Disentangling CO2 fluxes: direct measurements of mesocosm‐scale natural abundance 13CO2/12CO2 gas exchange, 13C discrimination,and labelling of CO2 exchange flux components in controlled environments

A ¹³C/¹²C mass spectrometer was interfaced with a open gas exchange system including four growth chambers to investigate CO₂ exchange components of perennial ryegrass (Lolium perenne L.) stands. Chambers were fed with air containing CO₂ with known δ¹³C (δ_CΟ2?2.6 or ?46.8‰). The system did not fractionate C isotopes and no extraneous CO₂ leaked into chambers. The on‐line ¹³C discrimination (Δ) of ryegrass stands in light was independent of δ_CΟ2 when δ_CΟ2 was constant. The δ of CO₂ exchanged by the stands in light (δ_Nd) and darkness (δ_Rn) differed by 0.7‰, suggesting some Δ in dark respiration at the stand‐level. However, Δ decreased by ～ 10‰ when δ_CΟ2 was switched from ?46.8 to ?2.5‰, and increased by ～ 10‰ following a shift from ?2.6 to ?46.7‰ due to isotopic disequilibria between photosynthetic and respiratory fluxes. Isotopic imbalances were used to assess (non‐photorespiratory) respiration in light and the replacement of the respiratory substrate pool(s) by new photosynthate. Respiration was partially inhibited by light, but increased during the light period and decreased in darkness, in association with temperature changes. The labelling kinetics of respiratory CO₂ indicated the existence of two major respiratory substrate pools: a fast pool which was exchanged within hours, and a slow pool accounting for ～ 60% of total respiration and having a mean residence time of 3.6 d.     

A dynamic leaf gas‐exchange strategy is conserved in woody plants under changing ambient CO2: evidence from carbon isotope discrimination in paleo and CO2 enrichment studies

Rising atmospheric [CO₂], c_a, is expected to affect stomatal regulation of leaf gas‐exchange of woody plants, thus influencing energy fluxes as well as carbon (C), water, and nutrient cycling of forests. Researchers have proposed various strategies for stomatal regulation of leaf gas‐exchange that include maintaining a constant leaf internal [CO₂], c_i, a constant drawdown in CO₂ (c_a ? c_i), and a constant c_i/c_a. These strategies can result in drastically different consequences for leaf gas‐exchange. The accuracy of Earth systems models depends in part on assumptions about generalizable patterns in leaf gas‐exchange responses to varying c_a. The concept of optimal stomatal behavior, exemplified by woody plants shifting along a continuum of these strategies, provides a unifying framework for understanding leaf gas‐exchange responses to c_a. To assess leaf gas‐exchange regulation strategies, we analyzed patterns in c_i inferred from studies reporting C stable isotope ratios (δ¹³C) or photosynthetic discrimination (?) in woody angiosperms and gymnosperms that grew across a range of c_a spanning at least 100 ppm. Our results suggest that much of the c_a‐induced changes in c_i/c_a occurred across c_a spanning 200 to 400 ppm. These patterns imply that c_a ? c_i will eventually approach a constant level at high c_a because assimilation rates will reach a maximum and stomatal conductance of each species should be constrained to some minimum level. These analyses are not consistent with canalization toward any single strategy, particularly maintaining a constant c_i. Rather, the results are consistent with the existence of a broadly conserved pattern of stomatal optimization in woody angiosperms and gymnosperms. This results in trees being profligate water users at low c_a, when additional water loss is small for each unit of C gain, and increasingly water‐conservative at high c_a, when photosystems are saturated and water loss is large for each unit C gain.     

Arbuscular mycorrhizal colonization on carbon economy in perennial ryegrass: quantification by 13CO2/12CO2 steady-state labelling and gas exchange

Effects of the arbuscular mycorrhizal fungus (AMF) Glomus hoi on the carbon economy of perennial ryegrass (Lolium perenne) were investigated by comparing nonmycorrhizal and mycorrhizal plants of the same size, morphology and phosphorus status. Plants were grown in the presence of CO2 sources with different C isotope composition (delta13C -1 or -44). Relative respiration and gross photosynthesis rates, and belowground allocation of C assimilated during one light period ('new C'), as well as its contribution to respiration, were quantified by the concerted use of 13CO2/12CO2 steady-state labelling and 13CO2/12CO2 gas-exchange techniques. AMF (G. hoi) enhanced the relative respiration rate of the root + soil system by 16%, inducing an extra C flow amounting to 3% of daily gross photosynthesis. Total C flow into AMF growth and respiration was estimated at < 8% of daily gross photosynthesis. This was associated with a greater amount of new C allocated belowground and respired in mycorrhizal plants. AMF colonization affected the sources supplying belowground respiration, indicating a greater importance of plant C stores in supplying respiration and/or the participation of storage pools within fungal tissues. When ontogenetic and nutritional effects were accounted for, AMF increased belowground C costs, which were not compensated by increased photosynthesis rates. Therefore the instantaneous relative growth rate was lower in mycorrhizal plants.     

