Modeling magnesium,phosphorus and potassium uptake by loblolly pine seedlings using a Barber-Cushman approach

The Barber-Cushman mechanistic nutrient uptake model, which has been utilized extensively to describe and predict nutrient uptake by crop plants, was evaluated for its ability to predict K, Mg, and P uptake by loblolly pine (Pinus taeda L.) seedlings. Sensitivity analyses were also used to investigate the impact of changes in soil nutrient supply, root morphological, and root uptake kinetics parameters on simulated nutrient uptake. Established experimental techniques were utilized to define the 11 parameters needed to model uptake by 1-0 seedlings of K, Mg, and P from a modified A horizon soil (Lilly series). Model predictions of K and P uptake over a 180-d growth period were underestimated by 6 and 11%, respectively. Estimates of Mg uptake were underestimated by 62%. While the level of agreement between predicted and observed K and P values was quite acceptable, analysis of parameter values and results of sensitivity analyses both indicated that the model underestimation of Mg uptake was the result of applying an I_max value developed under relatively low Mg concentration to a situation in which the functional I_max would be much higher due to the dominance of passive versus active uptake. Overall results of sensitivity analyses indicate that under the circumstances investigated, I_max, was the primary variable controlling plant uptake of K, Mg, and P. The dominance of this term over others was due to the relatively high C_li values for all three nutrients. Reducing (-50%) or increasing (+ 100%) other soil supply, root morphological, and remaining root uptake kinetics values did not substantially alter model estimates of nutrient uptake.     

Influence of phosphate status on phosphate uptake kinetics of maize (Zea mays) and soybean (Glycine max)

To obtain plants of different P status, maize and soybean seedlings were grown for several weeks in flowing nutrient solution culture with P concentrations ranging from 0.03–100 µmol P L^-1 kept constant within treatments. P uptake kinetics of the roots were then determined with intact plants in short-term experiments by monitoring P depletion of a 3.5 L volume of nutrient solution in contact with the roots. Results show maximum influx, I_max, 5-fold higher in plants which had been raised in solution of low compared with high P concentration. Because P concentrations in the plants were increased with increase in external P concentration, I_max was negatively related to % P in shoots. Michaelis constants, K_m, were also increased with increased pretreatment P concentration, only slightly with soybean, but by a factor of 3 with maize. The minimum P concentration, C_min, where net influx equals zero, was found between 0.06 and 0.3 µmol L^-1 with a tendency to increase with pretreatment P concentration. Filtration of solutions at the end of the depletion experiment showed that part of the external P was associated with solid particles.It was concluded that plants markedly adapt P uptake kinetics to their P status, essentially by the increase of I_max, when internal P concentration decreases. Changes of K_m and C_min were of minor importance.     

Regulation of nitrogen uptake on the whole plant level

The largest part of nitrogen requirements of crops is mostly covered by nitrate. The uptake of this ion is thermodynamically uphill and thus dependent on metabolism. This article considers regulation of N uptake in higher plants putting emphasis on NO₃ ^- and the whole plant level.In field conditions the transport rate depends on the concentration at the root surface in Michaelis-Menten-Kinetics. Maximum net influx of NO₃ ^- (I_max) was often reported at concentrations of 100 M NO₃ ^- and even lower. There are indications that for unrestricted growth the NO₃ ^- concentration at root surface has to be in the order of magnitude allowing I_max if plants are not able to compensate for lower NO₃ ^- concentrations by increasing root surface per unit of shoot.I_max is not a constant but depends for a given variety on N status of plants, the availability of NO₃ ^- and plant age. The decrease of I_max with increasing plant age is closely related to relative growth rate as long as the relationship between N demand and new growth is linear and the root:shoot ratio keeps constant. It seems that I_max is a meaningful physiological characteristic of NO₃ ^- uptake reflecting absolute N demand. There is evidence that shoot demand is linked to NO₃ ^- uptake of the root through an amino acid transport pool cycling in the plant via phloem and xylem.The N demand of a crop depends on increase of dry mass and might not be linear if the critical level of nitrogen in plant dry matter changes during crop development or if retranslocation of nitrogen from older leaves to meristematic tissue occurs. Radiation and temperature drive plant growth and thus N demand of crops. These relationships can be described by mathematical models.     

