Showing papers in "Journal of Experimental Botany in 1997"

The molecular biology of leaf senescence

Abstract: Senescence is a complex, highly regulated, developmental phase in the life of a leaf that results in the co-ordinated degradation of macromolecules and the subsequent mobilization of components to other parts of the plant. The application of molecular biology techniques to the study of leaf senescence has, in the last few years, enabled the isolation and characterization of a large range of cDNA clones representing genes that show increased expression in senescing leaves. The analysis of these genes and identification of the function of the encoded proteins will allow a picture of the complex processes that take place during senescence to be assembled. To date, genes encoding degradative enzymes such as proteases and nucleases, enzymes involved in lipid and carbohydrate metabolism and enzymes involved in nitrogen mobilization have all been identified as senescenceenhanced genes. A variety of other genes of no obvious senescence-related function have also been identified; their role in senescence may be less predictable and, possibly, more interesting. The combined action of several internal and external signals may be involved in the induction of senescence. Analysis of the regulatory mechanisms controlling the expression of senescence-induced genes will allow the signalling pathways that are involved in the regulation of senescence to be elucidated. Experiments with transgenic plants and mutants are already shedding light on the role played by cytokinins and ethylene in regulating senescence in leaves.

Photomorphogenesis, photosynthesis, and seed yield of wheat plants grown under red light-emitting diodes (LEDs) with and without supplemental blue lighting

Abstract: Red light-emitting diodes (LEDs) are a potential light source for growing plants in spaceflight systems because of their safety, small mass and volume, wavelength specificity, and longevity. Despite these attractive features, red LEDs must satisfy requirements for plant photosynthesis and photomorphogenesis for successful growth and seed yield. To determine the influence of gallium aluminium arsenide (GaAlAs) red LEDs on wheat photomorphogenesis, photosynthesis, and seed yield, wheat (Triticum aestivum L., cv. 'USU-Super Dwarf') plants were grown under red LEDs and compared to plants grown under daylight fluorescent (white) lamps and red LEDs supplemented with either 1% or 10% blue light from blue fluorescent (BF) lamps. Compared to white light-grown plants, wheat grown under red LEDs alone demonstrated less main culm development during vegetative growth through preanthesis, while showing a longer flag leaf at 40 DAP and greater main culm length at final harvest (70 DAP). As supplemental BF light was increased with red LEDs, shoot dry matter and net leaf photosynthesis rate increased. At final harvest, wheat grown under red LEDs alone displayed fewer subtillers and a lower seed yield compared to plants grown under white light. Wheat grown under red LEDs+10% BF light had comparable shoot dry matter accumulation and seed yield relative to wheat grown under white light. These results indicate that wheat can complete its life cycle under red LEDs alone, but larger plants and greater amounts of seed are produced in the presence of red LEDs supplemented with a quantity of blue light.

The Cohesion-Tension theory of sap ascent: current controversies

Abstract: In recent years, the Cohesion-Tension (C-T) theory of sap ascent in plants has come under question because of work published by Professor Ulrich Zimmermann and colleagues at the University of Wurzburg, Germany. The purpose of this review is to (1) state the essential and testable elements of the C-T theory, (2) summarize the negative evidence for the C-T theory, and (3) review critically the positive evidence for the C-T theory and the evidence that the ScholanderHammel pressure bomb measures xylem pressure potential (Px) correctly, because much of the evidence for the C-T theory depends on pressure bomb data. Much of the current evidence negates the conclusions drawn by Zimmermann from studies using the xylem pressure probe (XPP), but it is not yet clear in every instance why the XPP results disagree with those of other methods for estimating xylem pressure. There is no reason to reject the XPP as a useful new tool for studying xylem tensions in the range of 0 to -0.6 MPa. Additional research is needed to test the C-T theory with both the XPP and traditional methods.

Use of centrifugal force in the study of xylem cavitation

Abstract: Two methods were evaluated for using centrifugal force to measure the occurrence of cavitation as a function of negative pressures in xylem. The general protocol was to measure the hydraulic conductivity of xylem segments (stem or root pieces) before and after centring them on a centrifuge rotor and spinning them about their long axis to generate negative xylem pressure. The percentage decrease in conductivity from the initial to final measurement was used to quantify the embolism resulting from cavitation during spinning. In one approach, segments were spun with their ends exposed to air. This method could only be used when xylem conduits were much shorter than the segment. Results from an angiosperm (Betula occidentalis) and a gymnosperm (Abies lasiocarpa) corresponded to previous observations of embolism caused by air dehydration where negative pressure was measured with the pressure chamber. Results also agreed with embolism caused by injection of air into the xylem, in support of the air-seeding hypothesis for cavitation. In a second approach, segments were spun in a rotor designed to keep the segment ends immersed in water during spinning. This gave the same results as for non-immersed segments. Immersing the segment ends allowed measurements on any material, regardless of conduit length, as demonstrated for roots of B. occidentalis. The chief advantage of the centrifugal force method is the rapidity and precision with which any desired xylem pressure can be imposed.

