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Analyses of chlorophyll fluorescence were undertaken to investigate the alterations in photosystem II (PSII) function during senescence of wheat (Triticum aestivum L. cv. Shannong 229) leaves. Senescence resulted in a decrease in the apparent quantum yield of photosynthesis and the maximal CO2 assimilation capacity. Analyses of fluorescence quenching under steady-state photosynthesis showed that senescence also resulted in a significant decrease in the efficiency of excitation energy capture by open PSII reaction centers (F'v/F'm) but only a slight decrease in the maximum efficiency of PSII photochemistry (F'v/F'm). At the same time, a significant increase in non-photochemical quenching (qN) and a considerable decrease in photochemical quenching (qP) were observed in senescing leaves. Rapid fluorescence induction kinetics indicated a decrease in the rate of QA reduction and an increase in the proportion of QB-non-reducing PSII reaction during senescence. The decrease in both F'v/F'm and qP explained the decrease in the actual quantum yield of PSII electron transport ((φPSII). We suggest that the modifications in PSII function, which led to the down-regulation of photosynthetic electron transport, would be in concert with the lower demand for ATP and NADPH in the Calvin cycle which is often inhibited in senescing leaves. [ABSTRACT FROM AUTHOR]

The intracellular distribution of serine hydroxymethyltransferase (EC 2.1.2.1) was studied in young wheat (Triticum aestivum L. cv. Starke II) leaves by fractionation of protoplasts and further purification of peroxisomes and chloroplasts. Essentially all of the activity in wheat leaves was located in the mitochondria. Within the mitochondria the enzyme was mainly in the matrix as shown by centrifugation of sonicated wheat mitochondria. In the C4 plants, Zea mays (L. cv. Earliking), Panicum miliaceum and Panicum maximum (cv. Australia) belonging to different C4 types, serine hydroxymethyltransferase was almost exclusively found in bundle sheath cells. The location of this enzyme in leaves is consistent with its role relative to glycine decarboxylation during photorespiration. [ABSTRACT FROM AUTHOR]

Ears of wheat plants (Triticum aestivum L. cv. Kolibri), which were given different and uniform K+-nutrition in two experiments, were cut at 2, 4 and 6 weeks after anthesis at 15 cm below the ear. These detached ears were fed 30 mM (experiment 1) or 15, 30, 60 or 90 mM 86Rb-K2malate (experiment 2) and 146 mM [14C]-sucrose. After a pulse period of 6 and 4 h, respectively, the ears were transferred to identical non-labeled solutions for additional 0, 4, 8 or 20 h. About 50% of the K+ and sucrose supplied was absorbed by detached ears. This rate declined with plant age and decreasing transpiration. Within the 6 and 4 h uptake period less than 7% of the absorbed K+ of the absorbed K+, but 20% of the sucrose taken up were incorporated into the grain. During the chase period labeled K+ in the grain increased to 15% and 14C even to 50% of total tracer uptake. Incorporation of labeled K+ into the grain was not affected by the previous K+ nutrition of the plant and was proportional to the K+ concentration in the uptake solution. Transition of K+ from xylem into phloem during its acropetal transport is assumed. No evidence was found that the grain itself could control its uptake of K+. [ABSTRACT FROM AUTHOR]

The activities of several enzymes related to amino acid metabolism were investigated in senescing detached wheat leaves (Triticum aestivum L. cv. Diplomat) in light and darkness and after kinetin treatment. Glutamine synthetase and glutamate synthase activities rapidly declined in darkness. In light, the decline of glutamate synthase activity was retarded, while the activity of glutamine synthetase remained high and even increased transitorily. Kinetin treatment counteracted the decline of the activities of both enzymes. The activity of glutamate dehydrogenase markedly increased during senescence, particularly in light, and kinetin treatment lowered its activity. The activities of glutamate-oxaloacetate and glutamate-pyruvate amino- transferases and of NADP-dependent isocitrate dehydrogenase also increased in detached wheat leaves in light. Kinetin treatment prevented the rise of these enzyme activities. In darkness, the activities of glutamate-oxaloacetate aminotransferase and NADP-dependent isocitrate dehydrogenase decreased slowly while the decline of glutamate-pyruvate aminotransferase activity was more rapid. The activity of NAD-dependent malate dehydrogenase decreased both in light and, more rapidly, in darkness. The pattern of changes of the enzyme activities provides an explanation for the amino acid transformations and the flow of amino nitrogen into transport metabolites in senescing leaves. [ABSTRACT FROM AUTHOR]

Diurnal variations of in vitro and in vivo (intact tissue assay) nitrate reductase (EC 1.6.6. 1) activity and stability were examined in leaves of wheat (Triticum aestivum L. cv. Runar), oat (Avena sativa L. cv. Mustang) and barley (Hordeum vulgare L. cv. Agneta and cv. Gunilla). Nitrate reductase activity was generally higher for wheat than for oat and barley. However, the diurnal variations of nitrate reductase activity and stability were principally the same for all species, e.g. the high activity during the photoperiod was associated with low stability. All species showed a rapid (30-60 min) increase in the in vitro and in vivo activity when the light was switched on. When light was switched off the in vitro activity decreased rapidly whereas decrease in in vivo activity was slower. These experiments support the hypothesis that an activation/deactivation mechanism is involved in the regulation of diurnal variations in nitrate reductase activity. Red light enhanced nitrate reductase activity in etiolated wheat and barley leaves. In green leaves, however, the daily increase in nitrate reductase activity was not induced by a brief red light treatment. Indications of different regulation mechanisms for the diurnal variations of nitrate reductase activity among the cereals were not found. [ABSTRACT FROM AUTHOR]

Seedlings of winter wheat (Triticum aestivum L. cv. Hildur) were grown at 18°C for 7 days in darkness in a complete growth medium in the presence or absence of 1 mM KCl to produce roots with different ion contents (high and low K+, respectively). The roots were homogenized, the 3 000 g, 10 000 g, 30 000 g (further fractionated by two-phase partitioning) and 100 000 g pellets isolated, and their surface charge densities (σ) determined by the use of 9-aminoacridine fluorescence. The average σ for all membrane fractions weighted for protein content was the same (-18 mC m-2) for low and high K+ roots. The K+, Na+, Mg2+ and Ca2+ content of roots was determined and used to calculate an average σ following the procedure of Bérczi et al. [Physiol. Plant. 61: 529-534 (1984)]. The predicted value (-11 mC m-2) does not deviate much from the experimentally determined value. It is concluded as a useful working hypothesis that the average surface charge density is constant and that the ionic content of plant cells is regulated such that the average surface potential is constant. [ABSTRACT FROM AUTHOR]

High temperature stress adversely affects wheat growth in many important production regions, but the mode of injury is unclear. Wheat (Triticum aestivum L. cv. Newton) was grown under controlled conditions to determine the relative magnitude and sequences of responses of source and sink processes to high temperature stress during grain development. Regimes of 25°C day/15°C night, 30°C day/20°C night, and 30°C day/25°C night from 5 days after anthesis to maturity differentially affected source and sink processes. High temperatures accelerated the normal decline in viable leaf blade area and photosynthetic activities per unit leaf area. Electron transport, as measured by Hill reaction activity, declined earlier and faster than other photosynthetic processes at the optimum temperature of 25/15°C and at elevated temperatures. Changes in RUBP carboxylase activities were similar in direction but smaller in magnitude than changes in photosynthesic rate. Increased protease activity during senescence was markedly accentuated by high temperature stress. Specific protease activity increased 4-fold at 25/15°C and 28-fold at 35/25°C from 0 to 21 days after initiation of temperature treatments. Grain-filling rate decreased from the lowest to the highest temperature, but the change was smaller than the decrease in grain-filling duration at the same temperatures. We concluded that a major effect of high temperature is acceleration of senescence, including cessation of vegetative and reproductive growth, deterioration of photosynthetic activities, and degradation of proteinaceous constituents. [ABSTRACT FROM AUTHOR]

