不同环境条件下光合作用对光响应模型的研究进展

光合作用对光的响应模型是研究植物在不同环境条件下光合特性的有力数学工具，可为定量描述植物光合速率对光合有效辐射的响应提供理论依据。本文基于植物光合作用对光响应经验模型的常用数学表达式特征，综述了这些模型的优势及其在实际应用中可能遇到的问题。在此基础上探讨了光合作用对光响应机理模型在描述植物的原初光反应以及光合生理生态方面的优势，并对该模型的发展进行了展望。光合作用主要由原初反应、同化力形成和碳同化构成，任何一个过程的变化均可直接影响植物的光化学效率和碳同化能力。原初反应主要涉及光能吸收、激子共振传递、量子能级跃迁和退激发等与光能吸收传递相联系的、纯粹的物理过程。光合作用对光响应经验模型难以解释植物的非光化学淬灭(NPQ)随光强的增加一直非线性增加，也难以回答植物的捕光色素分子吸收过量的光能且不能及时地用于光化学反应时，单线态叶绿素分子的寿命将延长等现象。与此同时，光合作用对光响应机理模型拟合得到的参数不仅可以反映植物的原初光反应特征，还可以描述植物捕光色素分子的物理特性，如处于激发态的捕光色素分子数(N_k)、捕光色素分子的有效光能吸收截面(σ_ik     

不同CO2浓度下大豆叶片的光合生理生态特性

CO₂是光合作用的原料和底物,影响着光合作用的进程和光合产物的数量.利用Li-6400-40B同时测量大豆叶片在不同CO₂浓度(300、400、500和600 μmol·mol^-1)下的光合电子传递速率和光合作用对光的响应曲线,并用构建的光合作用对光响应机理模型拟合这些光响应曲线,获得大豆叶片一系列的光合参数、生理生态参数和捕光色素分子的物理参数.结果表明: 电子利用效率、最大电子传递速率和最大净光合速率随CO₂浓度的升高而增加;光补偿点和暗呼吸速率随CO₂浓度的升高而下降;光能利用效率和内禀(瞬时)水分利用效率随CO₂浓度的升高而增加,不同CO₂浓度下的最大光能利用效率和最大内禀(瞬时)水分利用效率之间存在显著差异,但不同CO₂浓度下的最大羧化效率的差异不显著.CO₂浓度的大小对光合作用中原初光反应存在一定程度的影响,即高CO₂浓度有利于减小捕光色素分子处于最低激发态的最小平均寿命,以提高光能传递的速度及增加大豆光合电子流的利用效率.     

不同CO2浓度下大豆叶片的光合生理生态特性

CO₂是光合作用的原料和底物,影响着光合作用的进程和光合产物的数量.利用Li-6400-40B同时测量大豆叶片在不同CO₂浓度(300、400、500和600 μmol·mol^-1)下的光合电子传递速率和光合作用对光的响应曲线,并用构建的光合作用对光响应机理模型拟合这些光响应曲线,获得大豆叶片一系列的光合参数、生理生态参数和捕光色素分子的物理参数.结果表明: 电子利用效率、最大电子传递速率和最大净光合速率随CO₂浓度的升高而增加;光补偿点和暗呼吸速率随CO₂浓度的升高而下降;光能利用效率和内禀(瞬时)水分利用效率随CO₂浓度的升高而增加,不同CO₂浓度下的最大光能利用效率和最大内禀(瞬时)水分利用效率之间存在显著差异,但不同CO₂浓度下的最大羧化效率的差异不显著.CO₂浓度的大小对光合作用中原初光反应存在一定程度的影响,即高CO₂浓度有利于减小捕光色素分子处于最低激发态的最小平均寿命,以提高光能传递的速度及增加大豆光合电子流的利用效率.     

光合电子流对光响应的机理模型及其应用

光合电子流对光响应的机理可以揭示植物光合电子流与光强、植物捕光色素分子物理特性之间的关系。该文讨论了光合电子流对光响应的机理模型的特性以及捕光色素分子的物理性质, 并利用此模型拟合了山莴苣(Lagedium sibiricum)、一年蓬(Erigeron annuus)和紫菀(Aster tataricus)的光合电子流对光响应的曲线。由此模型不仅可以得到植物的最大光合电子流、饱和光强、初始斜率等参数, 还可以获得捕光色素分子有效光能吸收截面和处于最低激发态的捕光色素分子数对光的响应关系。结果表明: 随光强的增加, 山莴苣的捕光色素分子的有效光能吸收截面下降最快, 紫苑的下降速度最慢; 山莴苣处于最低激发态的捕光色素分子数增长速度最快, 紫苑的增长速度最小。捕光色素分子的有效光能吸收截面随光强增加而下降、处于最低激发态的捕光色素分子数随光强增加而增加的特性将减少其光能的吸收和激子的传递, 因而有利于减少强光对植物产生的光伤害。     

