Showing papers in "Australian Journal of Plant Physiology in 1993"
TL;DR: Although acclimation to low irradiance reduced the photosynthetic capacity per unit nitrogen by 12%, the considerable reorganisation of proteins within the thylakoids increased potential daily photosynthesis by 20% over that which would have been gained by a 'sun' leaf.
Abstract: Nitrogen redistribution between and within leaves was examined in a plot of lucerne (Medicago sativa L. cv. Aurora) in relation to potential canopy photosynthesis. The canopy was sampled during regrowth after cutting and just prior to flowering. As leaves were progressively shaded by the newly produced leaves, nitrogen content fell and photosynthetic acclimation occurred. The rate of acclimation in the canopy was the same as occurred following a step change to 23 or 6% sunlight. The profile of leaf nitrogen content was stable with respect to leaf area index and independent of time of sampling. Optimal profiles of nitrogen distribution between leaves, photosynthetic rate per unit chlorophyll and nitrogen partitioning within leaves were calculated from the relationships between photosynthesis and nitrogen in conjunction with the light environment of the preceding 3 days. The optimal nitrogen content of the leaves should vary in proportion to the relative daily irradiance at each leaf. The observed distribution achieved 88% of the potential daily photosynthesis, while a uniform nitrogen distribution yielded only 80%. Photosynthetic acclimation and nitrogen partitioning within each leaf both responded to daily irradiance similarly to the calculated optimum except at the two extremes. At the top of the canopy, photosynthetic rate per unit of chlorophyll did not increase as much as the calculated optimum, while at the base of the canopy, nitrogen partitioning failed to fall as much as the calculated optimum. This may reflect the constraints on the flexibility of the photosynthetic system. Nitrogen redistribution between leaves made a dramatic contribution to increasing the potential photosynthesis by the canopy. Although acclimation to low irradiance reduced the photosynthetic capacity per unit nitrogen by 12%, the considerable reorganisation of proteins within the thylakoids increased potential daily photosynthesis by 20% over that which would have been gained by a 'sun' leaf. However, in terms of canopy photosynthesis, which is dominated by leaves intercepting most of the light, acclimation contributed only a few per cent to the potential daily canopy photosynthesis.
206 citations
TL;DR: It is proposed that the influence of heating on final grain dry weight is attributable to the observed reductions of soluble starch synthase activity.
Abstract: Ears of wheat were exposed for up to 10 days during the grain-filling stage to high temperature (35oC) and activities of five enzymes in the sucrose to starch pathway were compared to those in ears maintained at lower temperature (21oC day/16oC night). Two cultivars of wheat known to differ in their post-anthesis tolerance of high temperature were compared. On a per grain basis, the activity of sucrose synthase and of ADPglucose pyrophosphorylase in ears maintained at 21/16oC throughout did not change greatly between days 16 and 32 after anthesis, whereas UDPglucose pyrophosphorylase and soluble starch synthase activities declined with advancing development. Soluble starch synthase activity in grains of heated ears was decreased within 1 day to about one- half of the value in unheated grains, and 3 days' additional heating did not reduce the activity much further. Insoluble starch synthase activity was not significantly reduced by heating. Compared to soluble starch synthase, ADPglucose pyrophosphorylase activity was more slowly affected and decreased to a lesser extent by heat. Sucrose synthase and UDPglucose pyrophosphorylase activities were either not affected or only slightly reduced; part of this reduction could be due to advanced development at the higher temperature. In recovery experiments ears were heated for brief periods and then returned to 21/16oC for a few days. ADPglucose pyrophosphorylase and soluble starch synthase activities recovered in the cooler conditions but the other two enzymes generally only maintained or lost further activity. From a comparison of the activities of these enzymes with the rate of starch deposition, and by taking into account the effects of heating, it is proposed that the influence of heating on final grain dry weight is attributable to the observed reductions of soluble starch synthase activity.
195 citations
TL;DR: In this paper, the role of ABA as a long-distance signal in controlling growth by root-to-shoot communication is unclear: the concentrations found in xylem sap can affect stomatal conductance, but seem too low to affect leaf expansion.
Abstract: Hormones appear to be important in controlling plant growth in soils of low water potential, particularly in changing the root:shoot ratio as the soil dries or becomes saline, and in communicating soil conditions to the leaves. This review has necessarily focused on abscisic acid (ABA), as there is little information about the role of other hormones in controlling growth in dry or saline soils. ABA is partly responsible for the differential response of root and shoot growth to dry soils. In dry soil it maintains root growth and inhibits shoot growth. However, when applied to well-watered plants, it usually inhibits root and shoot growth, showing that plants in dry soil respond quite differently from well-watered plants. ABA affects the rate of cell expansion in plants in dry soils: it maintains cell expansion in roots and inhibits that in leaves. It may also affect the rate of cell production, but little is known about this. The role of ABA as a long-distance signal in controlling growth by root-to-shoot communication is unclear: the concentrations found in xylem sap can affect stomatal conductance, but seem too low to affect leaf expansion. Yet drought and salinity generally affect leaf expansion before they affect leaf conductance. A possible solution to this puzzle is that ABA is transported in xylem sap in a complexed form, or that another compound in xylem sap stimulates the synthesis or activity of ABA in leaves, or affects leaf expansion independently of ABA.
191 citations
TL;DR: Based on a comparison of the response of many species to different irradiances during growth, it is generally the case that the proportion of thylakoid nitrogen increases for leaves grown under lower irradiance so as to maintain a constant ratio of photosynthetic capacity to total leaf nitrogen.
