Correlative Aspects of Imposition of Dormancy in Caryopses of Aegilops kotschyi

Caryopses of Aegilops kotschyi Boiss. from different positions within the same spikelet differ greatly in their degree of dormancy. Imposition of this dormancy pattern is a correlative phenomenon within the spikelet, inasmuch as the uppermost developing caryopsis is least dormant and its development is associated with the dormancy status imposed on the lower (one or two) caryopses. Differences in pigmentation of the caryopses follow a corresponding pattern.     

Proton- and sodium-coupled phosphate transport systems and energy status of Yarrowia lipolytica cells grown in acidic and alkaline conditions

In this study we have used a newly isolated Yarrowia lipolytica yeast strain with a unique capacity to grow over a wide pH range (3.5–10.5), which makes it an excellent model system for studying H⁺- and Na⁺-coupled phosphate transport systems. Even at extreme growth conditions (low concentrations of extracellular phosphate, alkaline pH values) Y. lipolytica preserved tightly-coupled mitochondria with the fully competent respiratory chain containing three points of energy conservation. This was demonstrated for the first time for cells grown at pH 9.5–10.0. In cells grown at pH 4.5, inorganic phosphate (P_i) was accumulated by two kinetically discrete H⁺/P_i-cotransport systems. The low-affinity system is most likely constitutively expressed and operates at high P_i concentrations. The high-affinity system, subjected to regulation by both extracellular P_i availability and intracellular polyphosphate stores, is mobilized during P_i-starvation. In cells grown at pH 9.5–10, P_i uptake is mediated by several kinetically discrete Na⁺-dependent systems that are specifically activated by Na⁺ ions and insensitive to the protonophore CCCP. One of these, a low-affinity transporter operative at high P_i concentrations is kinetically characterized here for the first time. The other two, high-affinity, high-capacity systems, are derepressible and functional during P_i-starvation and appear to be controlled by extracellular P_i. They represent the first examples of high-capacity, Na⁺-driven P_i transport systems in an organism belonging to neither the animal nor bacterial kingdoms. The contribution of the H⁺- and Na⁺-coupled P_i transport systems in Y. lipolytica cells grown at different pH values was quantified. In cells grown at pH values of 4.5 and 6.0, the H⁺-coupled P_i transport systems are predominant. The contribution of the Na⁺/P_i cotransport systems to the total cellular P_i uptake activity is progressively increased with increasing pH, reaching its maximum at pH 9 and higher. Received: 15 December 2000/Revised: 14 May 2001     

Roles of Soluble Sugars in Protecting Phytochrome- and Gibberellin A3-Mediated Germination Control in Skotodormant Lettuce Seeds

Skotodormant seeds of Lactuca sativa Grand Rapids imbibed in darkness for 10 days (10-day DS) germinated poorly upon terminal treatment with red light (R) or gibberellin A₃ (GA₃). Soluble sugars in the imbibition solutions influenced the depth of skotodormancy. Ten-day DS seeds, imbibed in 50–500 mm sucrose or 100–500 mm glucose and given terminal GA₃ germinated completely and germinated about 80% when imbibed in 100 mm galactose, mannose, lactose, or maltose. In contrast, terminal R applied to 10-day DS seeds caused only 20–50% germination. If given R at day 0 and imbibed for 10 days in darkness in 500 mm sucrose or glucose, seeds washed free of exogenous glucose or sucrose then germinated about 50% in darkness in water. These seeds responded to terminal R or GA₃ with complete germination. When seeds were given FR at day 0, germination responses following terminal R or GA₃ were significantly lower when the duration of DS was increased from 7–10 day DS to 15 days. In 10-day DS seeds given initial FR and imbibed in either solutions of 50 or 100 mm sucrose and KNO₃, either terminal R or GA₃ treatment gave complete or near complete germination. It is concluded that seed exposure to certain soluble sugars and/or nitrate during a 10-day DS protected certain substrates and thereby extended the sensitivity of the seeds to terminal R or GA₃ treatment. The study provides substantial evidence for nonhormonal factors associated with light and GA action in the control of seed skotodormancy. Received October 30, 1996; accepted April 22, 1997     

Biochemical Changes Associated With Brassica juncea Seed Development. IV. Acid and Alkaline Phosphatases

