Simultaneous oxygen production and nitrogenase activity of an ammonia-excreting mutant of the cyanobacterium Anabaena variabilis in a co-culture with wheat

Summary A mutant strain of Anabaena variabilis, strain SA-1 that supported growth of wheat plants in a hydroponic co-culture in nitrogen (N) free medium also produced enough oxygen (O₂) to support root respiration. The steady-state concentration of net O₂ in the co-culture was dependent on incident light intensity. At an incident photosynthetic photoflux (PPF) of 1000 mol photons·m^–2·s^–1, net O₂ evolution by the co-culture in the root zone reached a maximum value of about 220 mol O₂ evolved·h^–1·mg chlorophyll (Chl)^–1. The O₂ concentration in the rhizosphere of the co-culture stayed above the ambient air level. O₂ uptake in the dark by strain SA-1-supplemented wheat roots washed free of cyanobacterium was higher than the root respiration of nitrate-grown plants. Nitrate-grown plants required aeration for maximum growth while the wheat-cyanobacterial co-culture can be cultured without aeration. These results show that O₂ produced by strain SA-1 can be used to supply the O₂ needs for root respiration of wheat. Respiration reduced net O₂ evolution by the mutant SA-1, decreasing the partial pressure of O₂ at the sites of cyanobacterial attachment to the roots. This led to an increase in the specific activity of nitrogenase of the co-culture at the high light intensities used to support wheat growth. This activity of about 30 mol ethylene produced·mg Chl^–1·h^–1 was three-fold higher than the activities obtained with the free-living strain SA-1 assayed at the same light intensity. In the co-culture, ammonia produced by the mutant strain SA-1 was not detectable. The NH _inf4 ^sup+ produced by strain SA-1 was used by the wheat plants and, under these conditions, the total N content of the plants reached as high as 85% of the total N content of nitrate-grown plants. In the co-culture system the metabolism of wheat and the cyanobacterium complemented each other, leading to higher plant growth in N-free medium. Offprint requests to: M. Gunasekaran     

Estimating soil respiration using spectral vegetation indices and abiotic factors in irrigated and rainfed agroecosystems

Aims

Our aims were to identify the primary factors involved in soil respiration (R_s) variability and the role that spectral vegetation indices played in R_s estimation in irrigated and rainfed agroecosystems during the growing season.

Methods

We employed three vegetation indices [i.e., normalized difference vegetation index (NDVI), green edge chlorophyll index (CI_{green edge}) and enhanced vegetation index (EVI)] derived from the Moderate-resolution Imaging Spectroradiometer (MODIS) surface reflectance product as approximations of crop gross primary production (GPP) for R_s estimation. Different statistical models were used to analyze the dependencies of R_s on soil temperature, soil water content and plant photosynthesis, and accuracy of these models were compared in the irrigated and rainfed agroecosystems.

Results

The results demonstrated that a model based only on abiotic factors (e.g., soil temperature and soil water content) failed to describe part of the growing-season variability in R_s. Residual analysis indicated that R_s was influenced by a short-term gross primary production (GPP) and a longer-term (≥3 days) accumulated GPP in the irrigated and rainfed agroecosystems. Therefore, photosynthesis dependency of R_s should be included in the R_s model to describe the growing-season dynamics of R_s. Among the three VIs, CI_{green edge} showed generally better correlations with GPP at different cumulative times and canopy green leaf area index than EVI and NDVI. Adding the CI_{green edge} into the model considering only soil temperature and soil water content significantly improved the simulation accuracy of R_s.

Conclusions

Our results suggest that spectral vegetation index from remote sensing could be used to estimate R_s, which will be helpful for the development of a future R_s model over a large spatial scale.     

