The effect of the osmotic potential and specific ion concentration of the nutrient solution on the uptake and reduction of nitrate by barley seedlings

Summary Short-term absorption experiments were conducted with intact barley (Hordeum vulgare L.) seedlings to observe the effects of the osmotic potential (Ψ^π) and salt species on nitrate uptake andin vivo nitrate reduction. The experiments consisted of growing barley seedlings for 5 days in complete nutrient solutions salinized to (Ψ^π) levels of −0.6, −1.8, −3.0, −4.2, and −5.4 bars with NaCl, CaCl₂ or Na₂SO₄. After the absorption period, the seedlings were separated into shoots and roots, weighed, then analyzed for NO₃. The nutrient solutions were sampled for NO₃ analysis each day immediately before renewing the solutions. The accumulative loss of NO₃ from the solutions was considered to be uptake whereas NO₃ reduction was the difference between uptake and seedling content. Lowering the (Ψ^π) of the nutrient solutions resulted in decreased concentrations of NO₃ in the plant, little or no effect (except at the lowest (Ψ^π) level) on uptake, and increased nitrate reductase activity. Increased rates of NO₃ reduction were in particular associated with the Cl concentration of the nutrient solution.     

Effects of boron on somatic embryogenesis of Daucus carota

Although the micronutrient boron (B) apparently exerts no influences on the induction of somatic embryogenesis of cultured carrot petiole explants, strong influences on the development of somatic embryos were observed (0–8 mg l⁻¹ B). At lower B concentrations the development of roots is promoted with simultaneous retardation of shoot development and at higher B concentrations shoot development is favored at the expenses of the development of the root system. Parallel to this, the ratios of concentrations of endogenous indole-3-acetic acid to total cytokinins (with one exception) changes continuously from 4 (zero B) to 0.22 (8 mg l⁻¹ B). Analogies to morphogenetic reactions following the application of various ratios of auxins/cytokinins to the nutrient medium of cultured tissue (Skoog and Miller, 1957 and others) are suggested. An increase in the B concentration in the nutrient medium results generally in a reduced concentration of endogenous abscisic acid.     

Leaf water characteristics and drought acclimation in sunflower genotypes

The responses of leaf water parameters to drought were examined using three sunflower (Helianthus annuus L.) genotypes. Osmotic potential at full water saturation (π¹⁰⁰), apoplastic water fraction (AWF) and bulk elastic modulus (BEM) were determined by pressure-volume curve analysis on well watered or on water-stressed plants (−1.0 MPa Ψ₁ < −1.5 MPa) previously drought-pretreated or not. The drought-pretreated plants were subjected to a 7-day drought period (predawn leaf water potential reached −0.9 MPa) followed by 8 days of rewatering. In well watered plants, all genotypes in response to drought acclimation displayed a significantly decreased π¹⁰⁰ associated with a decrease in the leaf water potential at the turgor-loss point (decrease in Ψ_tlp was between 0.15 and 0.21 MPa, depending on the genotype). In two genotypes, drought acclimation affected the partitioning of water between the apoplastic and symplastic fractions without any effect on the total amount of water in the leaves. As a third genotype displayed no modification of AWF and BEM after drought acclimation, the decreased π¹⁰⁰ was only due to the net accumulation of solutes and was consistent with the adjustment of the photochemical efficiency observed previously in this genotype in response to drought acclimation. In water-stressed plants, the osmotic adjustment (OA) can increase further beyond that observed in response to the drought pretreatment. However, the maintenance of photosynthetic rate and stomatal conductance at low leaf water potentials not only depends on the extent of osmotic adjustment, but also on the interaction between OA and AWF or BEM. Adaptative responses of leaf water parameters to drought are thus quite contrasted in sunflower genotypes. This revised version was published online in June 2006 with corrections to the Cover Date.     

