Ermittlung kinetischer Parameter der sauren Phosphatasen intakter Zuckerrübenwurzeln bei variierter Phosphaternährung

Determination of kinetic parameters of acid phosphatases in intact sugar beet roots of variable phosphorus nutrition Organically bound phosphorus has to be hydrolysed before its P can be taken up by plants. Both microbes and plant roots possess phosphatases, which could be of importance especially in soils with low concentrations of inorganic phosphorus in the soil solution. This could be the reason why nutrient uptake models underestimate the P-uptake by plants when P-mobilization by the phosphatases of roots is not taken into consideration. Therefore the activity of acid phosphatases (P_ase) was determined to answer the following questions: 1) To which extent does the root bound acid phosphatase (P_ase) follow Michaelis-Menten kinetics? 2) By which of the four linear transformations of the Michaelis-Menten equation (Lineweaver/Burk, Hanes, Eadie/Hofstee, Eisenthal/Cornish-Bowden) can plausible values of V_max and K_m be determined? 3) Which effect has the P nutrition of the plant on these kinetic parameters? Sugar beet plants were grown in full nutrient solution containing 1 and 100 μM P respectively. The P_ase activity of the intact roots was measured at pH 5 using p-nitrophenylphosphate (25—15000 μM p-NPP). V_max values were calculated per m root length. Acid phosphatase activity principally followed Michaelis-Menten kinetics. Transformations and calculations of V_max and K_m after Eadie/Hofstee and Eisenthal/Cornish-Bowden suggested the existence of at least two enzyme systems (P_{ase 1}, P_{ase 2}). The following kinetic parameters were found: P_{ase 1}: P deficient plants: V_max: 43—45 nmol m^—1 min^—1, K_m: 31—37 μM NPP; P sufficient plants: V_max: 7 nmol m^—1 min^—1, K_m: 47—53 μM NPP. P_{ase 2}: P deficient plants: V_max: 230—293 nmol m^—1 min^—1, K_m: 1579—3845 μM NPP; P sufficient plants: V_max: 123—171 nmol m^—1 min^—1, K_m: 3027—7000 μM NPP. Thus plants with sufficient P nutrition have a lower affinity to P_org and a lower hydrolysis of P_org. For P nutrition of crops P_{ase 1} might be the most important enzyme.     

Einfluß von Sorte und Phosphordüngung auf den Phosphorgehalt und die Aktivität der sauren Phosphatasen von Weizen und Gerste. — Ein Beitrag zur Diagnose der P-Versorgung von Pflanzen —

Influence of cultivar and phosphorus application on P concentration and acid phosphatase activity in wheat and barley. — A contribution to the diagnosis of P supply of plants — Acid phosphatase activity compared to total P concentration was studied as a diagnostic criterion of the phosphorus nutritional status of wheat and barley. In a field experiment with wheat cv ‘Sperber’ the influence of P level on P_ase activity and P concentration was monitored at four developmental stages. Significant correlations with grain yield were found for P_ase activity at all four stages whereas P concentration was significantly correlated with grain yield in the early stage (DC 26–27) only. Genetic variability and influence of P supply on P_ase activity and P concentration was studied at low and at high P supply with 9 wheat and 23 barley cultivars grown for 4 and 5 weeks in pot experiments. The variability of P_ase activity caused by cultivars was higher than that caused by P supply. It is, therefore, concluded that the acid phosphatase activity is not suitable as a generally applicable tool for diagnosing P supply. In comparison, total P concentration of the plants was influenced to a higher degree by P supply than by cultivars. Total P is, therefore, more suitable as a criterion for diagnosing P supply of plants than P_ase activity. The samples, though, have to be collected in stages before DC 27.     

Mobilität und Pflanzenverfügbarkeit von Phosphor aus organischen und anorganischen Formen in der Rhizosphäre von Lolium perenne

