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.     

Kinetics of Maize Leaf Elongation : III. Silver Thiosulfate Increases the Yield Threshold of Salt-Stressed Plants, but Ethylene Is Not Involved

The involvement of ethylene in the short-term responses of maize (Zea mays L.) leaf elongation to salinity was investigated. Leaf elongation rates (LER) were monitored with linear variable differential transformers. Salinity (80 mm NaCl) rapidly inhibited LER. Pretreatment with 4 mm silver thiosulfate (STS), an inhibitor of ethylene action, decreased LER of salt-stressed plants, but not that of controls. Investigation of the growth parameters affected by the interaction of STS and salinity indicated that the yield threshold was increased and the cell wall extensibility was decreased. All other growth parameters controlling cell elongation were unaffected. Further investigation indicated that ethylene production may not be involved in this reponse because treatments with 10 μm aminoethoxyvinylglycine, an ethylene biosynthesis inhibitor, did not affect the growth of salt-stressed plants, and no increase in ethylene production was detected in salt-stressed plants compared with controls. No changes in sensitivity to ethylene were evident because LER of both control and salt-stressed plants were inhibited to the same extent with exposure to 1.2 ppm ethylene. The above evidence indicated that ethylene was not involved in the short-term LER responses of salt-stressed maize.     

Diurnal cycles of leaf extension in unsalinized and salinized Beta vulgaris

Abstract. Continuous high resolution measurement of sugar beet leaf extension over 5 d in growth chambers showed average leaf extension rates (LER) in darkness to be from three to six times those in light for plants growing in non-salinized media. The changes in LER in light-dark transitions occurred within seconds, a response which was more rapid than stomatal opening or closing. When the growth medium was salinized to 100 mol m⁻³ NaCl, LER's were reduced by about 50% in darkness and 90% in light, markedly increasing the ratio of dark to light LER.
A 2-d episode of root-zone salinity imposed midway through a 5-d period of measurement decreased LER and produced higher leaf temperatures. LER and diurnal leaf temperature patterns reverted to their pre-salinized levels when root-zone salinity was removed. Thus, the effects of short episodes of high sodium chloride in the growth medium appear to be reversible, suggesting a water stress mechanism of growth reduction rather than toxicity effects of salt.     

The relationship between leaf growth and ABA accumulation in the grass leaf elongation zone

Detached barley (Hordeum vulgare L.) shoots, maintained at different air temperatures and VPDs, were fed ABA via the sub-crown internode in a leaf elongation assay. Analysis of variance of leaf elongation rate (LER) showed significant effects of temperature (T), fed [ABA] and the interaction T × [ABA]. However, the interaction became non-significant when LER was modelled against the [ABA] of the elongation zone, [EZ-ABA] When detached barley shoots were fed sap from droughted maize (Zea mays L.) plants, sap [ABA] could not explain the growth inhibitory activity. Measurement of [EZ-ABA] accounted for this ‘unexplained’ growth inhibition. The detached shoot experiments indicated that [EZ-ABA], and not xylem sap [ABA], was an appropriate explanatory variable to measure in droughted plants. However, ABA accumulation in the elongation zone could not explain a 35% growth reduction in intact droughted plants; thus we considered an interaction of water status and ABA. Using a coleoptile growth assay, we applied mild osmotic stresses (ψ=0 to ?0.06 MPa) and 10^?4 mol m^?3 ABA. Individually, these treatments did not inhibit growth. However, osmotic stress and ABA applied together significantly reduced growth. This interaction may be an important mechanism in explaining leaf growth inhibition of droughted plants.     

Participation of Plant Hormones in Growth Resumption of Wheat Shoots Following Short-Term NaCl Treatment

The effects of sodium-chloride salinity on the leaf elongation rate, transpiration rate, cell sap osmolality, and phytohormone content in 7-day-old shoots of durum wheat (Triticum durum L.) were studied. Leaf growth was suppressed under the salinity stress and resumed 1 h after NaCl removal. The resumption of leaf growth coincided with a decrease in the transpiration rate due to the rapid ABA accumulation in the differentiation leaf zone. The increased IAA concentration in the growing leaf zone promoted the formation of the attraction signal. The authors concluded that the changes in phytohormonal status in wheat plants occurred already following short-term (up to 1 h) salinity and were directed to the maintenance of plant growth under these conditions.     