Changes in acid and alkaline phosphatase activities in cytoplasmic and wall-bound fractions of developing mustard (Brassica juncea) seed were studied. Growth was measured by seed dry weight and water content. Seed dry weight data were fitted to a cubic polynomial equation. Seed water content and dry matter accumulation was significantly correlated. Cytoplasmic acid and alkaline phosphatase activities were substantially less in the cytoplasmic fraction than the wall-bound fraction. Wall-bound acid phosphatase activity was low initially, but high levels were maintained after day 25, indicating a relationship with dry matter accumulation. The results suggest that acid phosphatase plays an important role during mustard seed development. Received February 19, 1998; accepted May 6, 1999     

Chloride Conductance and P i Transport are Separate Functions Induced by the Expression of NaPi-1 in Xenopus Oocytes

Expression of the protein NaPi-1 in Xenopus oocytes has previously been shown to induce an outwardly rectifying Cl⁻ conductance (G_Cl), organic anion transport and Na⁺-dependent P _i -uptake. In the present study we investigated the relation between the NaPi-1 induced G_Cl and P _i -induced currents and transport. NaPi-1 expression induced P _i -transport, which was not different at 1–20 ng/oocyte NaPi-1 cRNA injection and was already maximal at 1–2 days after cRNA injection. In contrast, G_Cl was augmented at increased amounts of cRNA injection (1–20 ng/oocyte) and over a five day expression period. Subsequently all experiments were performed on oocytes injected with 20 ng/oocytes cRNA. P _i -induced currents (Ip) could be observed in NaPi-1 expressing oocytes at high concentrations of P _i (≥ 1 mm P _i ). The amplitudes of Ip correlated well with G_Cl. Ip was blocked by the Cl⁻ channel blocker NPPB, partially Na⁺-dependent and completely abolished in Cl⁻ free solution. In contrast, P _i -transport in NaPi-1 expressing oocytes was not NPPB sensitive, stronger depending on extracellular Na⁺ and weakly affected by Cl⁻ substitution. Endogenous P _i -uptake in water-injected oocytes amounted in all experiments to 30–50% of the Na⁺-dependent P _i -transport observed in NaPi-1 expressing oocytes. The properties of the endogenous P _i -uptake system (K _m for P _i > 1 mm; partial Na⁺- and Cl⁻-dependence; lack of NPPB block) were similar to the NaPi-1 induced P _i -uptake, but no Ip could be recorded at P _i -concentrations ≤3 mm. In summary, the present data suggest that Ip does not reflect charge transfer related to P _i -uptake, but a P _i -mediated modulation of G_Cl. Received: 22 October 1997/Revised: 24 March 1998     

Inheritance of seed phytate and phosphorus levels in common bean (Phaseolus vulgaris L.) and association with newly-mapped candidate genes

Common bean (Phaseolus vulgaris L.) is an important, high-quality staple food that provides large amounts of protein and mineral micronutrients to the diets of people in many countries. Phytates are a storage form of organic phosphorus which is used by the plant in various stages of growth and development but can have certain anti-nutrient properties due to chelation of minerals such as iron and zinc. At the same time, phytates provide certain health benefits and therefore are the subject of both mutagenesis and breeding programs for functional foods. The objective of this study was to evaluate the quantitative trait loci (QTL) associated with seed phytate and seed phosphorus concentration and content on a per-seed basis and to develop functional molecular markers for genes from the phytic acid synthesis pathway. We used a well-characterized mapping population, DOR364?×?G19833, in three field experiments with three repetitions each and two levels of soil phosphorus fertilization, as well as a large set of previously and newly developed primer pairs for the genes myo-inositol (3)P₁ synthase, myo-inositol kinase and various inositol kinases. We identified an association of phytate concentration QTL with one of two paralogs of the myo-inositol (3)P₁ synthase gene family, located on linkage group b01 and expressed in common bean seed rather than in vegetative tissues. We also identified QTL for phytate concentration on linkage group b06 and phytate content on linkage groups b03, b04 and b10. We provide a synteny analysis based on common bean versus soybean genome comparisons of all the phytic acid pathway genes that were genetically mapped and indicate flanking markers that can be used for marker-assisted selection when the genes themselves are not polymorphic as PCR amplicons. We can conclude that natural variability in phytate levels is controlled by the seed-expressed myo-inositol (3)P₁ synthase gene (MIPS) as well as other loci in the common bean genome. This means that breeding of phytate levels in common bean must take into account allele variability at certain candidate genes, such as this seed MIPS gene, a recently cloned ABC trasnporter and additional QTL for the trait, which underlie the oligogenic inheritance for phytate concentration in common bean.     

