Variations in structural,biochemical, and physiological traits of photosynthesis and resource use efficiency in Amaranthus species (NAD-ME-type C4)

C₄ plants show higher photosynthetic capacity and productivity than C₃ plants owing to a CO₂-concentrating mechanism in leaves, which reduces photorespiration. However, which traits regulate the photosynthetic capacity of C₄ plants remains unclear. We investigated structural, biochemical, and physiological traits associated with photosynthesis and resource use efficiency in 20 accessions of 12 species of Amaranthus, NAD-malic enzyme-type C₄ dicots. Net photosynthetic rate (P_N) ranged from 19.7 to 40.5 μmol m^?2 s^?1. P_N was positively correlated with stomatal conductance and nitrogen and chlorophyll contents of leaves and was weakly positively correlated with specific leaf weight. P_N was also positively correlated with the activity of the C₃ enzyme ribulose-1,5-bisphoshate carboxylase/oxygenase, but not with the activities of the C₄ enzymes phosphoenolpyruvate carboxylase and NAD-malic enzyme. Structural traits of leaves (stomatal density, guard cell length, leaf thickness, interveinal distance, sizes of mesophyll and bundle sheath cells and the area ratio between these cells) were not significantly correlated with P_N. These data suggest that some of the biochemical and physiological traits are involved in interspecific P_N variation, whereas structural traits are not directly involved. Photosynthetic nitrogen use efficiency ranged between 260 and 458 μmol mol^?1 N s^?1. Photosynthetic water use efficiency ranged between 5.6 and 10.4 mmol mol^?1. When these data were compared with previously published data of C₄ grasses, it is suggested that common mechanisms may determine the variations in resource use efficiency in grasses and this dicot group.     

Structural and physiological responses of the C4 grass Sorghum bicolor to nitrogen limitation

Nitrogen (N) is one of the major nutrients influencing photosynthesis and productivity of C₄ plants as well as C₃ plants. C₄ photosynthesis operates through close coordination between mesophyll (M) and bundle sheath (BS) cells. However, how the development of structural and physiological traits in leaves of C₄ plants is regulated under N limitation remains uncertain. We investigated structural and physiological responses of leaves of the NADP-ME-type C₄ grass Sorghum bicolor to N limitation. Plants were grown under four levels of N supply (.05 to .6 g N per 5-L pot). Decreasing N supply resulted in decreases in net photosynthetic rate, stomatal conductance, leaf N and chlorophyll contents, and the activity ratio of phosphoenolpyruvate carboxylase to ribulose 1,5-bisphosphate carboxylase/oxygenase and increases in δ¹³C values and photosynthetic N use efficiency. Low-N leaves were thinner and had smaller photosynthetic cells, especially in M, resulting in lower M/BS tissue area ratio, and contained smaller and fewer chloroplasts. The BS chloroplasts in the low-N leaves accumulated abundant starch grains. The number of thylakoids per granal stack was reduced in M chloroplasts but not in BS chloroplasts. The low-N leaves had thicker cell walls, especially in the BS cells, which might be associated with less negative δ¹³C values, and fewer plasmodesmata in the BS cells. These data reveal structural and physiological responses of C₄ plants to N limitation, most of which would be related to cellular N allocation, light use, CO₂ diffusion and leakiness, and metabolite transport under N limitation.     

Effect of nitrogen supply on leaf appearance,leaf growth,leaf nitrogen economy and photosynthetic capacity in maize (Zea mays L.)

Leaf area growth and nitrogen concentration per unit leaf area, N_a (g m⁻² N) are two options plants can use to adapt to nitrogen limitation. Previous work indicated that potato (Solanum tuberosum L.) adapts the size of leaves to maintain N_a and photosynthetic capacity per unit leaf area. This paper reports on the effect of N limitation on leaf area production and photosynthetic capacity in maize, a C4 cereal. Maize was grown in two experiments in pots in glasshouses with three (0.84–6.0 g N pot⁻¹) and five rates (0.5–6.0 g pot⁻¹) of N. Leaf tip and ligule appearance were monitored and final individual leaf area was determined. Changes with leaf age in leaf area, leaf N content and light-saturated photosynthetic capacity, P_max, were measured on two leaves per plant in each experiment. The final area of the largest leaf and total plant leaf area differed by 16 and 29% from the lowest to highest N supply, but leaf appearance rate and the duration of leaf expansion were unaffected. The N concentration of expanding leaves (N_a or %N in dry matter) differed by at least a factor 2 from the lowest to highest N supply. A hyperbolic function described the relation between P_max and N_a. The results confirm the ‘maize strategy’: leaf N content, photosynthetic capacity, and ultimately radiation use efficiency is more sensitive to nitrogen limitation than are leaf area expansion and light interception. The generality of the findings is discussed and it is suggested that at canopy level species showing the ‘potato strategy’ can be recognized from little effect of nitrogen supply on radiation use efficiency, while the reverse is true for species showing the ‘maize strategy’ for adaptation to N limitation.     