Three complexes between phenols and amines (phenolamides) have been found in Cichorium intybus L., a plant with an absolute requirement of vernalisation followed by long days for flowering. Upon hydrolysis, these complexes (A, B and C) liberate aromatic amines whose exact identification is in progress, but which are closely related to dopamine, tyramine and serotonin, respectively. In a first series of experiments, phenolamides were studied in the buds of plants grown in the greenhouse under varying conditions. Only buds from plants which flower in long days contained large amounts of these compounds. Much smaller amounts were found in buds at the end of vernalisation (at 2-4°C) before long-day treatment as well as in buds kept in the vegetative state after vernalisation by being grown in short days (8 h light) or in total darkness. In a second series of experiments, phenolamides were studied in bud-forming calli induced in vitro on explains of tuberised root. After sixteen days of culture in continuous light, large quantities of phenolamide were found in the buds and calli of the upper part of the explant, whale the lower part which never produces buds contained much less. Buds formed under continuous light produce inflorescences m approximate]y one month. Various other culture conditions make it possible to maintain the explants in the vegetative state. This can be obtained by short-day conditions, or otherwise under continuous illumination by decreasing the sugar or increasing the NAA levels in the medium. After 13 days of culture, the phenolamide levels were much lower under all of these conditions, than under conditions favourable to floral induction. Compound C is absent or present in trace amounts in vegetative buds. The significance of the differences observed between floral and vegetative buds is supported by the sensitivity of the analytical techniques used. The accumulation of phenolamides in tissues of Cichorium intybus appears to be closely linked to floral induction. Under continuous light it begins very early in young buds and even in the calli that bear these buds. [ABSTRACT FROM AUTHOR]

Two analytical methods for measuring betaine were compared in a study of betaine accumulation in salt-stressed sorghum. Spectrophotometric determination of betaine as the p-bromophenacyl ester is highly sensitive and specific. However, a periodide assay was found to be more convenient for screening numerous plant samples without undue sacrifice in accuracy. The accumulation of betaine in grain sorghum [Sorghum bicolor (L.) Moench] cvs NK 265 and Double TX was measured in salt-stressed plants grown hydroponically and in the field and in drought-stressed potted plants. Neither drought nor mild salinity (-0.2 MPa) stress was effective in stimulating betaine accumulation. However, when the osmotic potential of the culture solution was lowered to -0.8 MPa, betaine levels in the shoots rose rapidly for 12 days after initiation of salination, and then declined, apparently because of dilution by plant growth. In young leaf blades, betaine was strongly accumulated up to 70-75 μmol (g dry weight)-1; the concentration in leaf sheaths was less than 6 μmol (g dry weight)-1. In the field, betaine levels in salt-stressed sorghum increased 6- to 7-fold over the basal level of the control plants. In a comparable study of two wheat species (Triticum aestivum L. cv. Probred and T. durum Desf. cv. 1000-D), betaine increased only 3- to 4-fold over unstressed plants. [ABSTRACT FROM AUTHOR]

Leaf cell protoplasts were isolated from wheat seedlings (Triticum aestivum L. cv. Urquie) after orthophosphate (Pi) treatment of the plant to determine the capacity for intracellular phosphate accumulation. Seedlings were treated with Pi concentrations near the phytotoxic level to maximize the Pi concentration in the leaf prior to protoplast isolation 1 day later. Both foliar and root treatment of seedlings with Pi increased the phosphate content of leaf protoplasts by approximately 20 μmol (mg chlorophyll)-1 over Pi levels in untreated controls. Phosphate-loaded protoplasts from treated seedlings had similar photosynthetic rates and starch content but 50% more soluble reducing sugar than protoplasts from untreated seedlings. Protoplast dark respiration decreased after treatments which increased protoplast potassium content. The results suggest that similar amounts of Pi can be accumulated by leaf cells of wheat after foliar or root application of Pi to the seedling without hindering Pi-sensitive processes such as photosynthesis and starch synthesis. [ABSTRACT FROM AUTHOR]

Seedlings of spring wheat (Triticum aestivum L. cv, Svenno) were cultivated at 20°C in continuous light or darkness with the roots in nutrient solutions for six days. The plants were starved for K+ during different periods of time to produce plants with various K+ status. In one cultivation light-grown plants were pretreated in darkness, and vice versa, before the uptake experiment. In all experiments, roots were put in a complete nutrient medium containing 2.0 mM K+ radiolabelled with 86Rb. The uptake time was varied (5, 60 or 120 min). The K+ concentration in the roots, [K+]root, increased during the course of the uptake experiments, especially in light and at initially low [K+]root. At the same time K+ (86Rb) influx in the roots decreased. The sigmoidal relationship obtained between K+ (86Rb) influx and [K+]root was affected by these changes, and Hill plots gave various Hill coefficients, nH, depending on the duration of the uptake experiments. nH from three apparently straight line segments of the same plot, in different [K+]root-intervals, indicated a failing degree of interaction between the binding sites as [K+]root increased For the dark-grown plants negative cooperativity could not be demonstrated. [ABSTRACT FROM AUTHOR]

Potassium uptake and release in roots and translocation to the shoots were studied in 14-day-old winter wheat (Triticum aestivum L. cv. Martonvásári 8 ) of different K status. Transport processes were measured in the growth solution for 5 h ensuring near-equilibrium conditions. The uptake showed three phases: (1) at low external K+ concentration it increased with increasing concentration and culminated at 0.1 mM;(2) between 0.1 and 1 mM it decreased and (3) it increased again above 1 mM. The release of K+ showed a constant low level below 1 mM while paralleling the uptake above that. The uncoupler 2,4-dinitrophenol inhibited uptake phases (1) and (2) , whereas it did not affect either phase (3) or K+ release. Translocation showed similar patterns. It is concluded that phases (1) and 2 depend on metabolic energy while phase (3) is mostly passive. It is emphasized that different types of regulation seem to operate in the transport mechanism: i.e. limitation by transport sites control by negative feedback and by K+/K + exchange, respectively. [ABSTRACT FROM AUTHOR]

The usefulness of the nitrate‐free in vivo nitrate reductase assay for the study of nitrate pools in wheat leaves was investigated. Leaf sections from 7‐day‐old wheat seedlings, exposed 24 h before harvest to 1.5, 3.0 or 5.0 mM KNO3 were used. After 2 to 4 h of incubation nitrite production ceased, reaching a plateau. The time required to reach the plateau and the level of the plateau increased with increasing endogenous nitrate content. At nitrite plateau the amount of nitrate left in the tissue was independent of the original nitrate content in the tissue. Addition of nitrate at plateau caused resumed nitrite production. It is concluded that nitrate was the limiting factor in nitrite production. Oxygen inhibited nitrate reduction and stimulated further assimilation of nitrite. A considerable initial leakage of nitrate from tissue to the assay medium, followed by a slower continuous leakage, was observed throughout the incubation. N2‐flushing or inclusion of Triton X‐100 in the assay medium increased nitrite production by making more nitrate available for reduction. These treatments also increased the leakage of nitrate. At plateau levels the amount of nitrate left in the tissue was dependent on the oxygen tension in the assay medium. Under low oxygen tension nearly all nitrate in the tissue was available for reduction. Nitrite production at plateau is not a useful index for a metabolic nitrate pool and nitrate left in the tissue is not a useful index for a nitrate storage pool because both parameters are highly dependent on the oxygen tension in the assay medium. Further, in view of the considerable leakage, the nitrate‐free in vivo nitrate reductase assay cannot be used to detect two separate nitrate pools in wheat leaves. [ABSTRACT FROM AUTHOR]

When wheat aleurone layers (Triticum aestivum L. var. Potam S‐70) incubated in medium containing gibberellic acid were exposed to ethylene, the synthesis and release of amylase were enhanced relative to layers incubated in the presence of mercuric perchlorate. Exogenous ethylene stimulated gibberellic acid‐induced amylase synthesis by approximately 2.2‐fold. The ethylene‐mediated stimulation of amylase formation was dependent upon the tissue being exposed to the gas during the lag phase of gibberellic acid action. Ethylene appeared to promote only quantitative changes in amylase synthesis and release, since the isoelectric focusing patterns of amylase is enzymes were not significantly altered by ethylene. Ethylene had no effect on the incorporation of [methyl‐14C]choline into aleurone phospholipids, but stimulated the accumulation of [U‐14C]adenine into poly(A) RNA of gibberellic acid‐treated tissue by about 80%. [ABSTRACT FROM AUTHOR]

