The Utilization of P--N Compounds by Plants: I. Root-mediated Hydrolysis of Phosphodiamidate

The uptake and utilization of sodium phosphodiamidate, a compoundcontaining covalent P—N bonds, was investigated. Growthanalysis showed that this compound could serve as the sole sourceof phosphorus for tomato plants grown in culture solutions,although the growth rate of plants supplied with phosphodiamidatewas slightly less than that of plants utilizing diammonium phosphate.Chromatographic analysis of xylem sap showed that phosphodiamidatewas not transported in the unhydrolysed form in the sap of tomatoplants supplied with this compound. Tomato plants supplied withphosphodiamidate as the sole source of both phosphorus and nitrogenassimilated some nitrogen in a form other than unhydrolysedphosphodiamidate. Comparison with plants supplied with diammoniumphosphate showed that the plants receiving phosphodiamidatehad lower nitrogen contents, suggesting that the rate of hydrolysisof the compound may have been limiting nitrogen assimilationby the plants. Measurements of the hydrolysis of phosphodiamidatein culture solutions in the absence of plants showed that theplants assimilated more nitrogen than that released by normalchemical hydrolysis of the compound occurring in the absenceof plants. The excess nitrogen assimilated, over and above thatproduced by normal chemical hydrolysis, could not be accountedfor solely by the uptake of unhydrolysed phosphodiamidate asthis would require the concomitant uptake of more phosphorusthan was actually present in the plant. Thus it was inferredthat the presence of the plant roots in the culture solutionsomehow caused extra hydrolysis of the phosphodiamidate.     

The use of 3-amino-1,2,4-triazole to investigate the short-term effects of oxygen toxicity on carbon assimilation by Pisum sativum seedlings

To study the in vivo short-term effect of hydrogen peroxide on plant metabolism, 2 mol m^?3 3-amino-1,2,4-triazole, a catalase inhibitor, was applied through the transpiration stream to Pisum sativum seedlings, and gas exchange characteristics, ascorbate peroxidase, glutathione reductase and catalase activities, and levels of hydrogen peroxide and formate were determined. Carbon dioxide assimilation rates were inhibited after the addition of aminotriazole: photorespiratory conditions exacerbated this inhibition. Carbon dioxide response curves showed that aminotriazole reduced both the RuBP regeneration rate and the efficiency of the carboxylation reaction of Rubisco. Catalase activity was completely inhibited 200 min after the application of this inhibitor, but no concomitant increase in H₂O₂ concentration was found. Under enhanced photorespiratory conditions, H₂O₂ concentrations increased. This suggests that under normal environmental conditions hydrogen peroxide is metabolized via alternative mechanisms. The aminotriazole treatment had no effect on the ascotbate peroxidase and glutathione reductase activities, but caused a substantial increase in the formate pool size. These results suggest that hydrogen peroxide is metabolized by reacting with glyoxylate to produce formate and CO₂. The increased production of formate may reduce the flow of carbon through the normal photorespiratory pathway and may also be used anaplerotically as a precursor of products of 1-C metabolism other than serine. This would prevent the return of photorespiratory carbon to the RPP pathway, leading to a smaller RuBP pool size which would in turn result in a decrease in carboxylation conductance (carboxylation efficiency) and regeneration rate of RuBP.     

The Utilization of P--N Compounds by Plants II. The Role of Extracellular Root Phosphatases

Barley plants were grown on culture solutions containing sodiumphosphodiamidate as the sole source of both phosphorus and nitrogen.The compound hydrolysed more rapidly in solutions on which plantswere growing than in a similar solution without plants, supportingthe hypothesis of a root-mediated hydrolysis. Histochemicaltests provided corroborative evidence for root-mediated hydrolysisof phosphodiamidate and indicated that some of this hydrolyticactivity was probably associated with the cell wall. Purifiedroot cell wall preparations catalysed the hydrolysis of phosphodiamidate,the activity being firmly bound to the cell wall. Some evidencewas obtained to show that this activity was enzymic in nature,rather than a general surface catalysis. Competitive inhibitionby ß-glycerophosphate suggested that the hydrolysisof the phosphodiamidate was brought about by a non-specificphosphatase, rather than a phosphoamidase as such. The activitywas shown to be constitutive. It was concluded that the mainmechanism by which plants utilize phosphodiamidate as a nutrientsource involves hydrolysis of the compound at the root surface,probably by a constitutive, non-specific phosphatase, and subsequentuptake of the phosphate and ammonia produced by hydrolysis.     

