施氮和大气CO2浓度升高对小麦旗叶光合电子传递和分配的影响

采用开顶式气室盆栽培养小麦,设计2个大气CO₂浓度（正常：400 μmol·mol^-1;高：760 μmol·mol^-1）、2个氮素水平（0和200 mg·kg^-1土）的组合处理,通过测定小麦抽穗期旗叶氮素和叶绿素浓度、光合速率（P_n）-胞间CO₂浓度（C_i）响应曲线及荧光动力学参数,来测算小麦叶片光合电子传递速率等,研究了高大气CO₂浓度下施氮对小麦旗叶光合能量分配的影响.结果表明：与正常大气CO₂浓度相比,高大气CO₂浓度下小麦叶片氮浓度和叶绿素浓度降低,高氮处理的小麦叶片叶绿素a/b升高.施氮后小麦叶片PSⅡ最大光化学效率（F_v/F_m）、PSⅡ反应中心最大量子产额（F_v′/F_m′）、PSⅡ反应中心的开放比例（q_p）和PSⅡ反应中心实际光化学效率（Φ_PSⅡ）在大气CO₂浓度升高后无明显变化,虽然叶片非光化学猝灭系数（NPQ）显著降低,但PSⅡ总电子传递速率（J_F）无明显增加;不施氮处理的F_v′/F_m′、Φ_PSⅡ和NPQ在高大气CO₂浓度下显著降低,尽管F_v/F_m和q_P无明显变化,J_F仍显著下降.施氮后小麦叶片J_F增加,参与光化学反应的非环式电子流传递速率(J_C)明显升高.大气CO₂浓度升高使参与光呼吸的非环式电子流传递速率(J₀)、Rubisco氧化速率（V₀）、光合电子的光呼吸/光化学传递速率比（J₀/J_C）和Rubisco氧化/羧化比（V₀/V_C）降低,但使J_C和Rubisco羧化速率（V_C）增加.因此,高大气CO₂浓度下小麦叶片氮浓度和叶绿素浓度降低,而增施氮素使通过PSⅡ反应中心的电子流速率显著增加,促进了光合电子流向光化学方向的传递,使更多的电子进入Rubisco羧化过程,P_n显著升高.     

氮素对高大气CO2浓度下小麦叶片光合作用的影响

通过测定小麦拔节期叶片的光合气体交换参数和光强-光合速率（P_n）响应曲线,研究了氮素对长期高大气CO₂浓度（760 μmol·mol^-1）下小麦叶片光合作用的影响.结果表明：在长期高大气CO₂浓度下,增施氮肥能提高小麦叶片P_n、蒸腾速率（T_r）和瞬时水分利用效率（WUE_i）;与正常大气CO₂浓度相比,高大气CO₂浓度下小麦叶片的P_n和WUE_i增加,气孔导度（G_s）和胞间CO₂浓度(C_i）降低.随光合有效辐射的增强,高大气CO₂浓度下小麦叶片的P_n和WUE_i均高于正常大气CO₂浓度处理,G_s则较低,而C_i和T_r无显著变化.高氮水平下小麦叶片G_s与P_n、T_r、WUE_i呈线性正相关,G_s与C_i在正常大气CO₂浓度下呈线性负相关,但高大气CO₂浓度下二者无相关性;低氮水平下小麦叶片的G_s与P_n、WUE_i无相关性,而与C_i和T_r呈线性正相关,表明高大气CO₂浓度下低氮水平的小麦叶片P_n由非气孔因素限制.     