Abstract: Acclimation by the photosynthetic system to the gradient in irradiance through a leaf canopy was investigated with a plot of lucerne (Medicago sativa L. cv. Aurora). The aims were to determine the extent to which acclimation occurred in a natural canopy and to quantify the changes in the partitioning of nitrogen within the leaf that are associated with acclimation. The canopy grew up around light sensors placed at 10 cm height increments which logged the irradiance at 1 min intervals for the 4 days that preceded sampling. Photosynthetic capacity was measured with leaf disc oxygen electrodes and the chlorophyll, soluble protein and nitrogen contents of the leaves were determined. Daily irradiance declined exponentially down through the canopy. Nitrogen content and photosynthetic capacity both declined down through the canopy. Photosynthetic acclimation by the lower leaves was evident from the lower chlorophyll a/b ratios and reduced photosynthetic capacity per unit chlorophyll. The lower photosynthetic capacity per unit of chlorophyll was offset by an increased proportion of leaf nitrogen present in the thylakoids. Consequently, the photosynthetic capacity per unit leaf nitrogen was nearly independent of irradiance. Based on a comparison of the response of many species to different irradiances during growth, it is generally the case that the proportion of thylakoid nitrogen increases for leaves grown under lower irradiance so as to maintain a constant ratio of photosynthetic capacity to total leaf nitrogen. However, the ratio of photosynthetic capacity to total leaf nitrogen varies widely between species.
181 citations
TL;DR: Xylem sap contained considerable amounts of bound ABA, the level of which increased during total root drying and decreased again after rewatering, and level of cytokinins, zeatin (t-Z)+zeatin riboside ( t-ZR) and isopentenyladenine (2iP) + isopENTenyladenosine ( 2iPA), on the contrary, decreased duringRoot drying and increased again afterRewatering.
Abstract: Seedlings of rice cv. IR 36 were grown in soil in small pots with a horizontally divided root system: after 6-7 weeks, about 20% of the entire root system had protruded through the holes at the base of the pots and was kept in contact with nutrient solution. At this stage the plants were exposed to three different treatments: (a) the soil was kept watered and the protruding free roots were dried in air; (b) the free roots were kept moist and the soil left unwatered; (c) both soil and protruding roots were left unwatered for 30 h and then rewatered. During the first hours of treatment a and b, a decline in stomatal conductance was observed, whereas the stem water potential remained unchanged. The concentration of abscisic acid (ABA) in the xylem, however, increased. At later stages of treatment a and b, the stem water potential began to decrease with a parallel further increase of xylem ABA. Xylem sap contained considerable amounts of bound ABA, the level of which increased during total root drying and decreased again after rewatering. Level of cytokinins, zeatin (t-Z)+zeatin riboside (t-ZR) and isopentenyladenine (2iP) + isopentenyladenosine (2iPA), on the contrary, decreased during root drying and increased again after rewatering. The results are discussed with regard to a possible function of ABA and cytokinins as root-to-shoot signals.
172 citations
TL;DR: The response of grapevine plants to severe water deficit, imposed at a rate of about 0.16 MPa day-1, was studied in terms of leaf water relations characteristics, stomatal behaviour and gas exchange, and Photosynthesis was markedly reduced in water-stressed plants.
Abstract: The response of grapevine plants to severe water deficit (predawn leaf water potential of - 1.13 MPa), imposed at a rate of about 0.16 MPa day-1 was studied in terms of leaf water relations characteristics, stomatal behaviour and gas exchange. Carbohydrate status of leaves was also analysed in order to assess the contribution of soluble sugars as osmotic solutes during drought. Pressure/volume analysis showed an active osmotic adjustment in water-stressed leaves, which decreased osmotic potential at full turgor by 0.45 MPa and the apoplastic water fraction showed a reduction of 19% as compared to the well- watered plants. Cell wall elasticity was not significantly affected by water stress, and turgor loss point in stressed leaves was reached at lower water potential and relative water content values than in the well-watered controls. Photosynthesis was markedly reduced in water-stressed plants. However, well-watered and water-stressed leaves had similar concentrations of glucose and fructose. The concentrations of sucrose and starch decreased in water-stressed leaves. This accounted for a marked decrease in the ratio of leaf dry weight to area in droughted plants. The changes in concentrations of soluble carbohydrates could not account for the difference in osmotic potential between water-stressed and well-watered leaves.
138 citations
TL;DR: Within the cell cycle, hormone-dependent control points were identified at late G1 and at mitotic initiation by measurement of nuclear DNA content and the timing of activation of p34cdc2 and cell division, in Nicotiana suspension culture cells restimulated with hormone.
Abstract: Progress through the plant cell cycle involves changes of phosphorylation state and catalytic activity in p34cdc2 -like protein. The plant protein has protein kinase activity, binds p13suc1, copurifies with cyclin B-like protein and its catalytic activation at mitosis is shown, by a metaphase arresting mutant, to be tightly coupled to cyclin accumulation and breakdown. These properties correlate with findings in other laboratories that some plant cdc2 genes can complement the yeast cdc2 gene. Activation of p34cdc2 at mitosis is proposed to trigger preprophase band breakdown since these two events continue in cells inhibited with okadaic acid although completion of chromosome condensation and spindle formation, which normally precede breakdown, do not occur and are therefore not triggers. In plant development the level of this key division protein relative to other proteins declines in cells entering division to less than one-tenth of that in dividing cells and resumption of division follows restoration of the level in all tissues that have been tested, including cotyledon, stem pith and root. Hormonal induction of division revealed tissue-specific differences in effects of individual hormones on p34cdc2-like protein accumulation and its catalytic activation. In tobacco pith, auxin could induce synthesis of p34cdc2-like protein but cytokinin was necessary for its activation and for cell cycle activity. Within the cell cycle, hormone-dependent control points were identified at late G1 and at mitotic initiation by measurement of nuclear DNA content and the timing of activation of p34cdc2 and cell division, in Nicotiana suspension culture cells restimulated with hormone. Auxin was required at both control points, but cytokinin was required only at mitotic initiation. A direct effect of auxin on cdc2 gene expression is indicated by an increase of cdc2 mRNA levels within 10 min of exposing root tissue to 3-indolylacetic acid (IAA).