施氮量对利民33光合特性及产量的影响

在大田试验条件下设氮肥用量分别为N0（不施氮肥）、施氮120（N120）、200（N200）、280（N280）和360 kg/hm²（N360）5个处理,测定叶面积和叶绿素含量、净光合速率（Pn）、气孔导度（Gs）、胞间二氧化碳浓度（Ci）、蒸腾速率（Tr）和产量,探讨不同施氮量对利民33光合特性及产量的影响。研究结果表明,随施氮量的增加,利民33叶面积指数、叶绿素含量、净光合速率、光能利用率提高,进而提高产量;产量与净光合速率、气孔导度、蒸腾速率和光能利用率呈正相关,与胞间二氧化碳浓度呈负相关;产量与施氮量的回归方程为Y=-0.05x²+32.38x+6 351（R²=0.98）,最高产量施氮量为335.9 kg/hm²。     

间作条件下玉米与马铃薯的养分利用特征

采用田间小区试验,通过2行玉米间作2行马铃薯、4行玉米间作4行马铃薯及相应单作试验,在玉米拔节期和大喇叭口期比较植株氮磷钾浓度和养分吸收量,研究玉米间作马铃薯中养分吸收和肥料利用效率的变化。结果表明,玉米大喇叭口期,间作玉米植株氮钾浓度显著低于单作(F_氮=6.608,P_氮=0.015;F_钾=5.148,P_钾=0.028),且根、茎、叶中氮磷钾浓度都低于单作处理,部分差异达到了显著水平,根对氮和磷的利用贡献大一些,茎对钾的利用贡献大。间作玉米的养分吸收量没有优势,但间作玉米单株产量极显著高于单作,产量与玉米大喇叭口期植株氮钾浓度极显著和显著负相关,表明提高玉米氮钾利用效率可促进增产。     

利用隶属函数值法对玉米成熟期抗旱性的综合评价

以10个不同基因型玉米为试验材料,在宁夏中部干旱带,通过测定不同品种成熟期的K~+/Na~+、光合气体交换参数、农艺性状及产量性状等指标进行抗旱性鉴定,并利用主成分分析和隶属函数法对其抗旱性强弱进行综合评价。结果表明,干旱胁迫地和对照田中,各参试品种的光合特征参数、K~+/Na~+及形态性状等均有明显差异,且干旱胁迫条件下的净光合速率(P_n)、株高、产量均显著低于对照(P0.05)。同一品种不同器官的K~+/Na~+值表现为茎基茎叶根,表明植株体内的离子吸收和运输具有选择性,茎基具有贮存K~+的作用,根具有贮存Na~+的功能。不同基因型玉米品种的抗旱能力根据综合评价值(D值)大小,综合评价其抗旱性强弱顺序为A111-3/H242HoViY1北21/A58A3/A18高WY2宁3/1522H237/A18桂青贮1号H237/1506A3/A18。     

Induced polyploidy in inter‐subspecific maize hybrids to reduce heterosis breakdown and restore reproductive fertility

The aim of this study was to reduce the breakdown of hybrid vigour by crossing maize × teosinte subspecies through induced tetraploidy and to improve the biomass and reproductive fertility of the induced tetraploids. To achieve this, seeds of maize breeding lines selected from an open‐pollinated population ‘Sargodha‐2002’ and a maize × teosinte cross were germinated in colchicine solution (0·25, 0·5 or 1·0%) until they had a thick radical and protruded plumule. Colchicine at 0·5% induced the highest number of tetraploids with the lowest number of chimeric plants. The induced tetraploids of maize and maize × teosinte crosses showed a significant (P ≤ 0·05) increase in leaf area (5 and 14%), total soluble solids (20 and 18%), leaf oil percentage (42 and 12%) and leaf crude protein contents (10 and 14%) in leaves relative to the diploid subspecies. Moreover, induced tetraploidy also arrested hybrid vigour by slowing down the decay of heterosis in progenies (4x) for plant height, leaf biomass and leaf oil percentage. Induced maize × teosinte hybrid tetraploids selected for frequent bivalent chromosome pairing resulted in higher seed setting cob^?1 (increased 58%), pollen fertility (increased 59%) and seed setting percentage (increased 100%) than the quadrivalent variant.     