Zygotic embryogenesis versus somatic embryogenesis

Abstract: This review will summarize molecular and genetic analyses aimed at identifying the mechanisms underlying the sequence of events during plant zygotic embryogenesis. These events are being studied in parallel with the histological and morphological analyses of somatic embryogenesis. The strength and limitations of somatic embryogenesis as a model system will be discussed briefly. The formation of the zygotic embryo has been described in some detail, but the molecular mechanisms controlling the differentiation of the various cell types are not understood. In recent years plant molecular and genetic studies have led to the identification and characterization of genes controlling the establishment of polarity, tissue differentiation and elaboration of patterns during embryo development. An investigation of the developmental basis of a number of mutant phenotypes has enabled the identification of gene activities promoting (1) asymmetric cell division and polarization leading to heterogeneous partitioning of the cytoplasmic determinants necessary for the initiation of embryogenesis (e.g. GNOM), (2) the determination of the apical-basal organization which is established independently of the differentiation of the tissues of the radial pattern elements (e.g. KNOLLE, FACKEL, ZWILLE), (3) the differentiation of meristems (e.g. SHOOT-MERISTEMLESS), and (4) the formation of a mature embryo characterized by the accumulation of LEA and storage proteins. The accumulation of these two types of proteins is controlled by ABA-dependent regulatory mechanisms as shown using both ABA-deficient and ABA-insensitive mutants (e.g. ABA, ABI3). Both types of embryogenesis have been studied by different techniques and common features have been identified between them. In spite of the relative difficulty of identifying the original cells involved in the developmental processes of somatic embryogenesis, common regulatory mechanisms are probably involved in the first stages up to the globular form. Signal molecules, such as growth regulators, have been shown to play a role during development of both types of embryos. The most promising method for identifying regulatory mechanisms responsible for the key events of embryogenesis will come from molecular and genetic analyses. The mutations already identified will shed light on the nature of the genes that affect developmental processes as well as elucidating the role of the various regulatory genes that control plant embryogenesis.

REVIEW ARTICLE Zygotic embryogenesis versus somatic embryogenesis

Abstract: This review will summarize molecular and genetic analyses aimed at identifying the mechanisms underlying the sequence of events during plant zygotic embryogenesis. These events are being studied in parallel with the histological and morphological analyses of somatic embryogenesis. The strength and limitations of somatic embryogenesis as a model system will be discussed briefly. The formation of the zygotic embryo has been described in some detail, but the molecular mechanisms controlling the differentiation of the various cell types are not understood. In recent years plant molecular and genetic studies have led to the identification and characterization of genes controlling the establishment of polarity, tissue differentiation and elaboration of patterns during embryo development. An investigation of the developmental basis of a number of mutant phenotypes has enabled the identification of gene activities promoting (1) asymmetric cell division and polarization leading to heterogeneous partitioning of the cytoplasmic determinants necessary for the initiation of embryogenesis (e.g. GNOM), (2) the determination of the apical-basal organization which is established independently of the differentiation of the tissues of the radial pattern elements (e.g. KNOLLE, FACKEL, ZWILLE), (3) the differentiation of meristems (e.g. SHOOT-MERISTEMLESS), and (4) the formation of a mature embryo characterized by the accumulation of LEA and storage proteins. The accumulation of these two types of proteins is controlled by ABA-dependent regulatory mechanisms as shown using both ABA-deficient and ABA-insensitive mutants (e.g. ABA, ABI3). Both types of embryogenesis have been studied by different techniques and common features have been identified between them. In spite of the relative difficulty of identifying the original cells involved in the developmental processes of somatic embryogenesis, common regulatory mechanisms are probably involved in the first stages up to the globular form. Signal molecules, such as growth regulators, have been shown to play a role during development of both types of embryos. The most promising method for identifying regulatory mechanisms responsible for the key events of embryogenesis will come from molecular and genetic analyses. The mutations already identified will shed light on the nature of the genes that affect developmental processes as well as elucidating the role of the various regulatory genes that control plant embryogenesis.

Distribution of cadmium in shoot and root tissues of maize and pea plants : physiological disturbances

Abstract: Maize and pea plants were treated with 0.0 (control), 0.01 or 0.05 mM Cd in the growing medium for 11 d. Although the total Cd concentration was similar in shoot and root tissues of both species, pea plants showed more severe toxic symptoms. The fresh weight and percentage of water content of root and shoot decreased concomitantly to Cd supply. High Cd levels were found in the cell-wall fraction (Fraction I) and in Fraction IV (soluble) of maize plants, whereas Cd-treated pea accumulated more Cd in the soluble fraction. The protein concentration of Fraction IV of pea shoot and root significantly increased upon treatment with 0.05 mM Cd, whereas maize showed no effect. Furthermore, a previously not visible protein (∼12 kDa), appeared in Fraction IV of pea root grown with the highest Cd supply. Cadmium treatment, in general, notably enhanced the concentrations of 2-thiobarbituric acid reactive material (lipid peroxidation products) in pea fractions, presumably due to Cd-induced oxidative stress.