Photoacclimation in the Red Alga Porphyridium cruentum: Changes in Photosynthetic Enzymes, Electron Carriers, and Light-Saturated Rate of Photosynthesis as a Function of Irradiance and Spectral Quality

Acclimation of the photosynthetic apparatus to changes in the light environment was studied in the unicellular red alga Porphyridium cruentum (American Type Culture Collection No. 50161). Absolute or relative amounts of four photosynthetic enzymes and electron carriers were measured, and the data were compared with earlier observations on light-harvesting components (F.X. Cunningham, Jr., R.J. Dennenberg, L. Mustárdy, P.A. Jursinic, E. Gantt [1989] Plant Physiol 91: 1179-1187; F.X. Cunningham, Jr., R.J. Dennenberg, P.A. Jursinic, E. Gantt [1990] Plant Physiol 93: 888-895) and with measurements of photosynthetic capacity. P_max, the light-saturated rate of photosynthesis on a chlorophyll (Chl) basis, increased more than 4-fold with increase in growth irradiance from 6 to 280 μeinsteins·m⁻²·s⁻¹. Amounts of ferredoxin-NADP⁺ reductase, ribulose-1,5-bisphosphate carboxylase, and cytochrome f increased in parallel with P_max, whereas numbers of the light-harvesting complexes (photosystem [PS] I, PSII, and phycobilisomes) changed little, and ATP synthase increased 7-fold relative to Chl. The calculated minimal turnover time for PSII under the highest irradiance, 5 ms, was thus about 4-fold faster than that calculated for cultures grown under the lowest irradiance (19 ms). A change in the spectral composition of the growth light (irradiance kept constant at 15 μeinsteins·m⁻²·s⁻¹) from green (absorbed predominantly by the phycobilisome antenna of PSII) to red (absorbed primarily by the Chl antenna of PSI) had little effect on the amounts of ribulose-1,5-bisphosphate carboxylase, ATP synthase, and phycobilisomes on a Chl, protein, or thylakoid area basis. However, the number of PSI centers declined by 40%, cytochrome f increased by 40%, and both PSII and ferredoxin-NADP⁺ reductase increased approximately 3-fold on a thylakoid area basis. The substantial increase in ferredoxin-NADP⁺ reductase under PSI light is inconsistent with a PSI-mediated reduction of NADP as the sole function of this enzyme. Our results demonstrate a high degree of plasticity in content and composition of thylakoid membranes of P. cruentum.     

Morphological and physio-biochemical characterization of Brassica juncea L. Czern. & Coss. genotypes under salt stress

Abstract

Soil salinity is one of the major factors responsible for the low productivity of crop plants and has become an increasing threat for agriculture. In this context, the selection of tolerant genotype/s may be one of the remedies. Keeping this in view, the effect of NaCl (0–120 mM) stress on shoot length (SL) plant^?1, area (A) leaf^?1, leaf area index (LAI), fresh weight (FW) and dry weight (DW) plant^?1, stomatal conductance (g_s), net photosynthetic rate (P _N), total chlorophyll (Chl) content, malondialdehyde (MDA) content, sensitivity rate index (SRI), leaf- nitrogen (N), potassium (K) and sodium (Na) content, leaf-K/Na ratio, nitrate reductase (NR: EC.1.6.6.1) and ATP-sulphurylase (ATP-S: EC.2.7.7.4) activities and proline (Pro) and glycinebetaine (GB) content of ten genotypes of Brassica juncea L. was studied at 55 and 65 days after sowing (DAS). NaCl treatments decreased all the above parameters, except Pro, GB, MDA, Na and SRI at both stages. Salt stress resulted in accumulation of Pro and GB, in all genotypes. The magnitude of increase in both osmolytes (Pro and GB) was higher in genotype G₈ than the other genotypes. Salt stress increased MDA and Na content while it decreased Chl, N and K content and K/Na ratio, Chl content, NR and ATP-S activities in all genotypes. But the magnitude of increase in MDA and Na content and decrease in SL plant^?1, A leaf^?1, LAI, P _N, g_s, Chl content and NR and ATP-S activities in genotype G₈ was more than that of other genotypes. These results suggest that the salt-tolerant genotype may have better osmotic adjustment and protection from free radicals by increasing the accumulation of Pro and GB content with overproduction of N and K and higher K/Na, NR and ATP-S activities under salinity stress.     