Fluxes of Ca2+ (45Ca) and K+ (86Rb) were investigated in 12-day old intact plants of cucumber (Cucumis sativus L. var. Cilla) and 7-day old intact plants of wheat (Triticum aesrivum L. var. Svenno) which had been cultivated in nutrient solution. The rate of K+ uptake was affected by the external Ca2+ concentration, Which reduced the uptake of K+ when present above and below a certain level. Pretreatment of roots in high [Ca2+] during a 24 h period decreased the rate of both K+ and Ca2+ uptake. Reduced availability of Ca2+ in the nutrient solution during 24 h reduced the uptake of K+ and Ca2+. Separation of the Ca2+ and K+ retained in the roots, from the amounts transported to the shoots in plants pretreated with low [Ca2+], :showed that the influx of K+ retained in the roots decreased while Ca2+ influx increased. The transport of both ions towards the shoot was, however, reduced. Reduction of available Ca2+ in the nutrient solution was suggested to change the composition of the membranes, maintaining their permeability properties and metabolically regulated functions. Leakage of K+ (86Rb) from the vacuolar and cytoplasmic compartment of wheat roots increased at reduced Ca2+ concentration in the nutrient solution. [ABSTRACT FROM AUTHOR]

Effects of interrupted K+ supply on different parameters of growth and mineral cation nutrition were evaluated for spring wheat (Triticum aestivum L. cv. Svenno). K+ (2.0 mM) was supplied to the plants during different periods in an otherwise complete nutrient solution. Shoot growth was reduced before root growth after interruption in K+ supply. Root structure was greatly affected by the length of the period in K+ ‐free nutrient solution. Root length was minimal, and root branching was maximal within a narrow range of K+ status of the roots. This range corresponded to cultivation for the last 1 to 3 days, of 11 in total, in K+ ‐free nutrient solution, or to continuous cultivation in solution containing 0.5 to 2 mM K+. In comparison, both higher and lower internal/external K+ concentrations had inhibitory effects on root branching. However, the differing root morphology probably had no significant influence on the magnitude of Ca2+, Mg2+ and Na+ uptake. Uptake of Ca2+ and especially Mg2+ significantly increased after K+ interruption, while Na+ uptake was constant in the roots and slowly increased in the shoots. The two divalent cations could replace K+ in the cells and maintain electroneutrality down to a certain minimal range of K+ concentrations. This range was significantly higher in the shoot [110 to 140 μmol (g fresh weight)−1] than in the root [20 to 30 μmol (g fresh weight)−1]. It is suggested that the critical K+ values are a measure of the minimal amount of K+ that must be present for physiological activity in the cells. At the critical levels, K+ (86Rb) influx and Ca2+ and Mg2+ concentrations were maximal. Below the critical K+ values, growth was reduced, and Ca2+ and Mg2+ could no longer substitute for K+ for electrostatic balance. In a short‐term experiment, the ability of Ca2+ to compete with K+ in maintaining electroneutrality in the cells was studied in wheat seedlings with different K+ status. The results indicate that K+, which was taken up actively and fastest at the external K+ concentration used (2.0 mM), partly determines the size of Ca2+ influx. [ABSTRACT FROM AUTHOR]

Protein kinase activity was associated with chromatin in wheat (Triticum aestivum L. cv. Mukakomugi) embryos. The kinase activity did not change significantly during germination, whereas the activity of poly (ADP‐ribose) synthetase decreased significantly. The protein kinase activity in chromatin was inhibited by NAD, NADH, and ADP‐ribose, and was enhanced by treatment of the chromatin with snake venom phosphodiesterase or soybean trypsin inhibitor. The activity in chromatin was not stimulated by cyclic AMP. Different subfractions of the histones, H1 and H2, were mainly phosphorylated in germ and 3 day‐germinated seedling chromatins. The histones, H3 and H4, seemed unable to accept phosphate from ATP in the in vitro reaction system. Different acidic non‐histone chromosomal proteins were phosphorylated in germ and 3‐day‐germinated seedling chromations, and germ‐specific and seedling‐specific acidic non‐histone chromosomal proteins seemed unable to accept phosphate from ATP. [ABSTRACT FROM AUTHOR]

The dynamics of stomatal resistance and osmotic adjustment in response to plant water deficits and stage of physiological development was studied in the leaves of spring wheat (Triticum aestivum L., GWO 1809). Plants were germinated and grown in pots in a growth chamber at the Duke University Phytotron to four physiological stages of development (4th leaf, 7th leaf, anthesis, and soft dough), during which time stomatal resistance, total water potential and osmotic potential were measured on the last fully developed leaf of water stressed and non-stressed plants. Pressure potential was obtained by difference. Stomatal closure of the abaxial and adaxial surfaces were independent of each other, each having a different critical total water potential. The total water potential required to close the stomata on the last fully developed leaf were different at different stages of physiological development, decreasing as the plants grew older. The development of osmoregulation in wheat allows the closure of stomata during the vegetative stage at a high total water potential, but insures that stomata remain open from anthesis through the ear filling period to a lower total water potential. [ABSTRACT FROM AUTHOR]

Uptake of Ca2+(45Ca) was investigated in plants of wheat (Triticum aestivum L. var. Svenno) and cucumber (Cucumis sativus L. var. Cilla) cultivated in a nutrient solution with various Ca2+ concentrations. The adsorption of Ca2+ was higher in cucumber roots than in wheat roots especially at lower Ca2+ levels in the external medium. The intracellular fraction of Ca2+ was less than 20% of the total Ca2+ in wheat roots and less than 10% of the total Ca2+ in cucumber roots. The uptake of Ca2+ in cucumber was about 40 times higher than in wheat. Transport of Ca2+ in the roots towards the endodermis is suggested to take place mainly in the apoplastic pathway regulated by the availability of negatively charged binding sites along the cell wall continuum. Further transport of Ca2+ towards the stele may involve diffusion of Ca2+ into the symplasm in the vicinity of the endodermis. An active extrusion of Ca2+ towards the stele or towards the external medium is suggested to play a role in the regulation of Ca2+ uptake. [ABSTRACT FROM AUTHOR]

The absorption and translocation of diclofop-methyl (methyl 2-[4(2',4'-dichlorophenoxy)phenoxy]propanoate) was examined by using a specially designed treatment apparatus that separated excised roots or roots of seedlings into four zones. [14C]-Diclofop-methyl was absorbed along the entire root length of both wheat (Triticum aestivum L.) and oat (Avena sativa L.). In both species, absorption was greatest in the apical region of the root. Absorption by the apical region of wheat roots was more than three times greater than the basal portions, and more than twice as great as the apical region of oat roots. Less than 5% of the absorbed diclofop-methyl was translocated in both wheat and oat roots. Diclofop-methyl and diclofop(2-[4(2',4' dichlorophenoxy)phenoxy]propanoic acid) were the predominant translocated forms. The absorption and translocation of amitrole (3-amino-1,2,4-triazole) were also examined. Amitrole was absorbed along the entire length of wheat roots and translocated primatily in the basipetal direction. The usefulness of the specially designed apparatus for biochemical and physiological studies is discussed. [ABSTRACT FROM AUTHOR]

Wheat (Triticum aestivum L. cv. Jubilar) coleoptile segments convert 1-aminocyclopropane-1-carboxylic acid (ACC) to ethylene. This process is totally inhibited by nitrogen atmosphere and severely inhibited by free radical scavengers (sodium benzoate, ferulic acid), inhibitors of reactive -SH groups (p-chlormercuribenzoate, iodoacetate), CoCl2 and EDTA. Indole-3-acetic acid, aminoethoxyvinyl glycine, cycloheximide, actinomycin D, pyridoxal phosphate and NADH have no effect on ACC conversion to ethylene. Some in vivo characteristics of this conversion suggest that it could be catalyzed by peroxidase. However, isoperoxidase B1 isolated from wheat seedlings was not able to catalyze in vitro conversion of ACC to ethylene under a wide range of reaction conditions. Therefore, it is concluded that peroxidase is not directly involved in ethylene biosynthesis. [ABSTRACT FROM AUTHOR]

