夏玉米光合特性及光响应曲线拟合

通过对光合-光响应曲线的研究来探索光合有效辐射与净光合速率的关系是一种非常重要的手段。合适的模型才能较好地反映植株的光合特性。分析由于生育期与叶位的不同而导致各模型拟合值与实测值差异的变化性以及不同光合有效辐射强度下各模型的适用性, 可为夏玉米(Zea mays)光合模型的选择和光合-光响应曲线模型的进一步优化提供参考。该研究运用3种典型的光合模型对夏玉米大喇叭口期与乳熟期上部叶、穗位叶与下部叶做光合-光响应曲线拟合, 对比各模型的拟合度以及对实测数据的反映情况, 并将光合有效辐射分为3段, 分析3种模型在每段的拟合残差。结果表明: 最大净光合速率(P_nmax)、饱和光强(I_sat)、暗呼吸速率(R_d)等光合特征参数值随叶位由上而下呈降低趋势, 乳熟期普遍小于大喇叭口期; 不同生育期和叶位3种模型均可以拟合, 直角双曲线模型、非直角双曲线模型对P_nmax、I_sat的拟合值与实测值有一定的偏差; 通过残差分析表明叶子飘模型的拟合值与实测值最为相符, 尤其对高光部分的拟合表现出独有的优越性。     

油蒿叶片氮分配对其最大净光合速率季节变异的影响

植物最大净光合速率的季节变异性及其氮调控机制是近年来植物生理生态领域研究热点,对荒漠植物光合最大净光合速率季节动态及其叶氮影响,特别是叶氮分配对最大净光合速率调控机制的了解仍非常有限。2018年5—10月在宁夏盐池毛乌素沙地,对当地主要建群种油蒿(Artemisia ordosica)进行生长季原位观测,测定其叶光合光响应曲线(A-PAR)、CO₂响应曲线(A-Ci)和叶氮含量,结合环境观测数据,分析A-PAR关键参数最大净光合速率(A_max)的季节变异和叶片氮分配相关参数对A_max的调控。结果表明,油蒿叶片A_max在生长期季节变异系数(Cv)为14%,在完全展叶中期,光合氮利用效率(PNUE)有最大值11.82μmolCO₂ gN^-1 s^-1,此时叶片氮素在光合系统中的分配比例最大,A_max有最大值29.48μmol CO₂ m^-2s^-1,油蒿光...     

亚热带10种蕨类植物光响应模型拟合及光合特性研究

通过合适的光响应模型来准确计算植物光合参数是研究植物光合特性的重要手段。为深入了解亚热带蕨类植物的光合特性,采用直角双曲线模型、非直角双曲线模型、指数模型和直角双曲线修正模型等4种常用光响应模型分别对10种亚热带蕨类植物进行光响应曲线拟合。通过分析决定系数(R~2)、均方根误差(RMSE)、平均绝对误差(MAE)、光饱和点(LSP)、光补偿点(LCP)、最大净光合速率(P_nmax)、暗呼吸速率(R_d)和表观量子效率(AQE)等9种参数,探讨亚热带蕨类植物的光响应模型拟合效果及其光合特性。结果表明：直角双曲线修正模型和非直角双曲线模型对本研究10种亚热带蕨类植物的拟合效果较好,其中直角双曲线修正模型的LSP拟合效果最好,非直角双曲线模型的拟合精度最高。光合参数的聚类分析和TOPSIS(Technique for Order Preference by Similarity to an Ideal Solution)综合评价法表明,10种亚热带蕨类植物的光合特性既具有相似性又存在物种特异性。黑鳞耳蕨(Polystichum makinoi(Tag...     

