Drought resistance and root morphology in sorghum

Two 2 m³ plots of soil were prepared to different water contents and each isolated from surrounding soil by impermeable plastic material. Nine sorghum varieties were germinated in the plots and allowed to grow without further watering. Time-to-wilt after emergence was measured, and several parameters relating to water flow of the seedling and nodal roots were determined. There was a good positive correlation between both seminal root and nodal root relative conductvity and time-to-wilt. In a second experiment, plants were germinated and grown in pots, and after two weeks of growth without further watering were inspected for survival in the unwilted state. The per cent survival was calculated. There was a negative correlation of seminal root relative conductivity with per cent survival, and a high negative correlation of the number of seminal roots with per cent survival. It is concluded that high relative conductivity indicates drought resistance if the plants are growing with less restricted roots as in open soil, while if the plants are grown in pots the reverse is the case. Experiments linking root conductivity with survival conducted in pots are poor predictors of performance in less restricted rooting conditions.     

Evidence for a periodic excretion of nitrogen by roots of grass-legume associations

Diurnal variation in ion content of the solution bathing roots of two plants growing together in sand culture was analysed for three pairs of grass-legume species (Lolium multiflorum andTrifolium pratense; Zea mays andGlycine hispida; Avena sativa andVicia sativa) and their monospecific controls. Biomass and nitrogen content of plants were determined. Ion concentration (NO ₃ ⁻ , NO ₂ ⁻ , NH ₄ ⁺ , and K⁺) and pH of root solutions were measured for Lolium-Trifolium plant pairs and controls at 6 hours intervals over 36 h, starting at 8 am within a circadian cycle. Root solutions were regularly depleted in NO ₃ ⁻ by the grasses (Lolium-Lolium control) throughout the cycle. For associations involving the legume (Lolium-Trifolium and Trifolium-Trifolium), NO ₃ ⁻ depletion was followed by NO ₃ ⁻ enrichment at night, from late afternoon to early morning; the enrichment was more marked for the Lolium-Trifolium association. Solutions which did not contain NO ₂ ⁻ ions, were enriched by trace amounts of NH ₄ ⁺ ions, largely depleted in K⁺ and alkalanized for all associations throughout the cycle. Repeating the experiment with the three pairs of species at the vegetative phase of development confirmed the previous results: NO ₃ ⁻ enrichment during the night for associations with legumes. When the experiment was repeated with older plants which had almost completed their flowering stage, depletion only was observed and no NO ₃ ⁻ enrichment. These data suggest that NO ₃ ⁻ enrichment results from N excretion from active nodulated roots of the legume, accounting for the increase in both biomass and nitrogen content of the companion grass in grass-legume association. The quantitative importance and periodicity of nitrogen excretion as well as the origin of nitrate enrichment are discussed.     

Radial transport of water across cortical sleeves of excised roots ofZea mays L.

The stationary radial volume flows across maize (Zea mays L.) root segments without steles (sleeves) were measured under isobaric conditions. The driving force of the volume flow is an osmotic difference between the internal and external compartment of the root preparations. It is generated by differences in the concentrations of sucrose, raffinose or polyethylene glycol. The flows are linear functions of the corresponding osmotic differences ( ) up to osmotic values which cause plasmolysis. The straight lines obtained pass through the origin. No asymmetry of the osmotic barrier could be detected within the range of driving forces applied ( =±0.5 MPa), corresponding to volume-flow densities of j_{v, s}=±7·10^–8 m·s^–1. Using the literature values for the reflection coefficients of sucrose and polyethylene glycol in intact roots (E. Steudle et al. (1987) Plant Physiol.84, 1220–1234), values for the sleeve hydraulic conductivity of about 1·10^–7 m·s^–1 MPa^–1 were calculated. They are of the same order of magnitude as those reported in the literature for the hydraulic conductivity of intact root segments when hydrostatic pressure is applied.Abbreviations and symbols a _s outer surface of sleeve segment - c concentration of osmotically active solute - j _{v, s} radial volume flow density across sleeve segment - Lp_s hydraulic conductivity of sleeves - Lp_r hydraulic conductivity of intact roots - _N thickness of Nernst diffusion layer - reflection coefficient of root for solute - osmotic value of bulk phase - osmotic coefficient     

Sulphate influx in wheat and barley roots becomes more sensitive to specific protein-binding reagents when plants are sulphate-deficient

