pH值对水稻幼苗吸收NO3——的影响

运用微电极技术研究了改变细胞内外pH值对常规籼稻(杨稻6号)NO3-吸收的影响。结果表明:1)未经NO3-培养的水稻根表皮细胞吸收NO3-会引起细胞膜电位瞬时的去极化或超级化。当水稻根系吸收NO3-时,表皮细胞膜电位改变的程度随外界NO3-浓度的增大而增大;在浓度为0.5.mmol/L时达到最大,而后表皮细胞膜电位的变化基本维持不变。2)外界不同pH(4.4、5.5、7.0)条件下,表皮细胞膜电位对NO3-的响应随着外界pH的升高而有所降低,并且表皮细胞膜电位对NO3-的响应在pH.8.0受抑制程度不大。3)5.mmol/L丁酸钠和3.mmol/L普鲁卡因可以使细胞内pH分别降低0.16个单位和升高0.2个单位;改变细胞内pH值对水稻吸收NO3-时引起表皮细胞膜电位的变化几乎没有明显的影响。4)不同pH条件下,培养2周左右的水稻地上部干鲜重差异显著,以pH.5.5时为最高;组织中NO3-和NH4+的含量也在pH.5.5时最高。由于细胞内pH是受系统严格调控的,具有极强的缓冲能力,因此,NO3-的吸收主要受外界pH的影响。酸性可以促进NO3-的内流;而微碱性增加了NO3-的外流,降低了NO3-的净吸收。     

Effect of nitrate and humic substances of different molecular size on kinetic parameters of nitrate uptake in wheat seedlings

The kinetic parameters of nitrate uptake (I_max, Km and C_min) were evaluated in young seedlings of Triticum durum L., cv. Appulo, exposed to nitrate and/or to soil‐extracted humic acids (HAs) of different molecular weight. The uptake was enhanced after induction at low levels of nitrate (50 μM KNO₃), while it was inhibited after induction at higher concentrations (2000 μM). The kinetic parameters of uptake were selectively influenced by pre‐treatment with HAs: total (TE) and, at a greater extent, low (LMS, < 3500 Da) molecular size humic fraction increased either the nitrate uptake rate (I_max) and the efficiency of the whole transport system (low Km and C_min), while an opposite result was evidenced in high molecular size (HMS, > 3500 Da)‐treated plants. An additive effect was shown when nitrate and humic substances were provided simultaneously: the uptake rate was enhanced in TE‐ and LMS‐treated plants, but was strongly delayed in HMS‐treated plants. Removal of nitrate and/or humic fractions de‐induced the system and NO₃^— uptake rate decreased. Exposure to HAs was not able to induce nitrate reductase activity in root and leaf tissues. Inhibitors of protein synthesis p‐fluorophenylalanine and cycloheximide reversed the positive effect of LMS fraction on nitrate uptake. This would support the hypothesis of a promoting effect of HAs on the molecular expression of proteins of the nitrate transport system.     

Light-sucrose interaction stimulates nitrate uptake by wheat roots

To improve nitrate uptake efficiency and minimize fertilizer waster, nitrate uptake control was investigated. In the previous work, we found that illuminating shoots can stimulate nitrate uptake by wheat (Triticum aestivum cv. “EM18”) roots, which is thought to be via a signal carried through phloem, maybe a sugar. We also found that sucrose could show features of a specific signal of nitrate uptake. A noninvasive technique method with a high-time resolution (minutes) has enabled us to make the comparison between illumination and sucrose supply. Interestingly, the results showed when plants were in the light, the stimulation by external sucrose was less than that in the dark, indicating light affected nitrate uptake co-operatively with externally supplied sucrose. The interaction between light and externally supplied sucrose showed that the mechanism of light effect on nitrate uptake involved sucrose. This work was useful for further investigating its molecular mechanism and ultimately improving nitrogen use efficiency.     