The effect of waterlogging on the growth and ethylene content of Eucalyptus robusta Sm. (Swamp Mahogany)

Summary When waterlogged over a period of 80 days plants of Eucalyptus robusta Sm. showed symptoms of leaf chlorosis, epinasty and premature abscission, reduction of stem elongation, stem hypertrophy and formation of adventitious shoots; chlorophyll content was reduced and soluble protein content of the upper leaves increased. Waterlogging doubled the rate of release of ethylene from roots and stems within 6 days, but had no effect on the ethylene concentration of leaves.     

5-Aminolevulinic acid improves photosynthetic gas exchange capacity and ion uptake under salinity stress in oilseed rape (Brassica napus L.)

Salinity is one of the major constraints in oilseed rape (Brassica napus L.) production. One of the means to overcome this constraint is the use of plant growth regulators to induce plant tolerance. To study the plant response to salinity in combination with a growth regulator, 5-aminolevulinic acid (ALA), oilseed rape plants were grown hydroponically in greenhouse conditions under three levels of salinity (0, 100, and 200 mM NaCl) and foliar application of ALA (30 mg/l). Salinity depressed the growth of shoots and roots, and decreased leaf water potential and chlorophyll concentration. Addition of ALA partially improved the growth of shoots and roots, and increased the leaf chlorophyll concentrations of stressed plants. Foliar application of ALA also maintained leaf water potential of plants growing in 100 mM salinity at the same level as that of the control plants, and there was also an improvement in the water relations of ALA-treated plants growing in 200 mM. Net photosynthetic rate and gas exchange parameters were also reduced significantly with increasing salinity; these effects were partially reversed upon foliar application with ALA. Sodium accumulation increased with increasing NaCl concentration which induced a complex response in the macro-and micronutrients uptake and accumulation in both roots and leaves. Generally, analyses of macro- (N, P, K, S, Ca, and Mg) and micronutrients (Mn, Zn, Fe, and Cu) showed no increased accumulation of these ions in the leaves and roots (on dry weight basis) under increasing salinity except for zinc (Zn). Foliar application of ALA enhanced the concentrations of all nutrients other than Mn and Cu. These results suggest that under short-term salinity-induced stress (10 days), exogenous application of ALA helped the plants improve growth, photosynthetic gas exchange capacity, water potential, chlorophyll content, and mineral nutrition by manipulating the uptake of Na⁺.     

Kinetics of Maize Leaf Elongation: I. INCREASED YIELD THRESHOLD LIMITS SHORT-TERM, STEADY-STATE ELONGATION RATES AFTER EXPOSURE TO SALINITY

The short-term responses of leaf elongation to salinity areinvestigated in this study. The kinetics of maize (Zea maysL.) leaf elongation were measured with Linear Variable DifferentialTransformers (LVDTs). After exposure to salinity (0 to 120 molm^–3 NaCl), leaf elongation rates (LER) declined rapidly.Within 4 h, LER had recovered and reached a new steady-statefor all salinity treatments. These rates were reduced by 10,20, and 60% of control rates by 40, 80 and 120 mol m^–3NaCl, respectively. Osmotic adjustment in the growing zone ofleaves was correlated with the recovery of LER after plant exposureto salinity. However, after 4 h of exposure, the osmolalityof the cell sap continued to increase without effect on steady-stateLER. Estimates of the apparent turgor in the growing zone indicatedthat turgor was no longer limiting LER of salt-stressed plantsafter 4 h. An in vivo technique was developed to apply a unidirectionalforce to intact growing leaves of maize to mimic increases inelongation force. Relative elongation rate (RER) were increasedby adding weights to the LVDT core to increase elongation force.Plots of RER as a function of elongation force gave estimatesof two growth coefficients: the yield threshold and the yieldingcoefficient, mL/(m + L), where m is the cell wall extensibilityand L is the hydraulic conductivity. RER as a function of elongationforce was determined immediately, 05, 4, and 21 h after plantswere salinized. Estimates of the growth coefficients indicatedthat the apparent yield threshold decreased immediately aftersalinization. However, when LER reached steady-state, the yieldthreshold of salt-stressed plants had increased above controlvalues and was the only limiting growth coefficient. There wereno significant effects of salinity on the yielding coefficients,cell wall extensibility or hydraulic conductivity. One of theadvantages of this in vivo technique over other methods is thatyield threshold, yielding coefficient, and cell wall extensibilitycan be determined without the confounding effects of woundingor osmotic stress. This technique may prove widely applicableto the study of other growth regulating factors. Key words: Salinity, leaf growth, Zea mays L     

