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The photosynthetic capacity of leaves is dramatically influenced by nitrogen (N) availability in the soil, as CO2 concentration in chloroplasts and photosynthetic biochemical capacity are related to leaf N content. The relationship between mesophyll conductance (gm) and leaf N content was expected to be shaped by leaf anatomical traits. However, the increased gm in mature leaves achieved by N top-dressing is unlikely to be caused by changes in leaf anatomy. Here, we assessed the impacts of N supply on leaf anatomical, biochemical, and photosynthetic features, specifically, the dynamic responses of leaf anatomy, biochemistry, and photosynthesis to N top-dressing in tobacco. Plant performance was substantially affected by soil N status. In comparison with the leaves of plants subjected to low N treatment, leaves of plants with high N treatment photosynthesized significantly more, due to higher CO2 diffusion conductance and photosynthetic biochemical capacity. The high gm in high N-treated leaves apparently related to modifications in the leaf anatomy; however, the rapid response of gm to N top-dressing cannot be fully explained by leaf anatomical modifications.

Water-limited environments confound efforts to intensify plant production needed to address the impending shortfall in global food production capacity (Fischer et al., 2014). The additional influence of anthropogenic global warming on the extent of water limitation in key production environments, not only through anticipated effects on precipitation, but also via increased evaporative demand associated with higher temperature and vapour pressure deficit (e.g. Lobell et al., 2013), further exacerbates this global challenge, although increased atmospheric CO2 concentration and changes in seasonal climate patterns and crop system management can generate off-setting benefits (Parent et al., 2018; Hunt et al., 2019; Hammer et al., 2020).

The extent to which rising atmospheric CO2 concentration has already influenced food production and quality is uncertain. Here, we analyzed annual field trials of autumn-planted common wheat in California from 1985 to 2019, a period during which the global atmospheric CO2 concentration increased 19%. Even after accounting for other major factors (cultivar, location, degree-days, soil temperature, total water applied, nitrogen fertilization, and pathogen infestation), wheat grain yield and protein yield declined 13% over this period, but grain protein content did not change. These results suggest that exposure to gradual CO2 enrichment over the past 35 years has adversely affected wheat grain and protein yield, but not grain protein content.

The rapid response of stomatal conductance (gs) to fluctuating irradiance is of great importance to maximize carbon assimilation while minimizing water loss. Smaller stomata have been proven to have a faster response rate than larger ones, but most of these studies have been conducted with forest trees. In the present study, the effects of stomatal anatomy on the kinetics of gs and photosynthesis were investigated in 16 Oryza genotypes. Light-induced stomatal opening includes an initial time lag (λ) followed by an exponential increase. Smaller stomata had a larger maximum stomatal conductance increase rate (Slmax) during the exponential increase phase, but showed a longer time lag and a lower initial stomatal conductance (gs,initial) at low light. Stomatal size was, surprisingly, negatively correlated with the time required to reach 50% of maximum gs and photosynthesis (T50%gs and T50%A), which was shown to be positively correlated with λ and negatively correlated with gs,initial. With a lower gs,initial and a larger λ, small stomata showed a faster decrease of intercellular CO2 concentration (Ci) during the induction process, which may have led to a slower apparent Rubisco activation rate. Therefore, smaller stomata do not always benefit photosynthesis as reported before; the influence of stomatal size on dynamic photosynthesis is also correlated with λ and gs,initial., Smaller stomata have a faster rate of increase of light-induced stomatal conductance, but have a longer time lag before reaching the exponential increase phase.

Exposure to an elevated CO2 concentration ([CO2]) generally decreases leaf N content per unit area (Narea) and stomatal density, and increases leaf thickness. Mature leaves can ‘sense’ elevated [CO2] and this regulates stomatal development of expanding leaves (systemic regulation). It is unclear if systemic regulation is involved in determination of leaf thickness and Narea—traits that are significantly correlated with photosynthetic capacity. A cuvette system was used whereby [CO2] around mature leaves was controlled separately from that around expanding leaves. Expanding leaves of poplar (Populus trichocarpa3P. deltoides) seedlings were exposed to elevated [CO2] (720 mmol mol 21 ) while the remaining mature leaves inside the cuvette were under ambient [CO2 ]o f 360 mmol mol 21 . Reverse treatments were performed. Exposure of newly developing leaves to elevated [CO2] increased their thickness, but when mature leaves were exposed to elevated [CO2] the increase in thickness of new leaves was less pronounced. The largest response to [CO2] was reflected in the palisade tissue thickness (as opposed to the spongy tissue) of new leaves. The Narea of new leaves was unaffected by the local [CO2] where the new leaves developed, but decreased following the exposure of mature leaves to elevated [CO2]. The volume fraction of mesophyll cells compared with total leaf and the mesophyll cell density changed in a manner similar to the response of Narea. These results suggest that Narea is controlled independently of the leaf thickness, and suggest that Narea is under systemic regulation by [CO2] signals from mature leaves that control mesophyll cell division.