豌豆种子吸胀过程中脱水耐性变化的时间模式

研究了豌豆种子吸胀过程中脱水耐性的变化模式。种子在吸胀初期迅速吸收水分,然后缓慢吸收直到平台期。电解质渗漏速率在吸胀初期增加直到11h,然后随着吸胀下降。在吸胀过程中,种子的萌发率逐渐增加,种子和胚轴的脱水耐性逐渐丧失,10％和50％的种子和胚轴被脱水致死的含水量明显增加。赤霉素和脱落酸处理改变豌豆种子的萌发特性,提高胚轴的脱水耐性。研究结果表明,吸胀的豌豆种子脱水耐性的丧失是一种数量性状,正常性种子吸胀后脱水耐性的变化能够作为种子顽拗性研究的模式系统。     

Development of the Recalcitrant (Homoiohydrous) Seeds of Avicennia marina: Anatomical, Ultrastructural and Biochemical Events Associated with Development from Histodifferentiation to Maturation

Early histodifferentiation of the embryo of Avicennia marina(Forssk) Vierh was characterized by the formation of endospermhaustoria Once the growth phase was initiated, subsequent embryodevelopment was extra-ovular The mature seed, therefore, wasenclosed by a pericarp originating entirely from the ovary wallGrowth and reserve deposition was not initiated until 45–50d after fruit set (DAFS), when respiratory activity had peakedWater content remained constant from the earliest stages ofembryogenesis to seed abscission and respiratory activity, althoughdeclining somewhat after the completion of histodifferentiation,remained relatively high throughout seed development The ultrastructureof the meristematic root primordia was indicative of metabolicactivity, remaining essentially similar in all respects fromthe end of histodifferentiation until the mature seeds wereabscised During this phase cotyledon cells became highly vacuolatedand the soluble sugars, which constituted the major nutrientreserves of mature seeds, increased considerably Seeds of A,marina initiate germination, without the requirement for additionalwater, as soon as they are shed It is proposed that the accumulationof soluble sugars, rather than insoluble complex reserves, isa major factor in the developmental strategy of these highlyrecalcitrant seeds Anatomy, Avicennia marina, biochemistry, homoiohydrous, recalcitrant, seed development/maturation, ultrastructure     

Xylem hydraulic characteristics of subtropical trees from contrasting habitats grown under identical environmental conditions

Five evergreen subtropical tree species growing under identical environmental conditions were investigated to establish which hydraulic properties are genotypically rigid and which show phenotypic plasticity. Maximum xylem-specific conductivity ( k _s) correlated well with the anatomical characteristics (conduit diameter and density) for the four angiosperms Tecomaria capensis , Trichilia dregeana , Cinnamomum camphora and Barringtonia racemosa ; the anatomy of the gymnosperm Podocarpus latifolius was not assessed. Huber values (functional xylem cross-sectional area : leaf area) varied inversely with k _s among species. Maximum leaf-specific conductivity was similar in the five unrelated species. Vulnerability of xylem to cavitation differed between species, as did the relationship between transpiration and water potential. Models of these parameters and isolated midday readings confirm that these trees operate at similar maximum leaf-specific conductivity ( k _l) values. The data are consistent with the hypothesis that conductivity characteristics ( k _l, k _s) are influenced by environment, whereas vulnerability to cavitation is genetically determined.     

种子顽拗性：最新评价

根据种子的脱水行为,可以把种子分为正常性、顽拗性和中间性种子三种类型。种子脱水耐性和脱水敏感性的鉴定是制定种子的贮藏策略和物种基因资源长期保存的依据。除了物种的内在特性外,种子的发育状态、脱水速率、脱水时和重新水合时的环境是影响种子脱水耐性的重要因子。种子的存活率、电解质渗漏速率和存活种子的萌发生长速率是衡量种子脱水耐性的良好的综合参数。种子的脱水耐性是一种数量性状,用“临界含水量”的概念来判断是不正确的,且在评价种子顽拗性中引起了一些混乱。本文还提出了一种全新的评价种子顽拗性的工作模式。