不同CO2浓度下番茄幼苗叶片的光能利用效率

光能利用效率(LUE)是评价植物叶片光能利用能力的重要参数,更是影响生态系统生产力大小和质量的主要因素.本文基于植物光合作用对光响应的机理模型推导出植物叶片的光能利用效率模型以及叶片的最大光能利用效率(LUE_max)和对应的饱和光强(I_L-sat)的数学表达式,并用光能利用效率模型研究了番茄幼苗叶片在CO₂浓度分别为350、450、550和650 μmol·mol^-1下的光能利用效率.拟合结果表明: 所推导的叶片光能利用效率模型可以很好地描述4种CO₂浓度条件下番茄幼苗叶片的光能利用效率;在这4种CO₂浓度条件下,番茄的LUE_max在光合有效辐射(I)为70～90 μmol·m^-2·s^-1时就可以达到;番茄在CO₂浓度为550和650 μmol·mol^-1时叶片的LUE_max和I_L-sat没有差异.产生这种现象的原因可能是由于番茄幼苗长时间处于较低光强下,番茄幼苗叶片的光合功能已经适应了低光强环境,以致于高CO₂浓度难以改变它的捕光色素分子的内秉特性,如有效光能吸收截面以及处于激发态和基态色素的比例等.     

不同CO2浓度下番茄幼苗叶片的光能利用效率

光能利用效率(LUE)是评价植物叶片光能利用能力的重要参数,更是影响生态系统生产力大小和质量的主要因素.本文基于植物光合作用对光响应的机理模型推导出植物叶片的光能利用效率模型以及叶片的最大光能利用效率(LUE_max)和对应的饱和光强(I_L-sat)的数学表达式,并用光能利用效率模型研究了番茄幼苗叶片在CO₂浓度分别为350、450、550和650 μmol·mol^-1下的光能利用效率.拟合结果表明: 所推导的叶片光能利用效率模型可以很好地描述4种CO₂浓度条件下番茄幼苗叶片的光能利用效率;在这4种CO₂浓度条件下,番茄的LUE_max在光合有效辐射(I)为70～90 μmol·m^-2·s^-1时就可以达到;番茄在CO₂浓度为550和650 μmol·mol^-1时叶片的LUE_max和I_L-sat没有差异.产生这种现象的原因可能是由于番茄幼苗长时间处于较低光强下,番茄幼苗叶片的光合功能已经适应了低光强环境,以致于高CO₂浓度难以改变它的捕光色素分子的内秉特性,如有效光能吸收截面以及处于激发态和基态色素的比例等.     

光系统II实际光化学量子效率对光的响应模型的比较

为了比较光系统II实际光化学量子效率(Φ_PSII)对光的响应机理模型(简称机理模型)、负指数模型和指数模型的优缺点, 用LI-6400-40B光合作用测定仪控制CO₂浓度和温度, 测量了剑叶金鸡菊(Coreopsis lanceolata)、黄荆(Vitex negundo)和大狼杷草(Bidens frondosa)的电子传递速率(ETR)对光的响应曲线(ETR-I)和Φ_PSII对光的响应曲线(Φ_PSII-I), 然后用这3个模型分别拟合了这些数据。拟合结果表明: 3个模型都可以较好地拟合这3种植物的ETR-I的响应数据和Φ_PSII-I的响应数据, 但由指数模型拟合ETR-I和Φ_PSII-I的响应数据得到相应的饱和光强(PAR_sat)和光系统II最大光能利用效率(F_v/F_m)之间存在显著差异, 且估算的饱和光强远低于实测值。由机理模型可知, Φ_PSII不仅与光强的函数有关, 还与植物的内禀特性有关, 即与天线色素分子的本征光能吸收截面、激子的传递效率、能级的简并度、光化学反应常数、热耗散常数和处于最低激发态的平均寿命等参数有关。此外, 由机理模型还可知, Φ_PSII随光强的增加而下降的原因是捕光色素分子的有效光能吸收截面随光强增加而降低。     

Construction of CO2-response model of electron transport rate in C4 crop and its application