138 citations
TL;DR: The study emphasises the need for improving the percentage of high density grains in the rice panicle to maximise the use of available assimilates and enhance the grain yield potential of the plant.
Abstract: Anthesis of spikelets in the panicle of a tall indica rice cultivar, Kajalghara, was completed within a period of 7 days. Development progressed in basipetal fashion from the spikelets of the uppermost primary branch of the panicle to the lowermost. According to the timing of onset of anthesis, spikelets of the panicle were segregated into seven groups. Dry mass accumulation and starch and assimilate (soluble carbohydrates and amino acids) contents of each group were studied from anthesis to grain maturity. Spikelets that anthesed earlier produced better quality grains and also had higher sink efficiency in converting soluble assimilates into reserve structural matter than those which anthesed later. These results rule out deficiency in supply of assimilates as a cause of partial filling of grains in the rice panicle. The study also emphasises the need for improving the percentage of high density grains in the rice panicle to maximise the use of available assimilates and enhance the grain yield potential of the plant.
125 citations
TL;DR: Analysis of cytokinin levels in developing wheat and rice grains in the period immediately following pollination showed large transient increases in zeatin and zeat in riboside which coincided with the period of maximum endosperm cell division reported by others.
Abstract: Cytokinin analysis by immunoaffinity chromatography (IAC), high-performance liquid chromatography (HPLC) and radioimmunoassay (RIA) or enzyme-linked immunosorption assay (ELISA) has been used to study two separate topics: the role of tRNA in bacterial cytokinin biosynthesis and the changes in cytokinin concentration which occur during cereal grain development. Transfer RNA isopentenylation in the gall-forming plant pathogen Agrobacterium tumefaciens is encoded by the chromosomal miaA locus. Mutation of miaA reduces tRNA isopentenylation significantly and preliminary data suggest that turnover of isopentenylated tRNA is responsible for low level secretion of free N6-isopentenyladenine (iP) by the bacteria. However, the major route of cytokinin biosynthesis by gall-forming plant patho- genic bacteria is not via tRNA turnover but by direct biosynthesis mediated by dimethylallylpyro- phosphate: 5'-AMP transferase (DMAPP :AMP transferase) encoded by such genes as ipt, tzs (from A, tumefaciens) or ptz (from Pseudomonas savastanoi). Analysis of cytokinin levels in developing wheat and rice grains in the period immediately following pollination showed large transient increases in zeatin (Z) and zeatin riboside (ZR) which coincided with the period of maximum endosperm cell division reported by others. Detailed analyses of maize kernels, where development can be staged readily, showed that Z and ZR concentrations peaked 9 days after pollination (DAP). During the period 8-10 DAP, cytokinin oxidase underwent a significant increase in specific activity, indicating that cytokinin catabolism was enhanced as endosperm cell division ended.
103 citations
TL;DR: High CO2 increased growth by stimulating the development of tiller buds that would otherwise have been inhibited in wheat plants, indicating that salt uptake was largely independent of water uptake, and that high CO2 did not increase growth by reducing the salt load.
Abstract: Wheat plants (Triticum aestivum cv. Matong and T. durum cv. Modoc) were grown at ambient and elevated CO2 (350 cm3 m-3 above ambient) in soil with or without 150 mol m-3 NaCl for 6 weeks. The increase in dry matter, leaf area and tillering under high CO2 was relatively greater under saline than non-saline conditions for both cultivars. Tillering was the primary component of growth affected by both salinity and high CO2. Salinity greatly reduced tillering and high CO2 partly reversed the effects of salinity. High CO2 increased dry matter accumulation of the salt-sensitive Modoc to a greater extent (+104%) than that of the more salt-tolerant Matong (+73%) in the salt treatment. Transpiration rates were greatly reduced by salinity for both cultivars. Under high CO2, increased leaf areas compensated for reduced transpiration rates per unit leaf area (i.e. greater stomatal closure), and total transpiration was little affected by CO2 level within each treatment. The more salt-tolerant Matong showed greater stomatal closure and higher transpiration efficiencies than the salt-sensitive Modoc under salinity. High CO2 reduced transpiration rate (per unit dry weight) by 40 to 50%, but did not significantly change the rate of sodium accumulation (per unit dry weight), indicating that salt uptake was largely independent of water uptake, and that high CO2 did not increase growth by reducing the salt load. Our results suggest that high CO2 increased growth by stimulating the development of tiller buds that would otherwise have been inhibited.
102 citations
TL;DR: The study confirms the multiplicity of photosynthetic responses and of different protective strategies that may account for the differential sensitivity of plants to supplementary UV-B radiation.
Abstract: To compare the effects of supplementary ultraviolet-B (UV-B) radiation on a tropical/subtropical and a temperate plant, two indica rice cultivars (Er Bai Ai and Lemont) and peas were exposed to supplementary UV-B radiation for 8 days (biologically effective irradiance of 0.68 W m-2, 12 h per day). Marked decreases occurred in the ratios of variable to maximum chlorophyll fluorescence yield and in the quantum yield of photosynthetic oxygen evolution over the 8 day treatment period. The greatest decline always occurred in pea leaves, while in rice, cv. Er Bai Ai was more susceptible to UV-B radiation than cv. Lemont. Compared with control plants, the content of soluble protein and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) protein decreased significantly after the UV-B treatment; the greatest decrease occurred in pea leaves, while the rate of decrease in rice was greater in cv. Er Bai Ai than cv. Lemont. Over the 8 day UV-B treatment period, the increase of UV-B-absorbing compounds was greater in rice leaves than in pea leaves, and greater in cv. Er Bai Ai than in cv. Lemont, although cv. Lemont was more UV-B tolerant. We suggest that the increase in these compounds is not the only indicator of resistance to UV-B damage; other factors, yet to be identified, may also confer UV-B tolerance. While leaf orientation may be related to the severity of UV-B damage, naturally near-horizontal pea leaves were still much more susceptible to UV-B damage compared with rice leaves which were held horizontally over the 8 day treatment period. Brown spots appeared on the upper surface of leaves of cv. Er Bai Ai after 2 days of UV-B treatment and showed a cumulative increase with the duration of exposure. Our study confirms the multiplicity of photosynthetic responses and of different protective strategies that may account for the differential sensitivity of plants to supplementary UV-B radiation.