Do pH changes in the leaf apoplast contribute to rapid inhibition of leaf elongation rate by water stress? Comparison of stress responses induced by polyethylene glycol and down‐regulation of root hydraulic conductivity

We have dissected the influences of apoplastic pH and cell turgor on short-term responses of leaf growth to plant water status, by using a combination of a double-barrelled pH-selective microelectrodes and a cell pressure probe. These techniques were used, together with continuous measurements of leaf elongation rate (LER), in the (hidden) elongating zone of the leaves of intact maize plants while exposing roots to various treatments. Polyethylene glycol (PEG) reduced water availability to roots, while acid load and anoxia decreased root hydraulic conductivity. During the first 30 min, acid load and anoxia induced moderate reductions in leaf growth and turgor, with no effect on leaf apoplastic pH. PEG stopped leaf growth, while turgor was only partially reduced. Rapid alkalinization of the apoplast, from pH 4.9 ± 0.3 to pH 5.8 ± 0.2 within 30 min, may have participated to this rapid growth reduction. After 60 min, leaf growth inhibition correlated well with turgor reduction across all treatments, supporting a growth limitation by hydraulics. We conclude that apoplastic alkalinization may transiently impair the control of leaf growth by cell turgor upon abrupt water stress, whereas direct hydraulic control of growth predominates under moderate conditions and after a 30-60 min delay following imposition of water stress.     

Biosynthesis, translocation, and accumulation of betaine in sugar beet and its progenitors in relation to salinity

Like other halophytic chenopods, sugar beet (Beta vulgaris L.) can accumulate high betaine levels in shoots and roots. N,N,N-trimethylglycine impedes sucrose crystallization and so lowers beet quality. The objective of this research was to examine the genetic variability and physiological significance of betaine accumulation in sugar beet and its relatives. Three cultivated genotypes of B. vulgaris and two genotypes of the wild progenitor B. maritima L. were grown with and without gradual salinization (final NaCl concentration = 150 millimolar). At 6 weeks old, all five genotypes had moderately high betaine levels in shoots and roots when unsalinized (averages for all genotypes: shoots = 108 micromoles per gram dry weight; roots = 99 micromoles per gram dry weight). Salinization raised betaine levels of shoots and roots 2- to 3-fold, but did not greatly depress shoot or root growth. The genotype WB-167—an annual B. maritima type—always had approximately 40% lower betaine levels in roots than the other four genotypes, although the betaine levels in the shoots were not atypically low.

The site and pathway of betaine synthesis were investigated in young, salinized sugar beet plants by: (a) supplying 1 micromole [¹⁴C]ethanolamine to young leaf blades or to the taproot sink of intact plants; (b) supplying tracer [¹⁴C]formate to discs of leaf, hypocotyl, and taproot tissues in darkness. Conversion of both ¹⁴C precursors to betaine was active only in leaf tissue. Very little ¹⁴C appeared in the phospholipid phosphatidylcholine before betaine was heavily labeled; this was in marked contrast to the labeling patterns in salinized barley. Phosphorylcholine was a prominent early ¹⁴C metabolite of both [¹⁴C]ethanolamine and [¹⁴C]formate in all tissues of sugar beet. Betaine translocation was examined in young plants of sugar beet and WB-167 by applying tracer [methyl-¹⁴C]betaine to a young expanded leaf and determining the distribution of ¹⁴C after 3 days. In all cases, extensive ¹⁴C translocation to young leaves and taproot sink occurred; neither in the fed leaf nor in sink organs were any ¹⁴C metabolites of betaine detected.