Response of Eucalyptus camaldulensis Dehnh., E. globulus Labill. ssp. globulus and E. grandis W.Hill to excess boron and sodium chloride

In a sand culture experiment we investigated the effects of boron (0.01, 0.19, 0.46 and 0.93 mol m⁻³ B, as H₃BO₃), sodium chloride (0, 100 and 200 mol m⁻³ NaCl) and combined B and NaCl, over 36 days, on growth, water use and foliar ion concentrations of nine week-old seedlings of three fast-growing, commercial eucalypts ( Eucalyptus camaldulensis Dehnh. , E. globulus Labill. ssp. globulus and E. grandis W.Hill.). Shoot dry weight was significantly reduced by high concentrations of NaCl (p < 0.001) and by B and NaCl in combination (p ≤ 0.05) but not by B alone. Root dry weight was significantly reduced by both NaCl (p < 0.001) and B (p < 0.001), but not by combined B and NaCl. Foliar B concentrations increased with higher concentrations of applied B and decreased with higher NaCl concentrations. Foliar Na concentrations were greater with higher NaCl concentrations, whereas B application had no significant effect on foliar Na concentrations. All three species accumulated relatively high B concentrations in leaves. Severe boron toxicity symptoms (BTS) were apparent only when leaf B concentrations exceeded 50 mol x 10⁻⁶ g⁻¹, but even at these high concentrations plant growth was only slightly reduced. E. camaldulensis showed least development of BTS, the lowest leaf B concentrations and least reduction in height growth due to B and NaCl. The results suggest that there was a correlation between both B tolerance and B accumulation in leaves and between tolerance to B and NaCl. This revised version was published online in June 2006 with corrections to the Cover Date.     

Water potential and sap flow rate in adult trees with moist and dry soil as used for the assessment of root system depth

Sap flow rate (Q_w) and leaf water potential (Ψ_w.leaf) in adult specimens of birch (Betula) and oak (Quercus) were measured under contrasting soil moisture conditions (Ψ_w.sofl). With sufficient soil moisture Q_w reached about 250 cm³h⁻¹ calculated per unit tree-trunk segment as given by 1 cm length of its circumference. In soil water-stress conditions (when Ψ_w.leaf = = −15 × 10⁵Pa), birch stopped transpiration and wilted. Oak transpired even when Ψ_w.leaf fell below −20 × 10⁵Pa. The relation between Q_w and Ψ_w.leaf was always linear and with various Ψ_w.soil differed in the slopes of regression lines only. Hydraulic conductance (K_wcu) with nonlimiting moisture conditions reached about 6 × 10⁻⁹m³ 10⁻⁵Pa⁻¹s⁻¹ and “conductivity” (“k_wa”) when calculated per leaf area unit reached about 23 m 10⁻⁵Pa⁻¹s⁻¹. K_wcu and “k_wa” were of about one half to nine times greater in birch than in oak. On the basis of relations between Ψ_w.soil at various depths, Ψ_w.leaf and Q_w (resp. K_w) it is possible to assess the maximal rooting depth and the effective depth where the maximum of absorption of roots occurs. It is to be seen that the root system macrostructure substantially participates in the drought avoidance of adult trees in a forest stand.     

Response of nodulating and non-nodulatingPisum sativum L. to nitrate

This study examined whether ‘Risnod2’ and ‘Risnod27’ non-nodulating mutants of pea (Pisum sativum L.) provided with increasing concentrations of nitrate could achieve a growth and nitrogen accumulation comparable to their parental N₂-fixing cv. Finale. In the cv. Finale, nodule number, nodule dry mass accumulation, total C₂H₂-reducing activity of nodulated roots (TAR) and estimated N₂ fixation were considerably inhibited at 5.0 and 10.0 mM root medium NO₃ ⁻ concentrations. In contrast a 0.63 mM level stimulated both the nodule dry mass and TAR. The cv. Finale N₂-fixing plants grown on 0 to 2.5 mM NO₃ ⁻ levels had higher shoot N concentrations than the Nod⁻ mutants, but within the 5.0 to 10.0 mM levels the Nod⁻ mutants approached or even overtopped the N concentration of the cv. Finale plants. Compared with a high positive correlation found in the Nod⁻ mutants, shoot N concentration in the cv. Finale was negatively correlated with the root medium NO₃ ⁻ concentration. The pattern of nitrogen content in shoot dry mass was very similar to that seen in the shoot dry mass accumulation. The Nod⁻ mutants grown on the 5.0 and/or 10.0 mM NO₃ ⁻ level had plant dry mass, shoot nitrogen concentration, shoot nitrogen content, and root/shoot dry mass ratio comparable with those of the nodulating cv. Finale grown on the same nitrate levels.     