Transgenic antisense tobacco plants with a range of reductions in sedoheptulose-1,7-bisphosphatase (SBPase) activity were used to investigate the role of photosynthesis in stomatal opening responses. High resolution chlorophyll a fluorescence imaging showed that the quantum efficiency of photosystem II electron transport (F(q)(')/F(m)(')) was decreased similarly in both guard and mesophyll cells of the SBPase antisense plants compared to the wild-type plants. This demonstrated for the first time that photosynthetic operating efficiency in the guard cells responds to changes in the regeneration capacity of the Calvin cycle. The rate of stomatal opening in response to a 30 min, 10-fold step increase in red photon flux density in the leaves from the SBPase antisense plants was significantly greater than wild-type plants. Final stomatal conductance under red and mixed blue/red irradiance was greater in the antisense plants than in the wild-type control plants despite lower CO(2) assimilation rates and higher internal CO(2) concentrations. Increasing CO(2) concentration resulted in a similar stomatal closing response in wild-type and antisense plants when measured in red light. However, in the antisense plants with small reductions in SBPase activity greater stomatal conductances were observed at all C(i) levels. Together, these data suggest that the primary light-induced opening or CO(2)-dependent closing response of stomata is not dependent upon guard or mesophyll cell photosynthetic capacity, but that photosynthetic electron transport, or its end-products, regulate the control of stomatal responses to light and CO(2).

The balance between the capacities of RuBP (ribulose-1,5-bisphosphate) carboxylation (V(cmax)) and RuBP regeneration (expressed as the maximum electron transport rate, J(max)) determines the CO(2) dependence of the photosynthetic rate. As it has been suggested that this balance changes depending on the growth temperature, the hypothesis that the seasonal change in air temperature affects the balance and modulates the CO(2) response of photosynthesis was tested. V(cmax) and J(max) were determined in summer and autumn for young and old leaves of Polygonum cuspidatum grown at two CO(2) concentrations (370 and 700 micromol mol(-1)). Elevated CO(2) concentration tended to reduce both V(cmax) and J(max) without changing the J(max):V(cmax) ratio. The seasonal environment, on the other hand, altered the ratio such that the J(max):V(cmax) ratio was higher in autumn leaves than summer leaves. This alternation made the photosynthetic rate more dependent on CO(2) concentration in autumn. Therefore, when photosynthetic rates were compared at growth CO(2) concentration, the stimulation in photosynthetic rate was higher in young-autumn than in young-summer leaves. In old-autumn leaves, the stimulation of photosynthesis brought by a change in the J(max):V(cmax) ratio was partly offset by accelerated leaf senescence under elevated CO(2). Across the two seasons and the two CO(2) concentrations, V(cmax) was strongly correlated with Rubisco and J(max) with cytochrome f content. These results suggest that seasonal change in climate affects the relative amounts of photosynthetic proteins, which in turn affect the CO(2) response of photosynthesis.

Crop species with the C4 photosynthetic pathway are generally characterized by high productivity, especially in environmental conditions favouring photorespiration. In comparison with the ancestral C3 pathway, the biochemical and anatomical modifications of the C4 pathway allow spatial separation of primary carbon acquisition in mesophyll cells and subsequent assimilation in bundle-sheath cells. The CO2-concentrating C4 cycle has to operate in close coordination with CO2 reduction via the Calvin-Benson-Bassham (CBB) cycle in order to keep the C4 pathway energetically efficient. The gradient in CO2 concentration between bundle-sheath and mesophyll cells facilitates diffusive leakage of CO2. This rate of bundle-sheath CO2 leakage relative to the rate of phosphoenolpyruvate carboxylation (termed leakiness) has been used to probe the balance between C4 carbon acquisition and subsequent reduction as a result of environmental perturbations. When doing so, the correct choice of equations to derive leakiness from stable carbon isotope discrimination (Δ(13)C) during gas exchange is critical to avoid biased results. Leakiness responses to photon flux density, either short-term (during measurements) or long-term (during growth and development), can have important implications for C4 performance in understorey light conditions. However, recent reports show leakiness to be subject to considerable acclimation. Additionally, the recent discovery of two decarboxylating C4 cycles operating in parallel in Zea mays suggests that flexibility in the transported C4 acid and associated decarboxylase could also aid in maintaining C4/CBB balance in a changing environment. In this paper, we review improvements in methodology to estimate leakiness, synthesize reports on bundle-sheath leakiness, discuss different interpretations, and highlight areas where future research is necessary.