     

Control of Leaf Expansion Rate of Droughted Maize Plants under Fluctuating Evaporative Demand (A Superposition of Hydraulic and Chemical Messages?)

We have analyzed the possibility that chemical signaling does not entirely account for the effect of water deficit on the maize (Zea mays L.) leaf elongation rate (LER) under high evaporative demand. We followed time courses of LER (0.2-h interval) and spatial distribution of elongation rate in leaves of either water-deficient or abscisic acid (ABA)-fed plants subjected to varying transpiration rates in the field, in the greenhouse, and in the growth chamber. At low transpiration rates the effect of the soil water status on LER was related to the concentration of ABA in the xylem sap and could be mimicked by feeding artificial ABA. Transpiring plants experienced a further reduction in LER, directly linked to the transpiration rate or leaf water status. Leaf zones located at more than 20 mm from the ligule stopped expanding during the day and renewed expansion during the night. Neither ABA concentration in the xylem sap, which did not appreciably vary during the day, nor ABA flux into shoots could account for the effect of evaporative demand. In particular, maximum LER was observed simultaneously with a minimum ABA flux in the droughted plants, but with a maximum ABA flux in ABA-fed plants. All data were interpreted as the superposition of two additive effects: the first involved ABA signaling and was observed during the night and in ABA-fed plants, and the second involved the transpiration rate and was observed even in well-watered plants. We suggest that a hydraulic signal is the most likely candidate for this second effect.     

Leaf water potential of differentially salinized plants

Water and osmotic potential energies were measured with thermocouple psychrometers, at intervals during a 4-week period, in growing leaves of bean (Phaseolus vulgaris, var. Blue Lake) and barley (Hordeum vulgare, var. Liberty) plants having roots equally split between 2 differentially salinized nutrient solutions. The osmotic potentials of plants with half their roots in saline solutions were about halfway between the osmotic potentials of plants grown in nonsaline solutions and those grown in saline solutions. By the end of the 4-week measurement period, the beans and barley were almost mature. The final dry weights of shoots of plants with half their roots in saline solutions were about halfway between the dry weights of the shoots of plants grown in nonsaline solutions and the dry weights of those in saline solutions. The results obtained showed that the degree of osmotic adjustment and the rate of growth were functions of the proportion of the root system exposed to saline conditions.     

Rapid and reversible modifications of extension capacity of cell walls in elongating maize leaf tissues responding to root addition and removal of NaCl

A creep extensiometer technique was used to provide direct evidence that short (20 min) and long-term (3d) exposures of roots to growth inhibitory levels of salinity (100mol m^-3 NaCl) induce reductions in the irreversible extension capacity of cell walls in the leaf elongation zone of intact maize seedlings (Zea mays L.). The long-term inhibition of cell wall extension capacity was reversed within 20 min of salt withdrawal from the root medium. Inhibited elongation of leaf epidermal tissues was also reversed after salt removal. The salt-induced changes in wall extension capacity were detected using in vivo and in vitro assays (shortly after localized freeze/thaw treatment of the basal elongation zone). The rapid reversal of the inhibition of wall extensibility and leaf growth after salt removal from root medium of long-term salinized plants, suggested that neither deficiencies in growth essential mineral nutrients nor toxic effects of NaCl on plasmamembrane viability were directly involved in the inhibition of leaf growth. There was consistent agreement between the scale, direction and timing of salinity-induced changes in leaf elongation growth and wall extension capacity. Rapid metabolically regulated changes in the physical properties of growing cell walls, caused by osmotic (or other) effects, appear to be a factor regulating maize leaf growth responses to root salinization.     

