Leaf Extension Research Articles

Growth Physiology of Spring Wheat Under Saline Conditions

Growth Physiology of Spring Wheat Under Saline Conditions

Differential effects of salinity on leaf elongation kinetics of three grass species

This study focuses on the inhibitory effect of salinity on the leaf extension of three different grass species: Hordeum jubatum L., Hordeum vulgare L. and Zea mays L. Leaf elongation rates (LER) were measured on the third leaf of the plants. NaCl was added to the hydroponic solution (0, 40, 80 and 120 mM) and changes in LER were measured over time with a displacement transducer. Salinity inhibited LER immediately in all three species, and a new, but lower steady-state LER was reached within 5 h. The decrease in LER was proportional to the salinity level. Differences in salt tolerance (% of control LER) were evident between genotypes within 5 h after salinization, but the relative salt tolerance of the plant at this stage was not necessarily indicative of the long-term salt tolerance of the species. In general, H. jubatum was more tolerant than maize, which was more tolerant than barley to these short-term salinity stresses. In contrast, barley is more salt tolerant than maize over the long term. The mechanisms of inhibition of LER by salinity, as tested by the applied-tension technique, varied with the species examined, affecting either the apparent yield threshold, the hydraulic conductance of the whole plant or both. The cell wall extensibility was not significantly affected by salinity in the three species tested in this study.

Relationship Between Leaf Extension Rate and Extension Duration for Determining the Final Leaf Length in Maize under Various Phosphorus Levels

Relationship Between Leaf Extension Rate and Extension Duration for Determining the Final Leaf Length in Maize under Various Phosphorus Levels

Sward height, structure and leaf extension rate of Lolium perenne pastures when grazed by overwintering barnacle geese

Abstract The grazing of agricultural pastures during winter and spring by geese is considered an important agricultural problem in parts of the U.K. This study describes the sward structure, leaf extension and senescence rates of Lolium perenne‐dominated pastures that are frequently grazed by barnacle geese (Branta leucopsis) during winter in South‐west Scotland, as well as the conducting of a field experiment that simulated grazing to investigate the effects of defoliation. Gross leaf extension and senescence rates were strongly related to temperature, daylength and average tiller size, resulting in positive values of net leaf extension rate per tiller for most of the winter. Total tiller length declined from November to January but increased from January to April. Sward height, however, declined consistently from October to April, suggesting that swards were becoming trampled by repeated visits by flocks of geese over this time. The structure of individual tillers was found to vary slightly over the winter, with tillers becoming more dominated by younger leaves towards the end of the winter. Experimental defoliation of tillers suggested that absolute leaf extension rates did not respond in an under‐ or over‐compensatory manner, even when tillers were nearly completely defoliated. The results suggested that sward structure and leaf extension rates are not unduly affected by repeated grazing by overwintering geese and that short‐term depletion and trampling are the main impacts.

Effect of partial root excision on shoot water relations

Removing 4 out of 5 serminal roots from 7-day-old wheat seedlings arrested leaf elongation for 1.5 h. This effect can be explained by an initial decrease in foliar water content resulting from the smaller root surface area available for water uptake. Subsequently, leaf hydration increased with time and came to equal that of intact plants within 2 h. The rehydration was seemingly effected by an increasing conductivity of the one remaining root axis, since transpiration of the partially de-rooted plants did not fall below that of controls. With time, leaf elongation resumed, but at a slower rate than in intact plants. This slower growth may be attributed to a decrease in leaf extensibility since this was found to be reduced when measured by a counterweight technique involving linear displacement transducers. Loss of extensibility was associated with decreased IAA concentration in the leaf elongation zone.

Nitric oxide signaling in plant-pathogen interactions.

Nitric oxide (NO), first characterized as an endothelium-derived relaxation factor, is involved in diverse cellular processes including neuronal signaling, blood pressure homeostasis, and immune response. Recent studies have also revealed a role for NO as a signaling molecule in plants. As a developmental regulator, NO promotes germination, leaf extension and root growth, and delays leaf senescence and fruit maturation. Moreover, NO acts as a key signal in plant resistance to incompatible pathogens by triggering resistance-associated hypersensitive cell death. In addition, NO activates the expression of several defense genes (e.g. pathogenesis-related genes, phenylalanine ammonialyase, chalcone synthase) and could play a role in pathways leading to systemic acquired resistance.

Physiological and molecular basis of susceptibility and tolerance of rice plants to complete submergence.

