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.     

土壤水分胁迫下沙漠葳的光合生理特征

以从美国西部引进的沙生灌木——沙漠葳（Chilopsis linearis）的2年生实生苗为材料,通过盆栽试验于7-9份进行轻度、中度和重度土壤水分胁迫处理并分析其光合生理响应特征.结果显示：（1）60 d中度或重度土壤水分胁迫使沙漠葳的净光合速率（Pn）、水分利用效率（WUE）和光补偿点（LCP）显著降低,暗呼吸速率（DRR）减弱,而气孔导度（Cs）增大,气孔限制值（Ls）变小.（2）Pn日变化在7月份的轻度和中度土壤水分胁迫下表现为双峰曲线,其余月份的胁迫处理均为单峰曲线,同期内各胁迫处理Pn峰值出现的时间基本相同,而且8月份各水分胁迫的Pn值显著高于其它月份;WUE的日变化趋势和Pn日变化相似,而且土壤水分胁迫越严重,其水分利用效率越低;各水分胁迫处理的Pn和Tr光响应拟合曲线均基本呈抛物线形或倒抛物线形.（3）在轻度和中度土壤水分胁迫的初期,Pn降低主要受气孔限制因素的影响,随着胁迫期的延长和胁迫的加重,Pn降低由气孔限制为主逐步转向以非气孔限制为主.研究表明,沙漠葳对干旱胁迫具有一定的适应能力,但长期中度或重度干旱胁迫都会影响沙漠葳的正常生长发育,使其光合生产力大大降低.     

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.     

What is the usual internal carbon dioxide concentration in C<Subscript>4</Subscript> species under midday field conditions?

The carbon dioxide concentrating system in C₄ photosynthesis allows high net photosynthetic rates (P _N) at low internal carbon dioxide concentrations (C _i), permitting higher P _N relative to stomatal conductance (g _s) than in C₃ plants. This relation would be reflected in the ratio of C _i to external ambient (C _a) carbon dioxide concentration, which is often given as 0.3 or 0.4 for C₄ plants. For a C _a of 360 μmol mol⁻¹ that would mean a C _i about 110–140 μmol mol⁻¹. Our field observations made near midday on three weedy C₄ species, Amaranthus retroflexus, Echinochloa crus-galli, and Setaria faberi, and the C₄ crop Sorghum bicolor indicated mean values of C _i of 183–212 μ mol mol⁻¹ at C _a = 360 μmol mol⁻¹. Measurements in two other C₄ crop species grown with three levels of N fertilizer indicated that while midday values of C _i at high photon flux were higher at limiting N, even at high nitrogen C _i averaged 212 and 196 μmol mol⁻¹ for Amaranthus hypochondriacus and Zea mays, respectively. In these two crops midday C _i decreased with increasing leaf to air water vapor pressure difference. Averaged over all measurement days, the mean C _i across all C₄ species was 198 μmol mol⁻¹, for a C _i/C _a ratio of 0.55. Prior measurements on four herbaceous C₃ species using the same instrument indicated an average C _i/C _a ratio of 0.69. Hence midday C _i values in C ₄ species under field conditions may often be considerably higher and more similar to those of C₃ species than expected from measurements made on plants in controlled environments. Reducing g _s in C₄ crops at low water vapor pressure differences could potentially improve their water use efficiency without decreasing P _N.     

Gas exchanges of an endangered species Syringa pinnatifolia and a widespread congener S. oblata

Net photosynthetic rate (P _N), transpiration rate (E), water use efficiency (WUE), stomatal conductance (g _s), and stomatal limitation (L_s) were investigated in two Syringa species. The saturation irradiance (SI) was 400 µmol m^-2s^-1 for S. pinnatifolia and 1 700 µmol m^-2s^-1 for S. oblata. Compared with S. oblata, S. pinnatifolia had extremely low g_s. Unlike S. oblata, the maximal photosynthetic rate (P _max) in S. pinnatifoliaoccurred around 08:00 and then fell down, indicating this species was sensitive to higher temperature and high photosynthetic photon flux density. However, such phenomenon was interrupted by the leaf development rhythms before summer. A relatively lower P _N together with a lower leaf area and shoot growth showed the capacity for carbon assimilation was poorer in S. pinnatifolia.This revised version was published online in March 2005 with corrections to the page numbers.     

