A coupled model of stomatal conductance, photosynthesis and transpiration

A model that couples stomatal conductance, photosynthesis, leaf energy balance and transport of water through the soil–plant–atmosphere continuum is presented. Stomatal conductance in the model depends on light, temperature and intercellular CO₂ concentration via photosynthesis and on leaf water potential, which in turn is a function of soil water potential, the rate of water flow through the soil and plant, and on xylem hydraulic resistance. Water transport from soil to roots is simulated through solution of Richards’ equation. The model captures the observed hysteresis in diurnal variations in stomatal conductance, assimilation rate and transpiration for plant canopies. Hysteresis arises because atmospheric demand for water from the leaves typically peaks in mid‐afternoon and because of uneven distribution of soil matric potentials with distance from the roots. Potentials at the root surfaces are lower than in the bulk soil, and once soil water supply starts to limit transpiration, root potentials are substantially less negative in the morning than in the afternoon. This leads to higher stomatal conductances, CO₂ assimilation and transpiration in the morning compared to later in the day. Stomatal conductance is sensitive to soil and plant hydraulic properties and to root length density only after approximately 10 d of soil drying, when supply of water by the soil to the roots becomes limiting. High atmospheric demand causes transpiration rates, LE, to decline at a slightly higher soil water content, θ_s, than at low atmospheric demand, but all curves of LE versus θ_s fall on the same line when soil water supply limits transpiration. Stomatal conductance cannot be modelled in isolation, but must be fully coupled with models of photosynthesis/respiration and the transport of water from soil, through roots, stems and leaves to the atmosphere.     

Expression of a chitinase transgene in rose (Rosa hybrida L.) reduces development of blackspot disease (Diplocarpon rosae Wolf)

Blackspot, caused by the Ascomycete fungus Diplocarpon rosae, is the most widespread and pernicious disease of cultivated roses. While some species of rose possess resistance to D. rosae, none of the modern-day rose cultivars are fully resistant to the pathogen. In the current study, Biolistic gene delivery was used to introduce a rice gene, encoding a basic (Class I), chitinase into embryogenic callus of the blackspot-susceptible rose (Rosa hybrida L.) cv. Glad Tidings. The plasmid used for transformation carried the neomycin phosphotransferase (nptII) gene facilitating the selection and regeneration of transgenic plants on medium containing 250 mg/l kanamycin. Southern analysis confirmed integration of 2–6 copies of the chitinase gene into the rose genome; gene expression was confirmed by enzyme assay. Bioassays demonstrated that expression of the chitinase transgene reduced the severity of blackspot development by 13–43%. This degree of resistance to the pathogen correlated with the level of chitinase expression in the transgenic rose plants. The introduction of disease defence genes into rose provides a method of producing blackspot-resistant rose cultivars sought by breeders and growers.     

Epidermal conductance, stomatal density and stomatal size among genotypes of Sorghum bicolor (L.) Moench

Abstract. The ability of a plant to survive severe water deficits depends on its ability to restrict water loss through the leaf epidermis after stomata attain minimum aperture. At this stage, the rate of water loss is regulated by the epidermal conductance (g_c). Low g_c would be a useful selection criterion to identify genotypes with enhanced survival capability. Consequently, variation in g_c among Sorghum bicolor (L.) Moench genotypes was evaluated. Since there is little conclusive evidence linking g _c with leaf waxiness, alternative hypotheses relating g _c to stomatal trails were also examined. Epidermal conductance varied from 6.3 to 17.6mmol m⁻² s⁻¹ among sorghum genotypes. It was unrelated to stomatal pore length which varied with genotype and to pore depth which was similar for all genotypes measured. However, g _c, increased with increasing stomatal density. This indicates that stomatal density plays a direct role in water loss even at very low conductances. The association of low stomatal density with low g _c is consistent with the hypothesis that at the smallest stomata aperture, water loss from the epidermis above guard cell teichodes becomes a significant source of leaf water loss. Since low g _c is directly related to crop survival under severe water deficits, it is recommended that genotypes with low g _c. be selected using the selection criterion of stomatal density.     