一种估测小麦冠层氮含量的新高光谱指数

提出了一种估测小麦冠层氮含量的新高光谱指数--微分归一化氮指数(FD-NDNI)。以FieldSpec Pro FR地物光谱仪采集拔节后至孕穗前小麦的冠层光谱190份,随机抽取142份作为训练集,其余48份作为预测集。将光谱以小波阈值去噪法去噪后,利用其525、570 与730 nm处的一阶导数值,采用差值、比值以及归一化的方法构建了12种光谱指数以实现小麦冠层氮含量的估测,并与mNDVI705、mSR以及NDVI705等22种常用指数进行了比较分析。发现指数FD-NDNI对小麦冠层氮含量的估测结果最佳,其估测模型(指数形式)校正集决定系数(C-R²)与预测集决定系数(P-R²)分别达0.818与0.811,优于mNDVI705等常用指数。进一步分析表明,在各指数中,FD-NDNI对叶面积系数最不敏感,可最有效地避免冠层郁闭度等因素对氮含量估测的影响。为优化结果,采用最小二乘支持向量回归算法(LS-SVR)对模型进行了改进,当模型惩罚系数C与RBF核函数参数g取得最优解6.4与1.6时,其C-R²与P-R²分别提高至0.846与0.838,具有比指数模型更高的精度。结果表明:FD-NDNI是小麦冠层氮含量估测的优选指数,LS-SVR为建模的优选算法。     

Evaluation of reflectance spectroscopy indices for estimation of chlorophyll content in leaves of a tropical tree species

The effectiveness of eight spectral reflectance indices for estimating chlorophyll (Chl) content in leaves of Eugenia uniflora L., a tropical tree species widely distributed throughout the world and a key species for ecosystem restoration projects, was evaluated. Spectral reflectance indices were tested using sun and shade leaves with a broad variation in leaf mass per area (LMA). Shortly after plants were exposed to chilling temperatures, there was a dramatic visible change in some sun leaves from green to red. Prior to testing Chl-related reflectance indices, the green and red leaves were separated according to the anthocyanin reflectance index (ARI). Slightly green to dark green leaves corresponded to an ARI value less than 0.11 (n = 107), whereas slightly red to red leaves corresponded to an ARI value greater than 0.11 (n = 35). To estimate leaf Chl, two simple reflectance indices (SR₆₈₀ and SR₇₀₅), two normalized difference indices (ND₆₈₀ and ND₇₀₅), two modified reflectance indices (mSR₇₀₅ and mND705), a modified Chl absorption ratio index (mCARI₇₀₅) and an index insensitive to the presence of anthocyanins (CI_re) were evaluated. Good estimates of leaf Chl content were obtained using the reflectance indices tested regardless of the presence of anthocyanins and changes in LMA. Based on the coefficients of determination (r ²) and the root mean square errors (RMS?_c) the best results were obtained with reflectance indices measured at wavelengths of 750 and 705 nm. Considering the performance of the models the best reflectance indices to estimate Chl contents in E. uniflora leaves with a broad variation in LMA and anthocyanin contents was SR₇₀₅ and mCARI₇₀₅.     

