Investigation on the Assimilation of Nitrogen by Maize Roots and the Transport of Some Major Nitrogen Compounds by Xylem Sap

Xylem sap was collected from nitrogen-starved maize plants and investigations were made on the nitrogen transported. It appears from the results that several pools for different amino acids exist, which have different relations to the transport of nitrogen taken up. While in maize roots Glu, Glu-NH₂ and Arg are transported directly from the place of their synthesis, the transported Asp and Ala predominantly derive from the pool of amino acids synthetized before application of nitrogen. An explanation of this observation is offered.     

Investigation on the Assimilation of Nitrogen by Maize Roots and the Transport of Some Major Nitrogen Compounds by Xylem Sap

The uptake and assimilation of nitrate and ammonia have been studied in Zea mays. Nitrogen-starved maize roots are capable of accumulating a potential capacity for nitrogen uptake and assimilation. Reestablishment of nitrogen supply leads to intense uptake, reaching 154 % of the reference variant level after 24 hours when nitrate is supplied, and 121 % when ammonia is supplied. After 24 hours the insoluble nitrogen fraction accounts for 80, 54 and 55 % of the total taken up in the PK + NO₃^-, PK + NH₄⁺ and NPK variants respectively.     

Investigation on the Assimilation of Nitrogen by Maize Roots and the Transport of Some Major Nitrogen Compounds by Xylem Sap

The amino acid and protein metabolism of roots of maize has been studied. The important role of the free amino acids and proteins of the roots as active agents in nitrogen assimilation is pointed out. Nitrogen supplied as nitrate is preferably incorporated into α-ketoglutaric acid, and then by trans-aminases transferred to other ketoacids. In the case of ammonia supply the function of a nitrogen-accumulating assimilation system leading to the formation of Arg, Glu-NH₂ and Asp-NH₂ is shown.     

Transport Processes in Vacuoles of Higher Plants

The vacuole is the largest compartment of a mature plant cell and serves as an internal reservoir of metabolites and nutrients. In the last years transport of solutes across the tonoplast has been intensively investigated. It was shown that two different proton pumps reside in the tonoplast. These pumps generate an electrochemical gradient which can be used as an energy-source to accumulate solutes. Cation uptake is driven by an H⁺ antiport mechanism. Anions are accumulated in response to the inside positive membrane potential. In addition, the existence of ion channels was shown using the patch clamp technique. The aim of this review is to compare and to discuss the present state of our knowledge of solute transport across the tonoplast.     

Modifications of Cortical Cell Walls in Roots of Seedless Vascular Plants

Forty-three species of seedless vascular plants were assessed for modifications to root cortical cell walls. All species except Lycopodium had an endodermis with distinct Casparian bands. Experiments with the apoplastic tracer berberine hemisulfate showed that walls of all root cortical cells in the two Lycopodium species tested were permeable to this tracer. Although most species examined lacked a hypodermis several Equisetum species had a hypodermis with modified walls. Three Selaginella species had distinct Casparian bands in this cortical cell layer. This layer, therefore, is an exodermis in Selaginella and its presence limited the inward diffusion of the apoplastic tracer berberine hemisulfate.     

药物诱导的玉米根尖凋亡细胞表面电特性研究(英文)

细胞凋亡过程中细胞表面膜的电位很可能会发生改变。本文首次报导：应用细胞电泳技术（ｃｅｌｌ ｅｌｅｃｔｒｏｐｈｏｒｅｓｉｓ）对细胞毒素类药物放线菌酮（ｃｙｃｌｏｈｅｘｉｍｉｄｅ）、放线菌素 Ｄ（ａｃｔｉｎｏｍｙｃｉｎ Ｄ）和秋水仙碱（ｃｏｌｃｈｉｃｉｎｅ）等诱导的植物凋亡细胞与正常细胞之间电泳迁移率（ＥＰＭ）的差异进行了比较,对引起的膜电位变化进行了定量分析。实验以玉米根尖分生组织为材料,制备原生质体,经过适当剂量的药物处理（Ｆｉｇ．１－Ｂ）,在尽量减少细胞膜被破坏的情况下（Ｆｉｇ．２）,观察到：三种细胞毒素类药物的作用有所不同,被诱导的植物凋亡细胞的膜表面Ｚｅｔａ电位绝对值比正常细胞的高（Ｆｉｇ．１－Ａ）。本研究提示细胞电泳可对凋亡细胞表面膜电位的变化进行定量分析,为细胞凋亡的检测在方法上提供了新思路。     