Mobility and availability of phosphorus from organic and inorganic forms in the rhizosphere of Lolium perenne 1.) In pots (sandwich-technique according to Helal and Sauerbeck), where the soil (A_p-horizon of an Alfisol-Udalf from loess) was separated into a root chamber and an outer chamber (up to 10 mm away from the root chamber), the phosphorus (P) availability of super-P, rock-P (Hyper-P) and organic P [myo-inositol-2-mono-P di(cyclohexylammonium) salt] was studied. P was applied at a rate of 50 mg P kg^?1 soil only in the outer chamber. This P fertilization resulted in significant increase of CAL- and water-extractable P in the treatment with organic P and super-P compared to the P₀ treatment. During 6 cuts of Lolium perenne organic P showed the same effect on the P-uptake of plants as super-P. Compared to the treatment without P, rock-P had no effect on the P-uptake. The higher P-uptake of Lolium perenne both in the super-P- and organic P-treatment was reflected at the end of the experiment only by the CAL-P content differences between root chamber and outer chamber. The highest activity of acid phosphatase, measured in air dried soil was found in the root chamber of the organic P treatment The content of EUF extractable organic P was significantly higher in the root chamber of the treatments with super-P and organic P than in the P₀- and rock-P-treatment. 2.) The P elution was studied with 2 soils (A_p-horizon of an Alfisol-Udalf from loess pH: 6.4; A_h-horizon of a basaltic brown soil pH: 5.4) under lab conditions in columns. In the super-P-and organic P-treatment leached P was mainly inorganic. In the soil with low pH value (5.4), P was leached in the treatment with super-P or organic P in a higher amount of organic P than in the soil with pH 6.4. The results of this experiment do not show in which form the applied organic P was moved to the roots or displaced in the soil.     

Wirkung der Phosphatdüngung in einem 40jährigen Dauerversuch auf einer Sandlöß-Braunschwarzerde in Halle

Effect of differential P fertilizer application over a period of 40 years in a long-term field experiment on a Phaeozem near Halle, Germany P fertilizer in three forms was applied at 0 (P₀), 15 (P₁) and 45 (P³) kg ha^?1 P per annum to a relation lucerne — lucerne — potatoes — winter rye — sugar beet — spring barley. The experiment was laid down in 1949 near Halle on Phaeozem soil containing 57 mg total P and 8.5 mg DL-P per 100 g soil (0–20 cm) and with pH 6.0 (0.1 M KCl). Small differences between treatments were first observed only after 20 years. Since then the effects of treatment increased but even after the 7th cycle (1982–1987) the maximum effect was <8 cereal units (CU) per ha. The crops most responsive to P were: lucerne (in the year of sowing), potatoes and sugar beet. Over the period 1977–1988, annual P removal exceeded P input by 23 and 9 kg ha^?1 P for treatments P₀ and P₁ respectively; on treatment P³, P input exceeded removal by 19 kg ha^?1 per annum. DL-P on treatment P₀ in 1979 was 3.5–4.0 mg per 100 g, insufficient to support high yields. A level of 5–6 mg DL-P per 100 g soil, as found in treatment P₁ is required for sustainable high yield. Change in total soil P content corresponded with the P balance (input — withdrawal) only on treatment P³. The lack of correspondence on treatments P₀ and P₁ suggests that crops obtained much of their P requirement from subsoil.     

Phosphorus Efficiency of Buckwheat (Fagopyrum esculentum)

The ability of buckwheat (Fagopyrum esculentum) roots to acquire phosphorus (P) was characterized by investigating P uptake, morphological features, and chemical changes in the rhizosphere. Over a range of nutrient solution P concentrations (5–500 μmol · L^?1), maximum shoot growth was achieved with a P supply between 5 and 100 μmol · L^?1. Root weight and root length, as well as length and frequency of root hairs, were higher at low P levels. Root surface and the root surface/shoot dry weight ratio reached high values. Though P uptake rates were only moderate (0.15 pmol · cm^?1 root · sec^?1), shoot P concentrations were high (1.8% of dry weight with 100 μM P) predominantly being inorganic (80%). Phosphorus efficiency was characterized by a high specific absorption rate (810 mmol P · kg^?1 root dry wt · d^?1) rather than by an efficient utilization for dry weight production. Root exudates of low-P plants had lower pH values than exudates of high-P plants and increased the solubility of FePO₄ and MnO₂ to a greater extent. Amounts of exuded organic acids and phenolics were low and could not account for the observed solubilization of FePO₄ and MnO₂. Enhanced hydrolysis of glucose-6-phosphate by exudates from low-P plants was due to an increased “soluble” acid phosphatase activity, and root surface phosphatase activity was also slightly enhanced with P deficiency. In the rhizosphere soil of buckwheat, some depletion of organic P forms was observed, and in pot trials with quartz sand, buckwheat utilized glucose- 6-phosphate as a P source at the same rate as inorganic P.     