基于高光谱遥感的小麦叶干重和叶面积指数监测

生物量和叶面积指数(LAI)是描述作物长势的重要参数, 叶干重和LAI的实时动态监测对小麦(Triticum aestivum)生长诊断和管理调控具有重要意义。为分析多种高光谱参数估算小麦叶干重和LAI的效果, 建立小麦叶干重和LAI的定量监测模型, 该研究连续3年采用不同小麦品种进行不同施氮水平的大田试验, 于小麦不同生育期采集田间冠层高光谱数据并测定叶片叶干重和LAI。试验结果显示, 小麦叶干重和LAI随施氮水平的提高而增加, 随生育进程呈单峰动态变化模式。小麦叶干重和LAI与光谱反射率间相关性较好的区域主要位于红光波段(590~710 nm, r<-0.60)和近红外波段(745~1 130 nm, r>0.69)。对于不同试验条件下的叶干重和LAI, 可以使用统一的光谱参数进行定量反演, 其中基于RVI (810, 560)、FD755、GM1、SARVI (MSS)和TC3等光谱参数的方程拟合效果较好。经不同年际独立试验数据的检验表明, 以参数RVI (810, 560)、GM1、SARVI (MSS)、PSSRb、(R_750-800/R_695-740) -1、VOG2和MSR705为变量建立的叶干重和LAI监测模型均给出较好的检验结果。因此, 利用关键特征光谱参数可以有效地评价小麦叶片生长状况, 尤其是光谱参数RVI (810, 560)、GM1和SARVI (MSS)可以对不同条件下小麦叶干重和LAI进行准确可靠的监测。     

Protozoan Grazing, Bacterial Activity, and Mineralization in Two-Stage Continuous Cultures

In two-stage continuous cultures, at bacterial concentrations, biovolumes, and growth rates similar to values found in Lake Vechten, ingestion rates of heterotrophic nanoflagellates (HNAN) increased from 2.3 bacteria HNAN⁻¹ · h⁻¹ at a growth rate of 0.15 day⁻¹ to 9.2 bacteria · HNAN⁻¹ · h⁻¹ at a growth rate of 0.65 day⁻¹. On a yeast extract medium with a C/N/P ratio of 100:15:1.2 (Redfield ratio), a mixed bacterial population showed a yield of 18% (C/C) and a specific carbon content of 211 fg of C · μm⁻³. The HNAN carbon content and yield were estimated at 127 fg of C · μm⁻³ and 47% (C/C). Although P was not growth limiting, HNAN accelerated the mineralization of PO₄-P from dissolved organic matter by 600%. The major mechanism of P remineralization appeared to be direct consumption of bacteria by HNAN. N mineralization was performed mainly (70%) by bacteria but was increased 30% by HNAN. HNAN did not enhance the decomposition of the relatively mineral-rich dissolved organic matter. An accelerated decomposition of organic carbon by protozoa may be restricted to mineral-poor substrates and may be explained mainly by protozoan nutrient regeneration. Growth and grazing in the cultures were compared with methods for in situ estimates. Thymidine incorporation by actively growing bacteria yielded an empirical conversion factor of 1.1 × 10¹⁸ bacteria per mol of thymidine incorporated into DNA. However, nongrowing bacteria also showed considerable incorporation. Protozoan grazing was found to be accurately measured by uptake of fluorescently labeled bacteria, whereas artificial fluorescent microspheres were not ingested, and selective prokaryotic inhibitors blocked not only bacterial growth but also protozoan grazing.     

Rhizoremediation of Trichloroethylene by a Recombinant, Root-Colonizing Pseudomonas fluorescens Strain Expressing Toluene ortho-Monooxygenase Constitutively