Leaf water relations characteristics of differently potassium fertilized and watered field grown barley plants

Seasonal leaf water relations characteristics were studied in fully irrigated spring barley (Hordeum distichum L. cv. Gunnar) fertilized at low (50 kg K ha⁻¹) or high (200 kg K ha⁻¹) levels of potassium applied as KCl. The investigation was undertaken from about 14 days before anthesis until the milk ripe stage in leaves of different position and age. Additionally, the effects of severe water stress on leaf water relations were studied in the middle of the grain filling period in spring barley (cv. Alis). The leaf water relations characteristics were determined by the pressure volume (PV) technique. Water relations of fully irrigated plants were compared in leaf No 7 with the water relations of slowly droughted plants (cv. Alis). Leaf osmotic potential at full turgor (ψ _π ¹⁰⁰ ) decreased 0.1 to 0.3 MPa in droughted leaves indicating a limited osmotic adjustment due to solute accumulation. The leaf osmotic potential at zero turgor (ψ _π ⁰ ) was about −2.2 MPa in fully irrigated plants and −2.6 MPa in droughted plants. The relative water content at zero turgor (R⁰) decreased 0.1 unit in severely droughted leaves. The ratio of turgid leaf weight to dry weight (TW/DW) tended to be increased by drought. The tissue modulus of elasticity (ε) decreased in droughted plants and together with osmotic adjustment mediated turgor maintenance during drought. A similar response to drought was found in low and high K plants except that the R⁰ and ε values tended to be higher in the high K plants. Conclusively, during drought limited osmotic adjustment and increase in elasticity of the leaf tissue mediated turgor maintenance. These effects were only slightly modified by high potassium application. The seasonal analysis in fully irrigated plants (cv. Gunnar) showed that within about 14 days from leaf emergence ψ _π ¹⁰⁰ decreased from about −0.9 to −1.6 MPa in leaf No 7 (counting the first leaf to emerge as number one) and from about −1.1 to −1.9 MPa in leaf No 8 (the flag leaf) due to solute accumulation. A similar decrease took place in ψ _π ⁰ except that the level of ψ _π ⁰ was displaced to a lower level of about 0.2 to 0.3 MPa. Both ψ _π ¹⁰⁰ and ψ _π ⁰ tended to be 0.05 to 0.10 MPa lower in high K than in low K plants. R⁰ was about 0.8 to 0.9 and was independent of leaf position and age, but tended to be highest in high K plants. The TW/DW ratio decreased from about 5.5 in leaf No 6 to 4.5 in leaf No 7 and 3.8 in leaf No 8. The TW/DW ratio was 4 to 10% higher in high K than in low K plants indicating larger leaf cell size in the former. The apoplastic water content (V_a) at full turgor constituted about 15% in leaf No 7. ε was maximum at full turgor and varied from about 11 to 34 MPa. ε tended to be higher in high K plants. Conclusively, in fully watered plants an ontogenetically determined accumulation of solutes (probably organic as discussed) occurred in the leaves independent of K application. The main effect of high K application on water relations was an increase in leaf water content and a slight decrease in leaf ψ_π. The effect of K status on growth and drought resistance is discussed.     