准确估算光合电子流对CO₂响应的变化趋势对深入了解光合过程具有重要意义。该研究在植物光合作用对CO₂响应新模型(模型I)的基础上构建了电子传递速率(J)对CO₂的响应模型(模型II), 并对用LI-6400-40便携式光合仪测量的玉米(Zea mays)和千穗谷(Amaranthus hypochondriacus)的数据进行了拟合。结果表明, 模型II可以很好地拟合玉米和千穗谷叶片J对CO₂浓度的响应曲线(J-C_a曲线), 得到玉米和千穗谷的最大电子传递速率分别为262.41和393.07 mmol·m ^-2·s ^-1, 与估算值相符合。在此基础上, 对光合电子流分配到其他路径进行了探讨。结果显示, 380 mmol·mol ^-1 CO₂浓度下玉米和千穗谷碳同化所需的电子流为247.92和285.16 mmol·m ^-2·s ^-1, 分配到其他途径的光合电子流为14.49和107.91 mmol·m ^-2·s ^-1(考虑植物CO₂的回收利用)。比较两种植物的其他途径光合电子流分配值发现, 两者相差6倍之多。分析认为这与千穗谷和玉米的催化脱羧反应酶种类以及脱羧反应发生的部位不同密切相关。该发现为人们研究C₄植物中烟酰胺腺嘌呤二核苷磷酸苹果酸酶型和烟酰胺腺嘌呤二核苷酸苹果酸酶型两种亚型之间的差异提供了一个新的视角。此外, 构建的电子传递速率对CO₂的响应模型为人们研究C₄植物的光合电子流的变化规律提供了一个可供选择的数学工具。     

植物光合作用模型参数的温度依存性研究进展

综述了植物光合作用与温度响应模型研究的进展,围绕光合作用生化模型的4个主要参数：胞间CO₂浓度、RuBP最大碳同化速率(V_{c max})的活化能、RuBP最大再生速率(J_max)的活化能和J_max/V_{c max},讨论了影响光合作用 温度响应曲线的内在机理.随着生长温度的升高,所有物种的V_{c max}活化能均呈增加趋势,而其他参数的变化因物种不同而存在明显差异,说明V_{c max}的活化能可能是决定光合作用温度依存性的首要参数.最后分析了研究中存在的问题并提出研究展望,认为应整合叶片与群落水平的光合作用模型,从叶面积、太阳辐射、冠层结构、冠层小气候和光合能力等方面研究植物群落对全球变化的响应机理.这对于人们理解和准确估算植物生长、群落碳收支和生态系统初级生产力具有重要意义.     

基于光响应机理模型的不同植物光合特性

为了研究不同植物的光合特性,用LI-6400-40B荧光仪同时测量了葎草(Humulus scandens)、白泡桐(Paulownia fortunei)、龙葵(Solanum nigrum)的电子传递速率和光合作用对光的响应曲线。利用植物光合电子流对光响应和光合作用对光响应的机理模型研究了这3种植物光合特性的差异及其产生的原因。结果表明:葎草的叶绿素含量最低,但其捕光色素分子的本征光能吸收截面最大;白泡桐的叶绿素含量是龙葵的1.2倍,但这两种植物的最大电子传递速率没有差异。产生这种差异的原因是:龙葵的捕光色素分子的本征光能吸收截面要比白泡桐大,而它处于激发态的最小平均寿命要比白泡桐短,且龙葵的电子利用效率要比白泡桐大。此外,捕光色素分子的有效光能吸收截面和处于激发态的捕光色素分子对光响应曲线的差异也有可能影响它们的光合特性。     

