Enhancement of NaCl tolerance in Arabidopsis thaliana by exogenous L-asparagine and D-asparagine

The tolerances of Columbia Arabidopsis thaliana (L.) Heynh. to NaCl, L-asparagine (L-Asn) and D-asparagine (D-Asn) during seedling establishment on sterile agar medium were determined. Germination and the establishment of upright seedlings with expanded green cotyledons were increasingly inhibited by NaCl concentrations from 20 to 180 m M and radicle growth was prevented at 225 m M NaCl. Tolerance of established seedlings to NaCl was similar at these concentrations. Seedling establishment was prevented at 20 m M L-Asn and 60 m M D-Asn, but L-Asn was not toxic to established seedlings. At lower concentrations, exogenous L- and D-Asn enhanced NaCl tolerance during germination and seedling establishment. Inhibition of seedling establishment by NaCl concentrations below 225 m M was reduced by the addition of L- and D-Asn to the medium. Maximal reduction of NaCl inhibition occurred between 2 and 4 m M for both L- and D-Asn. Higher concentrations of NaCl prevented establishment whether exogenous Asn was present or not. Reduction of NaCl inhibition occurred to the same extent whether L-Asn was presented simultaneously with the NaCl or preloaded for up to 24 h. The total seedling content of Na⁺ increased about 4-fold to 55 μg (mg dry weight)⁻¹ as the medium concentration of NaCl was increased from 9 μ M to 150 m M NaCl. Total K⁺ content declined about 80% from about 34 μg (mg dry weight)⁻¹ over the same range of NaCl concentrations. The Na⁺ uptake and K⁺ efflux by whole seedlings were similar whether or not NaCl tolerance was increased by exogenous Asn.     

Effects of desiccation on the 77°K fluorescence properties of the liverwort Porella navicularis and the isolated lichen green alga Trebouxia pyriformis

The uptake of the auxin type herbicide 2,4-D into rice seedlings ( Oryza sativa L. cv. Dunghan Shali) and its effects on the K⁺, NH⁺₄ and NO₃ ion uptake and the K⁺ content were investigated at different pH values. A short incubation of the roots in 0.01 m M 2,4-D caused a marked ion uptake inhibition only at low pH. The non-auxin type herbicide benthiocarb did not produce such an inhibitory effect. Lowering of the pH in the external medium led to an increased 2,4-D uptake by the roots. These results can be explained by the increased H⁺ permeability of the membranes, allowing a more rapid entrance of 2,4-D into the root cells, thereby inhibiting the active ion uptake. Rice roots not subjected to 2,4-D treatment responded to H⁺ stress with an increased anomalous K⁺ uptake and a decreased K⁺ content. With reference to the effects of pH changes on the ion and 2,4-D uptake, possible transport mechanism of NH⁺₄ and 2,4-D are briefly discussed.     

Net CO2 exchange of Pinus taeda shoots exposed to variable ozone levels and rain chemistries in field and laboratory settings

Net CO₂ exchange rates (CERs) were measured in seedlings of two loblotly pine ( Pinus taeda L.) families following 6- or 13-week exposures to ozone (charcoalfiltered or ambient air + O₃) and acid rain treatments (pH 3.3, 4.5 and 5.2). Ozone exposures (14 or 170 nl l⁻¹) were made in open-top chambers, and in continously stirred tank reactors (14, 160 or 320 nl l⁻¹) located in the field and laboratory, respectively. The CERs of whole shoots were measured in an open infrared gas analysis system at 6 levels of photosynthetic photon flux density (0, 33, 60, 410, 800 and 1660 μmol m⁻² s⁻¹). Treatment effects were not consistent between field- and laboratory-exposed seedlings. Ozone-treated field seedlings exhibited statistically significant reductions in light-saturated CER of 12.5 and 25% when measured at 6 and 13 weeks, respectively. Laboratory seedlings exhibited mixed responses to O₃, with one family showing reduced CER only after 6 weeks of O₃ exposure and the other only after 13 weeks (O₃ >160 nl l⁻¹ for both). After 13 weeks of exposure, pH 3.3, and 4.5 rain treatments enhanced light-saturated CER by an average of 52% over that observed in seedlings exposed to the pH 5.2 treatment. Enhanced CERs due to acid rain were of the same magnitude (3–5 μmol CO₂g⁻¹ s⁻¹) as ozone-induced CER reductions. No differences in dark respiration were detected between treatments. Although ozone and acid rain treatments altered seedling CER, the differences were not translated into altered final plant dry weights over the 13-week exposure period.     