Influence of salinity on Na+ and K+ accumulation, and gas exchange in Avicennia germinans

We analysed plant growth, ion accumulation, leaf water relations, and gas exchange of Avicennia germinans (L.) L. subjected to a long-term, controlled salinity gradient from 0 to 55 ‰. Growth and leaf area were affected by salinity higher than 10 ‰. As salinity increased, the predawn leaf water potential (Ψ_w) and leaf osmotic potential (Ψ_s) decreased. Leaf Ψ_w was at least −0.32 MPa lower than the Ψ_w of solution. Na⁺ and K⁺ ions explained about 78 % of decrease in Ψ_s. K⁺ tissue water concentration decreased by more than 60 % in all salinity treatments as compared with those grown at 0 ‰. Inversely, Na⁺ concentration in tissue water increased with nutrient solution salinity. The maximum net photosynthetic rate (P _N) and stomatal conductance (g _s) decreased by 68 and 82 %, respectively, as salinity increased from 0 to 55 ‰; the intercellular CO₂ concentration (C _i) followed the same trend. The P _N as a function of C _i showed that both the initial linear slope and upper plateau of the P _N vs. C _i curve were markedly affected by high salinity (40 and 55 ‰).     

Characterization of natural variation for zinc, iron and manganese accumulation and zinc exposure response in Brassica rapa L.

Brassica rapa L. is an important vegetable crop in eastern Asia. The objective of this study was to investigate the genetic variation in leaf Zn, Fe and Mn accumulation, Zn toxicity tolerance and Zn efficiency in B. rapa. In total 188 accessions were screened for their Zn-related characteristics in hydroponic culture. In experiment 1, mineral assays on 111 accessions grown under sufficient Zn supply (2 μM ZnSO₄) revealed a variation range of 23.2–155.9 μg g⁻¹ dry weight (d. wt.) for Zn, 60.3–350.1 μg g⁻¹ d. wt. for Fe and 20.9–53.3 μg g⁻¹ d. wt. for the Mn concentration in shoot. The investigation of tolerance to excessive Zn (800 μM ZnSO₄) on 158 accessions, by using visual toxicity symptom parameters (TSPs), identified different levels of tolerance in B. rapa. In experiment 2, a selected sub-set of accessions from experiment 1 was characterized in more detail for their mineral accumulation and tolerance to excessive Zn supply (100 μM and 300 μM ZnSO₄). In this experiment Zn tolerance (ZT) determined by relative root or shoot dry biomass varied about 2-fold. The same six accessions were also examined for Zn efficiency, determined as relative growth under 0 μM ZnSO₄ compared to 2 μM ZnSO₄. Zn efficiency varied 1.8-fold based on shoot dry biomass and 2.6-fold variation based on root dry biomass. Zn accumulation was strongly correlated with Mn and Fe accumulation both under sufficient and deficient Zn supply. In conclusion, there is substantial variation for Zn accumulation, Zn toxicity tolerance and Zn efficiency in Brassica rapa L., which would allow selective breeding for these traits.     

Interactive effects of elevated CO2 and drought stress on leaf water potential and growth in Caragana intermedia

We studied the responses of leaf water potential (Ψ_w), morphology, biomass accumulation and allocation, and canopy productivity index (CPI) to the combined effects of elevated CO₂ and drought stress in Caragana intermedia seedlings. Seedlings were grown at two CO₂ concentrations (350 and 700 μmol mol⁻¹) interacted with three water regimes (60–70%, 45–55%, and 30–40% of field capacity of soil). Elevated CO₂ significantly increased Ψ_w, decreased specific leaf area (SLA) and leaf area ratio (LAR) of drought-stressed seedlings, and increased tree height, basal diameter, shoot biomass, root biomass as well as total biomass under the all the three water regimes. Growth responses to elevated CO₂ were greater in well-watered seedlings than in drought-stressed seedlings. CPI was significantly increased by elevated CO₂, and the increase in CPI became stronger as the level of drought stress increased. There were significant interactions between elevated CO₂ and drought stress on leaf water potential, basal diameter, leaf area, and biomass accumulation. Our results suggest that elevated CO₂ may enhance drought avoidance and improved water relations, thus weakening the effect of drought stress on growth of C. intermedia seedings.     