不同覆盖模式对玉米叶片水分利用效率的影响

采用常规耕作（T1）、秸秆覆盖（T2）、起垄覆膜膜侧种植（T3）、起垄无膜（T4）、无垄覆膜（T5）对玉米叶片光合作用和叶片水平上的水分利用效率以及产量和产量构成因素方面进行分析研究。结果表明,玉米叶片的净光合速率（P_n）、蒸腾速率（T_r）、气孔导度（Cond）、胞间二氧化碳浓度（C_i）和水分利用效率（WUE）在不同覆盖模式下均存在显著差异,且起垄覆膜膜侧种植模式下的P_n、Cond和WUE为最大,T_r和 C_i较低。叶片水分利用效率与净光合速率和气孔导度呈极显著正相关,相关系数分别为0.94、0.98;与蒸腾速率和胞间二氧化碳浓度呈显著负相关,相关系数分别为0.92、0.96。 2018 年 T3 处理产量最高,达 10 095 kg/hm²,较 T1、T2、T4、T5 处理分别高 23.18%、0.55%、19.32 %、3.71%;2019年T3处理产量最高,达10 248 kg/hm²,较T1、T2、T4、T5处理分别高21.84 %、12.47%、15.69%、6.03%。     

Photosynthetic Characteristics of an Amphibious C4 Plant,Eleocharis retroflexa ssp. chaetaria

Abstract

Eleocharis retroflexa (Poir.) Urban ssp. chaetaria (Roem. & Schult.) T. Koyama, an amphibious leafless sedge, grows not only under terrestrial conditions but also under completely submerged aquatic conditions. We investigated the photosynthetic traits and structural features of the culms, which are the photosynthetic organs, in the terrestrial and submerged forms of this species and compared them with those of other amphibious species of Eleocharis which are known to change the photosynthetic modes. The culms of the terrestrial form had Kranz anatomy with well-developed Kranz (bundle sheath) cells and high levels of C₄ enzyme activity typical of the NAD-malic enzyme (NAD-ME) subtype of C₄ metabolism. They also had a δ ¹³C value typical of C₄ plants, indicating that the terrestrial form fixes carbon through the C₄ pathway. The culms of the submerged form had not only a Kranz-like anatomy but also revealed anatomical traits typical of leaves of submerged aquatic plants. The activities of the C₄ enzymes in the submerged form were lower than those in the terrestrial form, but were still in the range typical of G4 plants, ¹⁴C pulse-¹²C chase experiments with the submerged form indicated that almost all of the fixed ¹⁴C was incorporated into G4 compounds, and subsequently the raioactivity was transferred into C₃ compounds and sucrose. The submerged form showed no diurnal fluctuation in malate level. These data demonstrate that a C₄ metabolism is operative even in the submerged form. This unique amphibious C₄ plant provides an intriguing example of the physiological and ecological adaptability of C₄plants.     

玉米单交种光合生产能力杂种优势对氮肥的响应

以先玉335和郑单958及其亲本为材料,设不追氮和追氮150 kg/hm~2两个氮处理,研究玉米单交种光合生产能力杂种优势与产量杂种优势的关系及其对氮肥的响应。不追氮条件下,玉米单交种产量杂种优势主要来源于干物质积累量和LAI的杂种优势,叶片光合特性Pn、Gs、Ci和Tr均未表现出杂种优势。追氮150 kg/hm~2条件下,产量杂种优势显著提高,较不追氮高36.6%;除干物质积累量和LAI等群体数量性状仍具有杂种优势外,叶片光合特性Pn、Gs、Tr和SPAD值等个体功能性状也表现出明显的杂种优势。施氮条件下,干物质积累量和LAI的杂种优势没有提高,Pn、Gs、Tr和SPAD值的杂种优势分别较不追氮提高35.8%、55.3%、50.9%和19.6%。施氮条件下玉米单交种产量杂种优势的提高主要来源于叶片光合特性杂种优势的提高,与干物质积累量和LAI的杂种优势无关。     

Modeling Photosynthesis Decline of Excised Leaves of Sweet Corn Plants Grown with Organic and Chemical Fertilization