Trichloroethylene (TCE) was removed from soils by using a wheat rhizosphere established by coating seeds with a recombinant, TCE-degrading Pseudomonas fluorescens strain that expresses the tomA⁺ (toluene o-monooxygenase) genes from Burkholderia cepacia PR1₂₃(TOM_23C). A transposon integration vector was used to insert tomA⁺ into the chromosome of P. fluorescens 2-79, producing a stable strain that expressed constitutively the monooxygenase at a level of 1.1 nmol/min · mg of protein (initial TCE concentration, 10 μM, assuming that all of the TCE was in the liquid) for more than 280 cell generations (36 days). We also constructed a salicylate-inducible P. fluorescens strain that degraded TCE at an initial rate of 2.6 nmol/min · mg of protein in the presence of 10 μM TCE [cf. B. cepacia G4 PR1₂₃(TOM_23C), which degraded TCE at an initial rate of 2.5 nmol/min · mg of protein]. A constitutive strain, P. fluorescens 2-79TOM, grew (maximum specific growth rate, 0.78 h⁻¹) and colonized wheat (3 × 10⁶ CFU/cm of root) as well as wild-type P. fluorescens 2-79 (maximum specific growth rate, 0.77 h⁻¹; level of colonization, 4 × 10⁶ CFU/cm of root). Rhizoremediation of TCE was demonstrated by using microcosms containing the constitutive monooxygenase-expressing microorganism, soil, and wheat. These closed microcosms degraded an average of 63% of the initial TCE in 4 days (20.6 nmol of TCE/day · plant), compared to the 9% of the initial TCE removed by negative controls consisting of microcosms containing wild-type P. fluorescens 2-79-inoculated wheat, uninoculated wheat, or sterile soil.     

Variations in Sulfhydryl, Disulfide, and Protein Content during Synchronous and Asynchronous Growth of HeLa Cells

The cellular contents of protein-bound and nonprotein sulfhydry (—SH) and disulfide (—SS—) groups were measured in both asynchronous and synchronous HeLa S3 cultures. About 90% of these groups are associated with proteins, the majority in the —SH form. The content of protein-bound groups, and hence the total content of —SH and —SS— groups (28 × 10^-15 moles/cell, or 1.1 × 10^-6 moles/g protein on average), changes in parallel with the protein content (which varies between 2 and 4 × 10^-10 g/cell) as asynchronous populations pass from the lag through the exponential to the stationary phase of growth. The concentration of nonprotein —SH groups, in contrast, increases 10-fold during lag phase and decreases in stationary phase; it follows the protein concentration closely during the exponential phase, at a level of about 2.8 × 10^-15 moles/cell. In synchronous cultures the protein content doubles during the cell cycle, possibly in an exponential fashion. The total —SH and —SS— content also doubles, but the rate of increase appears to fluctuate. The concentrations of the protein-bound groups show 2- to 3-fold fluctuations per unit protein: protein-bound —SH groups and mixed —SS— linkages rise to maxima while protein-bound —SS— groups fall to a minimum at the G₁/S transition, and fluctuations in these groups occur again during G₂. In addition, the protein-bound —SH concentration falls continuously during the S phase. The nonprotein —SH concentration undergoes the largest (relative) fluctuations, dropping from 4 × 10^-15moles/cell in early G₁ to about 0.4 × 10^-15 moles/cell (of standard protein content) at the end of G₁, and then rising to 30 times this value by the end of S.     

Steady-State Effects of 2,5,2′,5′-Tetrachlorobiphenyl on Growth, Photosynthesis, and P Uptake in Selenastrum capricornutum

The steady-state effect of 2,5,2′,5′-tetrachlorobiphenyl (TCBP) on the green alga Selenastrum capricornutum was investigated in a P-limited two-stage chemostat system. The partition coefficient of this polychlorinated biphenyl congener was 5.9 × 10⁴ in steady-state cultures. At a cellular TCBP concentration of 12.2 × 10⁻⁸ ng · cell⁻¹, growth rate was not affected. However, photosynthetic capacity (P_max) was significantly enhanced by TCBP (56 × 10⁻⁹ μmol of C · cell⁻¹ · h⁻¹ versus 34 × 10⁻⁹ μmol of C · cell⁻¹ · h⁻¹ in the control). Photosynthetic efficiency, or the slope of the photosynthesis-irradiance curve, was also significantly higher. There was little difference in the cell chlorophyll a content, and therefore the difference in these photosynthetic characteristics was the same even when they were expressed on a per-chlorophyll a basis. Cell C content was higher in TCBP-containing cells than in TCBP-free cells, but approximately 36% of the C fixed by cells with TCBP was not incorporated as cell C. The maximum P uptake rate was also enhanced by TCBP, but the half-saturation concentration appeared to be unaffected.     