Lysimeterstudie zum Einfluss von Düngung und Bewirtschaftung auf die Ertragsbildung,den Wasserhaushalt und die Nährstoffauswaschung im Trockengebiet der Uckermark

Lysimeter study about the effect of fertilization and management on yield, water balance, and solute leaching in the dry region of the Uckermark Results are presented of a lysimeter study on the effects of fertilization and management on yield, seepage rate, and solute leaching in the dry region of the Uckermark (Northeast Germany) from 1992 to 1999. The crop rotation included sugar beet, summer barley, pea, and winter wheat with fertilization of 280 and 400 kg N ha^—1 within the crop rotation. Additionally to the mineral fertilization, sugar beet was fertilized with the sugar beet leaf or slurry. The average annual seepage rate was 13.7 mm, the nitrogen leaching was 4.3 kg N ha^—1, and the phosphorus leaching was 0.008 kg P ha^—1. Differences between the treatments were negligible and insignificant. The amount of the precipitation from April to September had a significant influence on yields. Furthermore, results of water use efficiency and the effect of different nitrogen rates on yields are presented.     

盐土改良肥对草甸盐土的改良效果和对甜菜经济效益的影响

[目的]研究甘肃省河西走廊盐土改良肥对草甸盐土改良效果和甜菜经济效益的影响,为该区甜菜产业可持续发展提供技术支撑。[方法]选择甘肃省酒泉市肃州区铧尖乡草甸盐土,采用田间试验方法开展研究。[结果]影响甜菜产量的原料依次是:盐土调控剂甜菜专用肥有机碳肥。盐土改良肥配方组合为:甜菜专用肥0.0586:盐土调控剂0.0623:有机碳肥0.8791。盐土改良肥施用量与草甸盐土孔隙度、团聚体、持水量、有机质、速效氮磷钾、甜菜农艺性状、经济性状和产量之间呈显著的正相关关系,与容重、pH值、全盐含量之间呈显著的负相关关系。经回归统计分析,盐土改良肥经济效益最佳施用量为39.64t/hm2,甜菜块根理论产量为96.68t/hm2。施用盐土改良肥与传统的抗盐丰盐碱土改良剂和沃丰隆盐碱土改良剂比较,施肥利润分别增加414.48,946.10%元/hm2。[结论]施用盐土改良肥,可以改善草甸盐土理化性质,提高酶活性和甜菜产量。     

Root-induced changes in the rhizosphere: Importance for the mineral nutrition of plants

Root-induced changes in the rhizosphere may affect mineral nutrition of plants in various ways. Examples for this are changes in rhizosphere pH in response to the source of nitrogen (NH₄-N versus NO₃-N), and iron and phosphorus deficiency. These pH changes can readily be demonstrated by infiltration of the soil with agar containing a pH indicator. The rhizosphere pH may be as much as 2 units higher or lower than the pH of the bulk soil. Also along the roots distinct differences in rhizosphere pH exist. In response to iron deficiency most plant species in their apical root zones increase the rate of H⁺ net excretion (acidification), the reducing capacity, the rate of Fe^III reduction and iron uptake. Also manganese reduction and uptake is increased several-fold, leading to high manganese concentrations in iron deficient plants. Low-molecular-weight root exudates may enhance mobilization of mineral nutrients in the rhizosphere. In response to iron deficiency, roots of grass species release non-proteinogenic amino acids (?phytosiderophores”?) which dissolve inorganic iron compounds by chelation of Fe^III and also mediate the plasma membrane transport of this chelated iron into the roots. A particular mechanism of mobilization of phosphorus in the rhizosphere exists in white lupin (Lupinus albus L.). In this species, phosphorus deficiency induces the formation of so-called proteoid roots. In these root zones sparingly soluble iron and aluminium phosphates are mobilized by the exudation of chelating substances (probably citrate), net excretion of H⁺ and increase in the reducing capacity. In mixed culture with white lupin, phosphorus uptake per unit root length of wheat (Triticum aestivum L.) plants from a soil low in available P is increased, indicating that wheat can take up phosphorus mobilized in the proteoid root zones of lupin. At the rhizoplane and in the root (root homogenates) of several plant species grown in different soils, of the total number of bacteria less than 1 % are N₂-fixing (diazotrophe) bacteria, mainly Enterobacter and Klebsiella. The proportion of the diazotroph bacteria is higher in the rhizosphere soil. This discrimination of diazotroph bacteria in the rhizosphere is increased with foliar application of combined nitrogen. Inoculation with the diazotroph bacteria Azospirillum increases root length and enhances formation of lateral roots and root hairs similarly as does application of auxin (IAA). Thus rhizosphere bacteria such as Azospirillum may affect mineral nutrition and plant growth indirectly rather than by supply of nitrogen.     