Changes in leaf photosynthetic parameters with leaf position and nitrogen content within a rose plant canopy (Rosa hybrida)

This paper deals with changes in leaf photosynthetic capacity with depth in a rose (Rosa hybrida cv. Sonia) plant canopy. Measurements of leaf net CO₂ assimilation (A_l) and total nitrogen content (N_l) were performed in autumn under greenhouse conditions on mature leaves located at different layers within the plant canopy, including the flower stems and the main shoots. These leaves were subjected (i) to contrasting levels of CO₂ partial pressure (p_a) at saturating photosynthetic photon flux density (I about 1000 μ mol m ^{? 2} s ^{? 1}) and (ii) to saturating CO₂ partial pressure (p_a about 100 Pa) and varying I, while conditions of temperature were those prevailing in the greenhouse (20–38 °C). A biochemical model of leaf photosynthesis relating A_l to intercellular CO₂ partial pressure (p_i) was parameterized for each layer of leaves, supplying corresponding values of the photosynthetic Rubisco capacity (V_lm) and the maximum rate of electron transport (J_m). The results indicated that rose leaves growing at the top of the canopy had higher values of J_m and V_lm, which resulted from a higher allocation of nitrogen to the uppermost leaves. Mean values of total leaf nitrogen, N_l, decreased about 35% from the uppermost leaves of flower stem to leaves growing at the bottom of the plant. The derived values of non‐photosynthetic nitrogen, N_b, varied from 76 mmol_N m ^{? 2}_leaf (layer 1) to 60 mmol_N m ^{? 2}_leaf (layer 4), representing a large fraction of N_l (50 and 60% in layer 1 and 4, respectively). Comparison of leaf photosynthetic nitrogen (N_p = N_l–N_b) and I profiles supports the hypothesis that rose leaves acclimate to the time‐integrated absorbed I. The relationships between I and N_p, obtained during autumn, spring and summer, indicate that rose leaves seem also to acclimate their photosynthetic capacity seasonally, by allocating more photosynthetic nitrogen to leaves in autumn and spring than in summer.     

表油菜素内酯对月季切花衰老的影响

0.5mg·L~(-1)表油菜素内酯明显缓解水分对月季切花的胁迫,促进切花体内水分平衡的改善和还原糖含量的增加,抑制花瓣溶质外渗,从而延缓切花衰老。     

Leaf nitrogen, photosynthesis, conductance and transpiration: scaling from leaves to canopies

A model is presented which solves simultaneously for leaf-scale stomatal conductance, CO₂ assimilation and the energy balance as a function of leaf position within canopies of well-watered vegetation. Fluxes and conductances were calculated separately for sunlit and shaded leaves. A linear dependence of photosynthetic capacity on leaf nitrogen content was assumed, while leaf nitrogen content and light intensity were assumed to decrease exponentially within canopies. Separate extinction coefficients were used for diffuse and direct beam radiation. An efficient Gaussian integration technique was used to compute fluxes and mean conductances for the canopy. The multilayer model synthesizes current knowledge of radiation penetration, leaf physiology and the physics of evaporation and provides insights into the response of whole canopies to multiple, interacting factors. The model was also used to explore sources of variation in the slopes of two simple parametric models (nitrogen- and light-use efficiency), and to set bounds on the magnitudes of the parameters. For canopies low in total N, daily assimilation rates are ～10% lower when leaf N is distributed uniformly than when the same total N is distributed according to the exponentially decreasing profile of absorbed radiation. However, gains are negligible for plants with high N concentrations. Canopy conductance, G_c should be calculated as G_c=Aσ(f_slg_sl+f_shg_sh), where Δ is leaf area index, f_si and f_sh are the fractions of sunlit and shaded leaves at each level, and g_si and g_sh are the corresponding stomatal conductances. Simple addition of conductances without this weighting causes errors in transpiration calculated using the ‘big-leaf’ version of the Penman-Monteith equation. Partitioning of available energy between sensible and latent heat is very responsive to the parameter describing the sensitivity of stomata to the atmospheric humidity deficit. This parameter also affects canopy conductance, but has a relatively small impact on canopy assimilation. Simple parametric models are useful for extrapolating understanding from small to large scales, but the complexity of real ecosystems is thus subsumed in unexplained variations in parameter values. Simulations with the multilayer model show that both nitrogen- and radiation-use efficiencies depend on plant nutritional status and the diffuse component of incident radiation, causing a 2- to 3-fold variation in these efficiencies.     