Partitioning of photosynthetic electron flow between CO<Subscript>2</Subscript> assimilation and O<Subscript>2</Subscript> reduction in sunflower plants under water deficit

In sunflower (Helianthus annuus L.) grown under controlled conditions and subjected to drought by withholding watering, net photosynthetic rate (P _N) and stomatal conductance (g _s) of attached leaves decreased as leaf water potential (Ψ_w) declined from −0.3 to −2.9 MPa. Although g _s decreased over the whole range of Ψ_w, nearly constant values in the intercellular CO₂ concentrations (C _i) were observed as Ψ_w decreased to −1.8 MPa, but C _i increased as Ψ_w decreased further. Relative quantum yield, photochemical quenching, and the apparent quantum yield of photosynthesis decreased with water deficit, whereas non-photochemical quenching (q_NP) increased progressively. A highly significant negative relationship between q_NP and ATP content was observed. Water deficit did not alter the pyridine nucleotide concentration but decreased ATP content suggesting metabolic impairment. At a photon flux density of 550 μmol m⁻² s⁻¹, the allocation of electrons from photosystem (PS) 2 to O₂ reduction was increased by 51 %, while the allocation to CO₂ assimilation was diminished by 32 %, as Ψ_w declined from −0.3 to −2.9 MPa. A significant linear relationship between mean P _N and the rate of total linear electron transport was observed in well watered plants, the correlation becoming curvilinear when water deficit increased. The maximum quantum yield of PS2 was not affected by water deficit, whereas q_P declined only at very severe stress and the excess photon energy was dissipated by increasing q_NP indicating that a greater proportion of the energy was thermally dissipated. This accounted for the apparent down-regulation of PS2 and supported the protective role of q_NP against photoinhibition in sunflower.     

Response of in vitro strawberry to antibiotics

By identifying antibiotics that had the least phytotoxic effects on explants during genetic transformation, we evaluated the effect of various antibiotics on callus induction and morphogenesis from leaf explants and in vitro growth of Fragaria × ananassa Duch. cv. Toyonaka. Results showed that kanamycin (Kan) significantly inhibited callus induction, bud differentiation and root morphogenesis while carbenicillin (Carb), cefotaxime (Cef) and an equal concentration of Cef and Carb up to 500 mg L⁻¹ had no significant effects on callus induction and shoot growth. Kan, even at 2.5 mg L⁻¹, significantly inhibited callus induction, shoot regeneration and root formation, while no shoots regenerated at concentrations above 15 mg L⁻¹. Rooting was completely inhibited in the presence of 50 mg L⁻¹ Kan. Cef had negative effects on shoot regeneration from leaf explants and in vitro growth of strawberry. Compared to Cef, Carb at ≤300 mg L⁻¹ significantly promoted shoot and root organogenesis. However, an equal concentration of Carb plus Cef could alleviate the negative effect of Cef on strawberry. Results from relative electrolyte leakage, root and antioxidant activities, O₂^·− production rate, H₂O₂, proline and MDA contents showed that Kan, Cef and Carb caused electrolyte leakage and triggered active enzymatic processes and metabolism. This offers a possible mechanism for the inhibition or stimulation of strawberry growth caused by these antibiotics.     