Development of pepper (Capsicum annuum) seed quality

Changes in seed quality in pepper (Capsicum annuum L.) were monitored during seed development and maturation in two seasons. Seed quality was assessed by a number of different tests, but principally by determining seed storage longevity in laboratory tests and seedling growth in glasshouse tests. Mass maturity (defined as the end of the seed-filling phase) occurred 49–53 days after anthesis (DAA) in 1989 (varying among fruit layers) and 53 DAA in 1990 when seed moisture contents were 51–53%. The onset of both germinability and desiccation tolerance occurred either just before or at mass maturity. Maximum potential longevity (assessed by the value of the seed lot constant K_i) was achieved 63–65 DAA, i.e. not until 10–12 days after mass maturity (DAMM), in both years. Seedling dry weights in the glasshouse growth tests were maximal later still - for seeds harvested 17–21 DAMM in 1989 and 17 DAMM in 1990; the effects on seedling weight arose from differences in times from sowing to emergence (P < 0.005) among different seed harvests, with no significant differences in subsequent relative growth rates (P > 0.25). Seed priming reduced mean germination times for seeds harvested at all stages of development, but had little effect on germination capacity and potential longevity, and did not affect the pattern of changes in potential longevity during seed development and maturation. The results contradict the hypothesis that seed quality is maximal at the end of the seed-filling phase and that viability and vigour begin to decline immediately thereafter.     

The effect of seed drying and gibberellin treatment on the germination performance of developing carrot seed

In a succession of seed harvests of carrot, the highest percentage and most rapid germination was obtained from seed harvested 51 days after anthesis (DAA) when dried by conditioning at 25°C and 60% RH for one week and from seeeds harvested 65 or 79 DAA with or without conditioning treatment. Seed from these harvests had reached maximum weight when dried, had embryos of maximum length and were considered mature. The germinaton of seed from these treatments was unaffected by a mixture of the gibberellins A₄ and A₇ (GA_4/7) applied in the incubation medium. Seed harvested 37 DAA also gave maximum percentage germination when it was both conditioned and incubated in GA_4/7 solution. Seed harvested earlier than this germinated poorly. Germination times of both mature and immature seed were reduced after storage for 18 months but there was no response to GA_4/7.Abbreviations ABA abscisic acid - DAA days after anthesis - GA gibberellin     

PHOSPHORUS AND THE GROWTH OF JUVENILE MACROCYSTIS PYRIFERA (PHAEOPHYTA) SPOROPHYTES1

The effect of phosphate (P_i) supply on growth rate and tissue phosphorus content of juvenile Macrocystis pyrifera (L.) C. Ag. sporophytes was examined. Sporophytes were batch cultured in aquaria with flowing recirculated seawater enriched by 30 μM nitrate. Each aquarium was supplemented with a different seawater P_i concentration, 0, 0.3, 1, 2, 3, and 6 μM. Sporophyte mean specific growth rates declined with time in all cultures presumably due to the normal developmental decrease in the proportion of meristematic tissue of each plant. Growth rate declines were more pronounced in cultures that were nutrient limited. Sporophyte growth was P-limited after two-week exposure to P_i less than 1 μM, corresponding to a tissue P concentration of less than 0.20% dry weight. Plants cultured at 6 μM P_i contained tissue P levels of 0.53% dry weight after three weeks. Luxury consumption and storage of P occurred.     