小麦叶片氮素状况与光谱特性的相关性研究

 系统分析了不同时相下两个小麦(Triticum aestivium)品种叶片含氮量及叶片氮积累量与冠层光谱反射特征的关系。结果表明,随施氮水平的增加,小麦冠层在可见光区的反射率逐渐降低,而近红外波段的反射率逐渐升高。小麦叶片氮素状况与比值指数或归一化指数显著相关,两个品种表现极为一致,可以用一个指数方程来拟合。分阶段建模并没有提高模型的精度,因此可以建立一个适用于整个生育时期的通用氮素诊断方程。叶片含氮量同光谱指数在整个生育期内的关系要优于叶片氮积累量的,其中,与叶片含氮量关系最佳的指数为红波段(660 nm)和蓝波段(460 nm)的组合(R2>0.80);与叶片氮积累量关系最佳的光谱指数为中红外波段(1 220 nm)与红波段(660 nm)的组合(R2>0.62)。     

Post-Flowering Nitrate Uptake in Wheat Is Controlled by N Status at Flowering,with a Putative Major Role of Root Nitrate Transporter NRT2.1

In bread wheat (Triticum aestivum L.), the simultaneous improvement of both yield and grain protein is difficult because of the strong negative relationship between these two traits. However, some genotypes deviate positively from this relationship and this has been linked to their ability to take up nitrogen (N) during the post-flowering period, regardless of their N status at flowering. The physiological and genetic determinants of post-flowering N uptake relating to N satiety are poorly understood. This study uses semi-hydroponic culture of cv. Récital under controlled conditions to explore these controls. The first objective was to record the effects of contrasting N status at flowering on post-flowering nitrate (NO₃ ^-) uptake under non-limiting NO₃ ^- conditions, while following the expression of key genes involved in NO₃ ^- uptake and assimilation. We found that post-flowering NO₃ ^- uptake was strongly influenced by plant N status at flowering during the first 300–400 degree-days after flowering, overlapping with a probable regulation of nitrate uptake exerted by N demand for growth. The uptake of NO₃ ^- correlated well with the expression of the gene TaNRT2.1, coding for a root NO₃ ^- transporter, which seems to play a major role in post-flowering NO₃ ^- uptake. These results provide a useful knowledge base for future investigation of genetic variability in post-flowering N uptake and may lead to concomitant gains in both grain yield and grain protein in wheat.     

施氮肥缓解臭氧对小麦光合作用和产量的影响

以小麦(Triticum aestivum)品种‘扬麦16’为试材, 利用开放式空气臭氧(O₃)浓度升高平台, 研究了增施氮(N)肥对O₃对小麦光合作用和产量影响的缓解作用。结果表明, O₃胁迫下灌浆期小麦的净光合速率(P_n)、气孔导度(G_s)、蒸腾速率(T_r)、叶绿素a (Chl a)、叶绿素b (Chl b)、类胡萝卜素(Car)、总叶绿素含量(Chl t)和可溶性蛋白的含量显著降低, 降幅分别为28.95%、31.79%、23.17%、58.89%、68.64%、22.89%、60.31%和32.00%; 胞间CO₂浓度(C_i)变化很小; 成熟期生物量和收获时产量也明显下降, 降幅分别为12.23%和12.63%; 而增施N肥可以增加小麦灌浆期的P_n、Chl a、Chl b、可溶性蛋白的含量, 进而增加小麦生物量和产量, 增幅分别为25.66%、83.05%、121.57%、30.33%、14.94%和10.67%, 而对C_i、G_s、T_r、Car含量无明显影响。O₃和N肥对小麦叶片的P_n、Chl t及可溶性蛋白含量有明显的交互作用。因此, 在大气O₃浓度升高条件下增施N肥对小麦O₃损伤有一定的缓解作用。     