Use of a model of photosynthesis and leaf microenvironment to predict optimal stomatal conductance and leaf nitrogen partitioning

Abstract. A model of photosynthesis (PGEN) is presented. The model assumes that optimal use is made of the leaf nitrogen available for partitioning between the carboxylase and thylakoid components. This results in predictions of Rubisco and chlorophyll concentrations very similar to those measured elsewhere. A function is incorporated which represents the detrimental effects of negative leaf water potentials on the Calvin cycle, producing a quantitative and mechanistic trade-off between CO2 entering, and H₂O leaving, the leaf. Thus, an optimal stomatal conductance and associated internal partial pressure of CO₂ exists for any given set of environmental conditions. The model calculates this optimal state for the leaf, which is its output. The model was subjected to changes in the following parameters: soil water potential, irradiance, ambient CO2 partial pressure, leaf temperature, leaf-to-air vapour pressure deficit, wind speed, atmospheric pressure, leaf nitrogen content, root dry weight and leaf width. These perturbations resulted in changes in predicted optimal conductance which were very similar to what has been observed. In general, as the capacity of the leaf to fix CO₂ increased, so did the predicted optimal conductance, with the internal partial pressure of CO2 being maintained close to 22Pa.     

An attempt to establish a synthetic model of photosynthesis-transpiration based on stomatal behavior for maize and soybean plants grown in field

A synthetic model of photosynthesis-transpiration was established based on a comprehensive consideration of models of CO₂ and H₂O fluxes controlled by stomata of plant leaves.The synthetic model was developed by introducing the internal conductance to CO₂ assimilation, g_ic, and the general equation of stomatal conductance model to H₂O diffusion, g_sw = g₀+a₁A_mf(D_s)/(C_s-Γ), into models of CO₂ and H₂O diffusion through the plant leaves stomata. In the above expression, g₀ and a₁ are coefficients, C_s ambient CO₂ concentration at leaf surface, Γ CO₂ compensation point, and f(D_s) the general function describing the response of stomatal conductance to humidity. Using the data observed in maize (Zea mays L.) and soybean (Glycine max Merr.) plants grown in the field, the parameters in the model were identified, and the applicability of the model was examined. The verification indicated that the developed model could be used to estimate net assimilation rate, transpiration rate, and water use efficiency with a high enough level of precision. The examination also showed that when f(D_s) = h_s or f(D_s) = (1+D_s/D₀)⁻¹ was employed, the estimation precision of the synthetic model was highest. In the study, the parameter g_ic was estimated by means of a linear function of Q_P because it was shown to be mostly correlated with photosynthetic photon flux, Q_P, among various environmental factors.     

Seasonal trends of light-saturated net photosynthesis and stomatal conductance of loblolly pine trees grown in contrasting environments of nutrition, water and carbon dioxide