Content and Potential Availability of Selected Nutrients in Field‐Applied Sugar Beet Factory Lime

Factory lime generated during sugar beet processing contains phosphorus (P). Factory lime collected from seven sugar beet factories in North Dakota and Minnesota had P concentrations ranging from 3470 to 7043 mg P kg^?1. Soil collected from two field trials one and two growing seasons after factory lime application was analyzed for pH changes and soil‐test P (STP). At one site, soil pH increased after both growing seasons, as did STP, indicating a continued release of P as the factory lime dissolved and reacted with the soil. At the second site, increased soil pH and STP were evident after one growing season, but a few months later the pH had no further change and STP decreased. The data suggest factory lime has the potential to supply P to a growing crop, but actual proportion of P available from the factory lime could not be quantified from these experiments.     

Beziehungen zwischen Bodenfruchtbarkeitskennziffern,Ertrag und Inhaltsstoffen von Zuckerrüben auf Löß-Schwarzerde

Relations between soil fertility parameters and sugar beet yield and quality characteristics respectively, on Haplic Chernozem soils In interpreting four “experiments without interference” (VoE) on a total of 435 plots the influence of soil fertility parameters (BFK_i) as organic matter, P, K, Mg, pH value, Cu, B and Mn on sugar beet yield and its nutrient contents on Haplic Chernozem sites under Ca immission has been analysed. “VoE” is a method applied to find specific field data by means of which relations within the multidimensional system soil - plant are ascertained by utilizing soil and yield parameter variations. With increasing pH value the nutrient contents in the soil decreased in the order the following sequence: Mn, Mg. Cu, B; and, resulting from that, the sugar beet yield decreased significantly. The coefficients of determination (B) of the triple linear regression between yield and BFK_i ranged from B = 0,21 to B = 0,33 and between nutrient contents and BFK_i from B = 0 to B = 0,41. Most of the regression functions contained “BFK_i boron” as a variable. It has been found that there were positive correlations between “BFK_i pH value” and “soluble ashes” and negative correlations between “BFK_i pH value” and P contents in sugar beet.     

Distribution of exudates of lupin roots in the rhizosphere under phosphorus deficient conditions

Abstract

The distribution of secretory acid phosphatase and organic acids enhanced by phosphorus deficiency in lupin rhizosphere was investigated using a rhizobox system which separated the rhizosphere soil into 0.5 mm fractions. In the soil fraction closest to the root surface, the lupin exudates displayed an acid phosphatase activity of 0.73 u g^?1 dry soil and citrate concentration of 85.2 μmol g^?1 dry soil, respectively. The increase of the acid phosphatase activity-induced an appreciable depletion of organic P in the rhizosphere, indicating that lupin efficiently utilized the organic P from soil through the enzyme activitye The sterile treatments demonstrated that the acid phosphatase in the rhizosphere was mainly derived from lupin root secretions. The secretory organic acids enhanced considerably the solubility of the inorganic P in three types of soil and a sludge. However, the secretory acid phosphatase and organic acids from lupin roots were only detected in a considerable amount in 0-2.5 mm soil fractions from root surface.     

Sugar beet (Beta vulgaris L.) response to diammonium phosphate and potassium sulphate under saline–sodic conditions