When young wheat (Triticum aestivum L.) or barley (Hordeum vulgare L.) plants were deprived of an external sulphate supply (-S plants), the capacity of their roots to absorb sulphate, but not phosphate or potassium, increased rapidly (derepression) so that after 3–5 d it was more than tenfold that of sulphate-sufficient plants (+S plants). This increased capacity was lost rapidly (repression) over a 24-h period when the sulphate supply was restored. There was little effect on the uptake of L-methionine during de-repression of the sulphate-transport system, but S input from methionine during a 24-h pretreatment repressed sulphate influx in both+S and-S plants.Sulphate influx of both+S and-S plants was inhibited by pretreating roots for 1 h with 4,4-diisothiocyanatostilbene-2,2-disulphonic acid (DIDS) at concentrations > 0.1 mol · m^-3. This inhibition was substantially reversed by washing for 1 h in DIDS-free medium before measuring influx. Longer-term pretreatment of roots with 0.1 mol·m^-3 DIDS delayed de-repression of the sulphatetransport system in-S plants but had no influence on+S plants in 3 d.The sulphydryl-binding reagent, n-ethylmaleimide, was a very potent inhibitor of sulphate influx in-S roots, but was much less inhibitory in +S roots. Its effects were essentially irreversible and were proportionately the same at all sulphate concentrations within the range of operation of the high-affinity sulphate-transport system. Inhibition of influx was 85–96% by 300 s pretreatment by 0.3 mol·m^-3 n-ethylmaleimide. No protection of the transport system could be observed by including up to 50 mol·m^-3 sulphate in the n-ethylmaleimide pre-treatment solution. A similar differential sensitivity of-S and+S plants was seen with p-chloromercuriphenyl sulphonic acid.The arginyl-binding reagent, phenylglyoxal, supplied to roots at 0.25 or 1 mol·m^-3 strongly inhibited influx in-S wheat plants (by up to 95%) but reduced influx by only one-half in+S plants. The inhibition of sulphate influx in-S plants was much greater than that of phosphate influx and could not be prevented by relatively high (100 mol·m^-3 sulphate concentrations accompanying phenylglyoxal treatment. Effects of phenylglyoxal pretreatment were unchanged for at least 30 min after its removal from the solution but thereafter the capacity for sulphate influx was restored. The amount of new carrier appearing in-S roots was far greater than in+S roots over a 24-h period.The results indicate that, in the de-repressed state, the sulphate transporter is more sensitive to reagents binding sulphydryl and arginyl residues. This suggests a number of strategies for identifying the proteins involved in sulphate transport.Abbreviations DIDS 4,4-diisothiocyanatostilbene-2,2-disulphonic acid - NEM n-ethylmaleimide - PCMBS p-chloromercuriphenyl sulphonic acid     

Effect of HCO - 3 concentration in the absorption solution on the energetic coupling of H+-cotransports in roots of Zea mays L.

The effect of HCO ₃ ^- on ion absorption by young corn roots was studied in conditions allowing the independent control of both the pH of uptake solution and the CO₂ partial pressure in air bubbled through the solution. The surface pH shift in the vicinity of the outer surface of the plasmalemma induced by active H⁺ excretion was estimated using the initial uptake rate of acetic acid as a pH probe (Sentenac and Grignon (1987) Plant Physiol. 84, 1367). Acetic acid and orthophosphate uptake rates and NO ₃ ^- accumulation were slowed down, while ⁸⁶Rb⁺ uptake and K⁺ accumulation rates were increased by HCO ₃ ^- . These effects were similar to those induced by 4-(2-hydroxyethyl)-1-piperazineethane sulfonic acid/2-amino-2-(hydroxymethyl)-1,3-propanediol (Hepes-Tris). They were more pronounced when the H⁺ excretion was strong, were rapidly reversible and were not additive to those of Hepes-Tris. The hypothesis is advanced that the buffering system CO₂/H₂CO₃/HCO ₃ ^- accelerated the diffusion of equivalent H⁺ inside the cell wall towards the medium. This attenuated the surface pH shift in the vicinity the plasma membrane and affected the coupling between the proton pump and cotransport systems.Abbreviations FW fresh weight - Hepes 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid - J_aa acetic acid influx - J_K ⁺ K⁺ influx - J_Pi orthophosphate influx - Mes 2-(N-morpholino)ethanesulfonic acid - pCO₂ CO₂ partial pressure - Tris 2-amino-2-(hydroxymethyl)-1,3-propanediol     

Voltage clamping with single microelectrodes: Comparison of the discontinuous mode and continuous mode using the Axoclamp 2A amplifier