氮素对高大气CO2浓度下小麦叶片光合作用的影响

通过测定小麦拔节期叶片的光合气体交换参数和光强-光合速率（P_n）响应曲线,研究了氮素对长期高大气CO₂浓度（760 μmol·mol^-1）下小麦叶片光合作用的影响.结果表明：在长期高大气CO₂浓度下,增施氮肥能提高小麦叶片P_n、蒸腾速率（T_r）和瞬时水分利用效率（WUE_i）;与正常大气CO₂浓度相比,高大气CO₂浓度下小麦叶片的P_n和WUE_i增加,气孔导度（G_s）和胞间CO₂浓度(C_i）降低.随光合有效辐射的增强,高大气CO₂浓度下小麦叶片的P_n和WUE_i均高于正常大气CO₂浓度处理,G_s则较低,而C_i和T_r无显著变化.高氮水平下小麦叶片G_s与P_n、T_r、WUE_i呈线性正相关,G_s与C_i在正常大气CO₂浓度下呈线性负相关,但高大气CO₂浓度下二者无相关性;低氮水平下小麦叶片的G_s与P_n、WUE_i无相关性,而与C_i和T_r呈线性正相关,表明高大气CO₂浓度下低氮水平的小麦叶片P_n由非气孔因素限制.     

施氮和大气CO2浓度升高对小麦旗叶光合电子传递和分配的影响

采用开顶式气室盆栽培养小麦,设计2个大气CO2浓度(正常:400 μmol.mol-1;高:760 μmol·mol-1)、2个氮素水平(0和200 mg·kg-1土)的组合处理,通过测定小麦抽穗期旗叶氮素和叶绿素浓度、光合速率(Pn)-胞间CO2浓度(C1)响应曲线及荧光动力学参数,来测算小麦叶片光合电子传递速率等,研究了高大气CO2浓度下施氮对小麦旗叶光合能量分配的影响.结果表明:与正常大气CO2浓度相比,高大气CO2浓度下小麦叶片氮浓度和叶绿素浓度降低,高氮处理的小麦叶片叶绿素a/b升高.施氮后小麦叶片PSⅡ最大光化学效率(Fv/Fm)、PSⅡ反应中心最大量子产额(Fv'/Fm')、PSⅡ反应中心的开放比例(qr)和PSⅡ反应中心实际光化学效率(φPSⅡ)在大气CO2浓度升高后无明显变化,虽然叶片非光化学猝灭系数(NPQ)显著降低,但PSⅡ总电子传递速率(JF)无明显增加;不施氮处理的Fv'/Fm'、φPSⅡ和NPQ在高大气CO2浓度下显著降低,尽管Fv/Fm和qp无明显变化,JF仍显著下降.施氮后小麦叶片JF增加,参与光化学反应的非环式电子流传递速率(Jc)明显升高.大气CO2浓度升高使参与光呼吸的非环式电子流传递速率(J0)、Rubisco氧化速率(V0)、光合电子的光呼吸/光化学传递速率比(J0/Jc)和Rubisco氧化/羧化比(V0/Vc)降低,但使Jc和Rubisco羧化速率(Vc)增加.因此,高大气CO2浓度下小麦叶片氮浓度和叶绿素浓度降低,而增施氮素使通过PSⅡ反应中心的电子流速率显著增加,促进了光合电子流向光化学方向的传递,使更多的电子进入Rubisco羧化过程,Pn显著升高.     

大气CO2浓度升高对香蕉光合作用及光合碳循环过程中叶氮分配的影响

生长在高CO₂浓度(700±5μl·L^-1)1周的香蕉叶片,其光合速率(Pn,μmol·m^-2·s^-1)为5.14±0.32,较生长在大气CO₂浓度(356±301μl·L^-1)的高22.1%,而生长在较高CO₂浓度下8周,叶片Pn较生长在大气CO₂浓度的低18.1%,表现香蕉叶片对较长期高CO₂浓度的驯化和光合作用抑制.生长在高CO₂浓度的香蕉叶片有较低光下呼吸速率(R_d),而不包括光下呼吸的CO₂补偿点则变幅较小.最大羧化速率(Vcmax)和电子传递速率(J)分别较生长在大气CO₂浓度的低30.5%和14.8%,根据气体交换速率计算的表观量子产率(α,mol CO₂·mol^-1光量子),生长在较高CO₂浓度下8周的叶片为0.014±0.01,而生长在大气CO₂浓度下的为0.025±0.005.较高CO₂浓度下叶片的表观量子产率降低44%.光能转换效率electrons·quanta^-1)亦从0.203降低至0.136.生长在较高CO₂浓度下香蕉叶片的叶氮在Rubicos分配系数(PR)、叶氮在生物力能学组分分配系数(P_B)和叶氮在光捕组分的分配系数(P_L)均较生长在大气CO₂浓度低,表明在高CO₂浓度下较长期生长(8周)的香蕉叶片多个光合过程受抑制,光合活性明显降低.     