Salinity and sodicity are prime threats to land resources resulting in huge economic and associated social consequences in several countries. Nutrient deficiencies reduce crop productivity in salt‐affected regions. Soil fertility has not been sustainably managed in salt‐affected arid regions. Few researchers investigated the crop responses to phosphorus and potassium interactions especially in saline–sodic soils. A research study was carried out to explore the effect of diammonium phosphorus (DAP) and potassium sulphate (K₂SO₄) on sugar beet (Beta vulgaris L.) grown in a saline–sodic field located in Kohat district of Pakistan. The crop was irrigated with ground water with EC_iw value of 2.17–3.0 dS/m. Three levels each of K₂O (0, 75 and 150 kg/ha) as K₂SO₄ and P₂O₅ (0, 60 and 120 kg/ha) as DAP were applied. The application of P significantly affected fresh beet and shoot yield while K fertilizers had significant effect on fresh beet yield and ratio of beet:shoot, while non‐significant effects on the fresh shoot were observed. The application of K₁ and K₂ promoted sugar beet shoot yield by 49.2 and 49.2% at P₁ and 64.4 and 59.7% at P₂, respectively over controls. In comparison with controls, fresh beet yield was increased (%) by 15 and 51, 45 and 84, and 50 and 58 for corresponding K₁ and K₂ at P₀, P₁ and P₂, respectively. Addition of P₁ and P₂ increased beet yield by 37 and 47% over control. The shoot [P] (mmol/kg) were achieved as 55.2, 73.6 and 84.3 at P₀, P₁ and P₂, respectively. The shoot [Mg] and [SO₄] tended to decrease with increasing P levels, while [SO₄] was markedly reduced at P₂. The effect of P on leaf [Na] was non‐significant, but increasing levels of K decreased [Na] substantially at P₀ and P₁, but there was no difference in the effect of K level on [Na] at P₂. Consequently, K application reduced leaf Na:K ratios. Fresh shoot yield was weakly associated with leaf [P] (R² = 0.53). The leaf Na:K ratio showed a negative relationship (R² = 0.90) with leaf [K]. A strongly positive relationship (R² = 0.75) was observed between leaf [K] and fresh beet yield. The addition of K₂SO₄ also enhanced [SO₄] and SO₄:P ratios in leaf tissues. The ratio of Na:K in the shoot decreased with increasing K application. These results demonstrated that interactions of K and P could mitigate the adverse effects of salinity and sodicity in soils. This would contribute to the efficient management of soil fertility system in arid‐climate agriculture.     

Sodium‐induced calcium deficiency in sugar beet during substitution of potassium by sodium

Functions of sodium (Na⁺) and potassium (K⁺) are closely associated. In some crops, Na⁺ is able to prevent or reduce considerably the occurrence of K⁺ deficiency. Sugar beet (Beta vulgaris L.) is a natrophilic crop, and positive effects of Na⁺ applications on yield were observed when K⁺ was sufficiently supplied. However, it is not known which specific function of K⁺ can limit the growth of sugar beet when K⁺ is substituted by an equivalent amount of Na⁺. Therefore, K⁺ substitution by Na⁺ was investigated for sugar beet in hydroponics. Surprisingly, no K⁺‐deficiency symptoms were observed. However, calcium (Ca²⁺) concentrations in the leaves were significantly decreased. Moreover, Ca²⁺ uptake and translocation through xylem sap were reduced in Na⁺‐treated plants. It is concluded that Ca²⁺ uptake by roots and its translocation via xylem sap primarily limit the possibility of K⁺ substitution by Na⁺ in sugar beet nutrition.     

低磷胁迫对不同基因型甜菜抗性生理特征的效应

选用对低磷胁迫抗性各异的3个甜菜品种:品20、品17和品14,在人工培养室内采用沙培试验法,研究了甜菜抗耐低磷胁迫的生理机制。结果表明,低磷胁迫限制了甜菜对磷的吸收,导致植株含磷量和生物产量显著下降;不同品种间差异显著,品14降幅最大,品17次之,品20最小。与足磷处理(P 100μmol/L)相比,低磷胁迫后甜菜品种间体内抗性生理特征差异显著。其中,品14和品17的丙二醛含量、脯氨酸含量、过氧化物酶活性均显著增加;品20的丙二醛含量、脯氨酸含量极显著下降,而过氧化物酶活性变化不显著。叶片中Mg2+-ATPase活性降低,不同品种降幅各异,从小到大依次为品20品17品14,其中品20与后两者间的差异均达显著。品20和品17体内的钙调素(CaM)含量显著增加,而品14变化不明显,其相对值从大到小依次为品20品17品14,差异显著,与品种自身抗磷胁迫能力顺序一致。不同磷素营养条件对甜菜抵御外界不良环境有较大影响,叶片在受到热伤害时,抗磷胁迫能力较弱的品14和品17在低磷胁迫时质膜损伤率显著增加;而抗磷胁迫能力较强的品20叶片质膜的损伤率显著下降,抗热能力得到改善。     