Effects of elevated CO<Subscript>2</Subscript> and nitrogen on wheat growth and photosynthesis

The effects of nitrogen [75 and 150 kg (N) ha⁻¹] and elevated CO₂ on growth, photosynthetic rate, contents of soluble leaf proteins and activities of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and nitrate reductase (NR) were studied on wheat (Triticum aestivum L. cv. HD-2285) grown in open top chambers under either ambient (AC) or elevated (EC) CO₂ concentration (350 ± 50, 600 ± 50 μmol mol⁻¹) and analyzed at 40, 60 and 90 d after sowing. Plants grown under EC showed greater photosynthetic rate and were taller and attained greater leaf area along with higher total plant dry mass at all growth stages than those grown under AC. Total soluble and Rubisco protein contents decreased under EC but the activation of Rubisco was higher at EC with higher N supply. Nitrogen increased the NR activity whereas EC reduced it. Thus, EC causes increased growth and P_N ability per unit uptake of N in wheat plants, even if N is limiting.     

小麦叶面积指数与冠层反射光谱的定量关系

在分析不同氮素水平下小麦叶面积指数（LAI）和冠层光谱反射率随生育期变化模式的基础上,确立了LAI与冠层光谱反射率及光谱参数的相关关系,提出了小麦LAI的敏感光谱参数及预测方程.结果表明,小麦LAI和近红外短波段（760～1 220 nm）反射率都随施氮量的增加呈上升趋势,可见光波段反射率则相反;从拔节期到成熟期,LAI和近红外短波段反射率均表现为先上升后下降的趋势,而可见光波段（460～710 nm）反射率随生育期的推进先降低后升高,以孕穗期反射率最低,近红外长波段区域（1 480～1 650 nm）反射率的变化与可见光部分相同.LAI与可见光波段反射率呈负相关,与近红外短波段反射率呈极显著正相关,其中以810 nm相关性最好.可以选择RVI(810,510)和DVI(810,560)作为反演小麦LAI的光谱参数.另外,在证明垂直植被指数PVI和转换型土壤调整指数TSAVI对LAI预测能力的同时,发现利用RVI(810,510)、DVI(810,560)和PVI 3个植被指数共同推算小麦LAI的准确度更高.     

The effect of elevated CO2 concentration on leaf chlorophyll and nitrogen contents in rice during post-flowering phases

The effect of elevated CO₂ concentration (CE) on leaf chlorophyll (Chl) and nitrogen (N) contents and photosynthetic rate (P_N) was evaluated during the post-flowering stages of rice grown at CE (570 ± 50 μmol mol⁻¹) in open top chamber (OTC), at ambient CO₂ concentration (∼ 365 μmol mol⁻¹) in OTC and at open field. Thirty-five day old seedlings were transplanted in OTCs or in field and allowed to grow till maturity. Chl and N contents were highest at the time of flowering and thereafter it started to decline. The rate of decline in Chl and N contents was faster in plants grown under CE mostly in later part of growth. Irrespective of treatment difference, flag leaf contained the highest amount of Chl and N than penultimate and third leaf. The higher P_N was observed in leaves under CE than in the leaves in other two growing conditions. Considering growth stage, P_N was the highest at flowering which reduced at the later part of growth due to degradation of Chl and N content of the leaf. Under CE it was 40.02 μmol m⁻² s⁻¹ at flowering and it reduced to only 14.77 μmol m⁻² s⁻¹ at maturity stage. The beneficial effect of CE in increasing leaf P_N may be maintained by applying extra dose of nitrogen at the later stages of plant growth.     