Salinity, oxidative stress and antioxidant responses in shoot cultures of rice

Abstract: When shoot cultures derived from salt-sensitive Oryza sativa var. Taipei 309 were grown at 25°C in medium containing 0.35 M NaCI, responses to possible oxidative stress in the early stages of exposure were observed. Overall levels of Mn-superoxide dismutase activity, Cu, Zn-superoxide dismutase activity and H 2 O 2 were significantly elevated. After 1 d there was a notable decline in tissue concentrations of GSH and a corresponding increase in GSSG. However, after a further day, concentrations of GSH and GSSG returned to concentrations normally encountered in control cultures. Activities of ascorbate peroxidase and catalase were similar whether the shoots were grown in the presence or absence of NaCI. In contrast, there was an early increase in glutathione reductase activity in NaCI-exposed cultures, and no indication of extensive increases in lipid peroxidation. Thus although some indications of oxidative stress accompany exposure of this salt-sensitive rice variety to salinity, mechanisms appear to exist within its shoot tissue to permit the tolerance of such oxidative stress.

Abscission of flowers and floral parts

Abstract: The abscission of inflorescences, flowers, petals, sepals, styles, and stamens is discussed, with emphasis on the anatomy and ultrastructure of the abscission zones, and the role of cell wall degrading enzymes and hormonal control Shedding of these parts is usually due to cell wall dissolution, but abscission of petals, stamens, and styles in some species occurs due to the forces generated by the growing fruit Flower abscission is clearly regulated by ethylene, whilst auxins apparently decrease the sensitivity to ethylene Petal, style and stamen abscission also seems to be controlled by endogenous ethylene Auxin is apparently involved in abscission of styles and stamens, but in petals its role is at yet unclear The ultrastructural data indicate high protein synthesis and high secretory activity of material toward cell walls of abscission zone cells The physiological evidence indicates a role of both polygalacturonase and cellulase in cell wall dissolution, whilst the role of other cell wall degrading enzymes is still unknown The physiological processes occurring in the walls of the separating cells should be distinguished from those relating to defence against microbial intrusion, such as deposition of lignin and suberin and tylose formation Experimentation using mutants and transgenic plants may aid in separating these processes Sequencing of the isoenzymes specific for the abscission zone and a search for abscission zone-specific promoters seems a requirement for the successful evaluation of the enzymes involved in cell wall degradation

Response of ovule development and post-pollen production processes in male-sterile tomatoes to chronic, sub-acute high temperature stress

Abstract: In order to determine the effects of high temperature on ovule development and reproductive processes subsequent to pollen production, nine day/night temperature combinations were imposed over a 9 month period as four separate experiments, each with three treatments, including one common treatment. In order to eliminate known effects of high temperatures on pollen production and stylar position, high temperature treatments were applied only to male-sterile tomatoes (Lycopersicon esculentum Mill.). Pollen was obtained from male-fertile plants given optimal growth conditions. This allowed comparison of mean daily temperatures from 25-29 °C; day/night temperature differentials (DIFs) of 2, 6, and 10 °C; day temperatures of 28, 30 and 32 °C at night temperatures of 22, 24, and 26 C; and night temperatures of 22, 24 and 26 °C at day temperatures of 28, 30 and 32 °C. Average weight per fruit and flower number did not demonstrate a consistent pattern of response to high temperature. Other reproductive characteristics (% fruitset, total number and weight of fruit per plant, and seediness index) decreased as mean daily temperature rose from 25 °C to 26 °C and from 28 °C to 29 °C. The primary parameter affecting these variables was mean daily temperature, with day temperature having a secondary role. Thus, in determining reproductive responses of tomato to temperatures within this range, day temperature, night temperature and DIFs do not need to be considered independently of their effect on mean daily temperature. If this relationship holds true in other species, and for pre-pollen production processes as well, modelling the effects of projected climate change should be simplified.

Effects of pollination on floral attraction and longevity

Abstract: The end of a flower's attraction to pollinators may be due to a range of visible cues such as permanent flower closure, a colour change, and withering or abscission of the petals. Floral attraction may be reduced by pollination. Pollination-induced conclusion of floral attraction is often due to a colour change or to flower closure. This may or may not be followed by a reduction in floral longevity, defined as the time to petal withering, wilting or shattering. In a few species floral longevity is increased following pollination-induced flower closure or a pollination-induced change in colour. Floral attraction, therefore, has to be distinguished from floral longevity. A literature survey shows that pollination rapidly reduces floral attraction in numerous orchids, but among other plant families only about 60 genera have been found to show pollination-induced shortening of floral attraction. Although only a few species have been investigated, it was invariably established that the effect of pollination is blocked by inhibitors of ethylene synthesis or ethylene perception, hence is mediated by ethylene. The flowers that cease to be attractive to pollinators, shortly following pollination, tend to be from families that are known mainly to comprise species in which flower longevity, petal colour, or flower closure, is sensitive to exogenous ethylene. This indicates that the effect of pollination on floral attraction is generally mediated by endogenous ethylene. Numerous species reportedly show a decrease in the period of floral attraction after exposure to ethylene, whereas only for a small number of species a decrease in the period of floral attraction induced by pollination has been observed. This discrepancy may be due to the greater attention that has been paid to the effects of ethylene. Nonetheless, the possibility remains that endogenous ethylene has a role in changing perianth form and colour in addition to signalling the occurrence of pollination.