Exposure to preindustrial, current and future atmospheric CO2 and temperature differentially affects growth and photosynthesis in Eucalyptus

To investigate if Eucalyptus species have responded to industrial-age climate change, and how they may respond to a future climate, we measured growth and physiology of fast- ( E. saligna ) and slow-growing ( E. sideroxylon ) seedlings exposed to preindustrial (290), current (400) or projected (650 μL L⁻¹) CO₂ concentration ([CO₂]) and to current or projected (current +4 °C) temperature. To evaluate maximum potential treatment responses, plants were grown with nonlimiting soil moisture. We found that: (1) E. sideroxylon responded more strongly to elevated [CO₂] than to elevated temperature, while E. saligna responded similarly to elevated [CO₂] and elevated temperature; (2) the transition from preindustrial to current [CO₂] did not enhance eucalypt plant growth under ambient temperature, despite enhancing photosynthesis; (3) the transition from current to future [CO₂] stimulated both photosynthesis and growth of eucalypts, independent of temperature; and (4) warming enhanced eucalypt growth, independent of future [CO₂], despite not affecting photosynthesis. These results suggest large potential carbon sequestration by eucalypts in a future world, and highlight the need to evaluate how future water availability may affect such responses.     

Photosynthetic acclimation to elevated atmospheric carbon dioxide and UV irradiation in Pinus banksiana

Pinus banksiana seedlings were grown for 9 months in enclosures in greenhouses at CO₂ concentrations of 350 or 750 μmol mol⁻¹ with either low (0.005 to 0. 3 W m⁻²) or high (0.25 to 0. 90 W m⁻²) ultraviolet-B (UV-B) irradiances. Total seedling dry weight decreased with high UV treatment but was unaffected by CO₂ enrichment. High UV treatment also shifted biomass partitioning in favor of leaf production. Both CO₂ and UV treatments decreased the dark respiration rate and light compensation point. High UV light inhibited photosynthesis at 350 but not at 750 μmol mol⁻¹ CO₂ due to a UV induced increase in ribulose-1, 5-bisphosphate carboxylase/oxygenase efficiency and ribulose-1, 5-bisphosphate regeneration. Stomatal density was increased by high UV irradiance but was unchanged by CO₂ enrichment.     

Different effects of supplied ammonium on glutamine synthetase activity in mustard (Sinapis alba) and pine (Pinus sylvestris) seedlings

The activity of glutamine synthetase (GS) in mustard ( Sinapis alba L.) and Scots pine ( Pinus sylvestris L.) seedlings was used as an index to evaluate the capacity to cope with excessive ammonium supply. In these 2 species GS activity was differently affected by the application of nitrogen compounds (NH₄⁺ or NO₃⁻). Mustard seedlings older than 5 days showed a considerable increase in GS activity after NH₄⁺ or NO₃⁻ application. This response was independent of the energy flux, but GS activity in general was positively affected by light. Endogenous NH₄⁺ did not accumulate greatly after nitrogen supply. In contrast, seedlings of Scots pine accumulated NH₄⁺ in cotyledons and roots and showed no stimulation of GS activity after the application of ammonium. In addition, root growth was drastically reduced. Thus, the pine seedlings seem to have insufficient capacity to assimilate exogenously supplied ammonium. NO₃⁻, however, did not lead to any harmful effects.     

Germination and seedling growth of cotton: salinity-calcium interactions

Abstract. The effects of NaCl salinity on germination and early seedling growth of cotton were studied. Germination was both delayed and reduced by 200 mol m⁻³ NaCl in the presence of a complete nutrient medium. Seedlings, 7–9 d old, were greatly reduced in fresh weight by salinity. The addition of supplemental Ca²⁺ (10 mol m⁻³ as SO₄²⁻ or Cl⁻) to the medium did not improve germination but, to a large degree, offset the reduction in root growth caused by NaCl. Roots growing in the high salt medium without supplemental Ca²⁺ appeared infected by microbes. The cation specificity of the beneficial Ca²⁺ effect on growth was ascertained by testing additions of MgSO₄ or KCl to the NaCl treatments. The contents of K⁴ and Ca²⁺ were reduced in both roots and shoots by the NaCl treatments. Supplemental Ca²⁺ partially offset this effect for K⁴ in the roots and for Ca²⁺ in both roots and shoots. Sodium contents were not affected by the supplemental Ca²⁺. It is concluded that the beneficial effect of high Ca²⁺ concentrations on root growth of cotton seedlings in a saline environment may be due to maintenance of K/Na-selectivity and adequate Ca status in the root.     