Repeated measures analysis was used to evaluate the effect of long-term CO₂ enhancement on seasonal trends of light-saturated rates of net photosynthesis (A_sat) and stomatal conductance to water vapour (g_sat) of 9-year-old loblolly pine (Pinus taeda L.) trees grown in a 2 × 2 factorial experimental design of nutrition and water. A significant interaction effect of CO₂ and nutrition on mean A_sat was observed for juvenile foliage. Also, juvenile foliage exposed to +350 μmol mol^?1 CO₂ had a higher rate of increase of A_sat between late summer and early autumn. This would lead to a greater potential for recharging carbohydrate reserves for winter. Mature foliage was affected by CO_sat, water and nutrient treatments in two ways. First, A_sat was significantly increased as a result of elevated CO₂ in January, a period when stomatal conductance was only 47% of the maximum observed rate. Secondly, the rate of increase of A_sat from winter to early spring was accelerated as a result of both nutrient + water and + 350 μmol mol^?1 CO₂ treatments. This accelerated response resulted in a greater potential for photosynthate production during the period when growth initiation occurred. Nutrient, water or carbon dioxide treatments did not significantly alter trends in g_sat for mature or juvenile foliage. A significant nutrition × CO₂ interaction was observed for the mature foliage, suggesting that g_sat increased with increasing CO₂ and nutrition. These results may have important consequences for the determination of the water use efficiency of loblolly pine. In spite of low g_sat in the winter to early spring period, there was a substantial gain in A_sat attributable to elevated CO₂ concentrations.     

A stomatal optimization theory to describe the effects of atmospheric CO2 on leaf photosynthesis and transpiration

Background and Aims

Global climate models predict decreases in leaf stomatal conductance and transpiration due to increases in atmospheric CO₂. The consequences of these reductions are increases in soil moisture availability and continental scale run-off at decadal time-scales. Thus, a theory explaining the differential sensitivity of stomata to changing atmospheric CO₂ and other environmental conditions must be identified. Here, these responses are investigated using optimality theory applied to stomatal conductance.

Methods

An analytical model for stomatal conductance is proposed based on: (a) Fickian mass transfer of CO₂ and H₂O through stomata; (b) a biochemical photosynthesis model that relates intercellular CO₂ to net photosynthesis; and (c) a stomatal model based on optimization for maximizing carbon gains when water losses represent a cost. Comparisons between the optimization-based model and empirical relationships widely used in climate models were made using an extensive gas exchange dataset collected in a maturing pine (Pinus taeda) forest under ambient and enriched atmospheric CO₂.

Key Results and Conclusion

In this interpretation, it is proposed that an individual leaf optimally and autonomously regulates stomatal opening on short-term (approx. 10-min time-scale) rather than on daily or longer time-scales. The derived equations are analytical with explicit expressions for conductance, photosynthesis and intercellular CO₂, thereby making the approach useful for climate models. Using a gas exchange dataset collected in a pine forest, it is shown that (a) the cost of unit water loss λ (a measure of marginal water-use efficiency) increases with atmospheric CO₂; (b) the new formulation correctly predicts the condition under which CO₂-enriched atmosphere will cause increasing assimilation and decreasing stomatal conductance.     

A model explaining genotypic and ontogenetic variation of leaf photosynthetic rate in rice (Oryza sativa) based on leaf nitrogen content and stomatal conductance

BACKGROUNDS AND AIMS: Identification of physiological traits associated with leaf photosynthetic rate (Pn) is important for improving potential productivity of rice (Oryza sativa). The objectives of this study were to develop a model which can explain genotypic variation and ontogenetic change of Pn in rice under optimal conditions as a function of leaf nitrogen content per unit area (N) and stomatal conductance (g(s)), and to quantify the effects of interaction between N and g(s) on the variation of Pn. METHODS: Pn, N and g(s) were measured at different developmental stages for the topmost fully expanded leaves in ten rice genotypes with diverse backgrounds grown in pots (2002) and in the field (2001 and 2002). A model of Pn that accounts for carboxylation and CO diffusion processes, and assumes that the ratio of internal conductance to g(s) is constant, was constructed, and its goodness of fit was examined. KEY RESULTS: Considerable genotypic differences in Pn were evident for rice throughout development in both the pot and field experiments. The genotypic variation of Pn was correlated with that of g(s) at a given stage, and the change of Pn with plant development was closely related to the change of N. The variation of g(s) among genotypes was independent of that of N. The model explained well the variation in Pn of the ten genotypes grown under different conditions at different developmental stages. Conclusions The response of Pn to increased N differs with g(s), and the increase in Pn of genotypes with low g(s) is smaller than that of genotypes with high g(s). Therefore, simultaneous improvements of these two traits are essential for an effective breeding of rice genotypes with increased Pn.     