苹果三维树冠的净光合速率分布模拟

构建三维树冠光合模型可模拟出叶片净光合速率(P_n)、气孔导度(G_s)和光能利用效率(LUE)在树冠内的三维分布。以17年生纺锤形\"富士\"苹果树(Malus domestica Borkh. cv. ‘Fuji’)为试材,通过实测确定三维树冠内叶片和辐射分布,根据不同部位叶片最大光合速率经验公式模拟叶片P_n 在三维树冠空间内分布,并据2007-2009年测定数据拟合相关模型参数。模拟表明,苹果树冠叶片P_n 和辐射的三维分布相似,在树冠上部P_n 三维分布比较平缓,然后随辐射的减少而迅速降低。高辐射条件下(PAR=1500 μmol·m^-2·s^-1),从树冠上部3 m处降到到1 m,平均相对辐射从71.18%降到8.05%,减少了89%,叶片平均P_n从15.05 μmol·m^-2·s^-1降到1.92 μmol·m^-2·s^-1,减少了87%。单位体积小室内的总净光合速率大小主要取决于叶面积密度,部分取决于P_n。G_s三维分布与P_n相似,而LUE分布与辐射相反,中下部高,上部低。根据光合机理模型、树冠内辐射和叶面积三维分布可模拟出苹果三维树冠内叶片的P_n、G_s和LUE分布,该模型参数少,可方便用于其它果树三维光合模型构建和果树整形修剪研究。     

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.     

黄土高原4种豆科牧草的净光合速率和蒸腾速率日动态及水分利用效率

在晴天条件下 ,研究了 4年生甘肃红豆草 (Onobrychis viciaefolia scop.cv.‘Gansu’)、沙打旺 (Astragalus adsurgens)、东方山羊豆 (Galega orientalis)和多年生香豌豆 (L athyruslatifolius)人工种群花期 (5月 31日 )和再生期 (7月 10日 )的净光合速率、蒸腾速率、气孔导度、水分利用效率以及土壤贮水量和水分利用特征。结果表明 ,自 5月 31日 (花期 )至 7月 10日 (再生期 ) ,4种牧草对土壤水分消耗由大到小依次为 :沙打旺 119.2 mm、多年生香豌豆 91.6 mm、山羊豆 81.9m m和红豆草 73.8m m。红豆草在花期和再生期的净光合速率分别为 12 .4 1和 9.0 6μ mol CO2 / (m2 · s) ,沙打旺为 10 .10和 7.0 1μ m ol CO2 / (m2 · s) ;红豆草在花期和再生期的日均蒸腾速率 8.13和 9.0 5 m m ol H2 O/ (m2· s) ,沙打旺刈割前和刈割后蒸腾速率分别为 7.4 0和 6 .5 4mmol H2 O/ (m2· s) ,属于高光合、高蒸腾型。而山羊豆和多年生香豌豆则属于低蒸腾、低光合类型 ,花期和再生期 ,山羊豆的日均光合速率分别为 4 .74和 4 .88μm ol CO2 / (m2· s) ,多年生香豌豆为 4 .4 1和 4 .6 4 μ mol CO2 / (m2· s) ,相应的蒸腾速率分别达到 3.75和 5 .4 2 m mol H2 O/ (m2 · s) ,4 .74和 4 .34m mol H2 O/ (m2 · s)。     

减少降水对长白山蒙古栎叶片生理生态特性的影响

以长白山针阔叶混交林优势树种蒙古栎为实验对象,研究了生长季(6-9月)减少降水30％(人工截留穿透雨30％)对蒙古栎叶光饱和光合速率、气孔导度、蒸腾速率和水分利用效率的影响.结果表明:在叶片生长初期(6月),减少降水使土壤含水量降低26.4％,减少降水条件下与对照条件下叶片的光饱和光合速率无明显差异,但明显下降的蒸腾作用使蒙古栎叶片的水分利用效率增加了40.3％;7-9月,较高的自然降水量导致不同降水处理条件下的土壤含水量无显著差异,因而叶片的光饱和光合速率、气孔导度、蒸腾速率和水分利用效率亦无明显影响;由此可见,无论是低雨量时期(6月)还是高雨量时期(7-9月),长白山地区降水减少30％不会影响成熟蒙古栎的光合能力;8月的高光合速率可能说明,较高的土壤高含水量并未形成对蒙古栎根系吸水的限制.研究结果认为,成熟蒙古栎能适应较广的土壤含水量范围,在未来气候变暖、降水量变化条件下仍能保持较强的生长和适应能力.     

Do all leaf photosynthesis parameters of rice acclimate to elevated CO2, elevated temperature,and their combination,in FACE environments?

Leaf photosynthesis of crops acclimates to elevated CO₂ and temperature, but studies quantifying responses of leaf photosynthetic parameters to combined CO₂ and temperature increases under field conditions are scarce. We measured leaf photosynthesis of rice cultivars Changyou 5 and Nanjing 9108 grown in two free‐air CO₂ enrichment (FACE) systems, respectively, installed in paddy fields. Each FACE system had four combinations of two levels of CO₂ (ambient and enriched) and two levels of canopy temperature (no warming and warmed by 1.0–2.0°C). Parameters of the C₃ photosynthesis model of Farquhar, von Caemmerer and Berry (the FvCB model), and of a stomatal conductance (g_s) model were estimated for the four conditions. Most photosynthetic parameters acclimated to elevated CO₂, elevated temperature, and their combination. The combination of elevated CO₂ and temperature changed the functional relationships between biochemical parameters and leaf nitrogen content for Changyou 5. The g_s model significantly underestimated g_s under the combination of elevated CO₂ and temperature by 19% for Changyou 5 and by 10% for Nanjing 9108 if no acclimation was assumed. However, our further analysis applying the coupled g_s–FvCB model to an independent, previously published FACE experiment showed that including such an acclimation response of g_s hardly improved prediction of leaf photosynthesis under the four combinations of CO₂ and temperature. Therefore, the typical procedure that crop models using the FvCB and g_s models are parameterized from plants grown under current ambient conditions may not result in critical errors in projecting productivity of paddy rice under future global change.     