Overexpression of light-dependent PORA or PORB in plants depleted of endogenous POR by far-red light enhances seedling survival in white light and protects against photooxidative damage

The structurally related light-dependent protochlorophyllide (Pchlide) oxidoreductases PORA and PORB mediate the only light-requiring step in chlorophyll (Chl) biosynthesis in higher plants. Correlative evidence suggests that some in vivo functions of PORA and PORB may be unique, including a postulated photoprotective role for PORA. For example, wild-type Arabidopsis thaliana seedlings grown in non-photooxidative far-red light (cFR) resemble those grown in white light (WL), but they are yellow and do not green normally thereafter in WL. This defect is accompanied by the absence of detectable PORA and reduced levels of PORB expression. Here, direct evidence is provided that the presence of POR, either as PORA or PORB, can confer photoprotection in plants. In contrast to the wild-type, the plastids of transgenic PORA- or PORB-overexpressing Arabidopsis seedlings grown in cFR possess extensive prolamellar bodies. Upon a subsequent shift to WL, POR-overexpressing seedlings develop thylakoid membranes, accumulate large amounts of Chl and are viable at fluence rates lethal to the wild-type. Intriguingly, the plastid membrane architectures of greening transgenic seedlings seem to depend on whether PORA or PORB has been overproduced. POR-overexpressing seedlings shifted from cFR to WL of fluence rates from 20 to 500 μE m^–2 sec^–1 accumulate substantially higher amounts of Chl than does the wild-type. Furthermore, the WL fluence rate that permits maximal Chl accumulation increases from 8 μE m^–2 sec^–1 in the wild-type to 125 μE m^–2 sec^–1 in transgenic seedlings. POR overexpression during growth in cFR also correlates with a fourfold decrease in the steady-state content of Pchlide, a potentially lethal photosensitizer.     

Mode of interference of chlorosis-inducing herbicides with peroxisomal enzyme activities

In rye leaves ( Secale cereale L. cv. Petkus "Kustro") bleached in the presence of the chlorosis-inducing herbicides aminotriazole, haloxidine, San 6706 or difunone in white light of 54.2 W m^-2 (5000 lx), catalase activity was very low. In addition, the activities of glycolate oxidase and hydroxypyruvate reductase were strongly diminished in treatments with San 6706 and difunone. The lowering of the peroxisomal enzyme activities was observed in red, but not in blue light and did not occur after treatment with the non-bleaching pyridazinone derivative San 9785. The deficiencies of the peroxisomal enzymes did not appear to be involved in the initiation of the chlorosis. Instead they are probably produced as secondary consequences of the bleaching. Low peroxisomal enzyme activities were also obtained without herbicide treatment by growing the leaves in an atmosphere of 2% O₂ and 3% CO₂, but in this case were not accompanied by an increased sensitivity of the Chl to photooxidative bleaching. The peroxisomal enzymes reached as high activities as in untreated controls when the herbicide-treated leaves were grown at a low light intensity of 0.106 W m^-2 (10 lx). After transfer of herbicide-treated leaves grown under 0.106 W m^-2 to 306 W m^-2 (30 000 lx), catalase was strongly inactivated, even at 0°C. In treatments with San 6706 and difunone the increase of the activities of glycolate oxidase and hydroxypyruvate reductase was either stopped, remaining unchanged, or the enzymes were slightly inactivated after exposure to 306 W m^-2 (30 000 lx). The observations suggest that the inactivation of peroxisomal enzymes results from photooxidative events in the chloroplasts.     