大气CO2浓度升高对干旱条件下冬小麦叶片光合适应的影响

大气CO₂浓度升高是全球气候变化的主要特征,但大气CO₂浓度长期升高条件下冬小麦叶片发生光合适应的机制尚不十分清楚。本研究以盆栽冬小麦‘郑麦9023’为试验材料,在人工气候控制室内设置2个CO₂浓度(400和600 μmol·mol^-1)、2个水分条件(田间持水量的80%±5%和55%±5%),测定拔节期和抽穗期的光合特征曲线、叶绿素荧光动力学参数、叶氮含量和收获后的籽粒产量等指标,探讨干旱条件下库源关系改变对叶片光合适应的影响。结果表明: 在小麦拔节期,干旱条件下CO₂浓度升高处理的小麦PSⅡ实际光化学效率没有显著增加,但通过提升最大电子传递速率和电子向光化学方向的传递比例,增强了Rubisco的羧化速率,从而提高了最大净光合速率;在抽穗期,功能叶最大电子传递速率和电子向光化学方向的传递比例虽然较高,但PSⅡ实际光化学转换效率降低,Rubisco羧化速率和丙糖磷酸利用效率下降,以致最大净光合速率降低。干旱条件下,CO₂浓度升高增加了小麦单茎生物量、单穗粒数和穗粒重,降低了不孕小穗数,提高了籽粒产量。土壤干旱条件下,CO₂浓度升高对收获期小麦单茎籽粒产量的促进作用可能主要来自于生长前期的光合产物积累。生长后期光合适应发生的主要原因是功能叶PSⅡ实际光化学转换效率和丙糖磷酸利用效率的降低,而不是最大电子传递速率、光化学方向的电子传递比例和新叶库强的变化。     

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.     

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。     

空气CO2增高条件下荔枝叶片光合作用和超氧自由基产率

研究结果表明,生长在77±5PaCO₂分压下30d的荔枝幼树,其光合速率较大气CO₂分压(39.3Pa)下的低23%,光下线粒体呼吸速率和不包含光下呼吸的CO₂补偿点亦略有降低.空气CO₂增高使叶片最大羧化速率(V_cmax)和最大电子传递速率(J_max)降低,表明大气增高CO₂分压下叶片的光系统I(PSI)能量水平较低,叶片超氧自由基产率亦降低39%,叶片感染荔枝霜疫霉病率则从生长在大气CO₂分压下的1.8%增至9.5%.可能较低光合和呼吸代谢诱致较低的超氧自由基产率,而使叶片易受病害侵染.叶片受病害侵染后表现为超氧自由基的激增.在全球大气CO₂分压增高趋势下须加强对荔枝霜疫霉病的控制.     

The effect of free air carbon dioxide enrichment (FACE) and soil nitrogen availability on the photosynthetic capacity of wheat

A simple system for free air carbon dioxide enrichment (FACE) was recently developed and it is here briefly described. Such a MiniFACE system allowed the elevation of CO₂ concentration of small field plots avoiding the occurrence of large spatial and temporal fluctuations. A CO₂ enrichment field experiment was conducted in Italy in the season 1993–1994 with wheat (cv. Super-dwarf Mercia). A randomized experimental design was used with the treatment combination CO₂ × soil N, replicated twice. Gas exchange measurements showed that photosynthetic capacity was significantly decreased in plants exposed to elevated CO₂ and grown under nitrogen deficiency. Photosynthetic acclimation was, in this case, due to the occurrence of reduced rates of rubP saturated and rubP regeneration limited photosynthesis. Gas exchange measurements did not instead reveal any significant effect of elevated CO₂ on the photosynthetic capacity of leaves of plants well fertilized with nitrogen, in spite of a transitory negative effect on rubP regeneration limited photosynthesis that was detected to occur in the central part of a day with high irradiance. It is concluded that the levels of nitrogen fertilization will play a substantial role in modulating CO₂ fertilization effects on growth and yields of wheat crops under the scenario of future climate change.     