The activity of glutamate dehydrogenase and the amounts of two antigens, bearing the bulk of this activity, were determined in wheat leaves (Triticum acstivum L. cv. Capitole) at different ages. These results were compared to those obtained with leaves deprived of light (darkness‐stressed) during three days, at different stages of leaf life. The glutamate dehydrogenase activity increased in the leaves during normal senescence of the whole plant. De novo synthesis of one of the glutamate dehydrogenase proteins was shown, which correlated with this, increase in activity. The considerable differences between She effects of darkness‐stress, according to the leaf age, may characterize three steps in the leaf life: a first step during which leaves were not affected by stress, a second one for which the stress effect was partially reversible, and a final one during which stress impact was irreversible. [ABSTRACT FROM AUTHOR]

Glutamate dehydrogenase extracted from wheat leaves (Triticum aestivum L. cv. Capitole) taken at two different physiological stages was analysed by electrophoretic and immune‐chemical techniques. Two NAD‐dependent antigens were identified which bear the balk of the glutamate dehydrogenase activity in the two extracts. The first enzyme was found in much larger amounts in young than in senescent leaves and the reverse situation was observed for the second antigen. The possible relationships between this antigenic polymorphism and the heterogeneity detected by isoelectric focusing from the two extracts were investigated. A charge heterogeneity (isoelectric points about 5.7 and 4.8) was found for the first antigen in both extracts. The second antigen appeared homogeneous (isoelectric point about 5.7) at least in senescent leaves. The last result indicates that two quite different antigens appear in the same isoelectric focusing zone. [ABSTRACT FROM AUTHOR]

The distribution of divalent cation stimulated ATPase activity in relation to the distribution of other enzyme activities was studied for membrane fractions from wheat roots (Tritium aestivum L. cv. Svenno). A homogenate from dark grown plants was fractionated by differential centrifugation at 1000 g, 10,000 g, 30,000 g and 60,000 g (1, 10, 30 and 60 KP fractions), followed by partition in an aqueous polymer two‐phase system, using polyethylene glycol 4000/dextran T500 concentrations of 5.7/5.7, 5.9/5.9, 6.1/6.1, 6.3/6.3 and 6.5/6.5% (w/w). The 30 KP fraction was also separated by counter‐current distribution id a 6.3/6.3% two‐phase system. Protein and activities of Ca2+, Mg2+, and Mn2+ stimulated ATPases. cytochrome oxidase, light induced absorbance change (LIAC) related to cyt b reductions, inosine diphosphatase and NADH dependent antimycin A insensitive cytochrome c reductase were measured. The partition of ATPase activities stimulated by Ca2+, Mg2+ or Mn2+ was similar at all polymer concentrations tested, indicating: a low cation specificity of the dominating ATPases. The distribution of ATPases. agreed with different marker enzymes in different centrifuge fractions. Divalent cation stimulated ATPases were evidently related to several of the organelles. In the different fractions the distribution of ATPase activity should then follow that of the marker enzyme of the dominant organelle. From studies with different polymer concentrations the 6.3/6.3‐system was selected for further separation of the membranes in the 30 KP fraction by counter‐current distribution. By this method one fraction was obtained, which probably consisted of plasmalemma and was free from mitochondrial material. Indications for plasmalemma in this fraction were a) similar partition as protoplasts and b) high LIAC activity. [ABSTRACT FROM AUTHOR]

Growth of the second leaf of susceptible wild oat (Avena fatua L.) was inhibited within 2 days after treatment with the herbicide, diclofop-methyl, in the 1-½ leaf stage. Leaf growth of resistant wheat (Triticum aestivum L.) was unaffected by diclofop-methyl. In wild oat, chlorosis developed 1 day after leaf growth was inhibited. Foliar absorption of diclofop-methyl was similar between wild oat and wheat with 67 and 61% of the recovered radioactivity from [14C]diclofop-methyl being absorbed by wild oat and wheat, respectively, after 4 days. Wild oat was equally sensitive to the methyl ester and acid forms of the herbicide when the compounds were injected into the stem. Wheat was unaffected by both forms when treated similarly. Very little diclofop-methyl and diclofop (combined total of 10 to 12% in wild oat and 5 to 7% in wheat) remained in plant tissues 2 days after leaf treatment in both susceptible and resistant plants. Therefore, the active form of the herbicide must inhibit growth of susceptible plants very rapidly and at relatively low concentrations. Diclofop-methyl was rapidly hydrolyzed to diclofop by wild oat and wheat, Wild oat predominantly conjugated diclofop to an ester conjugate hut wheat hydroxylated the 2,4-dichlorophenyl ring and formed a phenolic conjugate. The formation of the different conjugates between wild oat and wheat was the most significant difference in metabolism between the two species. Nearly 60 and 70% of the methanol-soluble radioactivity was present as water-soluble conjugates in wild oat and wheat, respectively, 4 days after treatment. [ABSTRACT FROM AUTHOR]

A field experiment on wheat (Triticum aestivum L.) cv. Shera grown at 120 kg N ha-1 was conducted. Half of the dose of fertilizer N was applied at the pre-sowing stage and the other half when the seedlings were one month old. The leaf blades were examined for their NO3- content and NO3- assimilatory activity at various stages of growth and development. Soil nitrate level at 50 cm depth was determined throughout the wheat growing season in terms of cencentration (μg/ml) and total amount (kg ha-1). The upper leaf blades were examined for their capacity to assimilate NO3-. Highly significant correlation between NR (nitrate reductase) activity and NO3- content in the leaf blades, NR activity and soil NO3-, and between soil NO3- and leaf blade NO3- was observed. Findings on low soil NO3- status during the reproductive phase and the capacity of the upper leaf blades to assimilate additional amounts of NO3-, point to the need for developing a programme of soil fertilizer application whereby all the leaf blades can utilize the NO3- optimally and thus result in greater N harvest. [ABSTRACT FROM AUTHOR]

A study was done to determine the effects of vesicular-arbuscular mycorrhizal (VAM) colonization on drought acclimation of host plants. Safflower (Carthamus tinctorius L. cv. S555) and wheat (Triticum aestivum L. cv. Anza) were grown under environmentally controlled conditions with or without the VAM fungus, Glomus etunicatum Becker and Gerd., and were either acclimated (by pre-exposing plants to a 10–11 day drought period) or unacclimated to drought. Plants from all treatments were then exposed to drought for 9 days, and plant water status and root water uptake were measured. To minimize interactions between drought and P uptake, growth periods were adjusted so that acclimated and unacclimated plants were similar in size when measurements were made. When wheat was acclimated to drought, osmotic adjustment occurred (leaf solute potentials of leaf tissue rehydrated to full turgor were approximately 0.5 MPa lower in acclimated than unacclimated plants); in safflower, osmotic adjustment was minimal when plants were acclimated. Consequently, acclimated wheat plants were able to tolerate drought better than unacclimated plants, and maintained higher leaf water potentials and relative water contents as soil water was depleted. For both safflower and wheat, acclimated plants had higher water use efficiency, and therefore produced more biomass when water availability was limited, than unacclimated plants. However, mycorrhizal colonization did not affect osmotic adjustment, plant water status, water use efficiency or water uptake in either plant species, and therefore had no effect on drought acclimation or resistance. [ABSTRACT FROM AUTHOR]

Effects of environmental conditions influencing photosynthesis and photorespiration on senescence and net protein degradation were investigated in segments from the first leaf of young wheat (Triticum aestivum L. cv. Arina) plants. The segments were floated on H2O at 25, 30 or 35°C in continuous light (PAR: 50 or 150 µmol m-2 s-1) in ambient air and in CO2 -depleted air. Stromal enzymes, including phosphoglycolate phosphatase, glutamine synthetase, ferredoxin-dependent glutamate synthase, phosphoribulokinase, and the peroxisomal enzyme, glycolate oxidase, were detected by SDS-PAGE followed by immunoblotting with specific antibodies. In general, the net degradation of proteins and chlorophylls was delayed in CO2-depleted air. However, little effect of CO2 on protein degradation was observed at 25°C under the lower level of irradiance. The senescence retardation by the removal of CO2 was most pronounced at 30°C and at the higher irradiance. The stromal enzymes declined in a coordinated manner. Immunoreactive fragments from the degraded polypeptides were in most cases not detectable. However, an insolubilized fragment of glycolate oxidase accumulated in vivo, especially at 25°C in the presence of CO2. Detection of this fragment was minimal after incubation at 30°C and completely absent on blots from segments kept at 35°C. In CO2-depleted air, the fragment was only weakly detectable after incubation at 25°C. The results from these investigations indicate that environmental conditions that influence photosynthesis may interfere with senescence and protein catabolism in wheat leaves. [ABSTRACT FROM AUTHOR]