桂林岩溶区青冈栎光合速率与环境因子关系初步研究

在自然条件下,对桂林岩溶区青冈栎光合速率与环境因子的关系研究表明,初夏季节青冈栎蒸腾速率日变化为"单峰"曲线,光合速率日变化为"双峰"型曲线,午间光合速率明显降低。青冈栎光合"午休"主要由气孔导度降低引起;通径分析表明,环境因子中空气相对湿度和空气温度对青冈栎光合速率直接影响最大;光合有效辐射对青冈栎光合速率直接影响不显著,而主要是通过其它因子间接影响光合速率。     

指数改进模型在大麦光合-CO2响应曲线中的适用性

光合-CO₂响应曲线是研究植物对CO₂响应的重要工具。常用的光合-CO₂响应曲线模型, 除直角双曲线修正模型外, 均无法准确地计算出CO₂饱和点和光合能力。该文利用新的指数改进模型拟合大麦(Hordeum vulgare)光合-CO₂响应曲线, 与直角双曲线修正模型、直角双曲线模型、指数模型、非直角双曲线模型进行比较, 并随机选取部分数据进行检验, 同时得到了各个光合-CO₂响应曲线模型的主要生理参数, 对这些数据进行了比较分析, 讨论了各模型之间的优缺点和准确合理性。结果表明, 指数改进模型在5个模型中具有最好的精确度和合理性, 能准确地描述出大麦的光合-CO₂响应曲线, 计算出CO₂饱和点为484 μmol∙mol^-1, 光合能力为25.9 μmol CO₂∙m^-2∙s^-1。     

岩溶石漠化治理优良先锋植物种类光合、蒸腾及水分利用效率的初步研究

针对西南岩溶石漠化地区干旱缺水的特点,采用Li-6400对其先锋树种任豆、金银花、狗骨木的光合、蒸腾及水分利用效率等特征进行分析,以揭示先锋树种利用水分生理生态学特征,从而提出植被恢复的对策。光响应模拟结果表明金银花和狗骨木符合Walker的非直线双曲线模型。蒸腾临界值狗骨木(幼树期)最小,金银花次之,任豆树较大,说明阳性树种任豆具有较大的蒸腾拉力,能够从土壤提取更多的水分,以减少岩溶石漠化地区强光引起的高温灼伤。任豆树、金银花、狗骨木具有同地带干性或沙漠优势植物的光合速率特征。任豆树有较耐受强光的能力,这与其在石漠化地区总是能够成为优势层片优势种的地位相符。三种植物光有效辐射与叶片蒸腾的呈极显著的线性关系;任豆树的蒸腾作用受气孔调节明显;狗骨木有较高的水分利用效率。上午9点左右和下午5∶00~6∶00是这些石漠化地区植物水分利用最高时间段。可以说这三种植物具有适应石漠化地区干旱和高温的生理生态学适应特征和避旱避高温策略,并保持旺盛的生物生产力。从植物固定碳水化合物效率并提高水分利用效率而言,三种植物套种,任豆树能够对藤灌植物适当遮荫,对加速狗骨木生长,提高金银花的产量将具有较好的效益。     

氮素形态对铁线莲光合特性及氮代谢的影响

为了解铁线莲的光合性能和氮素代谢的响应机制,对不同氮素形态配比下1 a生厚叶铁线莲(Clematis crassifolia)与天台铁线莲(C.paten ssp.tientaiensis)的生长、光响应曲线、A-Ci曲线和氮代谢相关酶活性进行了比较.结果表明,氮素形态配比显著影响铁线莲的生物量和叶绿素(Chl)含量,...     

桂西南岩溶区八种适生植物光合性状的变异与关联

为探究岩溶植物的光合生理适应机制，采用Li-6400XT便携式光合作用测量系统，对广西平果市岩溶区8种适生植物的叶片净光合速率(P_n)、气孔导度(G_s)、胞间CO₂浓度(C_i)、蒸腾速率(T_r)、水分利用效率(WUE)和气孔限制值(L_s)等光合特征参数进行了测定分析。结果表明：(1)6个光合特征参数在种内和种间均存在不同程度的变异，并且种内变异均大于种间变异。(2)G_s和T_r的变化主要来源于种间变异(46.72%～49.76%),而P_n、C_i、WUE和L_s变化主要来源于种内变异(48.66%～64.50%)。在生活型水平上，P_n、G_s和T_r的种内变异表现为常绿植物小于落叶植物，而C_i、WUE和L_s则相反。(3)各参数的种间变异均表现为落叶植...     