Characterization of phosphorus uptake in slow- and fast-growing southern pine seedlings grown in solution culture

The effects of low-P growth conditions on growth and net P acquisition were examined in two species of pine that are indigenous to P-deficient soils of the Atlantic Coastal Plain: pond pine (Pinus serotina Michx.), a moderately-fast growing pine, and a slow-growing seed source of loblolly pine P. taeda L.) from Texas. Short-term ³²P uptake experiments were conducted using intact nonmycorrhizal seedlings that had been grown for 7 weeks in continuously-flowing solution culture at 5 or 100 µM P. Growth and P uptake of pond pine were more responsive to a higher P supply than the slow-growing loblolly pine. Pond pine seedlings in the 100 µM P treatment were twice the size of those grown in 5 µM P and accumulated almost five times as much seedling P. In contrast, seedling biomass of loblolly pine increased by only 8% under high-P growth conditions, and seedlings accumulated twice as much P, reflecting the higher P concentrations in shoot and root tissues. Although rates of unidirectional influx of ³²P were 22 and 61% higher under low-P growth conditions in pond and loblolly pine, respectively, net uptake rates in seedlings from the 5 µM P treatment were over three times those of seedlings grown in 100 µM P. These results suggest that unidirectional efflux out of the root was controlling net uptake of P as much, if not more, than unidirectional influx. Efflux of³² P out of root tissue, particularly older tissue, decreased in seedlings grown under low-P conditions, possibly due to a reduction in the size of the phosphorus pool available for efflux, i.e. the soluble P_i pool. Over 75% of the total root P in both loblolly and pond pine seedlings grown in 100 µM P treatment was present as organic P, suggesting that organic P, particularly phytate, may represent important storage pools in roots of woody species. Within each species, higher rates of influx and net uptake in seedlings from the low-P treatment were associated with lower P concentrations in shoot and root tissue, and shoot FW:root FW ratios. Efflux may represent a short-term means of regulating net P uptake, while the demand for P created by growth and storage may represent a long-term regulation.     

Some regulatory aspects of [14C]methylamine influx intoPisum sativum L. cv. Feltham First seedlings

[¹⁴C]Methylamine influx intoPisum sativum L. cv. Feltham First seedlings showed Michaelis-Menten-type kinetics with apparentV _max=49.2 mol·g^-1 FW·h^-1 and apparentK _m=0.51 mM. The competitive interactions between ammonium and methylamine were most obvious when biphasic kinetics were assumed with saturation of the first phase at 0.05 mM. The inhibitor constant for ammonium (K _i)=0.027 mM. When [¹⁴C]methylamine was used in trace amounts with ammonium added as substrate, the influx of tracer showed Michaelis-Menten-type kinetics with apparentV _max=3.46 mol·g^-1 FW·h^-1 and apparentK _m=0.15 mM. The initial rate of net ammonium uptake corresponded with that found when [¹⁴C]methylamine was used to trace ammonium influx. The latter was also stimulated by high pH_o and inhibited by nitrate. Ammonium pretreatment±methionine sulphoximine or glutamine pretreatment of the seedlings inhibited subsequent [¹⁴C]methylamine influx, while methylamine or asparagine pretreatment stimulated [¹⁴C]methylamine influx. There was also a stimulatory effect of prior inoculation withRhizobium. The results are discussed in terms of current models for the regulation of ammonium uptake in plants.     

Kinetics of sulfate uptake by freshwater and marine species ofDesulfovibrio

Apparent half-saturation constants (K _m) and maximum uptake rates (V _max) for sulfate were determined in four species ofDesulfovibrio of freshwater and marine origin using a³⁵S-sulfate tracer technique. The lowerstK _m (5 M) was found in the freshwater speciesDesulfovibrio vulgaris (Marburg) and the highestK _m (77 M) in the marine speciesDesulfovibrio salexigens. Maximum specific rates of sulfate uptake (i.e.,V _max) were proportional to the growth rates observed in batch cultures. The halophilicDesulfovibrio salexigens did not change itsK _m andV _max between 1 and 6,000 M SO ₄ ^2- , and apparently did not induce a low-affinity uptake system at high sulfate concentrations. The low half-saturation constants measured for sulfate uptake explain why high rates of bacterial sulfate reduction occur in surface sediments of freshwater lakes, and why sulfate reduction can be a quantitatively important process in anaerobic carbon mineralization in low-sulfate environments. The results shows that extremely low sulfate concentrations must occur before sulfate reduction is completely outcompeted by methanogenesis.Abbreviations MPB methane producing bacteria - SRB sulfate reducing bacteria     

Changes in the kinetics of phosphate and potassium absorption in nutrient-deficient barley roots measured by a solution-depletion technique

From measurements of the rates of depletion of labelled ions from solution in the low concentration range, we described the phosphate and potassium uptake characteristics of the roots of intact barley plants in terms of the kinetic parameters, K _m and I _max (the maximum rate of uptake). In relatively young (13 d) and older (42 d) plants, cessation of phosphate supply for 4 d or more caused a marked increase in I _max (up to four times), without concomitant change in K _m, which remained between 5 and 7 M. By contrast, 1 d of potassium starvation with 14-d plants caused a decline in the K _m (i.e. an increased apparent affinity for potassium) from 53 M to 11 M, without alteration to I _max. After longer periods of potassium starvation, I _max increased (about two times) while the K _m remained at the same low value. Growth of shoots and roots were unaffected by these treatments, so that concentrations of ions in the tissues declined after 1 d or more of nutrient starvation, but we could not identify a characteristic endogenous concentration for either nutrient at which changes in kinetic parameters were invariably induced. The possible mechanisms regulating carriermediated transport, and the importance of changes induced in kinetic parameters in ion uptake from solution and soil are discussed.Symbol I _max the maximum rate of absorption at saturating concentrations     