The LT50 values and soluble carbohydrate levels in wheat (Triticum aestivum L.) crowns and leaves were monitored throughout autumn and winter in cultivars varying in freezing tolerance and snow mold resistance during 1993/1994 and 1994/1995 in the field at Sapporo, Japan. During the first stage of hardening, from sowing to mid-November, the pattern of accumulation of mono- and disaccharides was similar for all cultivars. During the second stage of cold hardening, from mid-November to mid-December, the greatest accumulation of mono- and disaccharides, without a corresponding increase in fructan, was observed among the freezing-tolerant cultivars; and levels of simple saccharides rapidly decreased under snow cover. Conversely, levels of mono- and disaccharides in snow mold-resistant cultivars were less than 70% of those in freezing-tolerant cultivars before snow cover and maintained low levels throughout winter, while polysaccharide levels in snow mold-resistant cultivars were about 120% of those in freezing-tolerant cultivars in December. Sugar metabolism during the winter was examined using 18 cultivars in 1994/1995. LT50 values were correlated to the greatest extent with total mono- and disaccharide and fructan content among wheat cultivars excluding snow mold-resistant cultivars in December. Snow mold-resistant cultivars tended to metabolize carbohydrates more slowly until the end of the snow cover period. This result suggested that the enzymatic metabolism of the synthesis of sugars and the conversion of fructan to cryoprotective sugars in response to low temperatures, especially subzero ones, might be different between the two contrasting types in resistance to winter stress. [ABSTRACT FROM AUTHOR]

The effects of high temperature on accumulation of the 70-kDa heat shock protein (HSP70) and nucleoside diphosphate kinase (NDK) as well as two other proteins that have roles in the biosynthesis of storage proteins were examined during grain development. An HSP70 homolog and a 17-kDa NDK were co-purified from wheat endosperm, their identity verified, and a cDNA for an HSP70 expressed in endosperm was isolated. Wheat plants (Triticum aestivum, cvs Butte and Vulcan) were heat shocked at 40°C or exposed to maximum daily temperatures of 37 or 40°C during early or mid-grain fill. Antibodies and cDNA probes for BiP, HSP70, NDK and PDI were used to examine the effect of high temperatures on the accumulation of protein and mRNA in the endosperm. HSP70 mRNA levels increased substantially when plants were exposed to heat shock or to a 1-day gradual increase to 40°C. The effects of a 5-day heat treatment on mRNA levels were more complicated and depended on the developmental stage of the grain. A treatment that began at 7 days post-anthesis (DPA) decreased the level of mRNA for HSP70, BiP, PDI and NDK, whereas a treatment that began at 14 DPA slightly increased mRNA levels. The same treatments increased the accumulation of HSP70 but did not affect BiP, PDI, or NDK protein levels. This is the first detailed report on the effects of heat on mRNA and protein levels for HSP70 in a developing seed storage tissue. [ABSTRACT FROM AUTHOR]

Biosynthesis and accumulation of seed storage proteins such as the wheat glutens depend on the activity of a variety of other proteins, including chaperones and foldases. cDNA probes and antibodies to two chaperone proteins and a foldase were used to follow mRNA and protein accumulation in developing grains of wheat (Triticum aestivum, cvs Cheyenne and Butte). Endosperm was separated from other grain components and protein accumulation was analyzed on a per mg fresh weight basis. The ER resident chaperone BiP (binding protein) and foldase PDI (protein disulfide isomerase) accumulated to maximal levels in the middle stage of endosperm development, a period of rapid cell expansion and storage protein accumulation, whereas levels of a cytosolic chaperone, HSP70, remained relatively constant throughout grain development. In contrast, nucleoside diphosphate kinase (NDK), a cytosolic enzyme needed for synthesis of nucleoside triphosphates, accumulated early in endosperm development during the period of nuclear division and cell formation. When analyzed as a fraction of total protein the relative abundance of all four proteins peaked early in grain development and then declined. Accumulation of mRNA for the four proteins also peaked early in grain development. Although BiP and PDI formed a declining percentage of total protein as storage protein accumulated, their pattern of accumulation was compatible with a proposed role as catalysts for storage protein folding and accumulation in the ER. [ABSTRACT FROM AUTHOR]

Split-root experiments were conducted to test the hypothesis that adjustments in lateral root initiation, as might occur in response to localized soil conditions, are determined by the sugar content of the root and do not depend on changes in the import of phloem-translocated phytohormones. Wheat (Triticum aesticum L. cv. Alexandria) seedlings were grown in hydroponics with their seminal roots divided between two compartments within the culture vessel. Two seminal roots of treated plants were supplied with standard nutrient solution supplemented with 50 mM glucose, whilst the remaining three roots received nutrient solution without glucose. Control plants had their roots divided in the same ratio, but both ‘halves’ received nutrient solution without glucose. Feeding glucose to one ‘half’ of the root system increased the frequency (number per unit length) of lateral root primordia in the fed axes. The increase was first observed 15 h after the start of treatment and was located within the apical 30 mm of root. At this time there was no significant treatment effect on the frequency of primordia in non-fed axes. The enhanced initiation of lateral roots in glucose-fed root tips was associated with an increase in their concentration of glucose and sucrose plus low molecular mass fructans. In contrast, there was a reduction in partitioning of 14C-photosynthate to these root tips compared to the non-fed roots of treated plants and controls. The results indicate that lateral root initiation can be stimulated by sugars in the absence of an increase in phloem translocation. It is proposed that proliferation of lateral roots in response to localized soil conditions, such as nutrient patches, may be signalled by an increase in sugar content of the tissue, rather than an altered flux of phytohormones or other material co-transported with sucrose in the phloem. [ABSTRACT FROM AUTHOR]

The Russian wheat aphid (RWA), Diuraphis noxia (Mordvilko) is one of the most destructive insect pests of cereals world‐wide. Although resistant cultivars have been bred, the biochemical mechanism of resistance is unknown. The aim of this work was to gain information on the mechanism of resistance which could contribute to more directed breeding of resistant cultivars in the future. The effect of RWA infestation on the inter‐ and intracellular β‐1,3‐glucanase activities was studied in different resistant wheat (Triticum aestivum L.) cultivars containing the Dn‐1 gene for RWA resistance and corresponding near‐isogenic susceptible cultivars. The activity was determined spectrophotometrically by measuring the release of glucose from laminarin. Infestation differentially induced the intra‐ and intercellular activities to much higher levels in resistant than susceptible cultivars within 48 h. According to immunological studies induced enzyme activities were due to increased protein levels. The intracellular β‐1,3‐glucanase contained about 8% exo‐activity. The exo‐activity made an insignificant contribution to the intercellular activity. The genetic background into which the resistance gene was bred did affect the level of activity that corresponded to the resistance performance. Seven apoplastic isoforms of β‐1,3‐glucanase, varying from acidic to basic, were resolved by isoelectric focusing. All isoenzymes were equally induced and no specific one could be linked to resistance or susceptibility. The RWA induced β‐1,3‐glucanase activity in resistant cultivars closely resembles defence responses during pathogenesis and seems to be part of a general defence response like the hypersensitive reaction (HR), which confers resistance to the RWA. This knowledge might be helpful in future to identify genes for RWA resistance. The increased β‐1,3‐glucanase activity after RWA infestation might serve as an additional measure to biochemically trace resistance in crosses during breeding. [ABSTRACT FROM AUTHOR]