TL;DR: The equations for net photosynthesis derived from pot studies were shown to explain leaf photosynthesis in a crop of the same cultivar over a wide range of NL and irradiance.
Abstract: Photosynthesis-irradiance response curves and leaf nitrogen contents were measured weekly by destructive sampling over the life cycles of leaves 10, 15, 20 and 25 of sunflower plants (cv. Prosol 35) grown in large pots in the open under optimum conditions of temperature and high irradiance. Individual leaf responses were adequately described by a hyperbola of three parameters, viz. Pmax, the rate of photosynthesis in saturating irradiance; R, the rate of dark respiration adjusted for temperature (30oC); and e, the apparent quantum efficiency of photosynthesis at low irradiance. Pmax (range 0-40 μmol CO2 m-2 s-1) and R (0-4 μmol CO2 m-2 s-1) were non-linearly related to nitrogen content per unit leaf area (NL) (range 0.3-2.9 g N m-2) across all leaf positions and for all leaf ages. e (mean value 0.050 mol mol-1, s.e. 0.001) was independent of NL. The equations for net photosynthesis derived from pot studies were shown to explain (r2 =0.80) leaf photosynthesis in a crop of the same cultivar over a wide range of NL and irradiance.
TL;DR: It is proposed that the channels and the feedback loops play a wide range of roles in regulating normal plant function, as well as in mediating disturbance of normal function by environmental stressors and various pathogens.
Abstract: Mechanosensory calcium-selective ion channels probably serve to detect not only mechanical stress but also electrical, thermal, and diverse chemical stimuli. Because all stimuli result in a common output, most notably a shift in second messenger calcium concentration, the channels are presumed to serve as signal integrators. Further, insofar as second messenger calcium in turn gives rise to mechanical, electrical, and diverse chemical changes, the channels are postulated to initiate regulatory feedbacks. It is proposed that the channels and the feedback loops play a wide range of roles in regulating normal plant function, as well as in mediating disturbance of normal function by environmental stressors and various pathogens. In developing evidence for the physiological performance of the channel, a model for a cluster of regulatory plasmalemmal proteins and cytoskeletal elements grouped around a set of wall-to-membrane and transmembrane linkers has proved useful. An illustration of how the model might operate is presented. It is founded on the demonstration that several xenobiotics interfere both with normal channel behaviour and with gravitropic reception. Accordingly, the first part of the illustration deals with how the channels and the control system within which they putatively operate might initiate gravitropism. Assuming that gravitropism is an asymmetric expression of growth, the activities of the channels and the plasmalemmal control system are extrapolated to account for regulation of both rate and allometry of cell expansion. Finally, it is discussed how light, hormones, redox agents and herbicides could in principle affect growth via the putative plasmalemmal control cluster or centre.
TL;DR: The utility of integrating data on stomatal conductance, leaf water potential and whole-plant hydraulic conductance to interpret whole plant adaptation to water stress is demonstrated, and two mechanisms by which genotypes differentially acclimate to water Stress are elucidated.
Abstract: A comparative study of soil-plant water relations was conducted on three grapevine cultivars (Vitis vinifera L. cvv. carignane, merlot, shiraz) to investigate their adjustment to short-term and long-term water stress under field conditions. Adjustment was a function of the relative stability of the internal plant water status on diurnal and seasonal scales. On a diurnal scale, stomatal closure in response to water vapour pressure directly contributed to this stability. Indirect evidence suggested an influence of the soil water status on the diurnal stomatal activity. On a seasonal scale, sufficient leaf hydration required high whole-plant hydraulic conductance. This was achieved by either daily stomatal regulation or limitation of leaf area. Physiological adjustment to water stress through stomatal control was well developed in cv. carignane, which originated in a Mediterranean environment. However, cv. shiraz, which was of mesic origin, apparently adjusted to water stress by reducing leaf area. Our study demonstrates the utility of integrating data on stomatal conductance, leaf water potential and whole-plant hydraulic conductance to interpret whole plant adaptation to water stress, and elucidates two mechanisms by which genotypes differentially acclimate to water stress.
TL;DR: The promoters of the auxin-responsive genes have been fused to the E. coli uidA gene which encodes β-glucuronidase and transferred into tobacco and/or Arabidopsis via Agrobacterium T-DNA to study the effects of organ, tissue, and developmental-specific expression of cytokinins and auxins on plant growth, development, and physiology.
Abstract: We have cloned and sequenced a number of auxin-responsive cDNAs and their corresponding genes from soybean and Arabidopsis. Each of these genes, with the exception of GH2/4, is transcriptionally regulated specifically by auxins within minutes after hormone application. The auxin-responsive mRNAs are induced some 3-60-fold depending on the type of mRNA analysed, the tissue examined, the dose and duration of auxin application, and the manipulation of the organ tested. Some of the mRNAs show rapid turnover kinetics. The mRNAs show distinct patterns of organ-specific, tissue-specific, and developmental-specific expression. The promoters of the auxin-responsive genes have been fused to the E. coli uidA gene which encodes β-glucuronidase (GUS) and transferred into tobacco and/or Arabidopsis via Agrobacterium T-DNA. These promoters and parts of these promoters have been used to follow the expression patterns and auxin-inducibility of the reporter genes in transgenic plants. We are attempting to identify minimal auxin-responsive elements and gravity-responsive elements within these promoters. We have also fused the auxin-inducible promoters to bacterial genes that encode cytokinin and auxin biosynthetic or conjugating enzymes to study the effects of organ, tissue, and developmental-specific expression of cytokinins and auxins on plant growth, development, and physiology.