Control of leaf cell elongation in barley. Generation rates of osmotic pressure and turgor, and growth-associated water potential gradients

In a previous study on the effects of N-supply on leaf cell elongation, the spatial distribution of relative cell elongation rates (RCER), epidermal cell turgor, osmotic pressure (OP) and water potential (Ψ) along the elongation zone of the third leaf of barley was determined (W. Fricke et al. 1997, Planta 202: 522–530). The results suggested that in plants receiving N at fixed relative addition rates (N-supply limitation of growth), cell elongation was rate-limited by the rate of solute provision, whereas in plants growing on complete nutrient solution containing excessive amounts of N (N-demand limitation), cell elongation was rate-limited by the rate of water supply or wall yielding. In the present paper, these suggestions were tested further. The generation rates of cell OP, turgor and Ψ along the elongation zone were calculated by applying the continuity equation of fluid dynamics to the previous data. To allow a more conclusive interpretation of results, anatomical data were collected and bulk solute concentrations determined. The rate of OP generation generally exceeded the rate of turgor generation. As a result, negative values of cell Ψ were created, particularly in demand-limited plants. These plants showed highest RCER along the elongation zone and a Ψ gradient of at least −0.15 MPa between water source (xylem) and expanding epidermal cells. The latter was similar to a theoretically predicted value (−0.18 MPa). Highest rates of OP generation were observed in demand-limited plants, with a maximum rate of 0.112 MPa · h⁻¹ at 16–20 mm from the leaf base. This was almost twice the rate in N-supply-limited plants and implied that the cells in the leaf elongation zone were capable of importing (or synthesising) every minute almost 1 mM of osmolytes. Potassium, Cl⁻ and NO₃ ⁻ were the main inorganic osmolytes (only determined for demand-limited plants). Their concentrations suggest that, unlike the situation in fully expanded epidermal cells, sugars are used to generate OP and turgor. Anatomical data revealed that the zone of lateral cell expansion extended distally beyond the zone of cell elongation. It is concluded that leaf cell expansion in barley relies on high rates of water and solute supply, rates that may not be sustainable during periods of sufficient N-supply (limitation by water supply: Ψ gradients) or limiting N-supply (limitation by solute provision: reduced OP-generation rates). To minimise the possibility of growth limitation by water and osmolyte provision, longitudinal and lateral cell expansion peak at different locations along the growth zone. Received: 15 October 1997 / Accepted: 12 March 1998     

Plant water relations as selection criteria for drought tolerance in mustard

Genotypes of mustard (B. juncea) were evaluated for concurrent changes in leaf water potential (Ψ), leaf osmotic potential (π), leaf turgor potential (P) and leaf relative water content (RWC) during moisture stress at reproductive stage of growth. The slope ‘b’ in the regression between Ψ and π varied from 0.43 to 0.97 and was positively correlated with P and RWC. The genotypes with ‘b’ around 0.7 were able to maintain P of about 0.5 MPa at Ψ of − 2.5 MPa and thus such value of ‘b’ seems to provide enough degree of tolerance against drought.     

Effects of liming and boron fertilization on boron uptake of Picea abies

The effects of liming on concentrations of boron and other elements in Norway spruce [Picea abies (L) Karst.] needles and in the mor humus layer were studied in long-term field experiments with and without B fertilizer on podzolic soils in Finland. Liming (2000+4000 kg ha^-1 last applied 12 years before sampling) decreased needle B concentrations in the four youngest needle age classes from 6–10 mg kg^-1 to 5 mg kg^-1. In boron fertilized plots the corresponding concentrations were 23–35 mg kg^-1 in control plots and 21–29 mg kg^-1 in limed plots. Both liming and B fertilizer decreased the Mn concentrations of needles. In the humus layer, total B concentration was increased by both lime and B fertilizer, and Ca and Mg concentrations and pH were still considerably higher in the limed plots than controls. Liming decreased the organic matter concentration in humus layer, whilst B fertilizer increased it.The results about B uptake were confirmed in a pot experiment, in which additionally the roles of increased soil pH and increased soil Ca concentration were separated by means of comparing the effects of CaCO₃ and CaSO₄. Two-year-old bare-rooted Norway spruce seedlings were grown in mor humus during the extension growth of the new shoot. The two doses of lime increased the pH of soil from 4.1 to 5.6 to 6.1, and correspondingly decreased the B concentrations in new needles from 22 to 12 to 9 mg kg^-1. However, CaSO₄ did not affect the pH of the soil or needle B concentrations. Hence the liming effect on boron availability in these soils appeared to be caused by the increased pH rather than increased calcium concentration.     