Cesium Chloride Density Gradient Study of Some Modifications Induced by Ultraviolet Radiation on the Denatured DNA Molecule of Phage T2

The behaviour of the modifications due to ultraviolet (UV) radiation on denatured phagic DNA has been investigated by looking at the changes of density in CsCl gradient, after irradiation with increasing doses of monochromatic light. The spectral range studied is 2300-2900 A, with a wavelength resolution Δλ = ±20 A. The observed effect is a gradual shift toward higher densities as the UV absorbed dose increases. The experimental results show an exponential law and the analysis of the action spectra indicates that thymine is the main factor responsible for the observed effect. An evaluation of the quantum yield for the thymine-dimers formation gives a constant value of (18 ± 3) × 10^-3 quanta^-1.     

Effects of Pyrocatechol on Some Metabolic Processes of Mature Sugar Beet (Beta vulgaris) Plants

Pyrocatechol (PC), 10^-2M, was applied to the foliage of mature plants of sugar beet (Beta vulgaris L.). Its effect on the activity of nitrate reductase, transaminase, invertase, phosphatases, sucrose synthetase, sucrose phosphate synthetase, and UDPG-pyrophosphorylase were determined 7, 14, and 21 days after treatment. Significant reductions in the activity of nitrate reductase, transaminase, invertase, and phosphatases (including phenyl phosphatase, glucose-1-, glucose-6-, fructose-6-phosphatase, and adenosine triphosphatase) in the treated plants occurred. On the other hand, activities of the enzymes of sucrose biosynthesis, uridine, diphosphate glucose pyrophosphorylase (UDPG-pyrophosphorylase), sucrose synthetase, and sucrose phosphate synthetase were significantly stimulated by the application of pyrocatechol. The results suggest that the growth inhibition following the application of PC to sugar beet plants may stem in part from an amino acid stress resulting from a PC-induced decrement in nitrate reductase and transaminase activity. Its application also creates an enzymatic condition favorable for sucrose biosynthesis and storage.     

高等植物尿素代谢及转运的分子机理

尿素广泛存在于自然界中,是易于被许多生物（如植物）利用的生长氮源。该文通过概述尿素在不同生命系统中存在的基础生理意义及各类型尿素转运蛋白,讨论了植物细胞中尿素合成与分解的各种途径及尿素在植物氮营养、代谢和运输中的生理作用。迄今为止,在植物中已发现了2类转运尿素的膜蛋白,即MIPs和DUR3,它们分别在低亲和力、高亲和力尿素运输中发挥潜在作用。异源表达结果表明MIPs介导了尿素的被动迁移：而AtDUR3则参与拟南芥根系对尿素的吸收。对MIPs和DUR3转运尿素的酶学特征、亚细胞作用位点和表达调控状况等的研究表明：它们的分子生物学功能与植物的氮营养及氮素再分配和利用相关。     

高等植物尿素代谢及转运的分子机理

尿素广泛存在于自然界中, 是易于被许多生物(如植物)利用的生长氮源。该文通过概述尿素在不同生命系统中存在的基础生理意义及各类型尿素转运蛋白, 讨论了植物细胞中尿素合成与分解的各种途径及尿素在植物氮营养、代谢和运输中的生理作用。迄今为止, 在植物中已发现了2类转运尿素的膜蛋白, 即MIPs和DUR3, 它们分别在低亲和力、高亲和力尿素运输中发挥潜在作用。异源表达结果表明, MIPs介导了尿素的被动迁移; 而AtDUR3则参与拟南芥根系对尿素的吸收。对MIPs和DUR3转运尿素的酶学特征、亚细胞作用位点和表达调控状况等的研究表明: 它们的分子生物学功能与植物的氮营养及氮素再分配和利用相关。     

Polar Transport of Auxin across Gravistimulated Roots of Maize and Its Enhancement by Calcium

The effect of Ca on the polar movement of [³H]indoleacetic acid ([³H] IAA) in gravistimulated roots was examined using 3-day-old seedlings of maize (Zea mays L.). Transport of label was measured by placing an agar donor block containing [³H]IAA on one side of the elongation zone and measuring movement of label across the root into an agar receiver block on the opposite side. In vertically oriented roots, movement of label across the elongation zone into the receiver was slight and was not enhanced by incorporating 10 millimolar CaCl₂ into the receiver block. In horizontally oriented roots, movement of label across the root was readily detectable and movement to a receiver on the bottom was about 3-fold greater than movement in the opposite direction. This polarity was abolished in roots from which the caps were removed prior to gravistimulation. When CaCl₂ was incorporated into the receivers, movement of label across horizontally oriented intact roots was increased about 3-fold in both the downward and upward direction. The ability of Ca to enhance the movement of label from [³H]IAA increased with increasing Ca concentration in the receiver up to 5 to 10 millimolar CaCl₂. With the inclusion of CaCl₂ in the receiver blocks, gravity-induced polar movement of label into receiver blocks from applied [³H]IAA was detectable within 30 minutes, and asymmetric distribution of label within the tissue was detectable within 20 minutes. The results indicate that gravistimulation induces a physiological asymmetry in the auxin transport system of maize roots and that Ca increases the total transport of auxin across the root.     