Rice plants are much damaged by several days of total submergence. The effect can be a serious problem for rice farmers in the rainfed lowlands of Asia, and runs contrary to a widespread belief amongst plant biologists that rice is highly tolerant of submergence. This article assesses the characteristics of the underwater environment that may damage rice plants, examines various physiological mechanisms of injury, and reviews recent progress achieved using linkage mapping to locate quantitative traits loci (QTL) for tolerance inherited from a submergence-tolerant cultivar FR13A. Progress towards identifying the gene(s) involved through physical mapping of a dominant tolerance locus on chromosome 9 is also summarized. Available physiological evidence points away from responses to oxygen shortage as being inextricably involved in submergence injury. An imbalance between production and consumption of assimilates is seen as being especially harmful, and is exacerbated by strongly accelerated leaf extension and leaf senescence that are ethylene-mediated and largely absent from FR13A and related cultivars. DNA markers for a major QTL for tolerance are shown to be potentially useful in breeding programmes designed to improve submergence tolerance.

Gibberellin-mediated proteasome-dependent degradation of the barley DELLA protein SLN1 repressor.

DELLA proteins are nuclear repressors of plant gibberellin (GA) responses. Here, we investigate the properties of SLN1, a DELLA protein from barley that is destabilized by GA treatment. Using specific inhibitors of proteasome function, we show that proteasome-mediated protein degradation is necessary for GA-mediated destabilization of SLN1. We also show that GA responses, such as the aleurone alpha-amylase response and seedling leaf extension growth, require proteasome-dependent GA-mediated SLN1 destabilization. In further experiments with protein kinase and protein phosphatase inhibitors, we identify two additional signaling steps that are necessary for GA response and for GA-mediated destabilization of SLN1. Thus, GA signaling involves protein phosphorylation and dephosphorylation steps and promotes the derepression of GA responses via proteasome-dependent destabilization of DELLA repressors.

Reactive oxygen species in the elongation zone of maize leaves are necessary for leaf extension.

The production and role of reactive oxygen species (ROS) in the expanding zone of maize (Zea mays) leaf blades were investigated. ROS release along the leaf blade was evaluated by embedding intact seedlings in 2',7'-dichlorofluorescein-containing agar and examining the distribution of 2',7'-dichlorofluorescein fluorescence along leaf 4, which was exposed by removing the outer leaves before embedding the seedling. Fluorescence was high in the expanding region, becoming practically non-detectable beyond 65 mm from the ligule, indicating high ROS production in the expansion zone. Segments obtained from the elongation zone of leaf 4 were used to assess the role of ROS in leaf elongation. The distribution of cerium perhydroxide deposits in electron micrographs indicated hydrogen peroxide (H(2)O(2)) presence in the apoplast. 2',7'-Dichlorofluorescein fluorescence and apoplastic H(2)O(2) accumulation were inhibited with diphenyleneiodonium (DPI), which also inhibited O*(2)(-) generation, suggesting a flavin-containing enzyme activity such as NADPH oxidase was involved in ROS production. Segments from the elongation zone incubated in water grew 8% in 2 h. KI treatments, which scavenged H(2)O(2) but did not inhibit O*(2)(-) production, did not modify growth. DPI significantly inhibited segment elongation, and the addition of H(2)O(2) (50 or 500 microM) to the incubation medium partially reverted the inhibition caused by DPI. These results indicate that a certain concentration of H(2)O(2) is necessary for leaf elongation, but it could not be distinguished whether H(2)O(2), or other ROS, are the actual active agents.

Drought-induced changes in xylem pH, ionic composition, and ABA concentration act as early signals in field-grown maize (Zea mays L.).

Early signals potentially regulating leaf growth and stomatal aperture in field-grown maize (Zea mays L.) subjected to drought were investigated. Plants grown in a field lysimeter on two soil types were subjected to progressive drought during vegetative growth. Leaf ABA content, water status, extension rate, conductance, photosynthesis, nitrogen content, and xylem sap composition were measured daily. Maize responded similarly to progressive drought on both soil types. Effects on loam were less pronounced than on sand. Relative to fully-watered controls, xylem pH increased by about 0.2 units one day after withholding irrigation (DAWI) and conductivity decreased by about 0.25 mS cm(-1) 1-3 DAWI. Xylem nitrate, ammonium, and phosphate concentrations decreased by about 50% at 1-5 DAWI and potassium concentration decreased by about 50% at 7-8 DAWI. Xylem ABA concentration consistently increased by 45-70 pmol ml(-1) at 7 DAWI. Leaf extension rate decreased 5 DAWI, after the changes in xylem chemical composition had occurred. Leaf nitrogen significantly decreased 8-16 DAWI in droughted plants. Midday leaf water potential and photosynthesis were significantly decreased in droughted plants late in the drying period. Xylem nitrate concentration was the only ionic xylem sap component significantly correlated to increasing soil moisture deficit and decreasing leaf nitrogen concentration. Predawn leaf ABA content in droughted plants increased by 100-200 ng g(-1) dry weight at 7 DAWI coinciding with a decrease in stomatal conductance before any significant decrease in midday leaf water potential was observed. Based on the observed sequence, a chain of signal events is suggested eventually leading to stomatal closure and leaf surface reduction through interactive effects of reduced nitrogen supply and plant growth regulators under drought.