几种光合作用光响应典型模型的比较研究

光响应曲线是判定植物光合效率的重要方法,通过曲线可以获得植物光合特性的相关生理参数,但不同模型提取的光响应参数和指标存在差异。本文选择直角双曲线、非直角双曲线和两种指数曲线模型,分别对3个品系常绿杨光响应数据进行拟合,提取了各光响应曲线模型的主要特征参数,对比分析了各模型参数问的差异,并对光饱和点（LSP）的不同计算方法进行了讨论。最后用巨尾桉光响应数据对分析结果作了进一步验证。结果表明,直角和非直角双曲线模型拟合的最大净光合速率（P＇max）、表观量子效率（a）及暗呼吸速率（Rd）值高于指数模型拟合值,且直角双曲线拟合的各参数均比非直角双曲线拟合的各参数的值大,而两指数模型各参数拟合值基本一致;在LSP计算方法中,用光通量200μmol·m^-2·s^-1以下的点拟合的Pn-I直线与其它模型相结合得到光饱和点的方法不可靠,会使计算结果明显偏小,用接近最大总光合速率Pmax一定比例的方法估计LSP也存在较大偏差,而P＇max由于消除了Rd的影响,计算光饱和点时各模型的估计比例相对固定,是一个比较理想的LSP估计方法,初步得出直角、非直角及指数模型用P＇max来估计光饱和点时应选取的比例分别为（78±1）％、（82±1）％及（96±1）％。     

岩溶植物光合-光响应曲线的两种拟合模型比较

用直角双曲线和非直角双曲线两种模型,对桂林岩溶区石山植物红背山麻杆（Alchornea trewioides）、九龙藤（Bauhinia championii）、青檀（Pteroceltis tatarinowii）和圆叶乌桕（Sapium rotundifolium）光合作用的光响应曲线进行拟合。结果表明,直角双曲线拟合的表观量子效率（α）、最大净光合速率（Pmax）、暗呼吸速率（Rd）、光补偿点（Lcp）和光饱和点（Lφ）数值均高于相应的非直角双曲线拟合结果;直角双曲线模型拟合的参数值大小和曲线变化趋势不符合实际情况;非直角双曲线模型拟合结果更符合生理意义。红背山麻杆和九龙藤具有较高的表观量子效率和较低的光补偿点,对光能的利用效率和耐荫性均强于圆叶乌桕和青檀。     

Surprising lack of sensitivity of biochemical limitation of photosynthesis of nine tree species to open‐air experimental warming and reduced rainfall in a southern boreal forest

Photosynthetic biochemical limitation parameters (i.e., V_cmax, J_max and J_max:V_cmax ratio) are sensitive to temperature and water availability, but whether these parameters in cold climate species at biome ecotones are positively or negatively influenced by projected changes in global temperature and water availability remains uncertain. Prior exploration of this question has largely involved greenhouse based short‐term manipulative studies with mixed results in terms of direction and magnitude of responses. To address this question in a more realistic context, we examined the effects of increased temperature and rainfall reduction on the biochemical limitations of photosynthesis using a long‐term chamber‐less manipulative experiment located in northern Minnesota, USA. Nine tree species from the boreal‐temperate ecotone were grown in natural neighborhoods under ambient and elevated (+3.4°C) growing season temperatures and ambient or reduced (≈40% of rainfall removed) summer rainfall. Apparent rubisco carboxylation and RuBP regeneration standardized to 25°C (V_cmax25°C and J_max25°C, respectively) were estimated based on AC_i curves measured in situ over three growing seasons. Our primary objective was to test whether species would downregulate V_cmax25°C and J_max25°C in response to warming and reduced rainfall, with such responses expected to be greatest in species with the coldest and most humid native ranges, respectively. These hypotheses were not supported, as there were no overall main treatment effects on V_cmax25°C or J_max25°C (p > .14). However, J_max:V_cmax ratio decreased significantly with warming (p = .0178), whereas interactions between warming and rainfall reduction on the J_max25°C to V_cmax25°C ratio were not significant. The insensitivity of photosynthetic parameters to warming contrasts with many prior studies done under larger temperature differentials and often fixed daytime temperatures. In sum, plants growing in relatively realistic conditions under naturally varying temperatures and soil moisture levels were remarkably insensitive in terms of their J_max25°C and V_cmax25°C when grown at elevated temperatures, reduced rainfall, or both combined.     