The absorption and translocation of amino acids and amides in rice plant

It has been well known that the inorganic nitrogen compounds used as the common nitrogen source for the growth of higher plants can be replaced by some organic nitrogen compounds such as amino acids or amides. According to GHOSH and BURRIS (1), who investigated the effect of some amino acids as the nitrogen source, alanine, asparagine, glutamate and histidine were better nitrogen sources than ammonia for clover and tomato plants. For tobacco, however, nitrate and ammonia were superior to all organic nitrogen compounds used. RATNER et al. (2). made the same kind of study by using corn and sunflower plants and reported that the plants could grow with glycine, aspartate, glutamate and arginine, but all of them were inferior to inorganic nitrogen as the nitogen source.     

The effect of paclobutrazol on the growth and uptake of Ca and K by apple seedlings 1

Abstract

Addition of paclobutrazol (PP333) at 0.2 ppm to a nutrient solution in which 11‐month‐old apple seedlings were growing reduced terminal growth, fresh weight increase (FWI), transpiration and Ca and K uptake per seedling. Total root surface area was not affected by PP333 treatment. Reduced Ca uptake preceded the reduction in transpiration and FWI. In the case of K these changes were parallel. This may indicate that different mechanisms were responsible for reduced Ca and K uptake in PP333‐treated plants. At the conclusion of the experiment control and treated plants were absorbing similar amounts of Ca per unit of FWI. At the same time the amount of K uptake per unit of FWI was significantly lower in treated than control plants.     

Heavy metal uptake by wheat seedlings grown in fly ash-amended soils

A bioassay technique was used to investigate heavy metals uptake by wheat seedlings grown in fly ash-amended soils. No negative effect of fly ash on the growth of wheat seedlings was found. The addition of the fly ash generally decreased the heavy metal concentration in the wheat seedlings. The total accumulation largely depended on the soil characteristics. The variation in the soil pH induced by the fly ash treatment could be considered the most important parameter that influenced heavy metals uptake. In the soil with a higher increase of pH (2 units) total accumulation of heavy metals decreased with increasing fly ash addition. In the other soils where the pH variation was lower (0.4 units), total accumulation of some heavy metals increased. These obtained data provide useful information which is required before the agronomic use of fly ash can be recommended in Italian soils.     

Humic substances alter the uptake and toxicity of nanodiamonds in wheat seedlings

Purpose

Detonation synthesis nanodiamonds (ND) are among the most widely applied nanoparticles due to their low cost of production and broad scope of applications. However, the fate and behavior of NDs in the environment are largely unknown. The behavior of NDs is greatly affected by humic substances (HSs), which comprise 50 to 80 % of natural organic matter in water and soil ecosystems. The uptake of detonation NDs by wheat seedlings and its toxicity were evaluated in the presence of seven HSs of different origins, including humic acids (HA, HS fraction soluble in alkali and insoluble in acid) and fulvic acids (FA, soluble in both alkali and acid).

Materials and methods

To monitor the uptake of NDs by plants, tritium-labeled NDs were produced. Liquid scintillation spectrometry and autoradiography were used to determine the amount of NDs absorbed by plants. The photosynthetic activity of the plants was measured using light response curves.

Results and discussion

After a 24-h exposure period, the ND content in the plant roots was 1720 μg g^?1. The introduction of HSs decreased the ND contents in the plant roots to 680–1570 μg g^?1 (except for peat FA, for which the ND content did not differ from the blank value). The observed phenomenon was probably related mainly to the influence of HSs on the zeta potential of the NDs, which shifted from positive to negative. Based on chlorophyll fluorescence evaluation, the toxicity of NDs did not inhibit photosynthesis during illumination in the physiological range. However, NDs were slightly toxic to wheat plants under excessive light, likely due to the inhibition of electron transport between Q _A and Q _B and the disruption of the formation of a thylakoid transmembrane potential.

Conclusions

The introduction of HA in a suspension of NDs obviously reduced the inhibiting effect of the NDs; however, the mitigating activities of FA were not so apparent. Our results demonstrate the urgent need for further studies of the influences of NDs on plant growth and development.