青海省三江源区人工草地生态系统CO2通量

 了解三江源人工草地净生态系统CO₂交换(Net ecosystem CO₂ exchange, NEE)的季节变化规律和主要生物因子及环境因子对这些过程的影响将有助于认识青藏高原人工草地生态系统碳循环、生态价值、功能,以及对三江源区的生态安全的重要意义。该研究利用涡度相关技术,于2005年9月1日至2006年8月31日对位于青海腹地的垂穗披碱草（Elymus nutans）人工草地的NEE及生物和环境因子进行观测, 阐明NEE及其组分的动态变化特征和影响因子。三江源区人工草地生态系统的日最大吸收量为2.38 g C·m^－2·d^－1,出 现在7月30日。日间最大吸收率和最大排放率都出现在8月,分别为-6.82和2.95μmol CO₂·m^－2·s^－1。在生长季, 白天的NEE主要受光合有效辐射(Photosynthe tically active rad iation, PAR)变化控制,同时又与叶面积指数和群落多样性交互作用,共同调节光合速率和光合效率的强度。最大光合同化速率为2.46～10.39μmol CO₂·m^－2·s^－1,表观初始光能利用率为0.013～0.070μmol CO₂·μmol^－1 PAR。 在碳交换日过程中,NEE并不完全随着 PAR的增加而增大,当PAR超过某一值（>1 200μmol ·m^－2·s^－1）时,NEE随PAR的增加而降低。受温度的影响,生长季的生态系统的呼吸商Q₁₀（1.8）小于非生长季节的 2.6）。 生态系统呼吸主要受温度的控制,同时也受到叶面积指数的显著影响。生长季昼夜温差大并不利于生态系统的碳获取。 三江源区人工草地生态系统是一个较强的碳汇,为-49.35 g C·m^－2·a^－1。     

Carbon dioxide assimilation in the flowerhead of Arctium

Summary The gas exchange of flowerheads was determined in Arctium tomentosum and A. lappa during their development. The light, temperature and CO₂ responses were used to estimate flowerhead photosynthesis and the in situ contribution of carbon assimilation to the carbon requirement of the plant for supporting a flowerhead. Changes in vapour pressure deficit had no effect on flowerhead photosynthesis rates and were not included in the model.In both species assimilatory capacity correlated with total bract chlorophyll content. Light, temperature and CO₂ response curves were very similar in form between species, differing only in absolute rates. During all stages of development, flowerheads always exhibited a net carbon loss, which was mainly determined by temperature. The respiration rate decreased in the light, the difference of CO₂ exchange in the dark and in the light was interpreted as photosynthesis. This rate was larger in A. lappa than in A. tomentosum. 30% of the total C requirement of A. lappa flowerheads was photosynthesized by its bracts, the total contribution offlowerhead photosynthesis in A. tomentosum was only 15%. The potential competitive advantages of variation in flowerhead photosynthesis are discussed.     

植物光合作用的三维特性研究进展

光合作用是地球上最重要的化学反应。虽然针对植物光合作用已经进行了广泛深入的研究, 但从三维层面探讨植物叶片光合功能及其调节作用的工作较少。叶片结构、光合机构组分、叶片内光能吸收和传递均具有明显的三维特性, 极大影响叶片内CO₂转运、叶肉细胞的电子传递和碳同化, 进而使叶片光合功能及其调控表现出复杂的三维特征。因此, 从三维角度分析叶片光合特性有助于理解光合作用机理, 也能够为提高植物光合作用效率提供理论支持。     

Effect of photon flux density on inorganic carbon accumulation and net CO2 exchange in a high-CO2-requiring mutant of Chlamydomonas reinhardtii

The effect of photon flux density on inorganic carbon accumulation and photosynthetic CO₂ assimilation was determined by CO₂ exchange studies at three, limiting CO₂ concentrations with a ca-1 mutant of Chlamydomonas reinhardiii. This mutant accumulates a large internal inorganic carbon pool in the light which apparently is unavailable for photosynthetic assimilation. Although steady-state photosynthetic CO₂ assimilation did not respond to the varying photon flux densities because of CO₂ limitation, components of inorganic-carbon accumulation were not clearly light saturated even at 1100 mol photons m^-2 s^-1, indicating a substantial energy requirement for inorganic carbon transport and accumulation. Steady-state photosynthetic CO₂ assimilation responded to external CO₂ concentrations but not to changing internal inorganic carbon concentrations, confirming that diffusion of CO₂ into the cells supplies most of the CO₂ for photosynthetic assimilation and that the internal inorganic carbon pool is essentially unavailable for photosynthetic assimilation. The estimated concentration of the internal inorganic carbon pool was found to be relatively insensitive to the external CO₂ concentration over the small range tested, as would be expected if the concentration of this pool is limited by the internal to external inorganic carbon gradient. An attempt to use this CO₂ exchange method to determine whether inorganic carbon accumulation and photosynthetic CO₂ assimilation compete for energy at low photon flux densities proved inconclusive.     