Ozone effects on wheat in relation to CO2: modelling short‐term and long‐term responses of leaf photosynthesis and leaf duration

A combined stomatal–photosynthesis model was extended to simulate the effects of ozone exposure on leaf photosynthesis and leaf duration in relation to CO₂. We assume that ozone has a short‐term and a long‐term effect on the Rubisco‐limited rate of photosynthesis, A_c. Elevated CO₂ counteracts ozone damage via stomatal closure. Ozone is detoxified at uptake rates below a threshold value above which A_c decreases linearly with the rate of ozone uptake. Reduction in A_c is transient and depends on leaf age. Leaf duration decreases depending on accumulated ozone uptake. This approach is introduced into the mechanistic crop simulation model AFRCWHEAT2. The derived model, AFRCWHEAT2‐O3, is used to test the capability of these assumptions to explain responses at the plant and crop level. Simulations of short‐term and long‐term responses of leaf photosynthesis, leaf duration and plant and crop growth to ozone exposure in response to CO₂ are analysed and compared with experimental data derived from the literature. The model successfully reproduced published responses of leaf photosynthesis, leaf duration, radiation use efficiency and final biomass of wheat to elevated ozone and CO₂. However, simulations were unsatisfactory for cumulative radiation interception which had some impact on the accuracy of predictions of final biomass. There were responses of leaf‐area index to CO₂ and ozone as a result of effects on tillering which were not accounted for in the present model. We suggest that some model assumptions need to be tested, or analysed further to improve the mechanistic understanding of the combined effects of changes in ozone and CO₂ concentrations on leaf photosynthesis and senescence. We conclude that research is particularly needed to improve the understanding of leaf‐area dynamics in response to ozone exposure and elevated CO₂.     

Chloride as a macronutrient increases water‐use efficiency by anatomically driven reduced stomatal conductance and increased mesophyll diffusion to CO2

Chloride (Cl^?) has been recently described as a beneficial macronutrient, playing specific roles in promoting plant growth and water‐use efficiency (WUE). However, it is still unclear how Cl^? could be beneficial, especially in comparison with nitrate (NO₃^?), an essential source of nitrogen that shares with Cl^? similar physical and osmotic properties, as well as common transport mechanisms. In tobacco plants, macronutrient levels of Cl^? specifically reduce stomatal conductance (g_s) without a concomitant reduction in the net photosynthesis rate (A_N). As stomata‐mediated water loss through transpiration is inherent in the need of C₃ plants to capture CO₂, simultaneous increase in photosynthesis and WUE is of great relevance to achieve a sustainable increase in C₃ crop productivity. Our results showed that Cl^?‐mediated stimulation of larger leaf cells leads to a reduction in stomatal density, which in turn reduces g_s and water consumption. Conversely, Cl^? improves mesophyll diffusion conductance to CO₂ (g_m) and photosynthetic performance due to a higher surface area of chloroplasts exposed to the intercellular airspace of mesophyll cells, possibly as a consequence of the stimulation of chloroplast biogenesis. A key finding of this study is the simultaneous improvement of A_N and WUE due to macronutrient Cl^? nutrition. This work identifies relevant and specific functions in which Cl^? participates as a beneficial macronutrient for higher plants, uncovering a sustainable approach to improve crop yield.     

Changes in vegetation phenology are not reflected in atmospheric CO2 and 13C/12C seasonality