The rate and extent of leaf thickness and area development are important determinants of whole plant photosynthetic capacity. The interactive effects of photon flux density (PFD), nitrogen supply and CO 2 concentration on leaf expansion rate were measured as well as final leaf size and thickness of soybean. Leaf thickness and final area were not correlated with leaf relative expansion rate (RER) suggesting that these parameters are controlled by different mechanisms and that final leaf dimensions are determined by the duration rather than the rate of leaf expansion. Carbohydrate supply did not explain the variation in leaf RER since RER increased with increasing CO 2 concentration, but decreased with increasing PFD. Leaf thickness and final area were related to resource supply but not in a simple fashion. Both positive and negative correlations between leaf thickness and carbohydrate and nitrogen concentrations were obtained depending on the environmental variable responsible for the variation. In contrast, there was a simple proportional relationship between whole plant relative growth rate and a correlate of leaf thickness (leaf water content per unit area), suggesting that leaf thickness responds to the balanced supply of all resources, in the same fashion as RGR, rather than to any individual resource.

Russell, 1954; Heath and Meidner, 1957) and since then this response has been both a continual source of papers, The stomatal response to CO 2 is important in under- and a stock environmental stimulus with which to assess standing stomatal physiology, and important in under- and investigate stomatal function. standing vegetation‐atmosphere exchanges at all There are two main reasons why the response of scales from the individual plant up to global vegetation. stomata to CO 2 has been of such particular interest over Despite the long history of experiments on stomatal the last three decades. Firstly, it is because the stomata responses to CO 2 there are still considerable uncer- appear to respond to the intercellular CO

The continuing increase in atmospheric carbon dioxide concentration ([CO 2 ]) and projections of possible future increases in global air temperatures have stimulated interest in the effects of these climate variables on agriculturally important food crops. This study was conducted to determine the effects of [CO 2 ] and temperature on rice (Oryza sativa L., cv. IR-30)

Etude des effets a long terme de CO 2 sur la photosynthese au niveau de la canopee chez le soja. Les sojas sont plantes en serre avec des concentrations en CO 2 variant de une a trois fois la concentration atmospherique. La reponse de la canopee a la lumiere solaire, a la temperature de l'air et aux effets a court terme du CO 2 est mesuree en fonction des effets a long terme du CO 2 sur la croissance. Les effets du CO 2 et de la temperature sur l'activite de la Rubisco et la concentration en RuBP sont egalement etudies

Acclimation to CO2 enrichment was studied in maize plants grown to maturity in either 350 or 700 µl l–1 CO2. Plants grown with CO2 enrichment were significantly taller than those grown at 350 µl l–1 CO2 but they had the same number of leaves. High CO2 concentration led to a marked decrease in whole leaf chlorophyll and protein. The ratio of stomata on the adaxial and abaxial leaf surfaces was similar in all growth conditions, but the stomatal index was considerably increased in plants grown at 700 µl l–1 CO2. Doubling the atmospheric CO2 content altered epidermal cell size leading to fewer, much larger cells on both leaf surfaces. The photosynthesis and transpiration rates were always higher on the abaxial surface than the adaxial surface. CO2 uptake rates increased as atmospheric CO2 was increased up to the growth concentrations on both leaf surfaces. Above these values, CO2 uptake on the abaxial surface was either stable or increased as CO2 concentration increased. In marked contrast, CO2 uptake rates on the adaxial surface were progressively inhibited at concentrations above the growth CO2 value, whether light was supplied directly to this or the abaxial surface. These results show that maize leaves adjust their stomatal densities through changes in epidermal cell numbers rather than stomatal numbers. Moreover, the CO2-response curve of photosynthesis on the adaxial surface is specifically determined by growth CO2 abundance and tracks transpiration. Conversely, photosynthesis on the abaxial surface is largely independent of CO2 concentration and rather independent of stomatal function., Enrique Olmos is grateful for a Mobility Grant of Researcher from the Spanish Government, Ministerio de Educacion y Ciencia (PR2004-0361)

the mechanism by which the synthetic growth retardant dammozide (?59) inhibits stomatal opening has been investigated, and its action compared with that of the naturally occurring growth hormone, abscisic acid. The inhibition of stomatal opening by low concentrations of daminozide in normal air is largely reversed in C02-free air, suggesting that the main effect is due to an increase in internal CO2 concentration. This is supported by observations of a raised level of CO2 compensation in treated leaves. The C02-independent light reaction of stomata does, however, also appear to be inhibited by daminozide. Higher concentrations cause damage to cells of the epidermis, and such effects could also be found in normal practical use of the growth retardant. The mechanism of action of daminozide on stomata has little in common with that of abscisic acid, which does not operate by increasing the internal CO2 concentration and does not induce cellular damage. Daminozide cannot be recommended for practical use as an antitrans pirant because of its mechanism of action, which is likely to cause a decrease in the efficiency of water use.