Transformation of wheat by the biolistic procedure is unpredictable and inefficient. To gain insight into the fate of transgenes introduced into wheat scutellar tissue by particle bombardment, the expression of the firefly luciferase gene was followed using low-light video imaging. Luciferase expression can be detected as early as 40 to 50 min after bombardment. After 48 h, the levels of transient expression in individual scutella fall rapidly, becoming undetectable 10 to 20 days later. Luciferase activity was observed to recover in a small percentage of the material and it was from this material that transformed plants, stably expressing luciferase, were recovered. We concluded that the calluses which recover luciferase expression are stably transformed and we have termed the transition period between transient and stable expression the transformation boundary. The percentage of scutella displaying luciferase activity, as measured at 30 days post-bombardment, was judged a realistic measure of the efficiency of the transformation procedure. The results of our experiments suggest that the selection and regeneration of plants were not major factors contributing to the poor transformation efficiencies associated with biolistic transformation. The results demonstrate that luciferase can be used to assess rapidly and quantify the efficiency of the transformation procedure without the need to produce transformed plants. This will allow different procedures to be rapidly assessed and compared and should provide valuable insight into the conditions required to improve the efficiency of DNA integration and stable expression in species recalcitrant to transformation. [ABSTRACT FROM AUTHOR]

Senescence-associated coordination in amounts of enzymes localized in different cellular compartments were determined in attached leaves of young wheat (Triticum aestivum L. cv. Arina) plants. Senescence was initiated at the time of full leaf elongation based on declines in total RNA and soluble protein. Removal of N from the growth medium just at the time of full leaf elongation enhanced the rate of senescence. Sustained declines in the amount of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco, EC 4.1.1.39), and a marked decrease in the rbcS transcripts, just after full leaf elongation indicated that Rubisco synthesis/degradation was very sensitive to the onset of senescence. Rubisco activase amount also declined during senescence but the proportion of rca transcript relative to the total poly A RNA pool increased 3-fold during senescence. Thus, continued synthesis of activase may be required to maintain functional Rubisco throughout senescence. N stress led to declines in the amount of proteins located in the chloroplast, the peroxisome and the cytosol. Transcripts of the Clp protease subunits also declined in response to N stress, indicating that Clp is not a senescence-specific protease. In contrast to the other proteins, mitochondrial NADH-glutamate dehydrogenase (EC 1.4.1.2) was relatively stable during senescence and was not affected by N stress. During natural senescence with adequate plant nitrate supply the amount of nitrite reductase (EC 1.7.7.1) increased, and those of glutamine synthetase (EC 1.4.7.1) and glutamate synthase (EC 6.3.1.2) were stable. These results indicated that N assimilatory capacity can continue or even increase during senescence if the substrate supply is maintained. Differential stabilities of proteins, even within the same cellular compartment, indicate that proteolytic activity during senescence must be highly regulated. [ABSTRACT FROM AUTHOR]

Although Al-induced iron chlorosis has been observed in many plants, the mechanisms responsible for this phenomenon are yet to be understood. We investigated the effect of Al on iron acquisition in a Strategy II plant, wheat (Triticum aestivum L.) using both Al-tolerant (Atlas 66) and -sensitive (Scout 66) cultivars. When iron was supplied as insoluble iron, ferric hydroxide, in the culture solution, both cultivars without Al treatment grew normally, while those with 100 µM AlCl3 developed chlorosis of the young leaves after 3 days of the treatment. A 21-h treatment with 100 µM AlCl3 in 0.5 mM CaCl2 solution (pH 4.5) decreased the amount of 2′-deoxymugineic acid (DMA) secreted by Fe-deficient Atlas 66 and Scout 66 plants by 85 and 90%, respectively. The amount of DMA secreted decreased with increasing external Al concentrations. Al treatment during the biosynthesis process caused the inhibition of that of DMA within 3 h. The secretion process was also found to be inhibited by Al, resulting in the biosynthesized DMA remaining in the roots. These results demonstrate the inhibition by Al of both biosynthesis and secretion of DMA attributed to Al-induced iron chlorosis. [ABSTRACT FROM AUTHOR]

In order to examine whether the decrease in gene expression of chloroplast DNA-encoded polypetides contributes to the inhibition of photosystem II (PSII) function during water stress, changes in transcript and template levels of chloroplast psbA and psbD genes (encoding the D1 and D2 reaction center proteins of PSII, respectively) were investigated in spring wheat leaves (Triticum aestivum L. cv. Longchun No. 10) using northern, Southern and dot blot analyses. The results of northern hybridization indicated that stressing wheat seedlings in polyethylene glycol (PEG) solutions with an osmotic potential of -0.5 MPa for 0, 24, 48 and 72 h, caused marked declines in the steady state levels of the psbA and psbD transcripts but did not alter their transcript processing patterns. RNA dot blot analysis further demonstrated that over the whole range of water stress investigated, the transcript levels of the two genes declined by 2- and 3-fold, respectively, relative to the same amount of total RNA. As total RNA decreased 3-fold during the process of stress, the transcript levels of psbA and psbD genes actually declined by 6- and 9-fold, respectively. These results suggest that water stress affects the expression of the psbA and psbD genes, possibly at the transcriptional level. Southern and DNA dot blot analyses consistently showed that water stress did not affect the template levels of either psbA or psbD genes, suggesting that the decreased abundance of psbA and psbD transcripts under water stress is not due to limited gene templates but likely a result of lowered gene transcriptional activity and/or changed mRNA stability. [ABSTRACT FROM AUTHOR]

The function of ver203, a gene related to vernalization in winter wheat, was investigated by expression of a complementary DNA as an antisense RNA in transgenic plants. A verc203:gus fusion-expression plasmid was constructed in pBI221, which contains a CaMV (cauliflower mosaic virus) 35S-promoter, a gus gene and a nos terminator. The construct was then introduced into the plant by the pollen-tube pathway. The results showed that heading was strongly inhibited in 6 of 326 vernalized antisense transgenic winter wheat plants, until both the vernalized control winter wheat and sense transgenic plants ripened. The hybridization analysis of DNA, amplification of the insert DNA sequences with PCR, northern blot analysis with double- and single-stranded probes, and detection of GUS activity by X-gluc assay gave strong positive results. This suggests that the VER203 protein plays an important role in controlling heading and flower development in winter wheat. [ABSTRACT FROM AUTHOR]

The effect of different N supply on amino acid export to the phloem was studied in young plants of wheat (Triticum aestivum L. cv. Klein Chamaco), using the exudation in EDTA technique. Plants were grown in a growth cabinet in pots with sand, and supplied with nutrient solutions of different NO3-concentrations. When plants were grown for 15 days with nutrient solutions containing 1.0, 3.0, 5.0, 10.0, 15.0 or 20.0 mM KNO3, the exudation rate of sugars from the phloem was unaffected by N supply, but sugars accumulated in the leaf tissue when the N supply was limiting for growth. On the other hand, the rate of exudation of amino acids was proportional to the NO3-concentration in the nutrient solution. When the supply of N to plants grown for 15 days with 15.0 mM NO3-was interrupted, the exudation of sugars was again unaffected, but there was a fast decrease in the amount of amino acids exudated, and of the concentration of amino acids and nitrogen in the tissues. Also, when 10-day-old plants grown without N were supplied with 15.0 mM NO3-there was a sharp increase in the exudation of amino acids, without changes in the amount of sugar exudated. The rate of exudation of amino acids to the phloem was independent of the concentration of free amino acids in the leaves in all three types of experiment. Asp was the most abundant amino acid in the leaf tissue, while Glu was the one most abundant in the phloem exudate. Asp and Ala were exported to the phloem at a rate lower than expected from their leaf tissue concentrations, indicating some discrimination. On the contrary, Glu showed a preferential export at low N supply.It is concluded that the rate of amino acid export from the leaf to the phloem is dependent on the N available to the plant. This N is used for synthesis of leaf protein when the supply is low, exported to the phloem when supply is adequate, and accumulated in the storage pool when supply is above plant demand. [ABSTRACT FROM AUTHOR]

Winter, as compared with spring cereals, possess better acclimation mechanisms that allow them to overwinter and survive freezing temperatures. This difference is genetically programmed and involves a complex genetic system. To understand the nature of this system and its regulation by low temperature, genes associated with freezing tolerance in wheat (Triticum aestivum L.) were identified and characterized. Among these, the wcs120 gene family encodes a group of proteins ranging in size from 12 to 200 Wa. As shown by biochemical, immunohistochemical, molecular and genetic analyses, this gene family is specific to the Poaceae, highly abundant and coordinately regulated by low temperature. Furthermore, accumulation of WCS protein is directly correlated with the development of freezing tolerance. These analyses also revealed a regulatory control of the vernalization process over low temperature gene expression in winter cereals. Recent studies suggest that the molecular mechanisms controlling the expression of these genes involve negative regulatory factors that are modulated by phosphorylation. [ABSTRACT FROM AUTHOR]