Utilization of phosphorus by pasture plants supplied with myo-inositol hexaphosphate is enhanced by the presence of soil micro-organisms

A range of pasture grass (Danthonia richardsonii and Phalaris aquatica) and legume (Medicago polymorpha, M. sativa, Trifolium repens and T. subterraneum) species showed limited capacity to obtain phosphorus (P) from inositol hexaphosphate (IHP), when grown in either sterile agar (pH 5.0 or 5.5) or sand-vermiculite media (pH 5.0). The total P content of shoots from IHP-supplied plants grown in agar was between 20% and 34% of that for seedlings supplied with an equivalent amount of P as inorganic phosphate (P_i), while in sand-vermiculite, the total P content of IHP-grown plants was between 5 and 10% of control plants. The poor ability of plants to utilize P from IHP resulted in significantly lower tissue P concentrations and, in general, reduced plant dry weight accumulation. In contrast, the P nutrition of plants supplied with IHP was significantly improved by inoculating media with either a cultured population of total soil micro-organisms or with a specific isolate of Pseudomonas sp., selected for its ability to release phosphate from IHP (strain CCAR59; Richardson and Hadobas, 1997 Can. J. Micro. 43, 509-516). In agar and sand-vermiculite media, respectively, the P content of IHP-grown plants increased with inoculation by up to 3.9- and 6.8-fold, such that the dry weight and P content of the plant material were equivalent to those observed for control plants supplied with P_i. However, the response to inoculation was dependent on the growth medium and the source of micro-organisms used. In sand-vermiculite, the cultured population of soil micro-organisms was effective when IHP was supplied at an equivalent level of P_i required for maximum plant growth. By comparison, inoculation of plants with the Pseudomonas strain was only effective at very high levels of IHP supply (×36), whereas in agar a response to inoculation occurred at all levels of IHP. The ability of pasture plants to acquire P from phytate was, therefore, influenced by the availability of IHP substrate, which was further affected by the presence of soil micro-organisms. Our results show that in addition to having an effect on the sorption characteristics of the growth media, soil micro-organisms also provided a source of phytase for the dephosphorylation of phytate for subsequent utilization of P_i by plants.     

Seed germination studies with Heteropogon contortus

Germination characteristics of Heteropogon contortus were investigated in germination cabinets and in the glasshouse using soil trays. Under the former a dual inhibitory system appeared to operate. On the one hand naked caryopses from seeds which were less than 4 months of age were inhibited from germination by a process apparently analogous to epicotyl dormancy. This could readily be broken by applying 1% gibberellic acid and partly broken by repeated wetting and drying. On the other hand between 4 and 12 months, naked caryopses germinated more readily than intact seeds (caryopsis plus glumes, lemmas, etc.). However, the dormancy factor apparently associated with these structures could not be detected when seed was germinated in soil in the glasshouse and it appeared that they are a necessary protection for the caryopsis under field conditions. The optimal temperature for germination was between 30 and 35°C constant. Alternating diurnal temperatures of less than this appeared to cause reduced germination, and no high-temperature stimulation was observed. Germination was not directly promoted by the passage of a normal grass fire.     

Identification of a cDNA/Protein Leading to an Increased P i -uptake in Xenopus laevis Oocytes

In a previous report we documented an increased Na⁺-dependent transport of inorganic phosphate (P _i ) in Xenopus laevis oocytes injected with mRNA isolated from rabbit duodenum (Yagci et al., Pfluegers Arch. 422:211–216, 1992; ref 24). In the present study we have used expression cloning in oocytes to search for the cDNA/mRNA involved in this effect. The identified cDNA (provisionally named PiUS; for P _i -uptake stimulator) lead to a 3-4-fold stimulation of Na⁺-dependent P _i -uptake (10ng cRNA injected, 3–5 days of expression). Na⁺-independent uptake of P _i was also affected but transport of sulphate and l-arginine (in the presence or absence of sodium) remained unchanged. The apparent K _m -values for the induced Na⁺-dependent uptake were 0.26 ± 0.04 mm for P _i and 14.8 ± 3.0 mm for Na⁺. The 1796 bp cDNA codes for a protein of 425 amino acids. Hydropathy analysis suggests a lack of transmembrane segments. In vitro translation resulted in a protein of 60 kDa and provided no evidence of glycosylation. In Northern blots a mRNA of ∼2 kb was recognized in various tissues including different intestinal segments, kidney cortex, kidney medulla, liver and heart. Homology searches showed no similarity to proteins involved in membrane transport and its control. In conclusion, we have cloned from a rabbit small intestinal cDNA library a novel cDNA encoding a protein stimulating P _i -uptake into Xenopus laevis oocytes, but which is not a P _i -transporter itself. Received: 31 July 1996/Revised: 16 October 1996     