TL;DR: It is concluded that browning of mango skin induced by the sap is predominantly catalysed by PPO in the skin and that this is unlikely to be prevented by heat treatment of the fruit.
Abstract: Severe sapburn occurs in mango fruit of the cultivar Kensington when sap contacts the fruit, resulting in browning and then blackening of the skin. Both the sap and skin of mango fruit contained considerable polyphenol oxidase (PPO) activity. The sap enzyme was not activated by SDS, was inhibited by hexadecyltrimethylammonium bromide, and was active with both para- and ortho-diphenol substrates. The skin enzyme was activated by SDS, was inhibited by salicylhydroxamic acid and polyvinylpyrrolidone, and was active only with ortho-diphenol substrates. These properties suggest that the sap PPO is a laccase-type enzyme (EC 1.10.3.2) whereas the skin contains the more common catechol oxidase-type PPO (EC 1.10.3.1). The skin enzyme had a temperature optimum at 30oC but the sap enzyme had maximum PPO activity at 75oC. Both enzymes were relatively thermostable, requiring more than 15 min at 80oC for 50% loss of activity. It is concluded that browning of mango skin induced by the sap is predominantly catalysed by PPO in the skin and that this is unlikely to be prevented by heat treatment of the fruit.
TL;DR: Pretreatment with waterlogging significantly improved growth (particularly for E. globulus) and reduced the concentrations of Na+ and Cl- in leaves under SW treatment and showed least growth reduction and symptom development with addition of S, W and SW in these experiments.
Abstract: Two experiments were carried out in a glasshouse in Canberra during autumn 1987 to determine the extent to which waterlogging would modify the response to salinity of E. camaldulensis Dehnh., E. tereticornis Smith, E. robusta Smith and E. globulus Labill. Seedlings, 4-5 months old, established in free-draining sand-filled pots, were treated with either 150 (experiment 1) or 100 (experiment 2) mol m-3 NaCl with and without waterlogging for approximately 4 weeks; a waterlogging pretreatment of 3 weeks was included in experiment 2. Salt × waterlogging (SW) significantly reduced seedling growth for all species whereas growth under NaCl (S) or waterlogging (W) treatment alone was not significantly affected, except for E. robusta. In addition SW significantly reduced leaf stomatal conductance and whole-plant water use and increased leaf and stem Na+ and Cl- concentrations. Pretreatment with waterlogging (experiment 2) significantly improved growth (particularly for E. globulus) and reduced the concentrations of Na+ and Cl- in leaves under SW treatment. E. camaldulensis showed least growth reduction and symptom development with addition of S, W and SW in these experiments.
TL;DR: It is suggested that there has been a progressive increase in the water-use efficiency (WUE) of these cereals since the Middle Ages, which has coincided with increases in atmospheric CO2 concentration.
Abstract: Carbon isotope discrimination (Δ) was determined for kernels of six-row barley and durum wheat cultivated in the north-western Mediterranean basin during the last seven millennia. Samples from the Neolithic, Bronze, Iron and Middle Ages came from different archaeological sites in Catalonia. Samples for the period 1910-20 and 1990 were also analysed. There was a slight decrease (P=0.10, carbonisation-corrected values) in Δ from the Neolithic to the Iron Age period and a much steeper decrease from the Middle Ages to 1910-20 (P<0.01). Since water-use efficiency and isotope discrimination are negatively correlated, from the pattern of change in Δ it is suggested that there has been a progressive increase in the water-use efficiency (WUE) of these cereals. Since the Middle Ages this has coincided with increases in atmospheric CO2 concentration. Thus, for a given relative humidity and air temperature, the estimated WUE (measured as the ratio of CO2 assimilation to transpiration) from the Neolithic to the Middle Ages samples ranged between 65-70% of present time WUE values, whereas WUE for 1910-20 was about 86% of present values.
TL;DR: A continuous supply of CK is required and modulation of CK flux into the leaf appears to be a sensitive signalling mechanism controlling the senescence of leaves and other parts.
Abstract: Substantial evidence has indicated that cytokinins (CKs) inhibit senescence of a variety of plant organs, especially leaves, but how, or even whether, CK operates as an antisenescence signal in the intact plant needed to be determined. Because soybean has a number of advantageous features, we chose to analyse CK regulation of leaf senescence in that species. Toward the end of podfill, the leaves become visibly yellow and drop off (monocarpic senescence) leading to the death of the plant. This process appears to be mainly under the control of the pods, specifically the seeds, and therefore also needs to be correlated with pod development. Supplying CK solutions in lieu of the roots in explants (a leaf with the associated pod[s] and subtending stem segment) indicates that the roots are the main source of CKs for the leaves. The main conduit from the roots to the leaves is the xylem, and the dominant CKs of the xylem sap are zeatin riboside and dihydrozeatin riboside. These are also the most active in preventing leaf senescence. Early in pod development, before seed fill, the CK flux through xylem drops markedly and this decline is reflected in a decrease in foliar CK activity. This decrease in xylem flux is necessary but not sufficient to cause leaf senescence; however, if the CK flux is maintained in explants, it will largely override the senescence-inducing influence of the pods. Depodding of soybean plants at an early stage prevents the leaf yellowing and abscission and also causes a large resurgence of CK flux in the xylem, while depodding in late podfill does neither. 3H-labelled CKs fed through the xylem flow primarily to the leaves where they are rapidly metabolised, mainly to adenine and its riboside. Thus, a continuous supply of CK is required and modulation of CK flux into the leaf appears to be a sensitive signalling mechanism controlling the senescence of leaves and other parts. The possible roles of the various forms of CK are discussed. In addition to the root-leaf signal (CKs) and seed-leaf signal, a pod-root signal is considered.