Copper is a potent sulfhydryl reagent which can also catalyse the generation of active oxygen. Since nitrate reductase (EC 1.6.6.1) is an SH-enzyme sensitive to oxidative environments, the relations among copper, active oxygen species and nitrate reductase (NR) activity are of interest. Foliar segments of wheat (Triticum aestivum cv. Oasis) were floated on CuSO4 solutions (up to 250 μM) for 24 h under continuous light. Copper decreased NR activity before affecting active oxygen generation as estimated by changes in oxidative parameters, including malondialdehyde, K+ leakage and chlorophyll degradation. Cysteine and Na-benzoate counteracted this decrease, suggesting an oxidative damage of the enzyme in leaves exposed to high copper levels. Copper-induced NR inactivation was further studied in the partially purified enzyme. Preincubation with CuSO4 inhibited NR. Copper inhibition was reversed by subsequent incubation with EDTA, indicating that the metal bonded to key -SH groups of the enzyme. In addition, an •OH-generating system (composed of CuSO4, ascorbate and H2O2) irreversibly decreased the activity of purified NR to a greater extent than copper alone. Our results show that copper affects nitrogen metabolism by diminishing NR activity, involving a direct effect on key SH-groups and an indirect effect via attack by active oxygen species induced by the metal. [ABSTRACT FROM AUTHOR]

The relationship between freezing tolerance (expressed as LT50, the lethal freezing temperature for 50% of plants) and the amount and physical state (as determined by spin‐lattice [T1] and spin‐spin [T2] relaxation times of protons) of water in crown tissue was examined in contrasting winter wheat (Triticum aestivum L.) varieties grown under field conditions from 1992 to 1994. During acclimation, the LT50 values decreased from around ‐7 to ‐17, ‐20 and ‐27°C in PI 173438, Chihokukomugi and Valuevskaya, respectively. Tissue water content decreased continuously through autumn to reach a plateau around 3 g H2O (g dry weight)‐1 in early winter when LT50 was still decteasing, and then gradually increased under snow cover. A significant negative correlation was found between mean minimum air temperatures and freezing tolerance prior to the establishment of continuous snow cover. In contrast, a positive association between mean minimum temperatures and crown tissue water content was significant only when air temperatures were above 0°C, as water content did not decrease further at sub‐zero temperatures. Seasonal changes in T1 were closely related to changes in freezing tolerance. T1 decreased until January even though water content stopped decreasing. Further tests on 15 field‐grown varieties confirmed a strong negative association between freezing tolerance and T1. The results suggest that cold hardening is comprised of two stages, with the transition occurring at ca 0°C. Development of hardiness was related to (1) a reduction in water content in the first stage (at minimum temperatures > 0°C), and (2) a change in physical state of water without much reduction in water content in the second stage. Varietal differences in hardiness thus arise due to changes in both water content and physical state of water. [ABSTRACT FROM AUTHOR]

Addition of aluminium chloride (50 μM Al) caused different effects on the transmembrane electrical potential (PD) of root cells in Al-tolerant wheat (Triticum aestivum) cv. Kadett and Al-sensitive cv. WW 20299. As changes in PD of plant cells may depend on transient fluxes of protons, potassium and/or calcium through cell membranes, the effect of Al was investigated on the cytosolic concentrations of these ions in protoplasts isolated from root tips of the same cultivars. The tetra[acetoxymethyl] esters of the fluorescent dyes bis-carboxyethyl-carboxyfluorescein, BCECE K+-binding benzofuran isophthalate, PBFI, and the stilbene chromophore Fura 2-AM were used to determine pH, K+ and Ca2+, respectively. Changes in fluorescence ratios. directly reflecting changes in [H+], [K+] and [Ca2+] in the cytosol, were determined by photometry fluorescence microscopy. Additions and removals of Al to and from both cultivars caused hyperpolarizations and depolarizations, respectively, but only in the sensitive cv, WW 20299 did the resting PD decrease gradually. Addition of Al to the protoplasts caused rapid changes in cytosolic pH, free [K+] and [Ca2+]. In both cultivars Al caused a transient oscillating increase in cytosolic [Ca2+] for 1 or 2 mm and a rapid pH-dependent change in cytosolic [K+]. At pH 5 the presence of K+ in the medium diminished the Al-induced decrease in cytosolic [K+]. Aluminium (50 μM) induced a transient increase in cytosolic [H+] (pH decreased) in both cultivars, but the cytosolic pH returned to its initial value only in the Al-tolerant cv. Kadett. In the Al-sensitive cv. WW 20299, repeated additions of Al caused a gradual decline in pH. Moreover, in the presence of 1 mM KCl, pH recovered completely in both cultivars. Since only the effect on pH differed in the two cultivars, the more toxic effect of Al on the cv. WW 20299 should be related to the change in pH. [ABSTRACT FROM AUTHOR]

Sodium/calcium interactions were compared in two species of wheat known to differ in sodium accumulation in the shoot and biomass reduction under saline conditions. Unidirectional uptake of 22Na to the roots was measured in 5, 50, 100 and 150 mM NaCl with four levels of calcium activity. Translocation of sodium to the shoot was measured as 22Na uptake over 7 days. The rate of sodium uptake into the roots did not vary between the salt-sensitive Triticum turgidum cv. Modoc and the tolerant Triticum aestivum cv. Kharchia, except at low calcium, when the rate of uptake was higher in Kharchia and more sensitive to inhibition by additional calcium. The rate of translocation to the shoot was similar in both species, however the shoot:root ratio was higher in Kharachia and so the concentration of sodium, on a shoot fresh weight basis, was more dilute in Kharchia. There was no evidence for differences in sodium transport processes between the species which might confer differential salinity tolerance. Translocation of calcium to the shoot was measured as 45Ca uptake in 5 and 150 mM NaCl with 4 levels of calcium activity. Translocation of calcium was more inhibits by sodium in the salt-sensitive Modoc, and this may reflect differences in calcium transport processes, which could account in part for the greater salt sensitivity of this species. [ABSTRACT FROM AUTHOR]

We have studied the effect of aluminum (Al) on lipid composition of plasma membranes from roots of an Al-resistant (PT741) and an Al-sensitive (Katepwa) cultivar of Triticum aestivum L. Several genotype-specific changes were observed in phospholipids and steryl lipids. While exposure to 20 μM AlCl3 for 3 days had no effect on total phospholipids in either genotype, the most abundant phospholipid, phosphatidylcholine, increased significantly in the Al-resistant PT741 with corresponding decreases in other phospholipids. In comparison, no change in phosphatidylcholine was observed in the Al-sensitive Katepwa. Aluminum also decreased steryl lipids (mainly free sterols) in PT741. Such changes were not observed in Katepwa. As a result of differential changes in lipid composition, the relative abundance of one lipid class to another changed. The ratio of steryl lipids to phospholipids decreased in PT741, with no change in Katepwa. While limited information on the relationship between membrane function and lipid composition makes it difficult to relate these changes to Al toxicity and resistance, changes observed only in the Al-resistant genotype could contribute to continued plant growth in the face of Al stress. [ABSTRACT FROM AUTHOR]

