both in light (R-Light) and in darkness (R-Dark), over a 20-40 degrees C temperature range and during two different seasons. RLight was assessed using the Laisk method. Respiration rates measured at a standard temperature (25 degrees C-R-25) were higher in warm-grown trees and in the warm season, related to higher total leaf nitrogen (N) investment with higher temperatures (both experimental and seasonal), indicating that leaf N concentrations modulated the respiratory capacity to changes in temperature. Once differences in leaf N were accounted for, there were no differences in R-25 but the Q(10) (i. e., short-term temperature sensitivity) was higher in late summer compared with early spring. The variation in R-Light between experimental treatments and seasons was positively correlated with carboxylation capacity and photorespiration. R-Light was less responsive to short-term changes in temperature than RDark, as shown by a lower Q(10) in R-Light compared with R-Dark. The overall light inhibition of R was similar to 40%. Our results highlight the dynamic nature of leaf respiration to temperature variation and that the responses of R-Light do not simply mirror those of R-Dark. Therefore, it is important not to assume that RLight is the same as R-Dark in ecosystem models, as doing so may lead to large errors in predicting plant CO2 release and productivity

Lower photorespiration in elevated CO2 reduces leaf N concentrations in mature eucalyptus trees in the field

Rising atmospheric CO2 concentrations is expected to stimulate photosynthesis and carbohydrate production, while inhibiting photorespiration. By contrast, nitrogen (N) concentrations in leaves generally tend to decline under elevated CO2 (eCO2), which may reduce the magnitude of photosynthetic enhancement. We tested two hypotheses as to why leaf N is reduced under eCO2: (a) A “dilution effect” caused by increased concentration of leaf carbohydrates; and (b) inhibited nitrate assimilation caused by reduced supply of reductant from photorespiration under eCO2 . This second hypothesis is fully tested in the field for the first time here, using tall trees of a mature Eucalyptus forest exposed to Free-Air CO2 Enrichment (EucFACE) for five years. Fully expanded young and mature leaves were both measured for net photosynthesis, photorespiration, total leaf N, nitrate (NO−3) concentrations, carbohydrates and NO−3 reductase activity to test these hypotheses. Foliar N concentrations declined by 8% under eCO2 in new leaves, while the NO−3 fraction and total carbohydrate concentrations remained unchanged by CO2 treatment for either new or mature leaves. Photorespiration decreased 31% under eCO2 supplying less reductant, and in situ NO−3 reductase activity was concurrently reduced (−34%) in eCO2 , especially in new leaves during summer periods. Hence, NO−3 assimilation was inhibited in leaves of E. tereticornis and the evidence did not support a significant dilution effect as a contributor to the observed reductions in leaf N concentration. This finding suggests that the reduction of NO−3 reductase activity due to lower photorespiration in eCO2 can contribute to understanding how eCO2-induced photosynthetic enhancement may be lower than previously expected. We suggest that large-scale vegetation models simulating effects of eCO2 on N biogeochemistry include both mechanisms, especially where NO−3 is major N source to the dominant vegetation and where leaf flushing and emergence occur in temperatures that promote high photorespiration rates

Leaf age and eCO2 both influence photosynthesis by increasing light harvesting in mature Eucalyptus tereticornis at EucFACE

Only a few previous studies have examined how photosynthetically active radiation absorptance, pigments and electron flow change in mature trees exposed to long-term increase in CO2 concentration. We investigated pigment concentrations, leaf optical properties and quantum yield of old and new leaves exposed to ambient (aCO2) and elevated (eCO2) CO2 treatments. Leaf absorptance was around 90% in E. tereticornis trees across both foliage age classes and CO2 treatments. New leaves had 15% higher quantum yield with increased absorptance within the blue spectrum than old leaves; while they reflected and transmitted more photons. In addition, young foliage had increased mass-based concentrations of chlorophyll and carotenoids; however, pigment concentrations were reduced when expressed on area-basis. Quantum yield was 9% higher in eCO2 than aCO2 across both foliage age classes. The CO2 effect was stronger in new leaves where the quantum yield was 17% higher in eCO2 than aCO2, but not different in old leaves between CO2 treatments. New leaves had higher transmittance of photons in eCO2 than aCO2, while there was no change in old leaves. Mass-based concentrations of chlorophyll and carotenoids were reduced in eCO2 compared to aCO2 while concentrations of anthocyanins were higher in response to CO2 treatment. There was a significant effect of Age x CO2 interaction on ratio a/b with larger eCO2-related reductions in old leaves (–5%) but no change in new leaves. Generally, new leaves were more efficient in utilizing the absorbed photons than old leaves, especially under eCO2 which resulted in more carbon fixation. This implies that leaves can adjust their light harvesting capacity to eCO2, particularly in younger leaves which have higher photosynthetic activity

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

Elevated CO2 does not affect stem CO2 efflux nor stem respiration in dry Eucalyptus woodland, but it shifts the vertical gradient in xylem [CO2]

To quantify stem respiration (RS) under elevated CO2 (eCO2), stem CO2 efflux (EA) and CO2flux through the xylem (FT) should be accounted for, because part of respired CO2 is transported upwards with the sap solution. However, previous studies have used EA as a proxy of RS, which could lead to equivocal conclusions. Here, to test the effect of eCO2 on RS, both EA and FT were measured in a free‐air CO2 enrichment experiment located in mature Eucalyptus native forest. Drought stress substantially reduced EA and RS, which were unaffected by eCO2, likely as a consequence of its neutral effect on stem growth in this phosphorus‐limited site. However, xylem CO2 concentration measured near the stem base was higher under eCO2, and decreased along the stem resulting in a negative contribution of FT to RS, whereas the contribution of FT to RS under ambient CO2 was positive. Negative FT indicates net efflux of CO2 respired below the monitored stem segment, likely coming from the roots. Our results highlight the role of nutrient availability on the dependency of RS on eCO2 and suggest stimulated root respiration under eCO2 that may shift vertical gradients in xylem [CO2] confounding the interpretation of EA measurements