Photosynthetic acclimation to elevated CO2 is dependent on N partitioning and transpiration in soybean

Physiological processes that modulate photosynthetic acclimation to rising atmospheric CO₂ concentration are subjects of intense discussion recently. Apparently, the down-regulation of photosynthesis under elevated CO₂ is not understood clearly. In the present study, the response of soybean (Glycine max L.) to CO₂ enrichment was examined in terms of nitrogen partitioning and water relation. The plants grown under potted conditions without combined N application were exposed to either ambient air (38 Pa CO₂) or CO₂ enrichment (100 Pa CO₂) for short (6 days) and long (27 days). Plant biomass, apparent photosynthetic rate, transpiration rate and ¹⁵N uptake and partitioning were measured consecutively after elevated CO₂ treatment. Long-term exposure reduced photosynthetic rate, stomatal conductance and transpiration rate. In contrast, short-term exposure increased biomass production of soybean due to increase in dry weight of leaves. Leaf N concentration tended to decrease with CO₂ enrichment, however such difference was not true for stem and roots.A close correlation was observed between transpiration rate and ¹⁵N partitioned into leaves, suggesting that transpiration plays an important role on nitrogen partitioning to leaves. In conclusion existence of a feed back mechanism for photosynthetic acclimation has been proposed. Down-regulation of photosynthetic activity under CO₂ enrichment is caused by decreasing leaf N concentration, and reduced rate of transpiration owing to decreased stomatal conductance is partially responsible for poor N translocation.     

Involvement of nitrogen and cytokinins in photosynthetic acclimation to elevated CO2 of spring wheat

Acclimation of photosynthetic capacity to elevated CO₂ involves a decrease of the leaf Rubisco content. In the present study, it was hypothesized that nitrogen uptake and partitioning within the leaf and among different aboveground organs affects the down-regulation of Rubisco. Given the interdependence of nitrogen and cytokinin signals at the whole plant level, it is also proposed that cytokinins affect the nitrogen economy of plants under elevated CO₂, and therefore the acclimatory responses. Spring wheat received varying levels of nitrogen and cytokinin in field chambers with ambient (370 μmol mol⁻¹) or elevated (700 μmol mol⁻¹) atmospheric CO₂. Gas exchange, Rubisco, soluble protein and nitrogen contents were determined in the top three leaves in the canopy, together with total nitrogen contents per shoot. Growth in elevated CO₂ induced decreases in photosynthetic capacity only when nitrogen supply was low. However, the leaf contents of Rubisco, soluble protein and total nitrogen on an area basis declined in elevated CO₂ regardless of nitrogen supply. Total nitrogen in the shoot was no lower in elevated than ambient CO₂, but the fraction of this nitrogen located in flag and penultimate leaves was lower in elevated CO₂. Decreased Rubisco: chlorophyll ratios accompanied losses of leaf Rubisco with CO₂ enrichment. Cytokinin applications increased nitrogen content in all leaves and nitrogen allocation to senescing leaves, but decreased Rubisco contents in flag leaves at anthesis and in all leaves 20 days later, together with the amount of Rubisco relative to soluble protein in all leaves at both growth stages. The results suggest that down regulation of Rubisco in leaves at elevated CO₂ is linked with decreased allocation of nitrogen to the younger leaves and that cytokinins cause a fractional decrease of Rubisco and therefore do not alleviate acclimation to elevated CO₂.     