The effects of three ranges of C02 concentration on growth, carbon distribution and loss of carbon from the roots of maize grown for 14 d and 28 d with shoots in constant specific activity 14C02 are described. Increasing concentrations of C02 led to enhancement of plant growth with the relative growth rate (RGR) of the roots affected more than the RGR of the shoots. Between 16% and 21% of total net fixed carbon (defined as 14C retained in the plant plus 14C lost from the root) was lost from the roots at all C02 concentrations at all times but the amounts of carbon lost per unit weight of plant decreased with time. Possible mechanisms to account for these observations are discussed.

Lehnherr, B. Mâchler, F. and Nosberger, J. 1985. Effect of C02 concentration during growth on a C02 concentrating mechanism in white clover as predicted from differential 14C02/12C02 uptake.— J. exp. Bot. 36: 1835-1841. White clover was grown at 20 and 100 Pa p(C02). The C02 response of net photosynthesis and differential uptake of 14CO, and 12C02 by leaves were measured at various temperatures and at various 02 and C02 partial pressures and compared with predictions from ribulose bisphosphate carboxylase/oxygenase kinetics. Discrepancies between the observed gas exchange characteristics for the leaves and those predicted from the enzyme kinetics were interpreted as being due to a C02 concentrating mechanism. Plants grown at 20 Pa p(C02) showed a higher affinity for C02 than plants grown at 100 Pa p(C02) when measured at 10 °C. No difference in affinity was found at 30 °C. The postulated C02 concentrating effect was greater in plants grown at low C02 than in plants grown at high C02 concentration and occurred only at low temperature and low C02 partial pressure. It is suggested that plants grown at the lower C02 partial pressure have a higher affinity for C02 due to a more efficient C02 concentrating system than plants grown at the higher C02 partial pressure.

Larigauderie, A., Roy, J. and Berger, A. 1986. Long term effects of high C02 concentration on photosynthesis of water hyacinth (Eichhornia crassipes (Mart.) Solms).—J. exp. Bot. 37: 1303-1312. The photosynthetic response to C02 concentration, light intensity and temperature was investigated in water hyacinth plants (Eichhortiia crassipes (Mart.) Solms) grown in summer at ambient C02 or at 10000 ?(mol(C02) mol 1 and in winter at 6000 /rmol(C02) mol"1. Plants grown and measured at ambient C02 had high photosynthetic rate (35 /

Unlike other leaves investigated, maize leaves were found to be able to exhaust atmospheric C02-content to zero concentration. This occurred at 20o C. with a light intensity of 100 f.c. and at 30o C. with a light intensity of 500 f.c. The influences of temperature, light intensity, and waterstrain on this property of maize leaves were investigated systematically and a permanent after effect of waterstrain on the leaves was found. Stomatal conductance measurements showed that maize stomata are sensitive to CO2-concentrations between zero and 100 p.p.m., a circumstance not yet reported for other leaves.

There have been many descriptions of circuits incorporating infra-red gas analysers that can be used for measurements of photosynthetic rates (Sestak, Catsky, and Jarvis, 1971). These can be classified into three groups simply described as closed, open, and semi-closed circuits (Heath, 1969). Although closed circuits are the simplest to construct, they have been criticized on the grounds that they do not allow steady-state measurements of photosynthesis (Heath, 1969). Of the other two, the open circuit has been most widely used. In this system the CO2 concentration is measured before it enters and after it leaves the assimilation chamber. Photo synthetic rate is determined from measurement of depletion of CO2 in the chamber and its accuracy is dependent upon the precise control and recording of flow rate as well as careful measurement of CO2 concentration. In addition, if rates of photosynthesis at varying CO 2 concentrations are to be measured then either pre-mixed gases of known concentration must be available, or an expensive system of gas mixing pumps used. In the semi-closed circuit, CO2 is added to maintain a constant concentration in the assimilation chamber with the gas analyser acting as a null-point sensor regulating the supply of CO 2. The advantage in using this system is that accurate measurements of flow rates in the circuit are not required and that the CO2 concen tration can be maintained at any level within the range of the infra-red gas analyser. However, great precision is needed in the injection of CO2 into the circuit, especially when small volumes of pure CO2 are used. Koller and Samish (1964) and Redshaw and Meidner (1970) have described systems for injecting CO2 into a closed circuit but both depend on manual adjustment of gas flow and the measurement of flow of CO2 or a C02/air mixture into the circuit. We describe here a system which does not require measurement of flow rate and is fully automated with respect to injection of very small doses of pure CO 2.