Effects of Salinity on the Extensibility and Ca Availability in the Expanding Region of Growing Barley Leaves

The growth of barley (Hordeum vulgare L.) leaves is reduced by salinity. We used the Instron extensometric technique to measure the reversible and irreversible compliance of the expanding regions of growing barley leaves from plants exposed to 1, 40, 80 and 120 mM NaCl in nutrient solution. Two barley cultivars differing in salinity resistance (cv ‘Arivat’ and cv ‘Briggs’) were compared over 5d of leaf growth. During the period of most active leaf expansion, salinity reduced reversible compliance and increased compliance in the leaf segments, although responses to salinity were complex and changed over the course of leaf expansion. Salinity increased irreversible compliance more in the salt-sensitive cultivar Arivat than in the more salt-tolerant cultivar Briggs. Elemental analysis of the basal leaf segments used for extensometry revealed an accumulation of Na and a depletion of Ca in segments from salinized plants, resulting in very high Na: Ca ratios in salinized expanding tissue. The concentrations of K and Mg in basal leaf tissue were elevated by salinity. Our data do support the hypothesis that the inhibition of leaf expansion by salinity stress is mediated by a decline in irreversible extensibility. We suggest that reduced Ca availability in expanding leaf tissue may contribute to growth reduction in salt-stressed barley seedlings.     

Characterization of Na+ exclusion mechanisms of salt-tolerant reed plants in comparison with salt-sensitive rice plants

Salt-tolerant reed plants ( Phragmites communis Trinius) and salt-sensitive rice plants ( Oryza sativa L. cv. Kinmaze) were grown in salinized nutrient solutions up to 50 m M NaCl, and growth, Na⁺ contents and kinetics of ²²Na⁺ uptake and translocation were compared between the species to characterize the salt tolerance mechanisms operating in reed plants. When both plants were grown under the same salinity, Na⁺ contents of the shoots were lower in reed plants, although those of the roots were quite similar. The shoot base region of both species accumulated Na⁺ more than the leaf blades did. Sodium-22 uptake and pulse-chase experiments suggested that the lower Na⁺ transport rate from root to shoot could limit excessive Na⁺ accumulation in the reed shoot. There was a possibility that the apparently lower ²²Na⁺ transport rate to the shoot of reed plants was due to net downward Na⁺ transport from shoot base to root.     

Correlation between Long-Day Flowering and Root Elongation in Pharbitis nil, Sfrain Kidachi

Long-day flowering of Pharbitis nil, dwarf strain Kidachi, at20?C was greatly influenced by the size of the culture vesseland the number of plants per vessel. The smaller the vessel,the greater the flowering response. The volume of nutrient solutionper plant was not decisive for long-day flowering. For instance,plants cultured singly in 200-ml beakers flowered, but thosecultured in 5,000-ml vessels (33?26?11.5 cm, 48 plants per vessel)did not, even though there was only about 100 ml of nutrientsolution per plant. Long-day flowering was always accompaniedby the suppression of root elongation, but not by a decreasein the dry weight of roots or shoots, or in the rate at whichthe leaf primordia appeared (plastochrone). Aeration of thenutrient solution or culture in vermiculite promoted root elongationeven in small vessels, thereby inhibiting long-day flowering.Thus the suppression of root elongation seems to be necessaryfor long-day flowering. Removal of the roots or cotyledons;however, suppressed long-day flowering even when root elongationwas inhibited by culture in small vessels. When plants werecultured at 24?C, suppression of root elongation (culture ina small vessel) did not induce long-day flowering; but, short-daytreatment induced flowering without suppressing root elongation. (Received April 19, 1982; Accepted June 24, 1982)     