Alleviation of Ultraviolet-B Radiation-Induced Growth Inhibition of Green Gram by Triadimefon

Supplementary UV-B (12.2 kJ m⁻² d⁻¹ UV-B_BE) provided to Vigna radiata for 2 h d⁻¹ suppressed the length of root, shoot and whole plants, number of leaves, total leaf area, leaf area index, specific leaf mass, fresh and dry mass of leaves and shoot, relative growth rate and net productivity. In unstressed green gram plants (10 kJ m⁻² d⁻¹ UV-B_BE), triadimefon (TRIAD) (20 mg dm⁻³) enhanced growth in all parameters over control. The growth promoting effect of TRIAD enabled the UV-B impacted plants to overcome the growth inhibitions to varying degrees indicating its protective potential against UV-B stress. This revised version was published online in July 2006 with corrections to the Cover Date.     

Soil solute concentration and water uptake by single lupin and radish plant roots

Application of computer assisted tomography to gamma and X-ray attenuation measurements and Na⁺-LIX microelectrodes were used to determine the spatial distributions of soil water content and Na⁺ concentrations respectively near single roots of eighteen day old lupin and radish plants. These quantities were monitored at root depths of 3, 6 and 9 cm and at zero, 2, 4, 6, and 8 hour intervals from the diurnal commencement of transpiration. The plants were subjected to two levels of transpirational demand and five Na⁺ soil solution concentration levels. Water extraction rates for the lupin and radish roots increased continuously with time but were substantially reduced with increasing Na⁺ concentration in the treatment. Water uptake was uniform along the length of the essentially constant diameter lupin roots but decreased along the tapering radish roots as the diameter and hence the surface area per unit length of the roots decreased. The accumulation of Na⁺ at the root surfaces of both plants increased gradually with time in a near linear fashion and was slightly higher under the higher transpiration demand. These increases were not exponential as would be expected with non-absorption by the roots and this is considered to be due to back diffusion at the relatively high water contents used. At these water contents matric potentials had a much smaller influence on transpiration than osmotic potentials. The relationships between leaf water potentials (Ψ₁) and osmotic potentials at the root surfaces were linear with the decreases in Ψ₁ almost exactly reflecting the decreases in Ψ_π indicating rapid plant adjustment. Leaf water potentials decreased progressively with time and the relationships between leaf water potential and the transpiration rate were also linear supporting the suggestion of constant plant resistances at any given concentration.     

Variation in phosphorus efficiency among 73 bread and durum wheat genotypes grown in a phosphorus-deficient calcareous soil