Summary The voltage clamp technique is a powerful method for studying the physiology of excitable membrane. This technique has made possible the determination of ionic responses generated by activation of either receptor-mediated or voltage-dependent processes. The development of the whole-cell, tight-seal voltage clamp method has allowed the analysis and examination of membrane physiology at the single cell level. The method allows the characterization of voltage-dependent ionic conductances both at the macroscopic (whole-cell) and at the microscopic (unitary conductance or single channel) level in cells less than 10 µm in diameter, a feat difficult to achieve with conventional fine-tipped micropipettes.In this paper, several methologies used for culturing neuronal and non-neuronal cells in the laboratory are described. A comparison between the two modes of voltage clamp using blunt-tipped patch-microelectrodes, the switching (discontinuous) and the non-switching (continuous) modes, of the Axoclamp-2A amplifier is made. Some results on membrane currents obtained from neuronal and non-neuronal cells using the single electrode whole-cell tight-seal voltage clamp is illustrated. The possible existence of two inactivating K⁺ currents, one dependent on Ca⁺⁺ the other is not, is discussed.     

Citric acid excretion and precipitation of calcium citrate in the rhizosphere of white lupin (Lupinus albus L.)

Abstract. White lupin ( Lupinus albus L.) was grown for 13 weeks in a phosphorus (P) deficient calcareous soil (20% CaCO₃, pH(H₂O)7.5) which had been sterilized prior to planting and fertilized with nitrate as source of nitrogen. In response to P deficiency, proteoid roots developed which accounted for about 50% of the root dry weight. In the rhizosphere soil of the proteoid root zones, the pH dropped to 4.8 and abundant white precipitates became visible. X-ray spectroscopy and chemical analysis showed that these precipitates consisted of calcium citrate. The amount of citrate released as root exudate by 13-week-old plants was about 1 g plant⁻¹, representing about 23% of the total plant dry weight at harvest. In the rhizosphere soil of the proteoid root zones the concentrations of available P decreased and of available Fe, Mn and Zn increased. The strong acidification of the rhizosphere and the cation/anion uptake ratio of the plants strongly suggests that proteoid roots of white lupin excrete citric acid, rather than citrate, into the rhizosphere leading to intensive chemical extraction of a limited soil volume. In a calcareous soil, citric acid excretion leads to dissolution of CaCO₃ and precipitation of calcium citrate in the zone of proteoid roots.     

Norway spruce somatic embryogenesis: high-frequency initiation from light-cultured mature embryos

Somatic embryos and rooted plantlets have been regenerated from light-initiated embryogenic callus derived from mature embryos of Picea abies. Under a 16 h photoperiod, mature zygotic embryos were cultured on a modified half-strength Murashige & Skoog medium without NH₄NO₃ and supplemented with 5 mM glutamine, 4.5 M N6-benzyladenine and 10.7 M naphthaleneacetic acid or 10 M 2,4-dichlorophenoxyacetic acid. White translucent embryogenic callus, proliferating from the callusing hypocotyl region after 3 weeks incubation, was isolated from the green non-embryogenic tissue and subcultured for over 12 months. Upon transfer of the embryogenic callus through a specific sequence of media, somatic embryos proceeded to mature, elongating and forming rings of cotyledonary leaves similar to those of zygotic embryos. Transferred to medium without growth regulators, the somatic embryos germinated and produced plantlets with green cotyledons, elongated hypocotyls and primary roots.     

In vitro regeneration of Gaillardia pulchella Foug.

Young leaf and internodal stem segments of Gaillardia pulchella, collected from wild species re-established in the greenhouse, were used to initiate callus on Murashige & Skoog medium supplemented with NAA (2.0 mgl⁻¹) and BA (0.4 mgl⁻¹). Callus formed after 10 to 14 days in the dark. Cultures were transferred to fresh medium and placed under lighted conditions where shoot formation occurred approximately 14 to 30 days after initiation. Callus sub-cultured at 14 to 21-day intervals continued to produce primordia for several weeks. Flowers were produced by regenerated shoots maintained on MS medium, but roots did not develop until the plantlets were transferred to soil conditions.     

Regeneration of plants from callus and embryos of ‘Royal’ apricot

Immature embryos of apricot (Prunus armeniaca L.) cv. Royal with a PF index of 25–100 were used to regenerate plants in vitro using two methods. In the first case, callus was initiated on MS medium with 4.5 M 2, 4-D plus 0.44 M BA and regeneration of shoots from the callus occurred on MS medium with 4.4 M BA plus 1.0 M 2, 4-D. In the second case, adventitious buds were directly regenerated from the cotyledons on MS medium with 4.4 M BA plus 1.0 M 2, 4-D.Abbreviations BA 6-benzyladenine - IBA dole-3-butyric acid - NAA -naphthylacetic acid - 2, 4-D 2, 4-dichlorophenoxyacetic acid - PF (embryo length/seed length) x 100