Summary

The transpiration or photosynthesis decline curve of excised plant leaves has been used to predict water stress tolerance ability. This study analyzed the characteristics of the declining photosynthetic rate in excised leaves of sweet corn (Zea mays L.) plants after treatment with effective microorganisms (EM), a mixed culture of naturally occurring beneficial microorganisms, organic fertilizer (Bokashi) and chemical fertilizer. The net photosynthetic rates (P _N) of intact leaves were measured using LI-6400 gas exchange system under 2000 μmol m^?2 s^?1 PPF. When P _N reached the maximum (P _max), the leaf was excised and P _N was continuously monitored. A declining P _N curve with time is obtained and modeled according to P _N = [P_max - P _res (1 - βt)] e^?α(t-τ) + P _res (1 - βt), where t is the time elapsed after the leaf was excised; τ is the time point when the stomata begin to close; P _res is the residual P _N when stomatal closure completed; and α and β are constants related to stomatal and residual conductances respectively. Photosynthesis maintenance ability (PMA) is expressed as 1/(α+ β). Leaves of plants fertilized with organic materials and/or treated with EM showed a higher PMA and P _N than the controls after the leaves were excised.     

玉米磷酸烯醇式丙酮酸羧化酶基因的研究进展

高等植物按照CO₂不同的同化途径,分为C₃、C₄和CAM植物。由于C₄植物具有高光效基因,使其在高温、高光照、高氧分压条件下具有比C₃植物更高的光合作用效率。目前,很多学者致力于研究玉米中的高光效基因-磷酸烯醇式丙酮酸羧化酶基因（PEPC）,并试图将此基因转入到C₃植物中,使C₃植物的光合特性得到一定改善,进而使其产量提高。本文综述将C₄高光效基因转入到C₃植物后,C₃植物的生理生化变化和对其光合作用的影响,对向C₃植物中导入pepc基因能否提高其光合作用效率和产量的潜在可能性进行探讨。     

Effects of Salinity Stress on the Structure of Bundle Sheath and Mesophyll Chloroplasts in NAD-Malic Enzyme and PCK Type C4 Plants

Abstract

The effect of NaCl stress on the structure of leaf chloroplasts was investigated in several NAD-Malic enzyme (NAD-ME) and phosphoenolpyruvate carboxykinase (PCK) type C₄ plant species. Seedlings of the monocot species, except Zoysia japonica, grown in 300 mL pots were subjected to salt stress by adding 50 mL of 3% NaCl solution per day to the soil for 5 d after the fourth leaf blades were fully developed. Z. japonica and the dicot species, Amaranthus tricolor, were also treated with 3% NaCl in a similar manner from 5 wk after germination. Salt stress negatively affected the growth, chlorophyll content and chloroplast structure in all the species. At the ultrastructure level, swelling of thylakoids and disruption of envelopes were more or less observed in mesophyll cell (MC) chloroplasts after salt treatment. The structure of bundle sheath cell (BSC) chloroplasts, on the other hand, was hardly damaged under salt condition although stromal and starch areas were considerably decreased. Furthermore, salinity induced granal development in BSC chloroplasts in most species; the number of thylakoids per granum, granal indices and appressed thylakoid density in salt-treated plants were generally higher than those in control. Since the similar responses have also been reported in all NADP-ME type C₄ species investigated in our previous study, the high sensitivity to salt stress in MC chloroplasts and the granal development in BSC chloroplasts by salinity were considered to be common phenomena in all three C₄ subtypes.     

Effects of atmospheric CO2 concentration and defoliation on the growth of Themeda triandra

The effects of elevated atmospheric carbon dioxide (CO₂) concentration (700 μmol mol^?1) on defoliated (three clippings at 3‐week intervals) and undefoliated plants were determined for the C₄ grass Themeda triandra, Forsk. The elevated CO₂ concentration significantly increased leaf regrowth following defoliation, and total leaf production was greatest in this treatment. Shoot biomass of undefoliated plants was also increased under the elevated CO₂ concentration treatment. The primary effect of the elevated CO₂ concentration in both defoliated and undefoliated plants was an increase in individual leaf length and mass of dry matter, linked to a higher leaf water content and increased photosynthetic rates at the canopy level. Photosynthetic down‐regulation at the leaf level occurred, but this was compensated for by increased assimilation rates and greater canopy leaf area at the elevated CO₂ concentration. Increases in leaf and sheath growth of defoliated plants in the elevated CO₂ concentration treatment were lost following a final 3‐week reversion to ambient CO₂ concentration, but occurred in plants exposed to the elevated CO₂ concentration for the final 3‐week period only. In conclusion, elevated atmospheric CO₂ concentration increases shoot growth via increased leaf extension, which is directly dependent on stimulation of concurrent photosynthesis. CO₂ responsiveness is sustained following moderate defoliation but is reduced when plants experience reduced vigour as a result of maturation or high frequency of defoliation.     