A greenhouse experiment was carried out to study the severity of phosphorus (P) deficiency symptoms on leaves, shoot dry matter production, and shoot concentration and content (the total amount per shoot) of P in 39 bread wheat (Triticum aestivum L.) and 34 durum wheat (Triticum durum L.) genotypes grown in a severely P-deficient calcareous soil with low (20mgPkg⁻¹ soil) and adequate (80mgPkg⁻¹ soil) P supply for 39 days. As the seed P concentration or content can affect plant performance under P-deficient conditions, the seeds of the genotypes used in the present study were also analyzed for P concentration. Phosphorus efficiency (relative shoot growth) of genotypes, calculated by the ratio of shoot dry matter production under low P to that under adequate P supply, significantly differed among the genotypes, and varied between 46.7% and 78.6%. Phosphorus efficiency ranged from 51% to 71% with an average of 61% for bread and from 47% to 79% with an average of 66% for durum wheat genotypes. There was no correlation between P efficiency ratio and P concentration of plants (R ²=0.0001), but P efficiency of all bread and durum wheat genotypes showed a very significant correlation with the P content (the total amount of P per shoot) (R ²=0.333^***). The relationship between the P efficiency and total amount of P per shoot was much more significant in bread (R ²=0.341^***) than in durum wheat (R ²=0.135^*). Like shoot P concentrations, also severity of visible leaf symptoms of P deficiency on older leaves, including leaf chlorosis and necrosis, did not correlate with P efficiency. In most cases, genotypes showing higher P efficiency had higher absolute shoot dry weight under P deficient conditions. Under P deficient conditions, the absolute shoot dry weight very significantly correlated with shoot P content (R ²=0.665^***), but the correlation between the absolute shoot dry weight and shoot P concentration tended to be negative. There was also variation in native seed P reserve of the genotypes, but this variation had no influence on the P efficiency. The results indicate that the total amount of P per shoot and shoot dry matter production at low P supply are most reliable parameters in ranking genotypes for P efficiency at early growth stage. In wheat germplasm tested in the present study, several wheat genotypes are available showing both very high P efficiency and very high shoot content and concentration of P suggesting that P acquisition ability should be most important mechanism for high P efficiency in such genotypes. On the other hand, there are also genotypes in the germplasm having more or less same P concentration or P content in shoot but differing substantially in P efficiency, indicating importance of P utilization at cellular level in P efficiency. All these results suggest that P efficiency mechanisms can be different from one genotype to other within a given plant species.     

Kinetic analysis of boron transport in Chara

The permeability of biological membranes to boric acid was investigated using the giant internodal cells of the charophyte alga Chara corallina (Klein ex Will. Esk. R.D. Wood). The advantage of this system is that it is possible to distinguish between membrane transport of boron (B) and complexing of B by plant cell walls. Influx of B was found to be rapid, with equilibrium between the intracellular and extracellular phases being established after approximately 24 h when the external concentration was 50 μM. The intracellular concentration at equilibrium was 55 μM, which is consistent with passive distribution of B across the membrane along with a small amount of internal complexation. Efflux of B occurred with a similar half-time to influx, approximately 3 h, which indicates that the intracellular B was not tightly complexed. The concentration dependence of short-term influx measured with ¹⁰B-enriched boric acid was biphasic. This was tentatively attributed to the operation of two separate transport systems, a facilitated system that saturates at 5 μM, and a linear component due to simple diffusion of B through the membrane. V _max and K _m for the facilitated transport system were 135 pmol m⁻²s⁻¹ and 2 μM, respectively. The permeability coefficient for boric acid in the Chara plasmalemma estimated from the slope of the linear influx component was 4.4 × 10⁻⁷cm s⁻¹ which is an order of magnitude lower than computed from the ether:water partition coefficient for B. Received: 14 August 2000 / Accepted: 16 September 2000     

Phosphorus concentration in barley (Hordeum vulgare L.) seed: Influence on seedling growth and dry matter production

The effect of phosphorus (P) concentration in barley seed on seedling growth has not been much investigated. Consequently, two experiments were conducted in the greenhouse to determine the effect of P concentration in barley seed (Hordeum vulgare L., cv. Empress) on the seedlings grown in sand-filled boxes receiving a culture solution without P. Seeds were selected with three P concentrations: high-P (113.0 mmol P kg⁻¹), medium-P (80.7 mmol P kg⁻¹) and low-P (54.9 mmol P kg⁻¹). At 21 days after sowing, the shoot and root yield or shoot height was the least with seedlings from low-P seed. In the other experiment, high-P and low-P seeds were wetted with distilled water or with a solution of 25.8 cmol L⁻¹ of NaH₂PO₄ for 24 h, and then grown for 31 days. Solution P had been imbibed by seeds whether low or high in native P, but only the imbibed P held by low native P seed benefited seedling dry matter accumulation and shoot elongation. The lack of benefit from seed-imbibed P on seedlings grown from high-P barley seed was associated with low recovery of the imbibed P in those seedlings.