植株叶片的光合色素构成对遮阴的响应

叶绿素在植株体内负责光能的吸收、传递和转化, 类胡萝卜素则行使光能捕获和光破坏防御两大功能, 它们在光合作用中起着非常重要的作用。该文综述了几大主要光合色素的分布和功能, 以及不同物种的色素含量和构成差异。阳生植物的叶黄素库较大, 但脱环氧化水平不及阴生植物。黄体素与叶黄素库的比值与植物的耐阴性呈正相关关系。由不同的遮阴源造成的遮阴环境, 光强和光质有很大的差异, 总体来说对植物生长的影响, 建筑物遮阴<阔叶林遮阴<针叶林遮阴。光强的改变可诱导类胡萝卜素的两大循环——叶黄素循环和黄体素循环。由光强诱导的叶绿素含量和叶绿素a/b比值的改变与该物种的耐阴性无关。短时间的遮阴不会对植物的生长造成危害, 叶黄素库的大小不仅与每天接受的光量子有关, 更与光量子在一天的分布有关, 因为光照和温度是协同作用的。光合作用或色素构成是蓝光、红光和远红光共同作用的结果, 不是某一种单色光所能替代的。我们总结了影响植物色素构成的内因和外因, 指出植物主要通过调整光反应中心和捕光天线色素蛋白复合体的比例, 以及两个光系统的比值来调整色素含量和构成以适应不同的光照条件, 提出了现存研究中存在的一些问题, 旨在为今后的相关研究提供建议。     

Regulation of Photosynthesis of C3 Plants in Response to Progressive Drought: Stomatal Conductance as a Reference Parameter

We review the photosynthetic responses to drought in field-growngrapevines and other species. As in other plant species, therelationship between photosynthesis and leaf water potentialand/or relative water content in field-grown grapevines dependson conditions during plant growth and measurements. However,when light-saturated stomatal conductance was used as the referenceparameter to reflect drought intensity, a common response patternwas observed that was much less dependent on the species andconditions. Many photosynthetic parameters (e.g. electron transportrate, carboxylation efficiency, intrinsic water-use efficiency,respiration rate in the light, etc.) were also more stronglycorrelated with stomatal conductance than with water statusitself. Moreover, steady-state chlorophyll fluorescence alsoshowed a high dependency on stomatal conductance. This is discussedin terms of an integrated down-regulation of the whole photosyntheticprocess by CO₂ availability in the mesophyll. A study with sixMediterranean shrubs revealed that, in spite of some markedinterspecific differences, all followed the same pattern ofdependence of photosynthetic processes on stomatal conductance,and this pattern was quite similar to that of grapevines. Furtheranalysis of the available literature suggests that the above-mentionedpattern is general for C₃ plants. Even though the patterns describeddo not necessarily imply a cause and effect relationship, theycan help our understanding of the apparent contradictions concerningstomatal vs. non-stomatal limitations to photosynthesis underdrought. The significance of these findings for the improvementof water-use efficiency of crops is discussed.     

Oxygen Exchange in Relation to Carbon Assimilation in Water-stressed Leaves During Photosynthesis

In a study on metabolic consumption of photosynthetic electronsand dissipation of excess light energy under water stress, O₂and CO₂ gas exchange was measured by mass spectrometry in tomatoplants using ¹⁸O₂ and ¹³CO₂. Under water stress, gross O₂ evolution(E_O), gross O₂ uptake (U_O), net CO₂ uptake (P_N), gross CO₂ uptake(TPS), and gross CO₂ evolution (E_C) declined. The ratio P_N/E_Ofell during stress, while the ratios U_O/E_O and E_C/TPS rose.Mitochondrial respiration in the light, which can be measureddirectly by ¹²CO₂ evolution during ¹³CO₂ uptake at 3000 µll^{–1 13}CO₂, is small in relation to gross CO₂ evolutionand CO₂ release from the glycolate pathway. It is concludedthat PSII, the Calvin cycle and mitochondrial respiration aredown-regulated under water stress. The percentages of photosyntheticelectrons dissipated by CO₂ assimilation, photorespiration andthe Mehler reaction were calculated: in control leaves morethan 50 % of the electrons were consumed in CO₂ assimilation,23 % in photorespiration and 13 % in the Mehler reaction. Undersevere stress the percentages of electrons dissipated by CO₂assimilation and the Mehler reaction declined while the percentageof electrons used in photorespiration doubled. The consumptionof electrons in photorespiration may reduce the likelihood ofdamage during water deficit.     