Uptake and Assimilation of NO(3) and NH(4) by Nitrogen-Deficient Perennial Ryegrass Turf

Assimilation of NO₃⁻ and NH₄⁺ by perennial ryegrass (Lolium perenne L.) turf, previously deprived of N for 7 days, was examined. Nitrogen uptake rate was increased up to four- to five-fold for both forms of N by N-deprivation as compared to N-sufficient controls, with the deficiency-enhanced N absorption persisting through a 48 hour uptake period. Nitrate, but not NH₄⁺, accumulated in the roots and to a lesser degree in shoots. By 48 hours, 53% of the absorbed NO₃⁻ had been reduced, whereas 97% of the NH₄⁺ had been assimilated. During the early stages (0 to 8 hours) of NO₃⁻ uptake by N-deficient turf, reduction occurred primarily in the roots. Between 8 and 16 hours, however, the site of reduction shifted to the shoots. Nitrogen form did not affect partitioning of the absorbed N between roots (40%) and shoots (60%) but did affect growth. Compared to NO₃⁻, NH₄⁺ uptake inhibited root, but not shoot, growth. Total soluble carbohydrates decreased in both roots and shoots during the uptake period, principally the result of fructan metabolism. Ammonium uptake resulted in greater total depletion of soluble carbohydrates in the root compared to NO₃⁻ uptake. The data indicate that N assimilation by ryegrass turf utilizes stored sugars but is also dependent on current photosynthate.     

Production of NO2- and N2O by Nitrifying Bacteria at Reduced Concentrations of Oxygen

Pure cultures of the marine ammonium-oxidizing bacterium Nitrosomonas sp. were grown in the laboratory at oxygen partial pressures between 0.005 and 0.2 atm (0.18 to 7 mg/liter). Low oxygen conditions induced a marked decrease in the rate for production of NO₂^-, from 3.6 × 10⁻¹⁰ to 0.5 × 10⁻¹⁰ mmol of NO₂^- per cell per day. In contrast, evolution of N₂O increased from 1 × 10⁻¹² to 4.3 × 10⁻¹² mmol of N per cell per day. The yield of N₂O relative to NO₂^- increased from 0.3% to nearly 10% (moles of N in N₂O per mole of NO₂^-) as the oxygen level was reduced, although bacterial growth rates changed by less than 30%. Nitrifying bacteria from the genera Nitrosomonas, Nitrosolobus, Nitrosospira, and Nitrosococcus exhibited similar yields of N₂O at atmospheric oxygen levels. Nitrite-oxidizing bacteria (Nitrobacter sp.) and the dinoflagellate Exuviaella sp. did not produce detectable quantities of N₂O during growth. The results support the view that nitrification is an important source of N₂O in the environment.     

Interactive Effects of Salinity and Ozone Pollution on Photosynthesis,Stomatal Conductance,Growth, and Assimilate Partitioning of Wheat (Triticum aestivum L.)

Plants of an Egyptian cultivar of wheat (Triticum aestivum L. cv. Giza 63) were exposed in open-top chambers (OTCs) for 8 h d^–1 for up to 75 d to a factorial combination of two levels of salinity (0 and 50 mM NaCl) and two levels of O₃ (filtered air and 50 mm³ m^–3). Exposure to 50 mm³ m^–3 O₃ significantly decreased stomatal conductance (g _s), net photosynthetic rate (P _N), and chlorophyll (Chl) content by 20, 25, and 21 %, respectively. This reduction resulted in a change in assimilate allocation in favour of shoot growth leading to a decrease in root to shoot ratio and eventually to a decrease in relative growth rate (RGR) of both root and shoot. There was a very large reduction in yield parameters, especially in the number of ears/plant and 1 000-grain mass. Soil salinity significantly reduced P _N and g _s by 17 and 15 %, respectively, while Chl content was increased by 17 %. Root growth was decreased leading to an increase in root/shoot ratio. Yield parameters were decreased due to salt stress. There was antagonistic interaction between salinity (50 mM NaCl) and O₃ (50 mm³ m^–3) showing that salinity effectively protects against the adverse effects of O₃ by increasing g _s during O₃ fumigation.