野生二棱大麦种子休眠型态与农艺性状及生态地理因素相关性研究

对来自以色列不同地区16个生态型野生二棱大麦种子的休眠型态与其农艺性状及起源地生态地里因素的相关性进行了研究。结果表明：高温（40℃）储藏可以打破种子的休眠;16个生态型种子在高温处理下的萌发率表现出显著差异,其休眠打破过程显示出不同型态的对数生长曲线：8个旱生生态型为S型,而8个湿生生态型为倒L型。休眠深度用实际达到最大萌发率的时间度量,最低休眠深度（15·6d）是来自湿润地区＂进化峡谷＂的生态型37-N,而最深休眠深度（103·1d）是来自干旱地区Ein-Zukim（死海附近）的生态型32-6。此外,对11个物候及农艺性状指标与休眠深度做斯皮尔曼秩相关分析,结果有9个显示出显著相关,尤其是粒重与休眠深度有极显著相关性。同时,休眠深度与起源地15个生态地理因素中的9个有显著相关,种子休眠主要受其起源地的地理位置、温度和水分条件等影响。可见,野生二棱大麦自然选择进化了休眠特性去应对干热环境而繁衍生息。本研究结果可用于进一步遗传研究和现代栽培大麦品种的改良。     

Relationship of endogenous abscisic Acid to sucrose level and seed growth rate of soybeans

It has been proposed that abscisic acid (ABA) may stimulate sucrose transport into filling seeds of legumes, potentially regulating seed growth rate. The objective of this study was to determine whether the rate of dry matter accumulation in seeds of soybeans (Glycine max L.) is correlated with the endogenous levels of ABA and sucrose in those sinks. The levels of ABA and sucrose in seed tissues were compared in nine diverse Plant Introduction lines having seed growth rates ranging from 2.5 to 10.0 milligrams dry weight per seed per day. At 14 days after anthesis (DAA), seeds of all genotypes contained less than 2 micrograms of ABA per gram fresh weight. Levels of ABA increased rapidly, however, reaching maxima at 20 to 30 DAA, depending upon tissue type and genotype. ABA accumulated first in seed coats and then in embryos, and ABA maxima were higher in seed coats (8 to 20 micrograms per gram fresh weight) than in embryos (4 to 9 micrograms per gram fresh weight. From 30 to 50 DAA, ABA levels in both tissues decreased to less than 2 micrograms per gram fresh weight. Levels of sucrose were also low early in development, less than 10 milligrams per gram fresh weight at 14 DAA. However, by 30 DAA, sucrose levels in seed coats had increased to 20 milligrams per gram fresh weight and remained fairly constant for the remainder of the filling period. In contrast, sucrose accumulated in embryos throughout the filling period, reaching levels greater than 40 milligrams per gram fresh weight by 50 DAA. Correlation analyses indicated that the level of ABA in seed coats and embryos was not directly correlated to the level of sucrose measured in those tissues or to the rate of seed dry matter accumulation during the linear filling period. Rather, the ubiquitous pattern of ABA accumulation early in development appeared to coincide with water uptake and the rapid expansion of cotyledons occurring at that time. Whole tissue sucrose levels in embryos and seed coats, as well as sucrose levels in the embryo apoplast, were generally not correlated with the rate of dry matter accumulation. Thus, it appears that, in this set of diverse soybean genotypes, seed growth rate was not limited by endogenous concentrations of ABA or sucrose in reproductive tissues.     

Caryopsis germination and seedling emergence in an inland dune dominant grass Leymus secalinus

Leymus secalinus (Georg.) Tzvel. (Poaceae) is a dominant sand dune grass inhabiting the Mu-Us Sandland, semiarid China. Freshly harvested caryopses (seeds) are in non-deep physiological dormancy (non-deep PD) because of low percentage and slow rate of germination. Experiments were conducted to examine the effects of temperature, cold stratification, caryopsis coat scarification or partial removal of endosperm and sand burial on caryopsis dormancy, germination and seedling emergence. Caryopsis germination was significantly influenced by duration of cold stratification, temperature and their interactions. After 8 weeks of cold stratification, caryopsis germination percentage at 30 °C reached to 90%, equally in light or darkness. Rate and percentages of germination were also hastened and increased by scarifying the caryopsis coat or by artificial removal of different proportions of the endosperm. However, seedling developmental characteristics were significantly influenced by the proportion of the endosperm that remained in the caryopses. Seedling emergence, caryopsis germination and enforced dormancy in sand were significantly affected by sand burial depth. As sand burial depth increased, caryopsis germination and seedling emergence decreased whereas caryopsis enforced dormancy increased. 1–2 cm was the optimal depths for caryopses germination and seedling emergence. Although there were still 30% caryopses germinated at 8 cm, the maximal burial depth for seedling emergence was only 4 cm. The partial germination strategy regulated by non-deep PD, temperature and sand burial ensures that only a few caryopses germinated each time and may reduce risks for seedling survival.     