Natural variation and morpho-physiological traits associated with water-soluble carbohydrate concentration in wheat under different nitrogen levels

Stem water-soluble carbohydrates (WSCs) are important plant traits influencing grain yields in wheat. However, the traits regulating WSCs storage, particularly under different nitrogen (N) levels are poorly addressed. This study investigated 35 morpho-physiological traits associated with the variation in WSC concentration (WSCc) in the main stem of eight wheat genotypes including two primitive genotypes under three N levels (0, 100, and 200 kg N ha⁻¹). 28 traits were significantly, positively or negatively, correlated with the WSCc in all N levels, and 22 of them were consistent across N levels. Majority of the traits were positively correlated with WSCc suggesting that multiple traits regulate WSCc in wheat plants. However, few traits such as root:shoot ratio (RS_R), stem nitrogen (S_N), leaf nitrogen (L_N), nitrogen per unit leaf area (N_LA), total vegetative mass (V_MASS), cellulose (C_L), and hemicellulose (H_C), were negatively correlated with WSCc. This suggests that plant N concentration was an important selective force driving WSCc in wheat. Indeed, a percent increase in leaf N concentration resulted in 28% lower WSCc. Direct selection estimated that higher V_MASS, flag-leaf width (FL_W), but lower RS_R was adaptive and resulted in higher WSCc in low N level. In contrast, lower V_MASS and FL_W were adaptive and resulted in higher WSCc in high N level. Higher cellulose and hemicellulose were associated with lower WSCc suggesting that a reduced carbon flux to stem structural compounds may improve WSCc in wheat plants. Together, these results demonstrate that a specific suit of trait changes that evolve under N-specific selection increase main stem WSCc but the adaptive value of these changes varies among traits and N levels.     

秋季耕作方式对旱地麦-玉二熟区小麦产量形成的影响

为明确秋季耕作方式对旱地麦-玉二熟体系小麦籽粒产量形成的影响,于2019年6月至2021年6月,在豫西典型旱作麦-玉二熟区的洛阳市孟津区小浪底镇明达村,选择前茬夏玉米免耕的田块,设置秋深松、秋免耕和秋旋耕3种耕作方式,分析了不同秋季耕作方式下两个试验年度的小麦籽粒产量及其构成因素和穗部性状以及2020-2021年度的小麦群体茎蘖数、孕穗后的旗叶净光合速率（P_n）和干物质积累转运特性。结果表明,秋季耕作方式对小麦产量、穗部性状、旗叶P_n和干物质积累转运特性均有显著的调节作用。与秋旋耕相比,秋深松的小麦茎蘖数除返青期外均显著增加,穗数增加5.74%~16.06%;秋深松虽然对小麦旗叶P_n无显著影响,但提高灌浆中后期的旗叶SPAD值,显著改善了干物质积累转运特性,从而使千粒重增加,其中2019-2020年度使穗长和小穗数分别提高7.50%和9.42%,最终使籽粒产量提高9.77%~19.09%。秋免耕后,小麦孕穗和抽穗期的叶片P_n、返青至开花期的干物质积累量、花前干物质转运量及其对籽粒产量的贡献率也较秋旋耕均显著降低,而花后干物质对籽粒产量贡献率提高7.35个百分点,从而保证了籽粒产量无显著下降。与秋免耕相比,秋深松下不同生育时期的茎蘖数均显著增加,孕穗至灌浆中后期的叶片P_n,灌浆中后期的旗叶SPAD值,返青-成熟期的干物质积累量,花前干物质转运量、转运效率及其对籽粒产量的贡献率分别提高7.16%~18.41%、10.27%、12.25%~19.97%、42.19%、4.02个百分点和11.95个百分点,其中2019-2020年度穗长和小穗数分别增加7.50%和8.41%,穗数和籽粒产量分别提高16.74%~17.63%和10.31%~23.63%。综上,秋深松不仅可增加小麦穗数,而且可改善旗叶光合及干物质积累转运特性,进而提高籽粒产量,是旱地麦-玉二熟区实现小麦高产的耕作方式。     