Ribulose-1,5-bisphosphate regeneration limitation in rice leaf photosynthetic acclimation to elevated CO2

Our previous study has demonstrated that both RuBP carboxylation limitation and RuBP regeneration limitation exist simultaneously in rice grown under free-air CO₂ enrichment (FACE, about 200 μmol mol⁻¹ above the ambient air CO₂ concentration) conditions [G.-Y. Chen, Z.-H. Yong, Y. Liao, D.-Y. Zhang, Y. Chen, H.-B. Zhang, J. Chen, J.-G. Zhu, D.-Q. Xu, Photosynthetic acclimation in rice leaves to free-air CO₂ enrichment related to both ribulose-1,5-bisphosphate carboxylase limitation and ribulose-1,5-bisphosphate regeneration limitation. Plant Cell Physiol. 46 (2005) 1036–1045]. To explore the mechanism for forming of RuBP regeneration limitation, we conducted the gas exchange measurements and some biochemical analyses in FACE-treated and ambient rice plants. Net CO₂ assimilation rate (A_net) in FACE leaves was remarkably lower than that in ambient leaves when measured at the same CO₂ concentration, indicating that photosynthetic acclimation to elevated CO₂ occurred. In the meantime the maximum electron transport rate (ETR) (J_max), maximum carboxylation rate (V_cmax) in vivo, and RuBP contents decreased significantly in FACE leaves. The whole chain electron transport rate and photophosphorylation rate reduced significantly while ETR of photosystem II (PSII) did not significantly decrease and ETR of photosystem I (PSI) was significantly increased in the chloroplasts from FACE leaves. Further, the amount of cytochrome (Cyt) f protein, a key component localized between PSII and PSI, was remarkably declined in FACE leaves. It appears that during photosynthetic acclimation the decline in the Cyt f amount is an important cause for the decreased RuBP regeneration capacity by decreasing the whole chain electron transport in FACE leaves.     

Effect of nitrogen-deficiency on midday photoinhibition in flag leaves of different rice (<Emphasis Type="Italic">Oryza sativa</Emphasis> L.) cultivars

Effects of nitrogen (N)-deficiency on midday photoinhibition in flag leaves were compared between two contrastive Japanese rice cultivars, a traditional japonica cultivar with low yield, cv. Shirobeniya (SRB), and a japonica-indica intermediate type with high yield, cv. Akenohoshi (AKN). Both cultivars were grown under high-N and low-N conditions. At midday, low-N supply resulted in more intensive reductions in net photosynthetic rate, stomatal conductance, maximal quantum yield of photosystem II (PSII) and quantum yield of PSII electron transport in SRB than in AKN, indicating that SRB was more strongly photoinhibited than AKN under low-N condition. At midday, the low-N plants of two cultivars showed higher superoxide dismutase (SOD) activities than the high-N plants. However, ascorbate peroxidase (APX) activity was maintained in AKN but significantly decreased in SRB under low-N condition (N-deficiency). In contrast, hydrogen peroxide (H₂O₂) content in SRB significantly increased under low-N condition, indicating that the susceptibility to midday photoinhibition in the low-N plants of SRB is related to the increased H₂O₂ accumulation. It is suggested that the midday depression in photosynthesis may be a result of oxidative stress occurring in the low-N plants in which antioxidant capacity is not enough to cope with the generation of H₂O₂. Therefore, H₂O₂-scavenging capacity could be an important factor in determining the cultivar difference of midday photoinhibition in flag leaves of rice under low-N condition.     