Transport Interactions between Paraquat and Polyamines in Roots of Intact Maize Seedlings

Interactions between absorption of paraquat and the polyamines putrescine, cadaverine, and spermine in roots of intact maize (Zea mays L. cv 3377 Pioneer) seedlings were examined. Concentration-dependent kinetics for paraquat and putrescine influx were similar and both kinetic curves could be resolved into a linear and a saturable component. The linear component was previously shown to represent cell wall/membrane binding. The saturable components for paraquat and putrescine uptake, which represent influx across the plasmalemma, had K_m values of 98 and 120 micromolar, respectively, and V_max values of 445 and 456 nanomoles per gram fresh weight per hour, respectively. Lineweaver-Burk transformation of the saturable component of paraquat influx in the presence of varying concentrations of putrescine indicated that the diamine competitively inhibited the saturable component of paraquat uptake. Reciprocal experiments similarly demonstrated that paraquat competitively inhibited the saturable component of putrescine uptake. Competitive inhibition of both paraquat and putrescine influx could also be demonstrated with the diamine cadaverine, which has a charge distribution similar to that of paraquat and putrescine. In contrast, the larger, tetravalent polyamine spermine appeared to noncompetitively inhibit the influx of paraquat and putrescine. These results strongly suggest that paraquat enters maize root cells via a carrier system that normally functions in the transport of diamines with a charge distribution similar to that of paraquat.     

Changes in Growth and Nitrogen Assimilation in Maize Plants Induced by NaCl and Growth Regulators

Experiments were conducted to determine the interactive effects of salinity and certain growth regulators on growth and nitrogen assimilation in maize (Zea mays L. cv. GS-2). 100 mM NaCl inhibited the biomass accumulation, chlorophyll and carotenoid contents in leaves, nitrate content and uptake and nitrate reductase activity. The application of kinetin, ascorbic acid and 10 and 50 μM abscisic acid in the first experiment and 50 and 100 μM abscisic acid in the second experiment induced a substantial increase in the above parameters, the effect was highest with abscisic acid in salinized as well as non-salinized plants. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effect of Water Stress and Mercuric Chloride on the Translational Diffusion of Water in Maize Seedling Roots

Water diffusion in maize roots (Zea mays L., cv. Donskaya 1) was investigated with a pulsed gradient NMR using mercuric chloride as an inhibitor of water channels in cell membranes. A novel operation program was applied that allowed selective evaluation of fractional amounts of water transported through various pathways—the apoplastic, symplasmic, and transmembrane routes. The blockage of water channels with HgCl₂ reduced the rates of water diffusion by a factor of 1.5–2. This effect was reversible and was removed by the addition of -mercaptoethanol. The coefficient of water diffusion changed with time elapsed after the HgCl₂ treatment. The effect of water stress on the rates of water diffusion was similar to that of HgCl₂. Remarkably, the water-stressed roots of maize seedlings were insensitive to the inhibitor of water channels. The results are interpreted in terms of redistribution of water flows among various routes in plant tissues. Water stress and mercuric chloride treatments decelerate the transmembrane water transport and promote water flow along the apoplastic pathway. These responses might arise from the reversible regulation of water movement along various transport pathways.     

Effect of Inhibitors of Auxin Transport and of Calmodulin on a Gravisensing-Dependent Current in Maize Roots

Some characteristics of the gravity sensing mechanism in maize root caps were investigated using a bioelectric current as an indicator of gravity sensing. This technique involves the measurement of a change in the current density which arises at the columella region coincidently with the presentation time. Two inhibitors of auxin transport, triiodobenzoic acid and naphthylphthalamic acid, blocked gravitropic curvature but not the change in current density. Two inhibitors of calmodulin activity, compound 48/80 and calmidazolium, blocked both curvature and gravity-induced current. The results suggest that auxin transport is not a component of gravity sensing in the root cap. By contrast, the results suggest that calmodulin plays an intrinsic role in gravity sensing.