西北绿洲氮磷配施对冬小麦产量及养分利用效率的影响

为了解氮磷配施对西北地区冬小麦产量和养分利用效率的影响.在甘肃凉州区黄羊镇甘肃农业大学试验农场开展了4个施肥处理(N165 P105:165 kg N·hm-2 +105 kg P2O5·hm-2;N165 P165:165 kg N·hm-2 +165kg P2O5·hm-2;N225 P105:225 kg N·hm-2+105 kg P2O5·hm-2;N225 P165:225 kg N·hm-2 +165 kg P2O5·hm-2)的大田试验.结果表明,4个施肥处理中,N165 P105、N225 P165和N165 P165三个处理间产量差异不显著,但均显著高于N225 P105;N165 P105是河西绿洲冬小麦高产节肥的最佳施肥处理.少施N肥有利于WUE的提高(7.89%),而氮磷合理配施才能获得较高的WUE.多施N、P肥可增加N(36.72%)和P(58.94%)的消耗量,但明显降低N(44.48%)、P(53.50%)利用效率,不同处理间N、P利用效率差异显著或极显著,但N、P肥在养分利用上彼此影响不大.因此,肥料的合理配施是提高养分利用效率、实现西北地区高产的主要途径.     

超级杂交稻叶片光合特性、光合氮素利用效率和产量对不同施氮量的响应

为探明超级杂交稻叶片光合特性、光合氮素利用效率和产量对不同施氮量的响应特征,以Q优6号(V1)、宜香优2115(V2)为材料,于2019年在贵州省黄平县进行了不同施氮量的田间试验.结果 表明,随着施氮量的增加,在孕穗期,最大电子传递速率(Jmax)和单位叶面积氮含量(Narea)呈上升趋势,表观光量子效率(AQE)、净...     

灌浆结实期高温胁迫对糯玉米子粒产量和叶片光合特性的影响

以苏玉糯5号和渝糯7号为材料,研究灌浆结实期高温胁迫对子粒充实、叶片光合特性和产量的影响。结果表明,高温胁迫下灌浆后期粒重增速缓慢,穗粒数减少,产量降低。高温胁迫使花后5 d光合速率下降,胞间CO₂浓度、气孔导度和蒸腾速率上升。随灌浆进程推进光合参数逐渐降低,高温胁迫处理下降幅较高,是粒重降低并造成产量损失的关键因素。与苏玉糯5号相比,渝糯7号在高温胁迫处理下光合参数降幅较低,光合速率和胞间CO₂浓度中后期较高,受高温胁迫影响较小。     

Correlation of Leaf Nitrogen,Chlorophyll and Rubisco Contents with Photosynthesis in a Supernodulating Soybean Genotype Sakukei 4

Abstract

Soybean requires more nitrogen (N) than gramineous crops because it accumulates a large amount of N in seeds, and its photosynthetic rate per leaf N is low. The supernodulating genotype Sakukei 4 has a superior symbiotic N₂ fixation capability, and thereby is potentially high-yielding. In our previous study, Sakukei 4 was characterized by having a superior ability to maintain high leaf N content and high photosynthetic rate. The objectives of this study were to know photosynthetic characteristics of Sakukei 4 in detail, especially, the responses to CO₂ concentration and light intensity, and to elucidate how the photosynthetic characteristics of Sakukei 4 are associated with the amounts of photosynthesis-related N compounds (chlorophyll and Rubisco). The three genotypes (Sakukei 4 - supernodulating cultivar derived from Enrei, Enrei - normally nodulating cultivar, En1282-non-nodulating line derived from Enrei) were grown at various N levels in this study. The CO₂ exchange rate (CER) in Sakukei 4 was higher than, or equal to that in Enrei at wide ranges of CO₂ concentrations (150-700 μmol mol-¹) and light intensities (200-1,500 μmol m-² s-¹ PPFD). Sakukei 4 had higher leaf N (N_l), chlorophyll (Chl_L) and Rubisco (Rub_L) contents per leaf area, but lower chlorophyll and Rubisco contents per leaf N content (Chl_L/N_l, Rub_L/N_l) than Enrei. The specific leaf weight (SLW) and leaf area trended to be lower in Sakukei 4 than in Enrei. These results indicate that the superior photosynthetic rate in Sakukei 4 is attributed to higher total N, chlorophyll and Rubisco contents per leaf area, but not to high rate of allocation of total N to these N compounds.