Increase of cotton cotyledon resistance to the herbicide endothall by abscisic acid

The herbicide endothall (7-oxabicyclo 2.2.1 heptane-2, 3-dicarboxylic acid) was applied to cotton ( Gossypium hirsutum L. cv. Acala SJ-1) cotyledon discs. Treatment with 10⁻⁴ M endothall for 24 h resulted in injury which was expressed by increased leakage of electrolytes, development of necrotic areas, increased level of polyphenols and tissue browning. We examined whether treatments which decrease chilling injury would also decrease injury caused by the herbicide. Tissue from seedlings grown at 28°C was more sensitive to endothall than that from seedlings grown at 15°C. Pretreatment with 10⁻⁵ M abscisic acid greatly decreased the leakage of electrolytes, necrotic areas, and tissue browning caused by endothall. Similar pretreatment did not prevent the increase of polyphenols caused by the herbicide. The treatment with abscisic acid was more effective in protection against the herbicide injury when applied several hours prior to the herbicidal treatment. This time requirement indicates that the mechanism by which abscisic acid induces resistance to the herbicide depends, at least partially, on active metabolism. We suggest that the increased resistance to herbicide stress by abscisic acid is another example of a common resistance mechanism to various stresses in which abscisic acid is involved.     

Herbicidal activity of 4-chloroindoleacetic acid and other auxins on pea, barley and mustard

The natural auxins, 4-chloroindoleacetic acid and its methyl ester have strong herbicidal effects on pea, Pisum sativum , a plant in which they occur naturally. The standard herbicide, 2,4-dichlorophenoxyacetic acid (2,4-D) is only 5 times more effective than 4-chloroindoleacetic acid. The I₅₀, the dose inhibiting yield by 50%, for 4-chloroindoleacetic acid and its methyl ester is 0.5 kg ha⁻¹ or 15 mg kg⁻¹ fresh weight, close to the concentration of 4-chloroindoleacetic acid methyl ester in maturing pea seeds. Naphthaleneacetic acid and indoleacetic acid are also inhibitory, but at much higher concentrations. In its inhibiting effect on white mustard, Sinapis alba , 4-chloroindoleacetic acid approximates that of 2,4-D, whereas in barley, Hordeum vulgare , it is a stronger herbicide than 2,4-D. All auxins tested killed white mustard at low doses, but none killed barley. Both 4-chloroindoleacetic acid and 2,4-D killed pea. The chloroindole auxins of pea may be the hypothetic death hormones or senescence factors that are secreted from the developing seeds into the parent plant which is strongly inhibited or killed and from which the nutrients are mobilized and translocated to the seeds. The action mechanism of auxin type herbicides may be to simulate the action of endogenous herbicides.     

The response of Norway spruce seedlings to simulated acid mist

Four pot experiments are reported in which Norway spruce ( Picea abies (L.) Karst) seedlings, of different nutrient status, were treated with acid mist for one growing season in open-top chambers (OTCs). Combinations of H⁺, SO₄²⁻, NH₄⁺ and NO₃⁻ were applied at different frequencies of application and supplying different doses of S and N kg ha⁻¹. Plant growth, visible injury, frost hardiness and nutrient status were observed. These experiments were undertaken to improve our understanding of the interaction of environmental factors such as nutrition and mist-exposure frequency on seedling response to N and S deposition.
Both acidity (pH 2·7) and SO₄²⁻ ions were necessary to induce visible injury. Mist containing SO₄²⁻, H⁺ and to a lesser extent NH₄⁺ significantly reduced winter frost hardiness. Increasing the misting frequency, and to a lesser extent the overall dose, increased the likelihood of acid mist causing visible injury and reducing frost hardiness. Post-planting stress, low N status and needle juvenility increased the likelihood of acid mist causing visible injury. Increased plant vitality, adequate N status and growth rate reduced the likelihood of acid-mist-induced reductions in frost hardiness.
Principles underlying the responses of spruce seedlings treated in controlled conditions to acid mist are discussed.     