A coupled model of stomatal conductance and photosynthesis for winter wheat

The model couples stomatal conductance (g _s) and net photosynthetic rate (P _N) describing not only part of the curve up to and including saturation irradiance (I _max), but also the range above the saturation irradiance. Maximum stomatal conductance (g _smax) and I _max can be calculated by the coupled model. For winter wheat (Triticum aestivum) the fitted results showed that maximum P _N (P _max) at 600 μmol mol⁻¹ was more than at 350 μmol mol⁻¹ under the same leaf temperature, which can not be explained by the stomatal closure at high CO₂ concentration because g _smax at 600 μmol mol⁻¹ was less than at 350 μmol mol⁻¹. The irradiance-response curves for winter wheat had similar tendency, e.g. at 25 °C and 350 μmol mol⁻¹ both P _N and g _s almost synchronously reached the maximum values at about 1 600 μmol m⁻² s⁻¹. At 25 °C and 600 μmol mol⁻¹ the I _max corresponding to P _max and g _smax was 2 080 and 1 575 μmol m⁻² s⁻¹, respectively.     

Xylem cavitation and hydraulic control of stomatal conductance in Laurel (Laurus nobilis L.)

The possible link between stomatal conductance (g_L), leaf water potential ( Ψ _L) and xylem cavitation was studied in leaves and shoots of detached branches as well as of whole plants of Laurus nobilis L. (Laurel). Shoot cavitation induced complete stomatal closure in air‐dehydrated detached branches in less than 10 min. By contrast, a fine regulation of g_L in whole plants was the consequence of Ψ _L reaching the cavitation threshold ( Ψ _CAV) for shoots. A pulse of xylem cavitation in the shoots was paralleled by a decrease in g_L of about 50%, while Ψ _L stabilized at values preventing further xylem cavitation. In these experiments, no root signals were likely to be sent to the leaves from the roots in response to soil dryness because branches were either detached or whole plants were growing in constantly wet soil. The stomatal response to increasing evaporative demand appeared therefore to be the result of hydraulic signals generated during shoot cavitation. A negative feedback link is proposed between g_L and Ψ _CAV rather than with Ψ _L itself.     

Xylem transport and shoot accumulation of lumichrome, a newly recognized rhizobial signal, alters root respiration, stomatal conductance, leaf transpiration and photosynthetic rates in legumes and cereals

* Root respiration, stomatal conductance, leaf transpiration and photosynthetic rates were measured in phytotron and field-grown plants following the application of 5 or 10 nM lumichrome, 10 nM ABA (abscisic acid) and 10 ml of 0.2 OD600 infective rhizobial cells. * Providing soybean and cowpea roots with their respective homologous rhizobia and/or purified lumichrome increased the concentration of this molecule in xylem sap and leaf extracts. Relative to control, rhizobial inoculation and lumichrome application significantly increased root respiration in maize, decreased it in lupin, but had no effect on the other test species. * Applying either lumichrome (10 nM), infective rhizobial cells or ABA to roots of plants for 44 h in growth chambers altered leaf stomatal conductance and transpiration in cowpea, lupin, soybean, Bambara groundnut and maize, but not in pea or sorghum. Where stomatal conductance was increased by lumichrome application or rhizobial inoculation, it resulted in increased leaf transpiration relative to control plants. Treating roots of field plants of cowpea with this metabolite up to 63 d after planting showed decreased stomatal conductance, which affected CO2 intake and reduction by Rubisco. * The effect of rhizobial inoculation closely mirrored that of lumichrome application to roots, indicating that rhizobial effects on these physiological activities were most likely due to lumichrome released into the rhizosphere.     