Photosynthetic acclimation in trees to rising atmospheric CO2: A broader perspective

Analysis of leaf-level photosynthetic responses of 39 tree species grown in elevated concentrations of atmospheric CO₂ indicated an average photosynthetic enhancement of 44% when measured at the growth [CO₂]. When photosynthesis was measured at a common ambient [CO₂], photosynthesis of plants grown at elevated [CO₂] was reduced, on average, 21% relative to ambient-grown trees, but variability was high. The evidence linking photosynthetic acclimation in trees with changes at the biochemical level is examined, along with anatomical and morphological changes in trees that impact leaf- and canopy-level photosynthetic response to CO₂ enrichment. Nutrient limitations and variations in sink strength appear to influence photosynthetic acclimation, but the evidence in trees for one predominant factor controlling acclimation is lacking. Regardless of the mechanisms that underlie photosynthetic acclimation, it is doubtful that this response will be complete. A new focus on adjustments to rising [CO₂] at canopy, stand, and forest scales is needed to predict ecosystem response to a changing environment.Abbreviations A/C_i photosynthesis as a function of internal [CO₂] - J_max maximum rate of electron transport - Rubisco ribulose-1,5-bisphosphate carboxylase/oxygenase - Vc_max maximum rate of carboxylation The U.S. Government right to retain a non-exclusive, royalty free licence in and to any copyright is acknowledged.     

The Effects of N Nutrition on the Water Relations and Gas Exchange Characteristics of Wheat (Triticum aestivum L.)

The purpose of this study was to characterize leaf photosynthetic and stomatal responses of wheat (Triticum aestivum L.) plants grown under two N-nutritional regimes. High- and low-N regimes were imposed on growth-chamber-grown plants by fertilizing with nutrient solutions containing 12 or 1 millimolar nitrogen, respectively. Gas-exchange measurements indicated not only greater photosynthetic capacity of high-N plants under well-watered conditions, but also a greater sensitivity of CO₂ exchange rate and leaf conductance to CO₂ and leaf water potential compared to low-N plants. Increased sensitivity of high-N plants was associated with greater tissue elasticity, lower values of leaf osmotic pressure and greater aboveground biomass. These N-nutritional-related changes resulted in greater desiccation (lowered relative water content) of high-N plants as leaf water potential fell, and were implicated as being important in causing greater sensitivity of high-N leaf gas exchange to reductions in water potential. Water use efficiency of leaves, calculated as CO₂ exchange rate/transpiration, increased from 9.1 to 13 millimoles per mole and 7.9 to 9.1 millimoles per mole for high- and low-N plants as water became limiting. Stomatal oscillations were commonly observed in the low-N treatment at low leaf water potentials and ambient CO₂ concentrations, but disappeared as CO₂ was lowered and stomata opened.     

Photosynthesis in leaves and siliques of winter oilseed rape (Brassica napus L.)

The rate of photosynthesis and its relation to tissue nitrogen content was studied in leaves and siliques of winter oilseed rape (Brassica napus L.) growing under field conditions including three rates of nitrogen application (0, 100 or 200 kg N ha^-1) and two levels of irrigation (rainfed or irrigated at a deficit of 20 mm). The predominant effect of increasing N application under conditions without water deficiency was enhanced expansion of photosynthetically active leaf and silique surfaces, while the rate of photosynthesis per unit leaf or silique surface area was similar in the different N treatments. Thus, oilseed rape did not increase N investment in leaf area expansion before a decline in photosynthetic rate per unit leaf area due to N deficiency could be avoided. Much less photosynthetically active radiation penetrated into high-N canopies than into low-N canopies. The specific leaf area increased markedly in low light conditions, causing leaves in shade to be less dense than leaves exposed to ample light. In both leaves and siliques the photosynthetic rate per unit surface area responded linearly to increasing N content up to about 2 g m^-2, thus showing a constant rate of net CO₂ assimilation per unit increment in N (constant photosynthetic N use efficiency). At higher tissue N contents, photosynthetic rate responded less to changes in N status. Expressed per unit N, light saturated photosynthetic rate was three times higher in leaves than in silique valves, indicating a more efficient photosynthetic N utilization in leaves than in siliques. Nevertheless, from about two weeks after completion of flowering and onwards total net CO₂ fixation in silique valves exceeded that in leaves because siliques received much higher radiation intensities than leaves and because the leaf area declined rapidly during the reproductive phase of growth. Water deficiency in late vegetative and early reproductive growth stages reduced the photosynthetic rate in leaves and, in particular, siliques of medium- and high-N plants, but not of low-N plants.     