Calcium ion dependency of ethylene production in segments of primary roots of Zea mays

We investigated the effect of Ca²⁺ on ethylene production in 2-cm long apical segments from primary roots of corn ( Zea mays L., B73 × Missouri 17) seedlings. The seedlings were raised under different conditions of Ca²⁺ availability. Low-Ca and high-Ca seedlings were raised by soaking the grains and watering the seedlings with distilled water or 10 m M CaCl₂, respectively. Segments from high-Ca roots produced more than twice as much ethylene as segments from low-Ca roots. Indoleacetic acid (IAA; 1 μ M ) enhanced ethylene production in segments from both low-Ca and high-Ca roots but auxin-induced promotion of ethylene production was consistently higher in segments from high-Ca roots. Addition of I-aminocyclopropane-I-carboxylic acid (ACC) to root segments from low-Ca seedlings doubled total ethylene production and the rate of production remained fairly constant during a 24 h period of monitoring. In segments from high-Ca seedlings ACC also increased total ethylene production but most of the ethylene was produced within the first 6 h. The data suggest that Ca²⁺ enhances the conversion of ACC to ethylene. The terminal 2 mm of the root tip were found to be especially important to ethylene biosynthesis by apical segments and, experiments using ⁴⁵Ca²⁺ as tracer indicated that the apical 2 mm of the root is the region of strongest Ca²⁺ accumulation. Other cations such as Mn²⁺, Mg²⁺, and K⁺ could largely substitute for Ca²⁺. The significance of these findings is discussed with respect to recent evidence for gravity-induced Ca²⁺ redistribution and its relationship to the establishment of asymmetric growth during gravitropic curvature.     

Protecting direct seeded grasses from herbicide application: can new extruded pellet formulations be used in restoring natural plant communities?

Restoration of native plant communities through direct seeding often experience low seedling establishment success rates, partly due to competition with invasive weed species. To improve seeding success, herbicides can be applied to control weed competition, however, this can have negative impacts on the seeded species. Activated carbon (AC) can be incorporated into newly developed seed enhancement technologies to adsorb herbicides and increase seedling tolerance. This study expands upon research completed to date, by developing new formulations of extruded pellets containing AC, aiming to provide increased protection to seeded species and increase herbicide selectivity. We tested six extruded pellet formulations, which included two pellet formula variations, and three quantities of AC, to examine the impact on emergence (without herbicide) and mortality (with herbicide) of Lolium rigidum Gaudin (annual ryegrass). Extruded pellet formulations containing a superabsorbent polymer (3%) and AC (10%) did not impede emergence (79%), in the absence of herbicide, similar to the non‐pelleted seeds (81%). This extruded pellet formulation increased seedling tolerance to Simazine (a pre‐emergent, soil applied herbicide) application, with mortality reduced from 96% in non‐pelleted seeds, and 77% in pellets containing no AC, to 22% in pellets containing AC. The results from this study demonstrate that AC extruded pelleting can be used as a restoration seeding technology by protecting seeds from the negative effects of pre‐emergent herbicide applications. Field evaluations with native seeds will mark an important step forward to ensure seed enhancement technology options, such as AC extruded pelleting, are available for restoring natural plant communities in restoration programs.     

In vivo and in vitro effects of boron on the plasma membrane proton pump of sunflower roots

The effect of boron excess and deficiency on H⁺ efflux from excised roots from sunflower ( Heliarahus annuus L. cv. Enano) seedlings and on plasma membrane H⁺-ATPase (EC 3.6.1.35) in isolated KI-washed microsomes has been investigated. When seedlings were grown in media with toxic levels of H₃BO₃ (5 m M ) or without added boron and exposed to light conditions, an inhibition of the capacity for external acidification by excised roots was observed as compared to roots from seedlings grown with optimal H₃BO₃ concentration (0.25 m M ). Toxic and deficient boron conditions also inhibited the vanadate-sensitive H⁺-ATPase of microsomes isolated from the roots. The mechanism of boron toxicity was investigated in vitro with microsorne vesicles. A strong effect of boron on the vanadate-sensitive, ATP-dependent H⁺ transport was found, but the vanadate-sensitive phospho-bydrolase activity was not affected. These results suggest that boron could exert an effect on the plasma membrane properties, directly or indirectly regulating, proton transport.     