Predicting biochemical acclimation of leaf photosynthesis in soybean under in-field canopy warming using hyperspectral reflectance

Traditional gas exchange measurements are cumbersome, which makes it difficult to capture variation in biochemical parameters, namely the maximum rate of carboxylation measured at a reference temperature (V_cmax25) and the maximum electron transport at a reference temperature (J_max25), in response to growth temperature over time from days to weeks. Hyperspectral reflectance provides reliable measures of V_cmax25 and J_max25; however, the capability of this method to capture biochemical acclimations of the two parameters to high growth temperature over time has not been demonstrated. In this study, V_cmax25 and J_max25 were measured over multiple growth stages during two growing seasons for field-grown soybeans using both gas exchange techniques and leaf spectral reflectance under ambient and four elevated canopy temperature treatments (ambient+1.5, +3, +4.5, and +6°C). Spectral vegetation indices and machine learning methods were used to build predictive models for V_cmax25 and J_max25, based on the leaf reflectance. Results showed that these models yielded an R² of 0.57–0.65 and 0.48–0.58 for V_cmax25 and J_max25, respectively. Hyperspectral reflectance captured biochemical acclimation of leaf photosynthesis to high temperature in the field, improving spatial and temporal resolution in the ability to assess the impact of future warming on crop productivity.     

Photosynthetic capacity and temperature responses of photosynthesis of rubber trees (Hevea brasiliensis Müll. Arg.) acclimate to changes in ambient temperatures

The aim of this study was to assess the temperature response of photosynthesis in rubber trees (Hevea brasiliensis Müll. Arg.) to provide data for process-based growth modeling, and to test whether photosynthetic capacity and temperature response of photosynthesis acclimates to changes in ambient temperature. Net CO₂ assimilation rate (A) was measured in rubber saplings grown in a nursery or in growth chambers at 18 and 28°C. The temperature response of A was measured from 9 to 45°C and the data were fitted to an empirical model. Photosynthetic capacity (maximal carboxylation rate, V _cmax, and maximal light driven electron flux, J _max) of plants acclimated to 18 and 28°C were estimated by fitting a biochemical photosynthesis model to the CO₂ response curves (A–C _i curves) at six temperatures: 15, 22, 28, 32, 36 and 40°C. The optimal temperature for A (T _opt) was much lower in plants grown at 18°C compared to 28°C and nursery. Net CO₂ assimilation rate at optimal temperature (A _opt), V _cmax and J _max at a reference temperature of 25°C (V _cmax25 and J _max25) as well as activation energy of V _cmax and J _max (E _aV and E _aJ) decreased in individuals acclimated to 18°C. The optimal temperature for V _cmax and J _max could not be clearly defined from our response curves, as they always were above 36°C and not far from 40°C. The ratio J _max25/V _cmax25 was larger in plants acclimated to 18°C. Less nitrogen was present and photosynthetic nitrogen use efficiency (V _cmax25/N _a) was smaller in leaves acclimated to 18°C. These results indicate that rubber saplings acclimated their photosynthetic characteristics in response to growth temperature, and that higher temperatures resulted in an enhanced photosynthetic capacity in the leaves, as well as larger activation energy for photosynthesis.     

Temperature dependence of two parameters in a photosynthesis model

The temperature dependence of the photosynthetic parameters V_cmax, the maximum catalytic rate of the enzyme Rubisco, and J_max, the maximum electron transport rate, were examined using published datasets. An Arrehenius equation, modified to account for decreases in each parameter at high temperatures, satisfactorily described the temperature response for both parameters. There was remarkable conformity in V_cmax and J_max between all plants at T_leaf < 25 °C, when each parameter was normalized by their respective values at 25 °C (V_cmax0 and J_max0), but showed a high degree of variability between and within species at T_leaf > 30 °C. For both normalized V_cmax and J_max, the maximum fractional error introduced by assuming a common temperature response function is < ± 0·1 for most plants and < ± 0·22 for all plants when T_leaf < 25 °C. Fractional errors are typically < ± 0·45 in the temperature range 25–30 °C, but very large errors occur when a common function is used to estimate the photosynthetic parameters at temperatures > 30 °C. The ratio J_max/V_cmax varies with temperature, but analysis of the ratio at T_leaf = 25 °C using the fitted mean temperature response functions results in J_max0/V_cmax0 = 2·00 ± 0·60 (SD, n = 43).     