Pathways for the permeation of Na+ and Cl− into protoplasts derived from the cortex of wheat roots

Abstract: Sodium permeation into cortex cells of wheat roots was examined under conditions of high external NaCI and low Ca 2+ Two types of K + inward rectifier were observed in some cells The time-dependent K + inward rectifier was Ca 2+ -sensitive, increasing in magnitude as external Ca 2+ was decreased from 10 mM to 01 mM, but did not show significant permeability to Na + However, the spiky inward rectifier showed significant Na + permeation at Ca 2+ concentrations of 1 and 10 mM In cells that initially did not show K + inward rectifier channels, fast and sometimes slowly activating whole-cell inward currents were induced at membrane potentials negative of zero with high external Na + and low Ca 2+ concentrations With 1 mM Ca 2+ in the external solution, large inward currents were carried by Rb + , Cs + , K + , Li + , and Na + The permeability sequence shows that K + , Rb + and Cs + are all more permeant than Na + , which is about equally as permeant as Li + When some K + was present with high concentrations of Na + the inward currents were larger than with K + or Na + alone About 60% of the inward current was reversibly blocked when the external Ca 2+ activity was increased from 003 mM to 27mM (half inhibition at 031mM Ca 2+ activity) Changes in the characteristics of the current noise indicated that increased Ca 2+ reduced the apparent single channel amplitude In outside-out patches inward currents were observed at membrane potentials more positive than the equilibrium potentials for K + and Cl - when the external Na + concentration was high These channels were difficult to analyse but three analysis methods yielded similar conductances of about 30 pS

Calcium channels in higher plant cells: selectivity, regulation and pharmacology

Abstract: Rapid influx of Ca 2+ into the cytosol from extracellular pools or intracellular stores via ion channels can have wide-ranging physiological consequences. In addition, influx of Ca 2+ across the plasma membrane is necessary for the large net accumulation of Ca 2+ essential for cellular integrity. In this paper, the properties of Ca 2+ channels in various plant membranes are reviewed, and compared with new results on the Ca 2+ channel from the plasma membrane of wheat roots (rca channel) described originally by Pineros and Tester (1995). The rca channel has been studied at the single channel level by incorporation of plasma membrane-enriched vesicles into planar lipid bilayers. It has a high affinity for Ca 2+ permeation (K m =99 μM) and a maximal conductance of 30 pS. It is highly selective for Ca 2+ over Cl - , but allows the movement both of other divalent cations (with a conductivity sequence: Ba 2+ > Sr 2+ > Ca 2+ > Mg 2+ > Mn 2+ ) and of monovalent cations. The affinity for K + permeation was 6 mM, and the maximal conductance was 164 pS. The permeability ratio, P Ca 2+/P K + ranged from 17 to 41, decreasing with increasing extracellular Ca 2+ . With physiologically reasonable membrane potentials and ionic conditions, the channel will catalyse Ca 2+ influx. At normal resting potentials (negative of about -135 mV) the channel remains largely closed, but activates rapidly upon depolarization. It is insensitive to ABA and Ins 1,4,5-P 3 , but the voltage-dependence for activation was shifted to more negative potentials upon addition of cytosolic ATP. The channel was inhibited by a range of trivalent cations (La 3+ , Al 3+ and Gd 3+ ) and by some organic Ca 2+ channel effectors (verapamil, diltiazem, ruthenium red), although it was insensitive to bepridil and 1,4 dihydropyridines [nifedipine and (+) and (-) 202-791], at least in the conditions described here. The properties of this channel are compared with those of other plant and animal Ca 2+ channels, and are shown to be consistent with its proposed physiological role of divalent cation uptake into roots.

Scaling to a common temperature improves the correlation between the photosynthesis parameters Jmax and Vcmax

Abstract: A strong correlation between the photosynthetic parameters J max and V cmax was found by Wullschleger (1993) in a survey of 109 plant species. Measurements were made at various leaf temperatures, but the temperature dependence of J max and V cmax differ. Once values for J max and V cmax in Wullschleger's analysis were adjusted to a common temperature, using an equation for the temperature dependence of these parameters, the slope of the linear regression for J max versus V cmax forced through the origin increased from 1.97 to 2.68, and r 2 increased from 0.79 to 0.87.