PHOTOACCLIMATION OF PHOTOSYNTHESIS IRRADIANCE RESPONSE CURVES AND PHOTOSYNTHETIC PIGMENTS IN MICROALGAE AND CYANOBACTERIA1

The photosynthesis‐irradiance response (PE) curve, in which mass‐specific photosynthetic rates are plotted versus irradiance, is commonly used to characterize photoacclimation. The interpretation of PE curves depends critically on the currency in which mass is expressed. Normalizing the light‐limited rate to chl a yields the chl a‐specific initial slope (α^chl). This is proportional to the light absorption coefficient (a^chl), the proportionality factor being the photon efficiency of photosynthesis (φ_m). Thus, α^chl is the product of a^chl and φ_m. In microalgae α^chl typically shows little (<20%) phenotypic variability because declines of φ_m under conditions of high‐light stress are accompanied by increases of a^chl. The variation of α^chl among species is dominated by changes in a^chl due to differences in pigment complement and pigment packaging. In contrast to the microalgae, α^chl declines as irradiance increases in the cyanobacteria where phycobiliproteins dominate light absorption because of plasticity in the phycobiliprotein:chl a ratio. By definition, light‐saturated photosynthesis (P_m) is limited by a factor other than the rate of light absorption. Normalizing P_m to organic carbon concentration to obtain P_m^C allows a direct comparison with growth rates. Within species, P_m^C is independent of growth irradiance. Among species, P_m^C covaries with the resource‐saturated growth rate. The chl a:C ratio is a key physiological variable because the appropriate currencies for normalizing light‐limited and light‐saturated photosynthetic rates are, respectively, chl a and carbon. Typically, chl a:C is reduced to about 40% of its maximum value at an irradiance that supports 50% of the species‐specific maximum growth rate and light‐harvesting accessory pigments show similar or greater declines. In the steady state, this down‐regulation of pigment content prevents microalgae and cyanobacteria from maximizing photosynthetic rates throughout the light‐limited region for growth. The reason for down‐regulation of light harvesting, and therefore loss of potential photosynthetic gain at moderately limiting irradiances, is unknown. However, it is clear that maximizing the rate of photosynthetic carbon assimilation is not the only criterion governing photoacclimation.     

C3和C4植物光合途径的适应性变化和进化

 高等植物大多为C₃植物, C₄植物和景天酸代谢(Crassulacean acid metabolism, CAM)植物是由C₃植物进化而来的。C₄途径的多源进化表明, 光合途径由C₃途径向C₄途径的转变相对简单。该文分析研究了植物光合途径的进化前景, 指出C₄植物是从C₃植物进化而来的高光效种类, 且地质时期以来降低的大气CO₂浓度和升高的大气温度以及干旱和盐渍化是C₄途径进化的外部动力。C₃植物的C₄途径的发现说明植物的光合途径并非是一成不变的, C₃和C₄植物的光合特征具有极大的可塑性, 某些环境的变化会引起植物光合途径在C₃和C₄途径之间转变。C₃植物具有的C₄途径是环境调控的产物, 是对逆境的适应性进化结果, 因而光合途径的转变也适用于干旱地区植被的适应性生存机理研究。该文还利用国外最新的C₄光合进化模型介绍了植物在进化C₄途径中所经历的7个重要时期(从分子基础到形态基础、结构基础, 再到物质代谢水平、光合酶活水平, 直到C3和C4途径协调运转时期, 最后达到形态与功能最优化阶段), 并结合全球气候变化的特点对国内外相关领域的研究进行了分析, 总结了植物光合途径的适应性转变和进化的研究成果, 为今后的相关工作提出建议。     

Effects of nitrogen supply on the acclimation of photosynthesis to elevated CO2

A common observation in plants grown in elevated CO₂ concentration is that the rate of photosynthesis is lower than expected from the dependence of photosynthesis upon CO₂ concentration in single leaves of plants grown at present CO₂ concentration. Furthermore, it has been suggested that this apparent down regulation of photosynthesis may be larger in leaves of plants at low nitrogen supply than at higher nitrogen supply. However, the available data are rather limited and contradictory. In this paper, particular attention is drawn to the way in which whole plant growth response to N supply constitutes a variable sink strength for carbohydrate usage and how this may affect photosynthesis. The need for further studies of the acclimation of photosynthesis at elevated CO₂ in leaves of plants whose N supply has resulted in well-defined growth rate and sink activity is emphasised, and brief consideration is made of how this might be achieved.Abbreviations A rate of CO₂ assimilation - C_i internal CO₂ concentration - PCR photosynthetic carbon reduction - Rubisco Ribulose 1,5-bisphosphate carboxylase/oxygenase - RuBP ribulose 1,5-bisphosphate