Within-canopy variation in the rate of development of photosynthetic capacity is proportional to integrated quantum flux density in temperate deciduous trees

The present study was undertaken to test for the hypothesis that the rate of development in the capacity for photosynthetic electron transport per unit area (J_max;A), and maximum carboxylase activity of Rubisco (V_cmax;A) is proportional to average integrated daily quantum flux density (Q_int) in a mixed deciduous forest dominated by the shade‐intolerant species Populus tremula L., and the shade‐tolerant species Tilia cordata Mill. We distinguished between the age‐dependent changes in net assimilation rates due to modifications in leaf dry mass per unit area (M_A), foliar nitrogen content per unit dry mass (N_M), and fractional partitioning of foliar nitrogen in the proteins of photosynthetic electron transport (F_B), Rubisco (F_R) and in light‐harvesting chlorophyll‐protein complexes (V_cmax;A ∝ M_AN_MF_R; J_max;A ∝ M_AN_MF_B). In both species, increases in J_max;A and V_cmax;A during leaf development were primarily determined by nitrogen allocation to growing leaves, increases in leaf nitrogen partitioning in photosynthetic machinery, and increases in M_A. Canopy differences in the rate of development of leaf photosynthetic capacity were mainly controlled by the rate of change in M_A. There was only small within‐canopy variation in the initial rate of biomass accumulation per unit Q_int (slope of M_A versus leaf age relationship per unit Q_int), suggesting that canopy differences in the rate of development of J_max;A and V_cmax;A are directly proportional to Q_int. Nevertheless, M_A, nitrogen, J_max;A and V_cmax;A of mature leaves were not proportional to Q_int because of a finite M_A in leaves immediately after bud‐burst (light‐independent component of M_A). M_A, leaf chlorophyll contents and chlorophyll : N ratio of mature leaves were best correlated with the integrated average quantum flux density during leaf development, suggesting that foliar photosynthetic apparatus, once developed, is not affected by day‐to‐day fluctuations in Q_int. However, for the upper canopy leaves of P. tremula and for the entire canopy of T. cordata, there was a continuous decline in N contents per unit dry mass in mature non‐senescent leaves on the order of 15–20% for a change of leaf age from 40 to 120 d, possibly manifesting nitrogen reallocation to bud formation. The decline in N contents led to similar decreases in leaf photosynthetic capacity and foliar chlorophyll contents. These data demonstrate that light‐dependent variation in the rate of developmental changes in M_A determines canopy differences in photosynthetic capacity, whereas foliar photosynthetic apparatus is essentially constant in fully developed leaves.     

Mechanisms of Enhancing Photosynthetic Gas Exchange in Date Palm Seedlings (<Emphasis Type="Italic">Phoenix dactylifera</Emphasis> L.) under Salinity Stress by a 5-Aminolevulinic Acid-based Fertilizer