High frequency transformation and regeneration of transgenic plants in the model legume Lotus japonicus

Abstract: The molecular analysis of plant genes involved in nodulation has been slowed by the inability to produce high numbers of transgenic legume lines. The high efficiency gene transfer and plant regeneration systems of the model legume Lotus japonicus is described. A collection of wild-type A. rhizogenes strains was tested for infectivity and the most virulent strains, 9402 and AR10, were selected for further use. Growth conditions for plantlets, induction of hairy roots and nodulation of composite plants were optimized for large-scale screening in Petri dishes. A cluster of 3-10 nodules was regularly formed on transgenic hairy roots 7-12 d after inoculation with the effective Rhizobium loti strain NZP2235. There were no apparent morphological differences between nodulation of hairy and wildtype roots. To test the applicability of the hairy root system for the trapping of symbiotic genes, transformation experiments with binary vectors possessing a 8-g/ucuronidas e {gus, uidA) or a luciferase [luc) reporter driven by a cauliflower mosaic virus (CaMV) 35S promoter were performed. The frequency of cotransfer of a binary T-DNA with a root-inducing (Ri) T-DNA was 70%. Positive expression suggests that gus and luc trap vectors can be used for gene tagging in L. japonicus. To open the possibility of searching for mutant phenotypes, a regeneration system has been developed enabling the regeneration of large numbers of transgenic plants from hairy root cultures in about 5-6 months. At the same time, the A. tumefaciens hypocotyl transformation regeneration in L. japonicus has been improved. This new version provides fertile transgenic plants in about 4 months.

Antioxidant enzyme responses to chilling stress in differentially sensitive inbred maize lines

Abstract: Antioxidant enzyme activities were determined at the first, third and fifth leaf stages of four inbred lines of maize (Zea mays L.) exhibiting differential sensitivity to chilling. Plants were exposed to a photoperiod of 16:8 L: D for one of three treatments: (a) control (25°C), (b) control treatment plus an exposure to a short-term chilling shock (11°C 1d prior to harvesting), and (c) long-term (11 °C constant) chilling exposure. Catalase (CAT; EC 1.11.1.6), ascorbate peroxidase (ASPX; EC 1.11.1.11), superoxide dismutase (SOD; EC 1.15.1.1), glutathione reductase (GR; EC 1.6.4.2), and monodehydroascorbate reductase (MDHAR; EC 1.6.5.4) activities were assessed. Reducing and non-reducing sugars and starch concentrations were determined as general metabolic indicators of stress. Reduced activities of CAT, ASPX, and MDHAR may contribute to limiting chilling tolerance at the early stages of development in maize. Changes in levels of sugar and starch indicated a more rapid disruption of carbohydrate utilization in comparison to photosynthetic rates in the chilling-sensitive line under short-term chilling shocks and suggested a greater degree of acclimation in the tolerant lines over longer periods of chilling.

Transport, synthesis and catabolism of abscisic acid (ABA) in intact plants of castor bean (Ricinus communis L.) under phosphate deficiency and moderate salinity

Abstract: Flows of abscisic acid (ABA) were investigated in wholeplants of castor bean [Ricinus communis) grown insand culture under either phosphate deficiency ormoderate salinity. Xylem transport of ABA inP-deficient plants was stimulated by a factor of 6whereas phloem transport was affected only veryslightly. ABA deposition into leaves of P-deficientplants was not appreciably different from the controlsbecause of strong net degradation in leaves. Sinceconjugation of ABA was strongly reduced in all organsof P-deficient plants ABA was presumably metabolizedmainly to phaseic acid and dihydrophaseic acid. Theincreased import of ABA occurred predominantly intofully differentiated but not senescent leaves andshowed a good correlation with the inhibition of leafconductance under P deficiency.As with low-P-plants salt stress increased ABA syn-thesis in roots and associated transport in the xylem.However, salinity caused a distinctly greater accumu-lation of ABA in the leaves, stem segments and theapex than in P-deficient plants. As opposed to P defi-ciency, ABA export in the phloem from the leaves wasstimulated by salinity. Modelling of ABA flows withinan individual leaf over its life cycle showed that younggrowing leaves imported ABA from both phloem andxylem, whereas the adult non-senescent leaves werea source of ABA and thus provided a potential shoot-to-root stress signal as well as an acceptor for recip-rocal signals from root to shoot. In senescing leavesABA flows and accumulation were somewhat retardedand ABA was lost in net terms by export from the leaf.Key words: Abscisic acid, phosphorus deficiency, saltstress, phloem and xylem transport.

Effects of P deficiency on assimilation and transport of nitrate and phosphate in intact plants of castor bean (Ricinus communis L.)