Ammoniakemissionen aus alternden Pflanzen und bei der Zersetzung von Ernterückständen

Ammonia emissions from senescing plants and during decomposition of crop residues NH₃ emissions from plant stands, measured under simulated environmental conditions with the wind tunnel method, ranged between 0.8 and 1.4% of the N content of the shoot, equivalent to 1.1 to 2.9 kg NH₃-N ha^?1. The highest emissions were observed in faba beans whereas the emissions in winter wheat, spring rape and white mustard were lower. The total NH₃ emissions were not affected by removing a part of the ears (sink reduction), but emissions occurred earlier, as did the plant senescence. This suggests that the NH₃ emissions are closely related to senescence. NH₃ emissions from decomposing crop residues ranged from 0.9 to 3.7% of the N content. The emissions from sugar beet leaves and potato shoots with high water content reached from 8.6 up to 12.6 kg N ha^?1, whereas the emissions from field bean straw with high dry matter and N content were relatively low. (3.1 kg N ha^?1, or 0.9% of the N content). The NH₃ emissions from sugar beet leaves were reduced by 81% by ploughing and 63% by mulching.     

Cu‐, Zn‐ und Cd‐Aneignungsvermögen von zwei Spinatgenotypen in Abhängigkeit von der P‐Versorgung und Wurzelexsudation

Cu, Zn, and Cd acquisition by two spinach cultivars depending on P nutrition and root exudation Within a spectrum of 11 spinach cultivars (cvs) differences in the Cu, Zn, and Cd contents of shoots had been noticed. The aim of this study was therefore to analyze in more detail the acquisition of Cu, Zn, and Cd by the most differing cultivars (Tabu and Monnopa) in dependence on P nutrition. The plants were grown in a low phosphorus Luvisol (pH 6.3; total contents Cu: 89, Zn: 297, Cd: 2.4 mg kg^—1) with two phosphorus levels in pots under natural conditions. For the determination of inflow, root length/shoot weight ratio and of the Cu, Zn, and Cd concentration in the soil solution (rhizosphere) plants were harvested 26 and 40 days after sowing. Root exudation of organic acids of the two cvs was measured 35 days after growing in quartz sand with different P supply. Both cultivars responded to P fertilizer by doubling their shoot weight. With increased P supply (0.68—0.77% P in shoot‐DM) both cultivars showed similar heavy metal contents in the shoot resulting from similar root length/shoot weight ratios (RSR) and net uptake rates of the three elements as well as the same element concentrations in the rhizosphere soil solution. Under P deficiency, however, cv. Tabu (0.52% P in shoot‐DM) showed in comparison with cv. Monnopa (0.48% P) higher Cu, Zn, and Cd contents of shoots although its RSR was smaller than that of cv. Monnopa. However, the inflow for Cu was higher and for Zn and Cd significantly higher compared with cv. Monnopa. This result of cv. Tabu corresponded with higher concentrations of Cu, Zn, and Cd of its rhizosphere soil solution, and its higher exudation rates of oxalate, citrate, and malate (3.9; 1.0; 0.7 nmol cm^—1 h^—1). The corresponding values for cv. Monnopa were: 1.7; 0.3; 0.4 nmol cm^—1 h^—1. The mobilization of Cu, Zn, and Cd by the excreted organic acids seems to be responsible for the higher Cu, Zn, and Cd inflow of cv. Tabu.     

Räumliche Variabilität der Phosphorgehalte im Oberboden landwirtschaftlich genutzter Flächen in kleinen Einzugsgebieten

Spatial variability of Phosphorus contents in topsoils of two small catchments under agricultural use The spatial variability of total phosphorus (P_t) contents was investigated in two small agriculturally used catchments in Saxony/Germany. The mean P_t concentration was 982 mg kg^–1, with 80 % of values between 560 and 1530 mg kg^–1. The largest P_t contents were found at the tops and foots of slope, and the smallest at the middle slope positions. At the scale of slopes (microscale) the spatial variability was caused by the relief and soil properties, whereas at the scale of catchments, agricultural management had the largest influence. These differences must be considered for the modeling of diffuse P inputs to surface waters.     

Evaluation of an acid phosphatase assay for detection of phosphorus deficiency in leaves of maize (Zea mays L.)