Photosynthetic gas exchange characteristics, salt uptake, pigment contents, and electrolyte leakage were examined in date palm seedlings (Phoenix dactylifera L.) subject to seawater treatments at 1-, 15-, and 30-mS cm⁻¹ salinity levels in the presence or absence of 0.08% ALA-based (5-aminolevulinic acid-based) functional fertilizer commercially known as Pentakeep-v. Date palm seedlings accumulated significant amounts of Na⁺ in the foliage with increasing salinity, about a threefold increase in the accumulated Na⁺ between the control and 30-mS cm⁻¹salinity treatment. Electrolyte leakage indicated a significant reduction in membrane integrity as salinity increased. A strong linear correlation was observed between the chlorophyll (chl) a/b ratio and assimilation rate throughout salinity treatments. The slope (b) and the correlation coefficient between the chl a/b ratio and assimilation suggested that salinity reduced assimilation predominantly via the reduction in chlorophyll a contents (r ² = 0.885 and b = 1.77, P < 0.05). Plants treated with Pentakeep-v showed a similar response with increasing salinity but at higher levels of both chl a/b ratios and assimilation rates. Mechanistic analysis of A:Ci response curves showed that photosynthetic gas exchange in seedlings of the date palm was significantly reduced with increasing salinity due to gas phase limitation (S _L) as evident by stomatal conductance (g _s) values. Salinity did not induce any change in the carboxylation efficiency of the rubisco enzyme (Vc,max), or in the rate of electrons supplied by the electron transport system for ribulose 1,5-bisphosphate (RuBP) regeneration (Jmax). Accelerated carbon loss through respiration has significantly contributed to the described reduction in assimilation and increased CO₂ compensation point (Γ). Only at the 30-mS cm⁻¹ salinity level did treatment with Pentakeep-v reduce Na⁺ accumulation in the leaves, and caused a reduction in K⁺ selective uptake, leading to a concomitant reduction in K⁺/Na⁺ ratios. Pentakeep-v significantly improved chl a contents in all treatments, which was subsequently reflected in total chlorophyll and chl a/b ratios. The non-gas-phase components of the photosynthetic process (biochemical factors limiting gas exchange) were significantly improved by Pentakeep-v applications. Specifically, Pentakeep-v enhanced the biochemical efficiency of carbon fixation (Vc,max) and the rate of electron transport required for RuBP regeneration (Jmax) by 37.4% and 17.8%, respectively, over untreated plants at a salinity level of 15 mS cm^–1. In addition, Pentakeep-v reduced S _L to values similar to those of control plants (9.07%) and lowered CO₂ compensation points by reducing respiratory CO₂ loss, with increasing salinity to 30 mS cm⁻¹. We, therefore, conclude that the ALA-based fertilizer Pentakeep-v improves salt tolerance in date palm seedlings by increasing photosynthetic assimilation. The latter is mediated via boosting light-harvesting capabilities of the treated plants by increasing chl a content and by reducing stomatal limitation to photosynthetic gas exchange.     

Growth and physiological changes in saplings of Minquartia guianensis and Swietenia macrophylla during acclimation to full sunlight

Low light availability under a forest canopy often limits plant growth; however, sudden increase in light intensity may induce photoinhibition of photosynthesis. The aim of this study was to evaluate the ecophysiological changes that occur in potted plants of Minquartia guianensis and Swietenia macrophylla during the acclimation process to full sunlight. We used six full-sun independent acclimation periods (30, 60, 90, 120, 150, and 180 days) and a control kept in the shade. Shading was obtained by placing plants under the canopy of a small forest. The F_v/F_m ratio, net photosynthetic rate (P _N), the maximum carboxylation velocity of Rubisco (V _cmax), maximum electron transport rate (J _max), specific leaf area (SLA), and growth were assessed at the end of each of the six acclimation periods. Plant exposure to full sunlight caused a sudden decrease in the F_v/F_m ratio (photoinhibition) particularly in Minquartia. Photooxidation (necrotic patches) of the leaf tissue was observed in upper leaves of Minquartia. The higher P _N values were observed in Swietenia under full sun, about 12 μmol(CO₂) m⁻² s⁻¹. V _cmax25 values were higher after 90 days of acclimation, about 14 μmol(CO₂) m⁻² s⁻¹ for Minquartia, and 35 μmol(CO₂) m⁻² s⁻¹ for Swietenia. At the end of a 180-d acclimation period J _max25 was 35 μmol(electron) m⁻² s⁻¹ for Minquartia and 60 μmol(electron) m⁻² s⁻¹ for Swietenia. SLA was higher in Swietenia than in Minquartia. In Minquartia, monthly rate of leaf production per plant (MRLP) was positive (0.22 leaf month⁻¹) after four months in the open. Whereas, in Swietenia MRLP was positive (0.56 leaf month⁻¹) after an acclimation period of two months. After six months in the open, height growth rates were 3.5 and 28 mm month⁻¹ for Minquartia and Swietenia, respectively. The greater acclimation capacity of Swietenia was associated to an enhanced photosynthetic plasticity under full sun. In Minquartia, transition to full-sun conditions and lack of physiological adjustment resulted in severe photoinhibition and loss of leaves.     