Young carotenoid‐deficient etioplasts, isolated from Norflurazon (NF)‐treated wheat seedlings, were used to study the role of coloured carotenoids in the binding and import reactions of different nuclear‐encoded plastid proteins. Plastids from control seedlings exhibited significantly higher import efficiencies than did plastids from NF‐treated plants. Etioplasts containing normal levels of carotenoids imported approximately 2000 and 800 molecules per plastid of the precursors of the small Rubisco subunit (pSS) and the Rieske FeS protein (pFeS), respectively. Plastids from NF‐treated plants imported approximately 100 and 70 pSS and pFeS molecules per plastid, respectively. In addition, a maximum binding capacity of NF‐treated plastids of 1200 protein molecules per plastid was observed for both pSS and pFeS when assayed at 25°C: and a maximum binding capacity of approximately 1300 molecules per plastid was noted at 4°C. For control plastids, a similar amount of binding, or approximately 1400 protein molecules per plastid, could only be observed if import was inhibited by low ATP concentrations at 4°C. When these plastids were washed and transferred to conditions promoting import at 25°C and 10 mM Mg‐ATP, close to 60% of the envelope‐associated precursor protein molecules were imported. These results indicate that control and NF‐treated young etioplasts contain similar amounts of binding sites for precursor proteins. However, only in the case of control plastids the binding was productive and lead to import and processing in the stroma upon transfer to conditions promoting import. Plastids isolated from wheat seedlings grown in weak red light and containing different amounts of carotenoids, were assayed for their ability to bind and import a protein with unusual import characteristics, the Chlamydomonas reinhardtii PsaF precursor of PSI (pPsaF) and transit peptide deletion constructs. The PsaF protein was imported in a transit peptide‐dependent manner into control etioplasts, whereas import of pPsaF into young wheat etioplasts isolated from NF‐treated plants was inhibited at low levels of plastid carotenoids. [ABSTRACT FROM AUTHOR]

Acetohydroxyacid synthase (AHAS, EC 4.1.3.18; also known as acetolactate synthase), which catalyses the first reaction common to the biosynthesis of the branched-chain amino acids, L-valine, L-Ieucine and L-isoleucine, and is the target of several classes of herbicides, has been studied in hydroponically-grown seedlings of wheat (Triticum aestivum L. cv. Vulcan). Enzyme activity was greater in leaves than roots, reaching a maximum between 4 and 6 days after germination. AHAS was associated with the chloroplasts after centrifugation in a density gradient. A preparation of the enzyme was obtained from wheat leaves which gave a single band after electrophoresis in native gels but was resolved by denaturing sodium dodecyl sulphate-polyacrylamide gel electrophoresis into three polypeptide bands of molecular mass 58, 57 and 15 kDa. The native molecular mass was approximately 128 kDa. AHAS had optimum activity at pH 7 and did not require the addition of flavin adenine dinucleotide (FAD), thiamine pyrophosphate (TPP) and MgCl2 for activity. The enzyme did not display typical hyperbolic kinetics, in that the double reciprocal plot of activity against pyruvate concentration was non-linear. The concentration of pyruvate that gave half of the maximum activity was 4 mM. Sulfonylurea and imidazolinone herbicides were potent inhibitors of wheat leaf AHAS, with 50% inhibition being observed at concentrations of 0.6 and 0.3μM for chlorsulfuron and metsulfuron methyl, respectively, and at 2.5, 5 and 10 μM for imazaquin, imazethapyr and imazapyr. Inhibition by both classes of compounds was reversed by removal of the inhibitor. Progress curves of product formation against time in the presence of the herbicides were non-linear, and based on the assumption that inhibition by the sulfonylureas was of the slow, tight-binding type, estimates of 0.17 and 0.1 nM were obtained for the dissociation constants of chlorsulfuron and metsulfuron methyl, respectively, from the steady-state enzyme-inhibitor complex. [ABSTRACT FROM AUTHOR]

The lack of information about the movement of aluminum (Al) across the plasma membrane presents a significant barrier to the elucidation of resistance mechanisms which may involve exclusion of Al from the symplast. An understanding of mechanistic aspects of exclusion requires the estimation of symplastic Al levels. Such measurements may be achievable through the use of a kinetic approach. A kinetic protocol was developed to characterize the accumulation and distribution of Al in various cellular compartments in roots of wheat (Triticum aestivum L.). The kinetics of uptake and desorption were similar when Al was supplied as AlK(SO4)2 or as AlCl3. When both salts were supplied at low concentration (50 µM), Al bound to a purified cell wall fraction could be reduced to less than 10–20% of non-exchangeable Al, if roots were washed for 30 min in citric acid following exposure. In contrast, when AlK(SO4)2 was supplied at a high concentration (200 µM), a strong linear phase of uptake into cell wall material was observed, which accounted for approximately 48% of non-exchangeable Al in roots. These results suggest that the use of low concentrations of Al in simple salt solutions is required to minimize accumulation of non-exchangeable Al in the apoplasm. A series of multiple-desorption experiments confirmed that citric acid was effective in removing Al from the cell wall compartment of roots exposed to Al for short periods (3 h). However, long exposures (48 h) appeared to create conditions conducive to the accumulation of non-exchangeable Al in the cell wall. In experiments where uptake from solutions containing 50 µM AlCl3 was followed by desorption in citric acid, non-exchangeable Al in microsomal membrane fractions represented less than 4% of total non-exchangeable Al. Thus, we can exclude the plasma membrane and cell wall as major sites for accumulation of non-exchangeable Al short exposure studies. Although we cannot provide unequivocal evidence for the localization of Al within the symplast, use of simple salt solutions followed by desorption in citric acid provides the best kinetic technique currently available for the quantitation of Al in the symplasm. [ABSTRACT FROM AUTHOR]

We have investigated the water use efficiency of whole plants and selected leaves and allocation patterns of three wheat cultivars (Mexipak, Nesser and Katya) to explore how variation in these traits can contribute to the ability to grow in dry environments. The cultivars exhibited considerable differences in biomass allocation and water use efficiency. Cultivars with higher growth rates of roots and higher proportions of biomass in roots (Nesser and Katya) also had higher leaf growth rates, higher proportions of their biomass as leaves and higher leaf area ratios. These same cultivars had lower rates of transpiration per unit leaf area or unit root weight and higher biomass production per unit water use. They also had higher ratios of photosynthesis to transpiration, and lower ratios of intercellular to external CO2 partial pressure. The latter resulted from large differences in stomatal conductance associated with relatively small differences in rates of photosynthesis. There was little variation between cultivars in response to drought, and differences in allocation pattern and plant water use efficiency between cultivars as found under well-watered conditions persisted under dry conditions. At the end of the non-watered treatment, relative growth rates and transpiration rates decreased to similar values for all cultivars. High ratios of photosynthesis to transpiration, and accordingly high biomass production per unit of transpiration, is regarded as a favourable trait for dry environments, since more efficient use of water postpones the decrease in plant water status. [ABSTRACT FROM AUTHOR]

We have investigated the effect of aluminum (Al) on the activity of glucose-6-phosphate dehydrogenase (G6PDH; EC 1.1.149) and 6-phosphogluconate dehydrogenase (6PGDH; EC 1.1.144) isolated from 5-mm root apices of 4-day-old. wheat (Triticum aestivum) cultivars differing in resistance to Al. Rapid increases in G6PDH and 6PGDH activities were observed in Al-resistant cultivars (PT741 and Atlas 66) during the first 10 h of treatment with 100 μM Al, while no change in the activity of either enzyme was observed in Al-sensitive cultivars (Katepwa and Neepawa) during a 24-h exposure to Al. The Al-induced increases in enzyme activities observed in the Al-resistant PT741 appear to reflect an induction of protein synthesis since the increases were completely abolished by 1 mM cycloheximide. No differences in G6PDH and 6PGDH activities were observed between the Al-sensitive and the Al-resistant genotypes when Al was supplied in vitro. Under these conditions, an increase in Al concentration from 0 to 1.4 mM caused a gradual decrease in activity of both enzymes, irrespective of the Al-resistance of whole seedlings. Aluminum-sensitive and aluminum-resistant cultivars also differed in the rate and extent of accumulation of slowly-exchanging Al in 5-mm root apices. During the first 6 h of Al treatment, Al accumulation was only 10% more rapid in Katepwa than in PT741. After 24-h exposure, accumulation in the A1-sensitive Katepwa, was two-fold higher. A decline in Al accumulation in a sIowly-exchanging compartment as well as a decrease in activities of G6PDH and 6PGDH were found in the Al-resistant PT741, when seedlings were transferred to Al-free treatment solutions after 16-h exposure to 100 μM Al. These results suggest that rapid induction of G6PDH and 6PGDH in S Al-resistant line PT741 by Al may play a role in the mechanism of Al resistance, possibly by regulation of the pentose phosphate pathway. [ABSTRACT FROM AUTHOR]