TL;DR: Leaf-energy-budget simulations indicated that the observed range of maximum leaf conductance at low v would generate up to 3.0oC variation in leaf temperatures under field conditions of low to moderate windspeed, which could cause variation in carbon isotope discrimination, which reflects long-term cI values.
Abstract: Twelve common bean (Phaseolus vulgaris L.) cultivars were grown under greenhouse conditions to study the response of net photosynthesis (A) and transpiration (E) to variation in the leaf-to-air humidity gradient (v). Large differences were observed between cultivars in maximum rates of A and E. The variation in A correlated with both leaf nitrogen content and specific leaf area. Thin leaves had higher nitrogen contents per unit dry weight, but thick leaves had higher nitrogen content per unit surface area. Photosynthetic nitrogen-use efficiency did not correlate with nitrogen content on either a mass or a surface area basis. Very little variation was found between cultivars in the sensitivity of total leaf conductance (g) to increasing v, when sensitivity was defined as the slope of ln(g) versus v (δln(g)/δv). No significant correlation existed between δln(g)/δv and variation in maximum conductance values. Much steeper slopes (greater sensitivity) were found in the response of stomatal conductance alone (gs) to the leaf-leaf surface humidity gradient (vs). The sensitivity of stomatal response correlated positively with variation in maximum conductance among cultivars, and, since stomatal conductance was in series with a fixed boundary layer conductance, this positive correlation made possible the uniform sensitivity of g(total) with respect to v(leaf-air) despite the wide variation in gmax. All cultivm reached their maximum E at very similar v values, and all showed a relatively constant E over a wide range of high v. The implications of this relative homeostasis in E are discussed in the context of possible hydraulic limitations on E. Considerable recent interest has focussed on the use of carbon isotope discrimination (Δ) as a useful screening character in crop breeding programmes, and Δ has been found to correlate positively with yield in P. vulgaris. We found that Δ measured on bulk leaf tissue, positively correlated with both Amax and gmax between bean cultivars, but did not correlate with instantaneous measures of intercellular CO2 (cI) when v was held constant across all measurements. This apparent discrepancy may be due, at least in part, to variation in leaf temperatures among cultivars under normal growing conditions. Leaf-energy-budget simulations indicated that the observed range of maximum leaf conductance at low v would generate up to 3.0oC variation in leaf temperatures under field conditions of low to moderate windspeed. Given the strong stomatal response to v, this variation in leaf temperature could cause variation in carbon isotope discrimination, which reflects long-term cI values. Such a mechanism of producing variation in cI would not be apparent in cI measurements taken under cuvette conditions where leaf temperature was held constant.
TL;DR: The results with isolated grains support a previous suggestion that the activity of soluble starch synthase at elevated temperature in wheat is an important factor limiting the accumulation of dry matter through effects on the synthesis of starch.
Abstract: Soluble starch synthase activity decreased in isolated wheat grains heated in vials at temperatures between 31 and 40oC; a decrease of more than 50% occurred at 35oC within 30 min. Sucrose synthase activity was not significantly affected by heating and ADPglucose pyrophosphorylase decreased to a lesser extent and more slowly than soluble starch synthase. Wheat grains which were heated for 1 h at 35oC and allowed to recover at 20oC exhibited partial recovery of soluble starch synthase activity within 4 h. These responses in isolated grains to heating resembled effects reported elsewhere in intact ears. Isolated grains were exposed to a range of temperatures for 1 h prior to incubation in [14C]sucrose at 20oC. Within the range 20-40oC, the effect of pretreatment temperature on the activity of soluble starch synthase was highly correlated with the incorporation of 14C into starch. Control coefficients close to unity indicated that the activity of soluble starch synthase imposed a high degree of control on the rate of starch synthesis in this system. These results with isolated grains support a previous suggestion that the activity of soluble starch synthase at elevated temperature in wheat is an important factor limiting the accumulation of dry matter through effects on the synthesis of starch.
TL;DR: The data indicate that both Cl- and Na+ can reduce assimilation rates in salt-stressed lemon leaves, possibly because of poor compartmentation within leaves of either ion.