Abstract: An experimentally-based modelling technique was applied to describe quantitatively the uptake, translocation, storage, and assimilation of NO 3 - and H 2 PO 4 - over a 9 d period in mid-vegetative growth of sand-cultured castor bean (Ricinus communis L.) which was fed 12 mM NO 3 - and either 0.5 or a severely limiting 0.005 mM H 2 PO 4 - . Model calculations were based on increments or losses of NO 3 - and reduced N or of H 2 PO 4 - and organic P in plant parts over the study period, on the concentrations of the above compounds in xylem and phloem sap, and on the previously determined flows of C and N in the same plants (Jeschke et al., 1996). Modelling allowed quantitative assessments of distribution of NO 3 - reduction and H 2 PO 4 - assimilation within the plant. In control plants 58% of total NO 3 - reduction occurred in leaf laminae, 40% in the root and 2% in stem and apical tissues. Averaged over all leaves more than half of the amino acids synthesized in laminae were exported via phloem, while the root provided 2.5-fold more amino acids than required for root growth. P deficiency led to severe inhibition of NO 3 - uptake and transport in xylem and even greater depression of NO 3 - reduction in the root but not in the shoot. Accentuated downward phloem translocation of amino acids favoured root growth and some cycling of N back to the shoot. In control plants H 2 PO 4 - was the principal form of P transported in xylem with young laminae acting as major sinks. At the stem base retranslocation of P in the phloem amounted to 30% of xylem transport. H 2 PO 4 - assimilation was more evenly distributed than NO 3 - reduction with 54% occurring in leaf laminae, 6% in the apical bud, 19% in stem tissues, 20% in the root; young tissues were more active than mature ones. In P-deficient plants H 2 PO 4 - uptake was severely decreased to 1.8% of the control. Young laminae were the major sink for H 2 PO 4 - . Considerable remobilization of P from older leaves led to substantial shoot to root translocation via phloem (50% of xylem transport). Young leaf laminae were major sites of H 2 PO 4 - assimilation (50%), followed by roots (26%) and the apical bud (10%). The remaining H 2 PO 4 - was assimilated in stem and mature leaf tissues. Old leaves exhibited 'negative' net assimilation of H 2 PO 4 - , i.e. hydrolysis of organic P exceeded phosphorylation. In young laminae of low P plants, however, rates of H 2 PO 4 - assimilation per unit fresh weight were comparable to those of the controls.

Dissecting complex physiological functions through the use of molecular quantitative genetics

Abstract: Testing possible associations between physiological and biochemical traits by comparing plant phenotypes and looking for correlations between them is unreliable. The development of molecular marker technologies offers powerful alternative methods to examine the relationships between traits. This review describes the genetical methods required to analyse possible associations between traits that are inherited in a quantitative manner using quantitative trait locus (QTL) analysis. The regulation of carbohydrate metabolism is chosen as an example of how QTL analysis can be used to identify key control factors in a series of processes, by identifying possible candidate genes for QTL effects on sucrose and starch metabolism. Methods are also described to study the association between physiological traits such as abscisic acid concentrations and stomatal conductance. Advantages and some limitations of QTL analysis over other methods currently in use by physiologists to test associations between traits are discussed.

Reconsidering the rhizotoxicity of hydroxyl, sulphate, and fluoride complexes of aluminium

Abstract: Recent years have witnessed some convergence of opinion regarding the identity of the rhizotoxic species of aluminium. AI04AI12(0H)24(H20)?2+ (AI13) and Al 3+ are almost certainly toxic, but no rhizotoxicity has been detected for AI-S04 (AISO4+ and AlfSOJj") or Al-F (e.g. AIF2+ and AIF2+). The status of AI-OH (e.g. AIOH2+ and AI(OH)2+) is uncertain because experimental results often appear to indicate AI-OH toxicity. In this article it is argued that this appearance of toxicity is the consequence of the relief of Al3 + toxicity by H + (and vice versa). Furthermore, this view provides an explanation for phenomena, such as Al stimulation of growth, unexplained by the hypothesis that AI-OH is toxic. It is concluded, therefore, that AI-OH is not toxic at achievable activities. The previous failure to detect toxicity from AI-S04 and AI-F also may have been a consequence of the low activities used in the experiments. New experiments again failed to reveal a toxicity for AI-SO4, but do indicate that AIF2+ and AIF2+ are toxic. Several alternative hypotheses for the apparent toxicity of Al-F were considered and rejected. Consequently, this report concludes that the following Al species are toxic to wheat roots in the following order: AI 13 >AI 3+ >AIF 2+ > AIF 2 +. The correlation between toxicity and charge is discussed in light of the fact that the binding strength of cations to plasma membrane surfaces generally increases with charge.

Modification of the root plasma membrane lipid composition of cadmium-treated Pisum sativum

Abstract: The effects of in vivo Cd treatments on pea root plasma membrane (PM) lipid composition were studied. In the long-term experiment, plants were supplied with Cd: moderate stress (10 fitA) or strong stress (50 //M) for 10 d. Growth of root and shoot was severely affected in 50 fiM Cd-treated plants, as evidenced by the approximately 7-fold reduction in their Relative Growth Increment (RGI)- Treatment with Cd (10 ftM) resulted in changes to the lipid composition of the pea root PM, including increases in the degree of unsaturation of phospholipid-associated fatty acids and in the relative amount of stigmasterol (30-42%). This change was accompanied by a reduction in sitosterol content (26.8 to 17.4 //g mg 1 protein). However, the sterol composition was not altered in plants treated with 50 //M Cd for 10 d. The content of phosphatidylethanolamine and phosphatidylcholine (major phospholipids present in pea root PM) decreased as Cd level increased, but the ratio between them remained unaffected. In the short-term experiment, plants exposed to Cd (50 fiM) accumulated less sitosterol (from 27.7 to 14.0 fig gmg' 1 protein) over 72 h, but no significant effect on other measured lipids was observed. The physiological repercussions of changes in plasma membrane lipid composition, as a result of Cd exposure are discussed.