Assimilation of Phytate-phosphorus by the Extracellular Phytase Activity of Tobacco (Nicotiana tabacum) is Affected by the Availability of Soluble Phytate

Phytate, the major organic phosphorus in soil, is not readily available to plants as a source of phosphorus (P). It is either complexed with cations or adsorbed to various soil components. The present study was carried out to investigate the extracellular phytase activities of tobacco (Nicotiana tabacum variety GeXin No.1) and its ability to assimilate external phytate-P. Whereas phytase activities in roots, shoots and growth media of P_i-fed 14-day-old seedlings were only 1.3–4.9% of total acid phosphatase (APase) activities, P starvation triggered an increase in phytase secretion up to 914.9 mU mg⁻¹ protein, equivalent to 18.2% of total APase activities. Much of the extracellular phytase activities were found to be root-associated than root-released. The plants were not able to utilize phytate adsorbed to sand, except when insoluble phytate salts were preformed with Mg²⁺ and Ca²⁺ ions for supplementation. Tobacco grew better in sand supplemented with Mg-phytate salts (31.9 mg dry weight plant⁻¹; 0.68% w/w P concentration) than that with Ca-phytate salts (9.5 mg plant⁻¹; 0.42%), presumably due to its higher solubility. We conclude that insolubility of soil phytate is the major constrain for its assimilation. Improving solubility of soil phytate, for example, by enhancement of citrate secretion, may be a feasible approach to improve soil phytate assimilation.     

Control of seed dormancy in Nicotiana plumbaginifolia: post-imbibition abscisic acid synthesis imposes dormancy maintenance

The physiological characteristics of seed dormancy in Nicotiana plumbaginifolia Viv. are described. The level of seed dormancy is defined by the delay in seed germination (i.e the time required prior to germination) under favourable environmental conditions. A wild-type line shows a clear primary dormancy, which is suppressed by afterripening, whereas an abscisic acid (ABA)-deficient mutant shows a non-dormant phenotype. We have investigated the role of ABA and gibberellic acid (GA₃) in the control of dormancy maintenance or breakage during imbibition in suitable conditions. It was found that fluridone, a carotenoid biosynthesis inhibitor, is almost as efficient as GA₃ in breaking dormancy. Dry dormant seeds contained more ABA than dry afterripened seeds and, during early imbibition, there was an accumulation of ABA in dormant seeds, but not in afterripened seeds. In addition, fluridone and exogenous GA₃ inhibited the accumulation of ABA in imbibed dormant seeds. This reveals an important role for ABA synthesis in dormancy maintenance in imbibed seeds. Received: 31 December 1998 / Accepted: 9 July 1999     

Electrophysiological Characterization of the Flounder Type II Na+/P i Cotransporter (NaPi-5) Expressed in Xenopus laevis Oocytes