     

腐植酸钠对镉胁迫小麦幼苗生物效应的研究

盆栽试验研究Cd胁迫下不同腐植酸钠处理(浸种、灌施、混施)对小麦幼苗生长及对污染元素(Cd)、营养元素(Cu、Zn、Fe、Mn)吸收和累积的影响结果表明,不同腐植酸钠处理可缓解重金属Cd元素的毒害作用,促进小麦幼苗干物质积累,且轻度污染土壤中可显著抑制小麦幼苗对Cd元素的吸收,并促进小麦幼苗对Cu、Zn、Fe、Mn营养元素的吸收和积累,随Cd污染程度加重,不同腐植酸钠处理抑制Cd吸收能力减弱并对Cu、Zn、Fe和Mn营养元素吸收产生不同影响,表现出较好的抗逆效应     

Induction of high affinity nitrate and ammonium uptake systems in wheat

Abstract

The ability of 7 day old wheat seedlings to take up nitrate or ammonium from hydroponic solution was measured. Seedlings were grown under fully aerated hydroponic conditions. The growth solution consisted of either 0.5 mM CaSO₄ alone or in combination with high nitrate (5 mM NO₃ ^‐ ), high ammonium (2 mM NH₄ ⁺) or modified 1/10 Hoaglands solution with nitrate N only (14 mM) or ammonium N only (2 mM). After washing the roots for one hour in CaSO₄, nitrate or ammonium uptake was measured with an ion selective electrode. Plants grown in high nitrate were unable to take up nitrate from a 0.1 mM external solution. Those grown in CaSO₄ were able to take up nitrate at the same external concentration (flux = 10.2 +/‐ 3.0 μmol nitrate/g dry wtlbh). The same result was seen for plants grown in high ammonium vs those grown in CaSO₄ (flux = 21.0 +/‐ 10.0 μmol/g dry wtlbh). Similar results were obtained when modified Hoagland's solution was substituted for the high N solutions. These data indicate that wheat roots possess both high and low affinity nitrate and ammonium uptake systems. The data further indicate that, for a given ion, the high and low affinity systems do not operate simultaneously under high N conditions. The high affinity system is switched off in the range of 1 mM for both ionic forms of N. Developmental studies show that the expression of the high affinity trait is reversible and may be induced (repressed) by conditioning for 24 h in low (high) N media. Plants grown in high N solutions showed efflux of the ion under assay conditions. Neither ion interfered with the induction/repression of the high affinity trait for the other under the conditions used in this study.     

土壤氨基酸生物有效性及其环境调控研究进展

土壤中种类繁多的小分子和大分子有机氮,是土壤氮素的重要组成成分。大多数植物可以直接吸收氨基酸乃至多肽和蛋白质,不是完全需要经过传统理论认知的微生物分解为无机氮的过程。植物根系具有吸收、转运和代谢外源吸收的有机氮的能力。土壤微生物是植物根系有机氮的主要竞争者,不同土壤中,参与竞争的微生物组成存在较大差异。环境对植物根系吸收和后续代谢有机氮都具有重要的调控作用。未来应着重于精准定量化分析土壤有机氮组成及含量,确定土壤有机氮对植物生长的长期效应,探索环境变化尤其是复杂环境变化对植物吸收利用有机氮的影响及其关键步骤,进一步确定土壤有机氮对植物氮营养的贡献。     

Nitrate assay by multiple wavelength spectroscopy and analysis of kinetic parameters of net nitrate uptake by barley seedlings

A method for determination of nitrate concentration and estimation of kinetic parameters of nitrate uptake by spectroscopy based on absorbances at multiple wavelengths has been developed to estimate nitrate uptake by barley (Hordeum vulgare L. cv. Steptoe) seedlings. Nitrate concentration in the nutrient solution was determined from the slope of the linear regression line of the absorbances to nitrate absorption coefficients at 12 wavelengths. Interference by root exudates was only due to an absorption component changing with wavelength in correlation with nitrate absorption. The standard error of the determination decreased in reverse proportion to the square root of the number of the wavelengths. A linear form of the net uptake equation, NUR = I_maxC/K_m+C‐E, could be expressed as NUR = (I_max ‐ E) ‐ K_m NUR/C ‐ K_mE I/C where NUR is net uptake rate, Imax is maximum influx, C is concentration, Km is the Michaelis constant, and E is an efflux constant. The method described here was used to determine the time course of nitrate depletion by barley seedlings from their nutrient solution. The isotherm of net nitrate uptake rates derived from the time course was analyzed after modifications based on the linear form of the net uptake equation. The analysis yielded highly significant results (P<0.0001).     