Hormonal signalling in cereal aleurone

Abstract: The cereal aleurone is an excellent model system for hormonal signalling in plants. When treated with gibberellins (GAs), cereal aleurone layers and aleurone protoplasts initiate signalling cascades that result in the synthesis and secretion of hydrolytic enzymes, most notably α-amylases. Abscisic acid (ABA) antagonizes the effects of GA and stimulates the production of ABA-up-regulated proteins. Receptors for GA and ABA have been localized to the aleurone cell PM, and evidence suggests that another ABA receptor functions within the cytoplasm. Cytosolic and membrane-bound second messengers have been identified and signal transduction pathways are beginning to be understood. Transacting factors that regulate the transcription of hormonally induced genes have been cloned and bring us closer to linking cytosolic signals to changes in gene expression. In this review, recent data pertaining to hormonal signalling in cereal aleurone are summarized. Relationships between signalling and effector molecules are discussed, and models for hormone-induced signalling pathways are proposed.

Sloughing of root cap cells decreases the frictional resistance to maize (Zea mays L.) root growth

Abstract: Root caps provide a protective layer in front of the meristem that protects the meristem from abrasion by soil particles. The continuous production and sloughing of the root cap cells may be an adaptation to decrease the friction at the soil-root interface by acting as a low-friction lining to the channel formed by the root. Experiments were performed which provide the first direct evidence that such cell sloughing decreases frictional resistance to root penetration. The penetration resistance (force per unit crosssectional area) to maize roots, which were pushed mechanically into the soil, was compared with the penetration resistance to growing roots and to 1 mm diameter metal probes (cone semi-angles of 7.5° or 30°). The pushed roots experienced only about 40% of the penetration resistance experienced by the 7.5° metal probe that was pushed into the soil at the same rate. Thus, the friction between the soil and the pushed root was much smaller than between the soil and the metal probe. The penetration resistance to the growing root was between 50% and 100% of that to the pushed root, indicating that the relief of friction and slower rate of soil compression were more efficient around the growing root. SEM examination of the surface of roots pushed or grown into the soil showed that numerous root cap cells had detached from the cap and slid for several millimetres relative to the root. The low friction properties of roots may be due largely to the low coefficient of friction between sloughing root cap cells, and may be decreased further by intracellular mucilage secretions.

Elevated CO2 reduces O3 flux and O3-induced yield losses in soybeans: possible implications for elevated CO2 studies

Abstract: Soybeans were grown for three seasons in open-top field chambers to determine (1) whether elevated CO2 (360 versus 700 //mol mol"1) alleviates some of the yield loss due to pollutant 0 3, (2) whether the partial stomatal closure resulting from chronic 03 exposure (charcoal-filtered air versus 1.5 x ambient concentrations) is a cause or result of decreased photosynthesis, and (3) possible implications of C0 2 /0 3 interactions to climate change studies using elevated CO2. Leaf conductance was reduced by elevated C02, regardless of O3 level, or by exposure to 03 alone. As. a result of these effects on conductance, high C0 2 reduced estimated midday 03 flux into the leaf by an average of 50% in charcoal-filtered air and 35% in the high 03 treatment. However, while exposure to O 3 reduced seed yields by 41% at ambient CO2 levels, the yield reduction was completely ameliorated by elevated CO2. The threshold midday 03 flux for yield loss appears to be 20-30 nmol m~2 s" 1 in this study. Although elevated CO2 increased total biomass production, it did not increase seed yields. A/C, curves show a large reduction in the stomatal limitation to photosynthesis due to elevated CO2, but no effect of O3. These data demonstrate that (1) reduced conductance due to O3 is the result, and not the cause, of reduced photosynthesis, (2) 700 //mol mol"' CO2 can completely ameliorate yield losses due to 03 within the limits of these experiments, and (3) some reports of increased yields under elevated CO2 treatments may, at least in part, reflect the amelioration of unrecognized suppression of yield by 03 or other stresses.

The development of the grape berry cuticle in relation to susceptibility to bunch rot disease

Abstract: Some physical and morphological factors of grape berry cuticle were investigated at different developmental stages of three clones of Vitis vinifera cv. Pinot noir. The surface morphology of grape berries was examined by scanning electron microscopy and cuticle anatomy was examined by light and transmission electron microscopy. During the period from flowering to maturity, the composition of the cuticular waxes changed, corresponding with an increase of waxy deposits and significant modifications of the wax surface morphology. The content in cutin per unit surface decreased more than 2.5-fold between berry set (16 d after anthesis) and veraison of the grape berries, and might predispose the grape berry to fungal infection. This result was correlated with the differentiation of the cuticle layers and particularly with a decrease in the thickness of the primary cuticle at harvest.

From signal to form: aspects of the cytoskeleton-plasma membrane-cell wall continuum in root hair tips.