Phosphorus transport to the xylem and its regulation by water flow

Summary The effects of water flow on phosphorus uptake by roots and on its subsequent translocation to shoots were separated by giving short-term pulses of ³²P-labelled nutrient to intact tomato plants. At the end of a 5 min pulse, all the ³²P taken up by the plants was confined to the roots. Only about half of this ³²P was later translocated to shoots; there was very little translocation after 4 hours.Experiments after long-term labelling showed that only a small part of the total P in the root is readily translocated to shoots. This P appears to be in part of the symplast and contributes about 75% of the P transported to the xylem sap. The rest is presumably derived by leakage from vacuoles.A slow rate of water flow reduced both uptake into the symplast and the translocation to the shoots of P which had already been absorbed by the roots. This was conclusively demonstrated by giving a ³²P pulse before reducing the rate of water flow; ³²P not translocated to shoots was partly retained by the roots and partly lost to the external solution. Water flow also accelerates transport to the xylem of previously-absorbed P in excised roots.It is concluded that the major effect of water flow on phosphorus transport to shoots occurs after phosphorus uptake by the roots, probably during radial transport to the xylem.     

Root cooling strongly affects diel leaf growth dynamics,water and carbohydrate relations in Ricinus communis

In laboratory and greenhouse experiments with potted plants, shoots and roots are exposed to temperature regimes throughout a 24 h (diel) cycle that can differ strongly from the regime under which these plants have evolved. In the field, roots are often exposed to lower temperatures than shoots. When the root‐zone temperature in Ricinus communis was decreased below a threshold value, leaf growth occurred preferentially at night and was strongly inhibited during the day. Overall, leaf expansion, shoot biomass growth, root elongation and ramification decreased rapidly, carbon fluxes from shoot to root were diminished and carbohydrate contents of both root and shoot increased. Further, transpiration rate was not affected, yet hydrostatic tensions in shoot xylem increased. When root temperature was increased again, xylem tension reduced, leaf growth recovered rapidly, carbon fluxes from shoot to root increased, and carbohydrate pools were depleted. We hypothesize that the decreased uptake of water in cool roots diminishes the growth potential of the entire plant – especially diurnally, when the growing leaf loses water via transpiration. As a consequence, leaf growth and metabolite concentrations can vary enormously, depending on root‐zone temperature and its heterogeneity inside pots.     

Performance of seminal and nodal roots of wheat in stagnant solution: K+ and P uptake and effects of increasing O2 partial pressures around the shoot on nodal root elongation

Roots of intact wheat plants were grown for 7-12 d in stagnant nutrient solution, containing 0.1% agar, to mimic the lack of convection in waterlogged soil. Net K+ and P uptakes by seminal and nodal roots were measured separately using a split root system. For seminal roots in stagnant solution, net uptakes as a percentage of aerated roots were between 0% and 16% for P, while K+ ranged between 15% uptake and 54% loss. For the more waterlogging-tolerant nodal roots, net uptakes in stagnant nutrient solution, as a percentage of aerated roots, were 31-73% for P and 69-115% for K+. Elongation rates of nodal roots in stagnant nutrient were about 35-43% of those for roots in aerated solution. This partial inhibition occurred in these nodal roots despite their 15% porosity (v/v). Elevation of O2 partial pressures around the shoots to 40 kPa and then to 80 kPa substantially accelerated nodal root elongation in stagnant solution, demonstrating that most of the inhibition seen with ambient O2 around the shoots was associated with a restricted O2 supply to these nodal roots. Thus, in wheat nodal roots, with a partial pressure of 20 kPa O2 around the shoots, O2 diffusion from the shoots did not completely relieve the restrictions on elongation resulting from stagnancy in the nutrient solution. These results contrast with those in the literature for rice, in which roots function efficiently in stagnant solutions (0.1% agar). So, when wheat roots are aerenchymatous there are still restrictions to O2 diffusion in the gas space continuum between the atmosphere and the functional tissues of the roots. This poor acclimation must have been due to inefficiency of the aerenchymatous axes, which may include persistence of anoxic steles, and/or restricted O2 diffusion in other parts of the gas space continuum, in either the shoots and shoot-root junction or in the root tip.