The Role of Hormones in Fast Growth Responses of Wheat Plants to Osmotic and Cold Shocks

The effects of nutrient-solution cooling and PEG addition to the nutrient solution on the phytohormone content, the rate of leaf growth, leaf extensibility under the influence of external mechanical action, osmotic potential, and transpiration were studied in seven-day-old wheat plants. Leaf growth rapidly ceased, and the transpiration rate was reduced in both treatments. Growth cessation induced by PEG was transient, and growth resumption was preceded by an increase in the leaf extensibility. The functional role of auxin accumulation in plant shoots in the control of extensibility as well as the relationship between the ABA accumulation and a decrease in the cytokinin content, on the one hand, and reduced transpiration, on the other hand, under stress conditions are discussed.

Effect of PEG-treatment on the leaf growth response and auxin content in shoots of wheat seedlings

Addition of Polyethylene glycol(PEG) to the nutrient medium of wheatseedlings led to a rapid cessation of leaf growth and shrinkage, measured withahighly sensitive growth sensor, followed by a partial restoration of extensionin the subsequent 40 min. This osmotic shock resulted in IAAaccumulation in the shoots. Leaf extensibility, measured by addition of 5g as a counterweight to an extensiometer, did not change for 20min after the start of the treatment, but then increased andbecametwice as high as the initial value. Changes in leaf extensibility coincidedwitha restoration of leaf growth. Leaf shrinkage, which was observed immediatelyafter addition of the osmoticum, suggests that the initial growth response wasdue to a water deficit, which led to a decrease in cell turgor. Howeverrestoration of growth during water stress could be due to changes in cell wallrheological properties, which could be triggered by IAA accumulation.

In planta regulation of extension of an endophytic fungus and maintenance of high metabolic rates in its mycelium in the absence of apical extension.

The fungus Neotyphodium lolii is an endophytic symbiont. It grows in the intercellular spaces of the perennial ryegrass Lolium perenne, producing secondary metabolites which enhance the fitness of the association over that of uninfected L. perenne. We report that the average number of hyphal strands in a given section of a leaf remains constant during the life of a leaf, indicating synchrony of leaf and hyphal extension, including cessation of hyphal extension when leaf extension ceases. We used a constitutively expressed reporter gene as an indicator of the mycelium's metabolic activity during and after hyphal extension. Reporter gene activity decreased when the mycelium stopped extending in liquid culture but not in planta. This indicates that in planta endophyte hyphae remain metabolically highly active when extension has ceased and throughout the life of the leaf they are colonizing. The behavior of the fungus in planta indicates the existence of signaling pathways which (i) synchronize the extension of leaf and hypha by regulating hyphal extension, (ii) suppress hyphal branching, and (iii) stop apical extension of fungal hyphae, without reducing the mycelium's metabolic activity. These signals may be crucial for the symbiosis, by allowing the endophyte to switch the focus of its metabolic activity from extension to the production of secondary metabolites.

Flooding induces a suite of adaptive plastic responses in the grass Paspalum dilatatum

Summary We studied individual responses to flooding in the perennial grass Paspalum dilatatum, a widespread species in the Flooding Pampa of Argentina, using plants established in species‐diverse grassland microcosms. Flooding effects were evaluated on root and leaf sheath anatomy and shoot morphological traits. Leaf water status and CO2 exchange rates were monitored in flooded and unflooded plants under changing, natural and controlled atmospheric conditions. Root porosity and leaf sheath aerenchyma increased with flooding. Leaf extension rates and tiller height were also higher in flooded plants, which resulted in a large fraction of the shoot architecture emerging above the water surface. Flooding enhanced stomatal conductance, leaf water potential and net photosynthesis, especially under conditions leading to high air‐vapour pressure deficits. Therefore, flooded plants experienced fewer water deficits during periods of high atmospheric evaporative demand. P. dilatatum showed tight regulation of water and carbon relations under severe soil‐oxygen deficiency, even in the presence of natural competitors. The suite of adaptive responses documented here might help to explain the observed increase in abundance of this species during extensive floods.