Assays based on acid phosphatase activity in plant tissue show promise for rapid diagnosis of phosphorus deficiency. This study was undertaken to evaluate a simple leaf disc assay for the detection of the phosphorus deficiency in maize leaves. Leaf discs were excised from 36 to 48 day old greenhouse‐grown maize plants which had been supplied with phosphate in solution culture at levels of 5, 25 or 75 yM and which had total P concentrations in the leaves ranging from 26 to 309 mmol kg^‐1 dry wt. For acid phosphatase activity determinations, discs were incubated 15 min. at 30°C with 5 mM paranitrophenol phosphate in a pH 5.8 acetate buffer. In plants which showed visual symptoms of P‐deficiency, acid phosphatase activity was 2 to 3 times as great as in P‐suf‐ficient plants. Results were inconsistent in plants which were only moderately P‐deficient. This study indicates that the leaf disc acid phosphatase assay should be useful in confirming visual diagnosis of acute phosphorus deficiency in maize, but may not be adequately sensitive to detect moderate P deficiency.     

Surface phosphatase activity of Sitka spruce mycorrhizas from a serpentine site

The p-nitrophenyl phosphatase activity of mycorrhizal roots of Sitka spruce from a phosphorus deficient site was inversely proportional to the concentrations of 0.5 n acetic acid-extractable phosphorus in the decaying organic matter from which the roots were removed. This is interpreted as the derepression of phosphatase synthesis under conditions of inorganic P deficiency and is discussed in relation to the possible roles of mycorrhizal surface phosphatase activity.     

根际pH对玉米利用磷酸单酯和双酯盐的影响

【目的】土壤有机磷在土壤全磷中占有很大比重,是植物潜在的有效磷源,但必须通过磷酸酶的水解作用释放出无机磷才能被植物利用。土壤中有机磷的主要形式为磷酸单酯和磷酸双酯。本研究中,我们探讨了无菌条件下不同形态的氮源引起的根际pH变化如何影响植物对这两种有机磷的活化利用。【方法】采用琼脂无菌培养体系种植玉米,向玉米植株供应两种形态的氮源和磷源, 氮源为硝态氮和铵态氮,磷源为植酸钙和卵磷脂,植酸钙属于磷酸单酯盐,卵磷脂属于磷酸双酯盐。不同的供氮形态会导致根际pH变化,进而研究根际pH变化对磷酸单酯盐和磷酸双酯盐的活化利用所产生的影响。【结果】当给玉米供应铵态氮时,根际pH从5.5降至4.0; 供应硝态氮时,根际pH升至6.6。测定玉米根际的琼脂中根系分泌的磷酸单酯酶和磷酸双酯酶活性发现,磷酸单酯酶活性在pH 6.0~7.0之间最高,磷酸双酯酶活性在pH 7.0~8.0之间达到最高。无论以植酸钙还是卵磷脂为有机磷源,相对于铵态氮处理,硝态氮处理能够使根际保持较高的磷酸单酯酶或磷酸双酯酶活性。有机磷的水解过程由磷酸酶活性和底物有效性两个因素控制,而植酸钙的水解受根际pH影响很大,在一定pH范围内,植酸钙的溶解度随根际pH值降低而升高,有效态磷浓度的增加,使得磷酸酶的底物有效性提高。在供应铵态氮时,根际pH值降低,玉米对植酸钙的利用效率高于硝态氮处理,尽管供硝态氮时磷酸单酯酶活性更高。同时,在供应铵态氮条件下,植株对植酸钙的利用率要显著高于卵磷脂,原因在于虽然磷酸双酯酶和磷酸单酯酶活性较低,由于植酸钙的溶解度较大,它的底物有效性更高。因此,植酸钙处理中植株的磷含量更高。相反,在供应硝态氮时,植酸钙溶解度减小而两种磷酸酶活性较高,卵磷脂处理中植株的磷含量更高。【结论】土壤中有机磷的水解过程由磷酸酶活性和有机磷底物有效性两个因素控制,酶活性与根际pH密切相关。本研究说明土壤有机磷的活化必须首先转化为溶解于水溶液中的状态,才能作为磷酸酶的底物被催化水解。我国长期施用化肥导致北方土壤大范围酸化,这种酸化无疑对土壤固有或随有机物料进入农田的有机磷的活化利用是具有重要贡献的,应该在北方土壤养分管理中应加以考虑。