Effects of nitrate on influx, efflux and translocation of potassium in young sunflower plants

Influx, efflux and translocation of K⁺(⁸⁶Rb) were studied in the roots of sunflower seedlings ( Helianthus annuus L. cv. Uniflorus) treated with 0–4.0 m M NO₃⁻ during a 9 day growth period or a 24 h pretreatment period. Roots treated with high levels of NO₃⁻ absorbed and translocated more K⁺(⁸⁶Rb) than seedlings treated with low levels of NO₃⁻. The content of K⁺ in the shoots was, however, higher in seedlings treated with low levels of NO₃⁻, indicating a low rate of retranslocation of K⁺ in those plants. K⁺(⁸⁶Rb) efflux was highest into the low-NO₃⁻ solutions. All effects on K⁺(⁸⁶Rb)-fluxes were more obvious in high-K plants than in low-K plants. The results are discussed in relation to the Dijkshoorn-Ben Zioni hypothesis for K⁺+ NO₃⁻-uptake and translocation in plants.     

Effect of herbicide diclofop-methyl on proton extrusion from Lolium multiflorum seedlings differing in resistance and in fungal endophyte (Neotyphodium sp.) infection

Lolium multiflorum (Italian ryegrass), an annual weed invading wheat and barley cropping systems, has evolved resistance to diclofop-methyl (DM) herbicide. Earlier studies on the mode of action of DM in susceptible L. multiflorum and L. rigidum populations have shown that herbicide promotes oxidative stress leading to senescence, a process reversible through the action of auxins. The disruption of cell membrane potential ( E _m ) appears to be correlated with DM phytotoxicity in susceptible populations, suggesting that the continuous H⁺ extrusion from plasmalemma to extracellular space is inhibited. L. multiflorum usually establishes a symbiotic relationship with fungal endophytes of the Neotyphodium genus. This fungus confers to host plants higher survival at sublethal dosages of DM, probably due to the production of auxinic compounds. Our goal was to characterize DM-resistant and DM-susceptible L. multiflorum populations infected (E+) and non-infected (E−) with endophytes, by studying the capacity of H⁺ bumping of plasmalemma of intact roots under DM selection. We correlated the effects of DM on H⁺ disruption with plant survival. DM inhibited acidification markedly more in susceptible than in resistant populations. Continued extrusion of H⁺ by DM-resistant cell membranes was positively related to plant survival and growth. There was no detectable difference in the capacity of bumping H⁺ between DM-susceptible E+ and E− seedlings, even though survival was higher in E+ plants. The basis for the differential response in H⁺ extrusion between resistant and susceptible populations of L. multiflorum is discussed.     

Production and organ distribution of succinate in rice seedlings during anoxia

Anaerobic production of succinate, a common feature in animals able to sustain anoxia, has seldom been reported in plants. By the use of ¹H-nuclear magnetic resonance spectroscopy we show here that succinate is produced by rice seedlings (Oryza sativa L. cv. Arborio) subjected to anoxic conditions. Starting from levels below I μmol (g fresh weight)⁻¹ in air, after 48 h of anoxia the levels of alanine, succinate and lactate had increased to 23.8, 5.2 and 1.0 μmol (g fresh weight) ⁻¹, respectively, in shoot tissues. Succinate was accumulated in shoots, notably in the coleoptiles, but not in roots of the rice seedlings, suggesting its involvement in rice coleoptile elongation under anoxia. Other possible functions of succinate production in rice seedling, an organism highly tolerant to anoxia, are discussed.     

Induction of nitrate reductase in needles of Scots pine seedlings by NOX and NO3−

The possibility to induce nitrate reductase (NR; EC 1.6.6.2) in needles of Scots pine ( Pinus sylvestris L.) seedlings was studied. The NR activity was measured by an in vivo assay. Although increased NR activities were found in the roots after application of NO₃⁻, no such increase could be detected in the needles. Detached seedlings placed in NO₃⁻ solution showed increasing NR activities with increasing NO₃⁻ concentrations. Exposure of seedlings to NO_x (70–80 ppb NO₂ and 8–12ppb NO) resulted in an increase of the NR activity from 10–20 nmol NO₂⁻ (g fresh weight)⁻¹ h⁻¹ to about 400 nmol NO₂⁻ (g fresh weight)⁻¹ h⁻¹. This level was reached after 2–4 days of exposure, thereafter the NR activity decreased to about 200 nmol NO₂⁻ (g fresh weight)⁻¹ h⁻¹. Analyses of free amino acids showed low concentrations of arginine and glutamine in NO_x-fumigated seedlings compared to corresponding controls.