Increased leaf area expansion of hybrid poplar in elevated CO 2. From controlled environments to open-top chambers and to FACE

We examined the response of hybrid poplar to elevated CO 2 in contrasting growth environments: controlled environment chamber (CE), open-top chamber (OTC) and poplar free air CO 2 enrichment (POPFACE) in order to compare short versus long-term effects and to determine whether generalisations in response are possible for this fast growing tree. Leaf growth, which for poplar is an important determinant of stemwood productivity was followed in all environments, as were the determinants of leaf growth—cell expansion and cell production. Elevated CO 2 (550–700 μmol mol −1, depending on environment) resulted in an increase in final leaf size for Populus trichocarpa× Populus deltoides ( Populus× interamericana) and P. deltoides× Populus nigra ( Populus× euramericana), irrespective of whether plants were exposed during a short-term CE glasshouse study (90 days), a long-term OTC experiment (3 years) or during the first year of a POPFACE experiment. An exception was observed in the closed canopy POPFACE experiment, where final leaf size remained unaltered by CO 2. Increased leaf extension rate was observed in elevated CO 2 in all experiments, at some point during leaf development, as determined by leaf length. Again the exception were the POPFACE experiment, where effects were not statistically significant. Leaf production and specific leaf area (SLA) were increased and decreased, respectively, on five out of six occasions, although both were only statistically significant on two occasions and interestingly for SLA never in the FACE experiment. Although both cell expansion and cell production were sensitive to CO 2 concentration, effects appeared highly dependent on growth environment and genotype. However, increased leaf cell expansion in elevated CO 2 was often associated with changes in the biophysical properties of the cell wall, usually increased cell wall plasticity. This research has shown that enhanced leaf area development was a consistent response to elevated CO 2 but that the magnitude of this response is likely to decline, in long-term exposure to elevated CO 2. Effects on SLA and leaf production suggest that CE and OTC experiments may not always provide good predictors of the ‘qualitative’ effects of elevated CO 2 in long-term ecosystem experiments.

Stomatal and leaf growth responses to partial drying of root tips in willow.

Root tips of intact willow (Salix dasyclados Wimm., Clone 81-090) plants were partially dried by exposure to ambient greenhouse air and then kept in water-vapor-saturated air for up to 3 days. The drying treatment increased abscisic acid (ABA) concentrations in both the root tips subjected to drying and in the xylem sap, while it reduced leaf stomatal conductance and leaf extension rate. Despite the decrease in stomatal conductance, leaf water potentials were unaffected by the root drying treatment, indicating that the treatment reduced hydraulic conductivity between roots and foliage. After roots subjected to drying were returned to a nutrient solution or excised, ABA concentrations in the remaining roots and in the xylem sap, stomatal conductance of mature leaves and extension rate of unfolding leaves all returned to values observed in control plants. The 4-fold increase in xylem sap ABA concentration following the root drying treatment was not solely the result of reduced sap flow, and thus may be considered a potential cause, not merely a consequence, of the observed reduction in stomatal conductance.

The response of Whitegrass [Cortaderia pilosa (D’Urv.) Hack.] to nitrogen nutrition

To investigate the potential for increasing Falkland Island Whitegrass [Cortaderia pilosa (D’Urv.) Hack.] pasture production through application of nitrogen (N) fertilizer, two hydroponic experiments were conducted. First, 5 mg l–1 N was supplied to plants as nitrate (NO3–), urea [CO (NH2)2], ammonium (NH4+), or a nine parts NH4+: one part NO3– mixture. At harvest, plants grown in a NO3– medium had about half the biomass of plants grown in a NH4+ medium. In the second experiment plants were supplied with 1, 3, 10, 30 or 100 mg l–1 NH4+‐N. Plants at 1 and 3 mg l–1 N had the largest biomass of young root and the lowest shoot–root ratios. Leaf extension rate was low in the 1 mg l–1 N treatment. Plants given 10 mg l–1 N had the greatest proportion of green shoot material but little root growth; while those at 100 mg l–1 N produced very little shoot and root biomass. Preferential assimilation of NH4+‐N and a low N requirement make Whitegrass well adapted to dominating vegetation on much of the Falkland Islands.