Abstract: An attempt was made to differentiate between the possible effects of high Cl- or Na+ ions on lemon leaf photosynthesis by treating grafted (Citrus limon (L.) Burm. f. cv. 'Taylor') plants with either NaCl or Na2SO4 to establish different foliar concentrations of Cl- or Na+. The rootstocks, Rangpur lime (C. reticulata var. austera hybrid?) and rough lemon (C. jambhiri), were chosen because Rangpur lime is known to be a good Cl- 'excluder' and rough lemon to be a poor Cl- 'excluder'. The grafted plants were initially treated for 7 weeks with nutrient solution (control) or nutrient solution containing 50 mol m-3 NaCl or 25 mol m-3 Na2SO4, after which time there were only marginal reductions in both photosynthetic rates and shoot growth, with the exception of 'Taylor' lemon on rough lemon roostock treated with NaCl where growth was affected more severely than the other treatments. Salinity levels were then doubled to 100 mol m-3 NaCl and 50 mol m-3 Na2SO4 and the plants treated for a further 8 weeks, causing significant increases in leaf Na+ and/or Cl- concentrations. Mature, 3-4-month-old leaves of 'Taylor' lemon on Rangpur lime rootstock treated with Na2SO4 for this period, contained c. 105 mol m-3 Na+ and c. 10 mol m-3 Cl- and had photosynthetic rates 60% lower than controls. Similar reductions in assimilation rate were seen in leaves on rough lemon roostock treated with NaCl containing c. 195 mol m-3 Cl- and c. 35 mol m-3 Na+. Smaller (35%) but significant reductions in assimilation rate were observed for trees on rough lemon roostock treated with Na2SO4, where the tagged leaves contained c. 40 mol m-3 Na+ and 50 mol m-3 SO42-. Leaf nitrogen concentrations were significantly reduced by treatment of trees with NaCl or Na2SO4 but, apart from small reductions in chlorophyll concentrations, there were no visible symptoms of nitrogen deficiency. Leaf turgor was not adversely affected. The data indicate that both Cl- and Na+ can reduce assimilation rates in salt-stressed lemon leaves, possibly because of poor compartmentation within leaves of either ion. Increases in leaf Na+ and Cl- accounted for 54-96% of the reduction in osmotic potential in 'Taylor' lemon leaves on trees treated with NaCl, whereas increases in Na+ and SO42- accounted for 33-71% of the reduction in osmotic potential in leaves on trees irrigated with Na2SO4. The greater increase in Cl- compared with the net increase of [Na+ + K+] in 'Taylor' leaves on rough lemon rootstocks was offset (65%) by reductions in malic and succinic acids. Proline increased significantly only in 'Taylor' leaves on Rangpur lime roostock treated with Na2SO4.
TL;DR: To study the way in which the gibberellins affect this association, it is necessary to clarify the molecular architecture of the structure that links cortical microtubules with the plasma membrane.
Abstract: Gibberellins control the direction of expansion of plant cells. They change the orientation of cellulose microfibrils by changing the orientation of cortical microtubules and, hence, the direction of cell expansion. When gibberellins change the orientation of cortical microtubules, they also change their stability. If the way in which gibberellins change the orientation of microtubules is identical to the way in which they change microtubule stability, then studies on the mechanism that regulates this stability should give us some clues to the mechanism that regulates the orientation of microtubules. With this possibility in mind, we undertook a series of studies on the stability of cortical microtubules. These revealed that the association of cortical microtubules with the plasma membrane is an important part of the mechanism for their stabilisation. Gibberellins seem to change the stability of microtubules by affecting their association with the plasma membrane. To study the way in which the gibberellins affect this association, it is necessary to clarify the molecular architecture of the structure that links cortical microtubules with the plasma membrane.
TL;DR: Seeds of Maranthes corymbosa Blume, a monsoon rain forest species of northern Australia, were sown under ambient or elevated CO2 concentrations in tropical Australia and the responses of plants adapting to different photon flux densities are compared.
Abstract: Seeds of Maranthes corymbosa Blume, a monsoon rain forest species of northern Australia, were sown under ambient or elevated CO2 concentrations in tropical Australia. Seedlings were grown under conditions of photon flux density, temperature and atmospheric vapour pressure deficit which followed ambient variations as closely as possible. Specific leaf area, chlorophyll, stomatal density, stomatal conductance and assimilation responses to photon flux density were measured after 30 weeks growth. Gas exchange characteristics were divided into morning and afternoon data sets and analysed separately. Stomatal density decreased and leaf area:dry weight ratio decreased in response to elevated CO2. In contrast there was no effect of elevated CO2 upon chlorophyll (total or ratio of a:b). Apparent quantum yield and rates of light saturated assimilation (Amax) increased in response to elevated CO2. There was a significant decline in apparent quantum yield for both treatments between morning and afternoon. Stomatal conductance (gs) declined in response to elevated CO2. There was no significant difference in gs between morning and afternoon for ambient grown trees, but gs declined significantly between morning and afternoon for elevated CO2 grown trees. Instantaneous transpiration efficiency (ITE) was higher for elevated CO2 grown trees compared with control trees. There was a significant increase in ITE between morning and afternoon data for ambient grown trees; in contrast a significant decline in ITE was observed for elevated CO2 grown trees between morning anf afternoon data sets. The slope of the regression between assimilation rate and stomatal conductance increased for plants grown under elevated CO2. These data are discussed and compared with the responses of plants adapting to different photon flux densities.
TL;DR: It is concluded that further research is needed to evaluate the potential of ash as a criterion in selecting for improved W, particularly during the early phases of a breeding programme when large populations are usually involved.
Abstract: Breeding and selection for higher transpiration efficiency (W) has been hampered by tedious and costly methodology. Rapid and less costly methods are needed for screening W in plant improvement programmes. We report the relationship of ash, silicon (Si) concentration, and Si uptake to W in crested wheatgrass (Agropyron desertorum [Fischer ex Link] Schultes), an important C3 range grass in western North America. Clones of crested wheatgrass were grown under three water levels in a field rainout shelter and as potted plants under two water levels in the field and greenhouse. Ash and Si concentrations were compared to previously determined values of shoot mass, transpiration, W, and carbon isotope discrimination (A). Ash and Si concentrations were not consistently related to A and W across all environments; however, ash concentration was positively correlated with A (r 69 •*, df = 22) and negatively correlated with W (r = - 0 .61**, df= 22) in the well-watered field environment. Across all environments and studies, the ranges in the coefficients of variation (CV, %) for clonal means were: W, 4-15; A, 1-4; ash concentration, 6-14; Si concentration, 13-30; and Si uptake, 21-33. The generally lower CV for W, A, and ash concentration suggest that these traits were more repeatable than Si concentration or uptake. Although a consistent relationship was not observed between Si and W and between ash and W, the correlations of ash and W from the well-watered field environment were encouraging. In view of the low cost for ash analysis, we conclude that further research is needed to evaluate the potential of ash as a criterion in selecting for improved W, particularly during the early phases of a breeding programme when large populations are usually involved. Later selections could be based on the more precise and accurate, but costly, A analysis.