The two electrode voltage clamp technique was used to investigate the steady-state and presteady-state kinetic properties of the type II Na⁺/P _i cotransporter NaPi-5, cloned from the kidney of winter flounder (Pseudopleuronectes americanus) and expressed in Xenopus laevis oocytes. Steady-state P _i -induced currents had a voltage-independent apparent K _m for P _i of 0.03 mm and a Hill coefficient of 1.0 at neutral pH, when superfusing with 96 mm Na⁺. The apparent K _m for Na⁺ at 1 mm P _i was strongly voltage dependent (increasing from 32 mm at −70 mV to 77 mm at −30 mV) and the Hill coefficient was between 1 and 2, indicating cooperative binding of more than one Na⁺ ion. The maximum steady-state current was pH dependent, diminishing by 50% or more for a change from pH 7.8 to pH 6.3. Voltage jumps elicited presteady-state relaxations in the presence of 96 mm Na⁺ which were suppressed at saturating P _i (1 mm). Relaxations were absent in non-injected oocytes. Charge was balanced for equal positive and negative steps, saturated at extremes of potential and reversed at the holding potential. Fitting the charge transfer to a Boltzmann relationship typically gave a midpoint voltage (V _0.5) close to zero and an apparent valency of approximately 0.6. The maximum steady-state transport rate correlated linearly with the maximum P _i -suppressed charge movement, indicating that the relaxations were NaPi-5-specific. The apparent transporter turnover was estimated as 35 sec⁻¹. The voltage dependence of the relaxations was P _i -independent, whereas changes in Na⁺ shifted V _0.5 to −60 mV at 25 mm Na⁺. Protons suppressed relaxations but contributed to no detectable charge movement in zero external Na⁺. The voltage dependent presteady-state behavior of NaPi-5 could be described by a 3 state model in which the partial reactions involving reorientation of the unloaded carrier and binding of Na⁺ contribute to transmembrane charge movement. Received: 11 March 1997/Revised: 3 June 1997     

Seed Dormancy,Seedling Establishment and Dynamics of the Soil Seed Bank of Stipa bungeana (Poaceae) on the Loess Plateau of Northwestern China

Studying seed dormancy and its consequent effect can provide important information for vegetation restoration and management. The present study investigated seed dormancy, seedling emergence and seed survival in the soil seed bank of Stipa bungeana, a grass species used in restoration of degraded land on the Loess Plateau in northwest China. Dormancy of fresh seeds was determined by incubation of seeds over a range of temperatures in both light and dark. Seed germination was evaluated after mechanical removal of palea and lemma (hulls), chemical scarification and dry storage. Fresh and one-year-stored seeds were sown in the field, and seedling emergence was monitored weekly for 8 weeks. Furthermore, seeds were buried at different soil depths, and then retrieved every 1 or 2 months to determine seed dormancy and seed viability in the laboratory. Fresh seeds (caryopses enclosed by palea and lemma) had non-deep physiological dormancy. Removal of palea and lemma, chemical scarification, dry storage (afterripening), gibberellin (GA₃) and potassium nitrate (KNO₃) significantly improved germination. Dormancy was completely released by removal of the hulls, but seeds on which hulls were put back to their original position germinated to only 46%. Pretreatment of seeds with a 30% NaOH solution for 60 min increased germination from 25% to 82%. Speed of seedling emergence from fresh seeds was significantly lower than that of seeds stored for 1 year. However, final percentage of seedling emergence did not differ significantly for seeds sown at depths of 0 and 1 cm. Most fresh seeds of S. bungeana buried in the field in early July either had germinated or lost viability by September. All seeds buried at a depth of 5 cm had lost viability after 5 months, whereas 12% and 4% seeds of those sown on the soil surface were viable after 5 and 12 months, respectively.     

Partial purification and characterization of phytases from pollen of lily (Lilium longiflorum Thunb.)

Summary Two phytases from lily pollen (Lilium longiflorum Thunb.) were partially purified and characterized. The first (pH optimum 5.0) was purified 40-fold from ungerminated pollen. The second (pH optimum 6.5) appeared during germination and was purified 68-fold from pollen germinated 2 h. Molecular weight of the first was 72 kD, and the second was 36 kD as determined by gel filtration. Both were active against phosphate esters other than phytate, although purification of the first reduced its activity against AMP and myo-inositol 2-P to 10% of activity against phytate. Phytase from germinated pollen (but not ungerminated) was inhibited by the sulfhydryl agent parahydroxy mercuribenzoate; P _i inhibited phytase from ungerminated but not germinated pollen. Such different catalytic and physical properties may reflect different biochemical functions.Abbreviations HPLC High performance liquid chromatography - DEAE diethyl aminoethyl - P _i orthophosphate - PP _i pyrophosphate - p-NPP para-nitrophenyl phosphate - pNP para-nitrophenol - MI myo-inositol - MI 2-P myo-inositol 2-P - MI penta P myo-inositol pentakisphosphate - PHMB para-hydroxy mercuribenzoate - PMSF phenyl methyl sulfonyl fluoride - AMP adenosine monophosphate - GMP guanosine monophosphate - EGTA ethylene glycol-bis (-aminoethyl ether) N, N, N, N-tetraacetic acid