The effect of freezing nights on photosynthesis, stomatal conductance, and internal CO2 concentration in seedlings of Engelmann spruce (Picea engelmannii Parry)

Abstract The effect of freezing night temperatures on net photosynthesis, stomatal conductance, and internal CO₂ concentration was investigated in unhardened seedlings of Engelmann spruce. Exposure to – 2.5°C in the dark for 10 h caused a slight and reversible reduction in gas-exchange parameters on the following days. Substantial and irreversible inhibition of photosynthesis occurred after exposure to -4°C or –5°C. Despite a parallel decline in stomatal conductance and net photosynthesis, exposure to a hard freeze caused a decrease in the stomatal limitation to gas exchange. Hard-freeze conditions (less than – 4°C) also caused a decrease in carboxylation efficiency and apparent quantum yield, indicating a freeze-induced failure of the dark reactions and electron transport. There was no significant difference in the photosynthetic response to freezing temperatures in different elevational populations of spruce, although acclimatory adjustments were observed. Gas exchange in seedlings grown under cool conditions (14°C day/9°C night) was less affected and recovered more rapidly after exposure to a hard freeze than in seedlings grown under warm conditions (24°C day/19°C night).     

Relationships between root permeability to water,leaf conductance and transpiration rate in sunflower (Helianthus annuus L.) cultivars

Summary The relations between leaf conductance (g_l) transpiration rate and root permeability to water (R_p) of three sunflower (Helianthus annuus L.) cultivars grown in a controlled environment cabinet are described.No differences in transpiration rates were found but it was shown that plants with low values of R_p have active stomatal closure with favourable consequences for water use efficiency under water limiting conditions.R_p was estimated by applying hydrostatic pressure on the root system. Values of R_p per unit root volume ranged from 0.34×10^–5 to 16.75×10^–5 (s MPa^–1). There were significant inter-cultivar differences (P<0.05) in R_p and g_l and an inverse correlation between R_p and the maximum values cf g_l within cultivars.Pressure applied on the root system is proposed as a useful tool for the determination of differences in the root permeability to water amongst sunflower cultivars.     

Effects of water vapour pressure deficit and stomatal conductance on photosynthesis,internal pressurization and convective flow in three emergent wetland plants

Internal pressurization and convective gas flow in emergent wetland plants is a function of the water vapour pressure deficit (WPD) and stomatal conductance (G _s) separating the external atmosphere from the internal aerenchyma. We have compared the effects of WPD and G _s under a range of light intensities on static pressures and convective flows in Phragmites australis, Typha orientalis and Baumea articulata. The capacity of the three species to generate flows per unit leaf area differed, being greatest in P. australisand lowest in B. articulata. In all three species, decreasing light intensity from full sunlight (2200 mol m^–2 s^–1 photosynthetically active photon flux density (PPFD)) to < 200 and < 10 mol m^–2 s^–1PPFD caused immediate decreases in photosynthetic assimilation, followed by more gradual decreases in transpiration and G _s. However, internal pressures and flows in the two low light intensities remained similar to values recorded in full sunlight. WPD was more significantly related to pressures and flows in P. australis and T. orientalis than G _s. In B. articulata, pressures increased at low G _s values but flow rates were unaffected, as predicted by earlier models describing pore size effects on pressures and flows. The data suggest that emergent macrophytes can maintain significant internal convection even at low light intensities, and this may be beneficial for nocturnal aeration, particularly in arid climates where the atmospheric humidity at night is low.     

Control of stomatal conductance by leaf water potential in Hymenoclea salsola (T. & G.), a desert subshrub

The role of leaf water potential in controlling stomatal conductance ( g _s) was examined in the desert subshrub Hymenoclea salsola . For plants operating at high irradiance, stomatal closure in response to high leaf-air humidity gradient ( D ) was largely reversed by soil pressurization. Stomatal re-opening eliminated, on average, 89% of the closure normally induced by high D . Transpiration rates ( E ) reached under these conditions were far higher than maximal rates normally observed at any point of the D response. In situ stem psychrometry indicated that water flux at all times conformed to a simple Ohm's-law analogy. Under conditions of high D, E increased substantially in response to soil pressurization. Stomatal regulation did not constrain E during this treatment, but did result in nearly constant minimum leaf water potentials.