Drought survival in Dactylis glomerata and Festuca arundinacea under similar rooting conditions in tubes

Drought survival in perennial forage plants involves different adaptative responses such as delay of dehydration through water uptake, limitation of water loss and tolerance of tissues to dessication. To compare the importance of these responses in contrasting cultivars of forage grasses at the whole plant level, we carried out two experiments under glasshouse conditions. Plants of cocksfoot (Dactylis glomerata L.) cultivars, cvs. Currie, Medly (both of Mediterranean origin) and Lutetia (of continental origin), and of tall fescue (Festuca arundinacea L.) cv. Centurion (Mediterranean) were grown in 60 cm-deep cylinders to eliminate the effect of differences of root depth on water availability whilst allowing severe drought to be imposed at a realistic rate. In both experiments, the cvs. were ranked similarly for plant survival, with high mortality for Centurion, low for the Mediterranean cocksfoots Currie and Medly, and intermediate for Lutetia. These differences could not be ascribed to water use during most of the drought period since water uptake and decrease in leaf extension were not significantly different between species and cultivars. However, resistant cvs. of cocksfoot were able to extract water for a longer period and at a lower soil water potential (Ψs) than other cvs. The critical Ψs at plant death was −3.8 and −3.6 MPa for Medly and Currie and −3.0-,−2.6 MPa for Lutetia and Centurion. Moreover, at a low soil water reserve (15–2%), membrane stability and water content were maintained for longer in enclosed immature leaf bases of cocksfoots cultivars, whereas the fescue Centurion exhibited accelerated lamina senescence and steady increase of membrane damage in surviving tissues. Therefore, it is proposed that the drought resistance of tall fescue in the field can mainly be ascribed to its ability to develop a deep root system. In cocksfoot, dehydration tolerance in surviving tissues and the ability of roots to extract water at low soil water potentials may, in addition to root depth, contribute significantly to plant survival under severe drought.

Responses of alpine shrubs to simulated environmental change during three years in the mid‐latitude mountain, northern Japan

Effects of artificial warming on phenology, individual leaf traits, vegetative growth, and reproduction of five alpine species (two deciduous and three evergreen shrubs) were investigated during three years in the mid‐latitude alpine, northern Japan. Eleven open‐top chambers (OTCs) were set up on a fellfield (1680 m a. s. l.) in the Taisetsu Mountains by which air temperature at plant height was increased by ca 2°C. Vaccinium uliginosum (deciduous shrub) showed earlier leaf emergence in every season and earlier flowering only in the first season in the OTCs. By contrast, acceleration of leaf emergence in the OTCs was not clear for other species, i.e. Arctous alpinus (deciduous shrub). Ledum palustre. V. vitis‐idaea, and Empetrum nigrum (evergreen shrub). Both deciduous species showed longer leaf life‐span in the OTCs every season. All evergreen species had higher leaf survival rates in the OTCs. indicating extension of leaf life‐span. Leaf nitrogen concentration and leaf mass per unit leaf area (mg cm −2) generally tended to decrease in the OTCs. Relationships between the individual leaf traits and cumulative air temperature during the leaf developing period were not clear. Total leaf production during the three seasons increased in the OTCs in A. alpinus. L. palustre. V. vitis‐idaea, and E. nigrum. All evergreen shrubs showed larger shoot growth in the OTCs but both deciduous shrubs did not show significant changes. In contrast to the vegetative growth, deciduous shrubs produced more flowers in the OTCs. Fruit production was not influenced by the OTCs for all species. The extension of photosynthetic period in the OTCs may contribute to the larger vegetative growth or flower production.

517 Heat Treatments and Controlled Atmospheres Maintain Quality and Reduce the Inner Leaf Extension or “Telescoping” of Minimally Processed Green Onions

Minimal processing of green onions (Allium cepa × A. fistulosum) involves trimming and removing damaged leaves, cutting of roots, and removal of the compressed stem. If the stem tissue is completely removed with the roots, the white inner leaf bases may extend, or “telescope,” during storage. Storage at 0 °C greatly retards extension growth, but storage at 5 °C results in unacceptable extension rates. To maintain high quality and to extend the shelf life of intact and minimally processed green onions, the potential benefits of heat treatments and controlled atmosphere storage were evaluated. Atmospheres of 0.1% to 0.2% O2 or 0.1% to 0.2% O2 containing 7.5% to 9.0% CO2 at 5 °C were the CA conditions that best maintained visual appearance and prolonged shelf life to more than 2 weeks in both intact and cut onions. No CA treatment completely controlled “telescoping” at 5 °C. Several heat treatment combinations (52.5 and 55 °C water for 4 and 2 min, respectively) of the white inner leaf bases were effective in controlling “telescoping” of cut green onions stored at 5 °C. The effective heat treatments resulted in higher average respiration rates during 12 days, but did not affect the visual quality or shelf life of the cut green onions. Total soluble sugars decreased in intact or cut green onions, but concentrations were maintained in heat -treated onions. Thiosulfinate concentrations did not vary importantly during 14 days at 5 °C, except for a reduction in heat-treated onions not stored under CA.