Effect of CO2-enrichnient on seedling physiology and growth of two tropical tree species

Seedlings of two tree species from the Atlantic lowlands of Costa Rica, Ochroma la-gopus Swartz, a fast-growing pioneer species, and Pentaclethra macroloba (Willd.) Kuntze, a slower-growing climax species, were grown under enriched atmospheric CO₂ in controlled environment chambers. Carbon dioxide concentrations were maintained at 350 and 675 μl 1⁻¹ under photosynthetic photon flux densities of 500 μol m⁻² s⁻¹ and temperatures of 26°C day and 20°C night. Total biomass of both species increased significantly in the elevated CO₂ treatment; the increase in biomass was greatest for the pioneer species, O. lagopus . Both species had greater leaf areas and specific leaf weights with increased atmospheric CO₂. However, the ratio of non-pho-tosynthetic tissue to leaf area also increased in both species leading to decreased leaf area ratios. Plants of both species grown at 675 μl 1⁻¹ CO₂ had lower chlorophyll contents and photosynthesis on a leaf area basis than those grown at 350 μl 1⁻¹. Reductions in net photosynthesis occurred despite increased internal CO₂ concentrations in the CO₂-enriched treatment. Stomatal conductances of both species decreased with CO₂-enrichment resulting in significant increases in water use efficiency.     

Net photosynthetic and transpiration rates in a chlorophyll-deficient isoline of soybean under well-watered and drought conditions

The gas exchange traits of wild type soybeans (cv. Clark) and a near-isogenic, chlorophyll-deficient line homozygous for the recessive allele y9 (y9y9) were compared under either well-watered or water-stress conditions. Mature leaves of y9 had a 65% lower chlorophyll content than wild type. However, the net photosynthetic rate (PN) of y9 leaves was only 20% lower than in the wild type, irrespective of water availability. Transpiration rates (E) were significantly higher in leaves of y9, compared to the wild type, either under well-watered or stress conditions. The higher E of y9 correlated with increased stomatal conductance, particularly in the abaxial epidermis, where more than 70% of the stomata were located. The combination of lower PN and increased E resulted in a significant decrease of water use efficiency in y9, at both water availability levels. The relative water content decreased in stressed leaves, much more in y9 than in wild type leaves, probably because of the higher E of the mutant line. This revised version was published online in June 2006 with corrections to the Cover Date.     

Effect of elevated CO2 on the stomatal distribution and leaf physiology of Alnus glutinosa

Variation in stomatal development and physiology of mature leaves from Alnus glutinosa plants grown under reference (current ambient, 360 μmol mol⁻¹ CO₂) and double ambient (720 μmol mol⁻¹ CO₂) carbon dioxide (CO₂) mole fractions is assessed in terms of relative plant growth, stomatal characters (i.e. stomatal index and density) and leaf photosynthetic characters. This is the first study to consider the effects of elevated CO₂ concentration on the distribution of stomata and epidermal cells across the whole leaf and to try to ascertain the cause of intraleaf variation. In general, a doubling of the atmospheric CO₂ concentration enhanced plant growth and significantly increased stomatal index. However, there was no significant change in relative stomatal density. Under elevated CO₂ concentration there was a significant decrease in stomatal conductance and an increase in assimilation rate. However, no significant differences were found for the maximum rate of carboxylation ( V _cmax) and the light saturated rate of electron transport ( J _max) between the control and elevated CO₂ treatment.     

Has photosynthetic capacity increased with 80 years of soybean breeding? An examination of historical soybean cultivars

Crop biomass production is a function of the efficiencies with which sunlight can be intercepted by the canopy and then converted into biomass. Conversion efficiency has been identified as a target for improvement to enhance crop biomass and yield. Greater conversion efficiency in modern soybean [Glycine max (L.) Merr.] cultivars was documented in recent field trials, and this study explored the physiological basis for this observation. In replicated field trials conducted over three successive years, diurnal leaf gas exchange and photosynthetic CO₂ response curves were measured in 24 soybean cultivars with year of release dates (YOR) from 1923 to 2007. Maximum photosynthetic capacity, mesophyll conductance and nighttime respiration have not changed consistently with cultivar release date. However, daily carbon gain was periodically greater in more recently released cultivars compared with older cultivars. Our analysis suggests that this difference in daily carbon gain primarily occurred when stomatal conductance and soil water content were high. There was also evidence for greater chlorophyll content and greater sink capacity late in the growing season in more recently released soybean varieties. Better understanding of the mechanisms that have improved conversion efficiency in the past may help identify new, promising targets for the future.