Abstract: Root hairs are excellent cells for the study of the exocytotic process that leads to growth in higher plants, because the exocytotic event takes place locally and because the cells are directly accessible for signals, drugs, fixatives, microinjection, and microscopic observation. Well-characterized lipochitooligosaccharides, signal molecules excreted by Rhizobium bacteria, induce root hair growth which can be recorded microscopically in a root hair deformation assay developed for Vicia sativa L. Root hair deformation is a morphogenetic process involving swelling of the hair tip and subsequent new hair outgrowth from that swelling. This response to the signal occurs at a specific developmental stage, namely when hairs are terminating growth. Thus, since polar growth can be triggered intentionally, the system allows the study of growth phenomena in higher plants at the cellular level. Furthermore, important advances are being made with molecular genetics that will allow the unravelling of the signal transduction pathways in root hair morphogenesis leading to growth. This paper first discusses cytological phenomena involved in the process of polar growth, such as cytoplasmic polarity, cytoplasmic streaming and the organization of actin filaments, the location of a spectrin-like antigen, the distribution of intracellular calcium, cortical microtubules and cell wall texture, endocytosis by means of coated pits, and physical aspects of the incorporation of exocytotic vesicles into the plasma membrane. In the second part, changes are discussed that occur in some of these phenomena when growth is influenced by growth regulators and mutations.

Developmental control of xylem hydraulic resistances and vulnerability to embolism in Fraxinus excelsior L.: impacts on water relations

Abstract: The hydraulic properties and leaf gas exchanges of Fraxinus excelsior L. branches differing by their age and their vertical crown position, but in comparable ambient air conditions (vapour pressure deficit and global radiation) were compared. The variations in leaflet water potential ψleaflet, leaflet stomatal conductance and transpiration rate, E, were small between different branches of the same crown. Whole branch hydraulic resistances (r branch ), and partitioning between leaf (r leaf ) and xylem resistance (r xylem ) were assessed with a high pressure flowmeter. r leaf represented 90% and 10% of r branch for upper and lower crown branches, respectively. The changes resulted from increases in r xylem caused by the formation of short shoot internodes mostly located in secondary axes. However, leaf area-specific branch resistances (r branch =r branch x LA) were nearly constant throughout the crown. This was consistent with the vertical variations in ψ leaflet because r branch x E represents the water potential drop from the trunk to the leaves. Because r xylem was higher, lower ψxylem values were predicted in lower crown rachises. However, rachises from lower crown branches were less vulnerable to embolism than in upper branches ( ψxylem at onset of embolism, ψ cav, were -3 and -2MPa, respectively). It was concluded that r xylem increased with branch age, but r* branch remained constant because LA decreased. As a consequence, E was maximized and ψxylem remained above ψcav, This suggested that, in Fraxinus, leaf gas exchanges and leaf areas were coupled with xylem hydraulic capacities probably through a control of bud activity.

Primary responses of root and leaf elongation to water deficits in the atmosphere and soil solution

Abstract: Plants experience drought by a limitation of water supply and by enhanced transpiration. Both processes tend to decrease the plant's water potential, but affect growth responses in the root and leaf differently. The evaluation of the underlying mechanisms leads to a discussion of recent studies on biophysical aspects of cell expansion at a cellular, tissue and organ level. Two processes enable roots to compensate rapidly effects of water deficits originating in the medium: (i) adjustment of the minimum pressure in cells required for expansion (yield threshold), and (ii) solute transport within the elongation zone. Limitations of root growth are discussed with respect to hydraulic, mechanical, and solute relations in the root elongation zone. It is argued that the variable nature of both the yield threshold and solute transport challenges the applicability of the Lockhart concept to determine growth-related parameters from steady conditions of turgor and growth. On a whole organ level, the attenuation of xylem pressure along the root is important for the differential response of root and leaf growth. Experimental evidence is presented for the hydraulic separation of the elongation zones, which is closely related to root development and functioning. The data obtained over the past few years have been used to extend mathematical models of growth and water transport in roots.

A patch clamp study of Na+ transport in maize roots

Abstract: The mechanisms mediating Na + transport in higher plant roots were investigated by applying the patch clamp technique to protoplasts isolated from the cortex and stele of maize roots. In the cortex, permeation of Na + through a time-dependent K + -selective inward rectifier was negligible. Instead, Na + influx into maize roots probably occurs via an instantaneously-activating current. This current was partially inhibited by extracellular Ca 2+ , but was insensitive to extracellular TEA + , Cs + and TTX. In outside-out patches, a plasma membrane ion channel was found which mediated an inward Na + current which, at least in part, underlies the whole-cell instantaneously-activating current. The unitary conductance of this channel was 15 pS in 102:121 mM Na + (outside:cytosol). Channel gating was voltage-independent and distinct from that observed for the inwardly rectifying K + -selective channel in the same cell type. Increasing extracellular Ca 2+ from 0.1 to 1 mM reduced the open probability and unitary conductance of this channel. In 102 mM Na + :123 mM K + (outside:cytosol) a P Na : P K of 2.1 was calculated. It is suggested that the plasma membrane Na + -permeable channel identified in the cortex of maize roots represents a pathway for low affinity Na + uptake by intact maize roots. In the stele, permeation of Na + through outwardly rectifying K + channels was found to be negligible and the channels are thus unlikely to be involved in the transport of Na + from the root symplasm.