TL;DR: Many of the effects on plant development are consistent with observations made following exogenous auxin and/or cytokinin application to plant tissues, and the availability of these genes offers a new approach to the study of plant physiology using transformation methodology.
Abstract: This review deals with the physiological and morphological effects of altering the auxin/cytokinin balance in transgenic plants by expressing specific genes from pathogenic bacteria. Genes which have been used to alter auxin levels or sensitivity in transgenic plants include the iaaM/iaaH genes from Agrobacterium tumefaciens and A. rhizogenes; gene 5 and possibly gene 6b from A. tumefaciens; the rol B and possibly the rol A gene from A. rhizogenes and the iaaL gene from Pseudomonas syringae subsp. savastanoi (P. savastanoi). Genes which have been used to alter cytokinin levels in transgenic plants include the ipt gene from A. tumefaciens and the rol C gene from A. rhizogenes. A variety of biochemical mechanisms have been identified which result in alterations to phytohormone levels following expression of these genes in transgenic plants. Many of the effects on plant development are consistent with observations made following exogenous auxin and/or cytokinin application to plant tissues, and the availability of these genes offers a new approach to the study of plant physiology using transformation methodology.
TL;DR: The results obtained suggest that the immanent risk of dioxygen toxicity associated with photosynthetic electron flow could be reduced in the presence of high CO2 levels, and highCO2 could promote oxidative processes which cause GSH oxidation and require an enhanced cellular ability to scavenge superoxide anion and hydrogen peroxide.
Abstract: The effects of progressively higher CO2 levels on the foliar antioxidant status were studied by growing soybean (Glycine max Merrill cv. Cresir) plants at decreasing distances from natural CO2 sources of geothermal origin in central Italy. When compared with neighbouring controls grown under normal CO2 concentration (C), soybean leaves grown at 2 × C, 7 × C and more than 20 × C showed a substantial reduction in the size of ascorbate pool and in the activity of Cu,Zn-superoxide dismutase; both the content of ascorbic acid and the activity of ascorbate peroxidase declined at 2 × C and 7 × C and recovered to the control values at 20 × C. The foliar titre of glutathione disulfide and the activities of glutathione disulfide reductase and Mn-superoxide dismutase progressively increased as CO2 concentration increased in ambient air. The results obtained suggest that the immanent risk of dioxygen toxicity associated with photosynthetic electron flow could be reduced in the presence of high CO2 levels. On the other hand, depending on both the CO2 exposure regimes and the cell compartment considered, high CO2 could promote oxidative processes which cause GSH oxidation and require an enhanced cellular ability to scavenge superoxide anion and hydrogen peroxide.
TL;DR: Considering the similarity of the lipo-oligosaccharides with chitin, it is shown that analogous molecules could occur in uninfected plants.
Abstract: Rhizobia, bacterial symbionts of leguminous plants, produce lipo-oligosaccharide signal molecules after induction of their nod genes by the plant host. The effects of these signals on plant developmental processes are discussed. Considering the similarity of the lipo-oligosaccharides with chitin, we discuss the possible basis of these effects and show some preliminary results which indicate that analogous molecules could occur in uninfected plants.
TL;DR: Although development in wheat is generally not source-limited in the traditional environments for cultivation, under high temperatures and shorter photoperiods development may be slowed by limiting source:sink ratios.
Abstract: This study was designed to examine whether development rate of wheat might be changed by radiation at high temperature and more generally whether development might be varied by changed source: sink ratio. In the genotype studied, when temperature was high (27/22oC) and especially when the photoperiod was short (9 h), leaf primordia were produced more slowly under low than high natural radiation, and this slowing was reflected in lower final spikelet numbers and in fewer grains per ear. Under the higher source: sink condition of low temperature (17/7oC), and under either short or long photoperiod, rates of primordia production were not measurably changed by radiation, but more light did result in more spikelets. The lengthening of plastochron intervals (thermal time between the appearance of leaf primordia) associated with low radiation did not translate into significantly increased phyllochron intervals (thermal time between emergence of leaves). However, phyllochron intervals were responsive to both photoperiod and temperature. Under short photoperiod, source limitation associated with high temperature extended the thermal time to ear emergence. Thus, an increase in temperature from 17/7oC to 27/22oC doubled the thermal time to heading from 1241 to 2571 oCd. Source limitation associated with low radiation also increased thermal time to heading by up to 400 oCd and calendar time by up to 17 days. Under long photoperiod, where development was rapid, radiation did not change the timing of developmental stages. It is concluded that although development in wheat is generally not source-limited in the traditional environments for cultivation, under high temperatures and shorter photoperiods development may be slowed by limiting source:sink ratios.
TL;DR: The tension-dependent activity of mechanosensory calcium-selective cation channels in excised plasmalemmal patches from onion bulb scale epidermis is modulated by pH in the physiologically meaningful range between 4.5 and 7.2.
Abstract: The tension-dependent activity of mechanosensory calcium-selective cation channels in excised plasmalemmal patches from onion bulb scale epidermis is modulated by pH in the physiologically meaningful range between 4.5 and 7.2. It is rapidly lowered by lowering pH and rapidly raised by raising pH. Channel activity is effectively inhibited by low levels of aluminium ions and activity can be partially restored by washing for a few minutes. We suggest that under normal conditions the sensitivity of the mechanosensory channels to pH of the wall free space plays important roles in regulation of plant activities such as growth. We further suggest that, when levels of acid and aluminium ions in the soil solution are high, they might inhibit similar sensory channels in cells of the root tip, thus contributing critically to the acid soil syndrome.