A simple model for phosphorus uptake kinetics of wheat seedlings

A simple model to compare and predict phosphorus (P) uptake behavior of plants may be useful to agronomists. A predictive equation based on Michaelis‐Menten kinetics was developed for this purpose. Kinetic parameters for use in the model were determined in an experiment using two cultivars of winter wheat grown for 21 days in 14 soil treatments, including seven P levels, each in fumigated and unfumigated soil. In another experiment, the same wheat cultivars were grown for 7, 14, or 21 days at one soil P level in both fumigated and unfiimigated soil. Using parameter values developed in the first experiment with multiple P levels and one time period, the model closely (R²=0.966, P<0.001) predicted P uptake in the second experiment with one P level over multiple time periods. The model could be a useful agronomic tool because of its simplicity and because no data need be collected from artificial growing conditions.     

A contribution to the repressing effect of ammonium on nitrate uptake

The instantaneous impairing effect of 0.5 mM NH₄+ on ¹³NO₃^? influx/net uptake and the early kinetics of ¹³NH₄+ influx are demonstrated in experiments with barley and squash seedlings. Barley was found to have a high constitutive NH₄+ uptake capacity in contrast to squash where NH₄+ pretreatment appeared to accelerate the uptake after a certain lag phase.     

Competition for amino acids between wheat roots and rhizosphere microorganisms and the role of amino acids in plant N acquisition

The direct uptake of organic nitrogen compounds from the soil solution by plant roots has been hypothesised to constitute a significant source of N to the plant particularly in N limiting ecosystems. The experiments undertaken here were designed to test whether wheat roots could out-compete the rhizosphere microflora for a pulse addition of organic N in the form of three contrasting amino acids, namely lysine, glycine and glutamate. Amino acids were added at a concentration reflecting reported soil solution concentrations (100 μM) and the uptake into either plant biomass or respiration or microbial biomass and respiration determined over a 24 h chase period. The results showed that the plant roots could only capture on average 6% of the added amino acid with the remainder captured by the microbial biomass. We therefore present direct in vivo evidence to support earlier work which has hypothesised that organic N may be of only limited consequence in high input agricultural systems. We suggest that this is a result of the higher concentrations of NO₃⁻ in agricultural soil solutions, the slow movement of amino acids in soil relative to NO₃⁻, the rapid turnover of amino acids by soil microorganisms, and the poor competitive ability of plant roots to capture amino acids from the soil solution.     

Nutrient uptake by wheat seedlings that differ in response to mixed nitrogen nutrition 1

In hydroponics, cereal crop growth is usually enhanced when the nutrient media contains both nitrate and ammonium compared to either form separately. Identification of genotypic variation between two spring wheat cultivars (Triticum aestivum cv. Len, and Triticum durum cv. Inbar) in their magnitude of growth response to mixed N permits a comparative evaluation of metabolic processes underlying enhanced growth from utilization of N mixtures. The objective of this study was to determine if the additional mixed N‐induced growth, and the difference in response between cultivars, is associated with the uptake or partitioning of N, P, or K during the seedling stage of development. Plants of Len and Inbar were grown hydroponicaily for 21 days in nutrient solutions containing N either as all nitrate or as a 50/50 mixture of nitrate and ammonium. The mean of three experiments showed that whole plant dry weight increased 19% for Len and 41 % for Inbar when the plants were supplied with the N mixture compared to only nitrate. With the N mixture, the increase in biomass was almost entirely due to enhanced shoot production, which was mainly the result of greater tiller formation. Both cultivars absorbed more N, P, and K when grown with mixed N, and the additional N and K uptake of appeared to be associated with the enhanced growth. This conclusion is based on the observation that Inbar absorbed proportionately more N and K, but less P, than Len when grown with mixed N nutrition. These data suggest that the increased dry matter production from the utilization of mixed N nutrition is not directly related to the additional absorption of P, but may be associated with enhanced N and K uptake.