Contrasting growth response of an N2-fixing and non-fixing forb to elevated CO2: dependence on soil N supply

With the ability to symbiotically fix atmospheric N₂, legumes may lack the N-limitations thought to constrain plant response to elevated concentrations of atmospheric CO₂. The growth and photosynthetic responses of two perennial grassland species were compared to test the hypotheses that (1) the CO₂ response of wild species is limited at low N availability, (2) legumes respond to a greater extent than non-fixing forbs to elevated CO₂, and (3) elevated CO₂ stimulates symbiotic N₂ fixation, resulting in an increased amount of N derived from the atmosphere. This study investigated the effects of atmospheric CO₂ concentration (365 and 700 mol mol^–1) and N addition on whole plant growth and C and N acquisition in an N₂-fixing legume (Lupinus perennis) and a non-fixing forb (Achillea millefolium) in controlled-chamber environments. To evaluate the effects of a wide range of N availability on the CO₂ response, we incorporated six levels of soil N addition starting with native field soil inherently low in N (field soil + 0, 4, 8, 12, 16, or 20 g N m^–2 yr^–1). Whole plant growth, leaf net photosynthetic rates (A), and the proportion of N derived from N₂ fixation were determined in plants grown from seed over one growing season. Both species increased growth with CO₂enrichment, but this response was mediated by N supply only for the non-fixer, Achillea. Its response depended on mineral N supply as growth enhancements under elevated CO₂ increased from 0% in low N soil to +25% at the higher levels of N addition. In contrast, Lupinus plants had 80% greater biomass under elevated CO₂ regardless of N treatment. Although partial photosynthetic acclimation to CO₂ enrichment occurred, both species maintained comparably higher A in elevated compared to ambient CO₂ (+38%). N addition facilitated increased A in Achillea, however, in neither species did additional N availability affect the acclimation response of A to CO₂. Elevated CO₂ increased plant total N yield by 57% in Lupinus but had no effect on Achillea. The increased N in Lupinus came from symbiotic N₂ fixation, which resulted in a 47% greater proportion of N derived from fixation relative to other sources of N. These results suggest that compared to non-fixing forbs, N₂-fixers exhibit positive photosynthetic and growth responses to increased atmospheric CO₂ that are independent of soil N supply. The enhanced amount of N derived from N₂ fixation under elevated CO₂ presumably helps meet the increased N demand in N₂-fixing species. This response may lead to modified roles of N₂-fixers and N₂-fixer/non-fixer species interactions in grassland communities, especially those that are inherently N-poor, under projected rising atmospheric CO₂.     

Gas exchange and photosynthetic performance of the tropical tree <Emphasis Type="Italic">Acacia nigrescens</Emphasis> when grown in different CO<Subscript>2</Subscript> concentrations

The photosynthetic responses of the tropical tree species Acacia nigrescens Oliv. grown at different atmospheric CO₂ concentrations—from sub-ambient to super-ambient—have been studied. Light-saturated rates of net photosynthesis (A _sat) in A. nigrescens, measured after 120 days exposure, increased significantly from sub-ambient (196 μL L⁻¹) to current ambient (386 μL L⁻¹) CO₂ growth conditions but did not increase any further as [CO₂] became super-ambient (597 μL L⁻¹). Examination of photosynthetic CO₂ response curves, leaf nitrogen content, and leaf thickness showed that this acclimation was most likely caused by reduction in Rubisco activity and a shift towards ribulose-1,5-bisphosphate regeneration-limited photosynthesis, but not a consequence of changes in mesophyll conductance. Also, measurements of the maximum efficiency of PSII and the carotenoid to chlorophyll ratio of leaves indicated that it was unlikely that the pattern of A _sat seen was a consequence of growth [CO₂] induced stress. Many of the photosynthetic responses examined were not linear with respect to the concentration of CO₂ but could be explained by current models of photosynthesis.