Northern terrestrial ecosystems have shown global warming‐induced advances in start, delays in end, and thus increased lengths of growing season and gross photosynthesis in recent decades. The tradeoffs between seasonal dynamics of two opposing fluxes, CO₂ uptake through photosynthesis and release through respiration, determine the influence of the terrestrial ecosystem on the atmospheric CO₂ and ¹³C/¹²C seasonality. Here, we use four CO₂ observation stations in the Northern Hemisphere, namely Alert, La Jolla, Point Barrow, and Mauna Loa Observatory, to determine how changes in vegetation productivity and phenology, respiration, and air temperature affect both the atmospheric CO₂ and ¹³C/¹²C seasonality. Since the 1960s, the only significant long‐term trend of CO₂ and ¹³C/¹²C seasonality was observed at the northern most station, Alert, where the spring CO₂ drawdown dates advanced by 0.65 ± 0.55 days yr^?1, contributing to a nonsignificant increase in length of the CO₂ uptake period (0.74 ± 0.67 days yr^?1). For Point Barrow station, vegetation phenology changes in well‐watered ecosystems such as the Canadian and western Siberian wetlands contributed the most to ¹³C/¹²C seasonality while the CO₂ seasonality was primarily linked to nontree vegetation. Our results indicate significant increase in the Northern Hemisphere soil respiration. This means, increased respiration of ¹³C depleted plant materials cancels out the ¹²C gain from enhanced vegetation activities during the start and end of growing season. These findings suggest therefore that parallel warming‐induced increases both in photosynthesis and respiration contribute to the long‐term stability of CO₂ and ¹³C/¹²C seasonality under changing climate and vegetation activity. The summer photosynthesis and the soil respiration in the dormant seasons have become more vigorous which lead to increased peak‐to‐through CO₂ amplitude. As the relative magnitude of the increased photosynthesis in summer months is more than the increased respiration in dormant months, we have the increased overall carbon uptake rates in the northern ecosystems.     

Increasing canopy photosynthesis in rice can be achieved without a large increase in water use—A model based on free‐air CO2 enrichment

Achieving higher canopy photosynthesis rates is one of the keys to increasing future crop production; however, this typically requires additional water inputs because of increased water loss through the stomata. Lowland rice canopies presently consume a large amount of water, and any further increase in water usage may significantly impact local water resources. This situation is further complicated by changing the environmental conditions such as rising atmospheric CO₂ concentration ([CO₂]). Here, we modeled and compared evapotranspiration of fully developed rice canopies of a high‐yielding rice cultivar (Oryza sativa L. cv. Takanari) with a common cultivar (cv. Koshihikari) under ambient and elevated [CO₂] (A‐CO₂ and E‐CO₂, respectively) via leaf ecophysiological parameters derived from a free‐air CO₂ enrichment (FACE) experiment. Takanari had 4%–5% higher evapotranspiration than Koshihikari under both A‐CO₂ and E‐CO₂, and E‐CO₂ decreased evapotranspiration of both varieties by 4%–6%. Therefore, if Takanari was cultivated under future [CO₂] conditions, the cost for water could be maintained at the same level as for cultivating Koshihikari at current [CO₂] with an increase in canopy photosynthesis by 36%. Sensitivity analyses determined that stomatal conductance was a significant physiological factor responsible for the greater canopy photosynthesis in Takanari over Koshihikari. Takanari had 30%–40% higher stomatal conductance than Koshihikari; however, the presence of high aerodynamic resistance in the natural field and lower canopy temperature of Takanari than Koshihikari resulted in the small difference in evapotranspiration. Despite the small difference in evapotranspiration between varieties, the model simulations showed that Takanari clearly decreased canopy and air temperatures within the planetary boundary layer compared to Koshihikari. Our results indicate that lowland rice varieties characterized by high‐stomatal conductance can play a key role in enhancing productivity and moderating heat‐induced damage to grain quality in the coming decades, without significantly increasing crop water use.     

Photosynthesis and the influence of CO2-enrichment on δ13C values in a C3 halophyte

Abstract Shifts in ?¹³C of the graminaceous C₃ halophyte Puccinellia nuttalliana (Schultes) Hitch. can be induced by salinization. To investigate this phenomenon, three approaches were taken: assay of carboxylases, CO₂-enrichment studies, and gas exchange analysis. Although ribulose-1,5-bisphosphate carboxylase activity decreased with salinity, phosphoenolpyruvate carboxylase activity did not increase and its levels were not atypical of C₃ plants. When plants were grown at four NaCl concentrations under atmospheres of 310 and 1300 cm³ m^?3 CO₂, the CO₂-enrichment enhanced the effects of salinity on ?¹³C. This is consistent with a biophysical explanation for salt-induced shifts in ?¹³C, whereby there is a steepening of the CO₂ diffusion gradient into the leaf. Gas exchange analysis indicated that intercellular CO₂ concentrations were depressed in the leaves of salt-affected plants. This resulted from a greatly decreased stomatal conductance coupled with only small effects on intrinsic photosynthetic capacity. Water-use efficiency was enhanced.