Nutrient and genotypic effects on CO2-responsiveness: photosynthetic regulation in Leucadendron species of a nutrient-poor environment

Four South African Leucadendron congenerics with divergent soil N and P preferences were grown as juveniles at contrasting nutrient concentrations at ambient (350 mol mol^-1) and elevated (700 mol mol^-1) atmospheric CO2 levels. Photosynthetic parameters were related to leaf nutrient and carbohydrate status to reveal controls of carbon uptake rate. In all species, elevated CO2 depressed both the maximum Rubisco catalytic activity (Vc,max, by 19-44%) and maximum electron transport rate (Jmax, by 13-39%), indicating significant photosynthetic acclimation of both measures. Even so, all species had increased maximum light-saturated rate of net CO2 uptake (Amax)) at the elevated growth CO2 level, due to higher intercellular CO2 concentration (ci). Leaf nitrogen concentration was central to photosynthetic performance, correlating with Amax, Vc,max and Jmax, Vc,max and Jmax were linearly co-correlated, revealing a relatively invariable Jmax:Vc,max ratio, probably due to N resource optimization between light harvesting (RuBP regeneration) and carboxylation. Leaf total non-structural carbohydrate concentration (primarily starch) increased in high CO2, and was correlated with the reduction in Vc,max and Jmax. Apparent feedback control of Vc,max and Jmax was thus surprisingly consistent across all species, and may regulate carbon exchange in response to end-product fluctuation. If so, elevated CO2 may have emulated an excess end-product condition, triggering both Vc,max and Jmax down-regulation. In Leucadendron, a general physiological mechanism seems to control excess carbohydrate formation, and photosynthetic responsiveness to elevated CO2, independently of genotype and nutrient concentration. This mechanism may underlie photosynthetic acclimation to source:sink imbalances resulting from such diverse conditions as elevated CO2, low sink strength, low carbohydrate export, and nutrient limitation.     

Leaf phenology and seasonal variation of photosynthesis of invasive <Emphasis Type="Italic">Berberis thunbergii</Emphasis> (Japanese barberry) and two co-occurring native understory shrubs in a northeastern United States deciduous forest

Early leafing and extended leaf longevity can be important mechanisms for the invasion of the forest understory. We compared the leaf phenology and photosynthetic characteristics of Berberis thunbergii, an early leafing invasive shrub, and two co-occurring native species, evergreen Kalmia latifolia and late leafing Vaccinium corymbosum, throughout the 2004 growing season. Berberis thunbergii leafed out 1 month earlier than V. corymbosum and approximately 2 weeks prior to the overstory trees. The photosynthetic capacity [characterized by the maximum carboxylation rate of Rubisco (V _cmax) and the RuBP regeneration capacity mediated by the maximum electron transport rate (J _max)] of B. thunbergii was highest in the spring open canopy, and declined with canopy closure. The 2003 overwintering leaves of K. latifolia displayed high V _cmax and J _max in spring 2004. In new leaves of K. latifolia produced in 2004, the photosynthetic capacity gradually increased to a peak in mid-September, and reduced in late November. V. corymbosum, by contrast, maintained low V _cmax and J _max throughout the growing season. In B. thunbergii, light acclimation was mediated by adjustment in both leaf mass per unit area and leaf N on a mass basis, but this adjustment was weaker or absent in K. latifolia and V. corymbosum. These results indicated that B. thunbergii utilized high irradiance in the spring while K. latifolia took advantage of high irradiance in the fall and the following spring. By contrast, V. corymbosum generally did not experience a high irradiance environment and was adapted to the low irradiance understory. The apparent success of B. thunbergii therefore, appeared related to a high spring C subsidy and subsequent acclimation to varying irradiance through active N reallocation and leaf morphological modifications.