A quantitative method for separating reaction components of CMP-sialic acid: Lactosylceramide sialyltransferase using Sep Pak C18 cartridges

A rapid procedure is described for the separation of CMP-sialic acid:lactosylceramide sialyltransferase reaction components using Sep Pak C₁₈ cartridges. The quantitative separation of the more polar nucleotide sugar, CMP-sialic acid, and its free acid from the less polar G_M3-ganglioside is simple and rapid relative to previously described methods. Recovery of G_M3 is optimized by the addition of phosphatidylcholine to the reaction mixture prior to the chromatographic step. Using rat liver Golgi membranes as a source of CMP-sialic acid: lactosylceramide sialyltransferase activity (G_M3 synthase; ST-1), the transfer of [¹⁴C] sialic acid from CMP-[¹⁴C] sialic acid to lactosylceramide can be quantified by this assay. The procedure is reliable and may be applicable to the isolation of ganglioside products in otherin vitro glycosyltransferase assays.Abbreviations G_M3 G_M3-ganglioside - II³NeuAc-LacCer NeuAc2-3Gal1-4Glc1-1Cer - G_D1a G_D1a-ganglioside, IV³NeuAc, II³NeuAc-GgOse₄Cer, NeuAc2-3Gal1-3GalNac1-4(NeuAc2-3)Gal1-4Glc1-1Cer - G_D3 G_D3-ganglioside, II³(NeuAc)₂LacCer, NeuAc2-8NeuAc2-3Gal1-4Glc1-1Cer - GgOse₄Cer asialo-G_M1 Gal1-3GalNAc1-4Gal1-4Glc1-1Cer - FucG_MI fucosyl-G_MI-ganglioside, Fuc1-2Gal1-3GalNAc1-4Gal1-4 Glc1-1Cer - ST-1 G_M3 synthase, CMP-sialic acid:lactosylceramide sialyltransferase - LacCer lactosylceramide, Gal1-4Glc1-1Cer - CMP-NeuAc cytidine 5-monophospho-N-acetylneuraminic acid - PC phosphatidylcholine - PMSF phenylmethylsulfonyl fluoride     

Seed source variation in 32P uptake in inus taeda L. seedlings: A possible regulation by efflux

Short-term ³²P uptake experiments were conducted with intact seedlings of loblolly pine (Pinus taeda L.) to examine possible seed source variation in net accumulation of ³²P in roots and shoots, and in rates of unidirectional influx. Seed source had a highly significant effect on biomass and P concentrations of shoots and roots. Seedlings from two seed sources representing fast-growing populations (a broadly-adapted and wet-site seed source) accumulated over 60% more total seedling P than smaller seedlings from a drought-hardy seed source, reflecting higher biomass and root P concentrations. Rates of unidirectional ³²P influx in seedlings from the drought-hardy seed source were more than twice the rates of the seedlings from the broadly-adapted seed source. However, after 24 h in labeled uptake solution, net accumulation of ³²P was similar, suggesting that rates of unidirectional efflux from roots of the drought-hardy seed source were also high. Although there were no significant differences in biomass and tissue P concentrations between the two fast-growing seed sources, rates of unidirectional influx in seedlings from the broadly-adapted seed source were 42% lower than rates in seedlings from the wet-site source. Yet, after 24 h in labeled uptake solution, net accumulation of ³²P in seedlings from the broadly-adapted seed source was 50% higher. Unidirectional efflux out of the root may regulate net uptake of P as much, if not more, than influx in loblolly pine seedlings-at least under high-P growth conditions. The results in this study do not support previous studies with herbaceous plants suggesting that fast-growing species typically exhibit higher rates of nutrient uptake than slow-growing species.     

Carbohydrate status in roots of two soybean cultivars: A possible parameter to explain different efficiencies concerning phosphate uptake

The carbohydrate levels in the roots of two soybean cultivars (Century, Woodworth) were determined during the early vegetative growth stage. Based on Michaelis-Menten kinetics Century was characterized as being more efficient at phosphate uptake when compared to Woodworth. The experiments were carried out at normal nutrition (+P treatment) as well as under a condition of moderate phosphate stress (–P treatment). The stress situation was created by withdrawal of phosphate from the nutrient solution for five or nine days. Compared to the +P-treatment the deprivation of phosphate led to a reduction of more than 50% in the phosphate content of the plant tissue.The carbohydrates in the roots exhibited differences between both cultivars with Century generally having higher sucrose levels. During the morning the +P treatment of Century exhibited 10 mg g^-1 and in the afternoon 16 mg g^-1 sucrose (plant dry weight basis). This compares to 8 mg g^-1 sucrose (morning) and 13 mg g^-1 sucrose (afternoon) for Woodworth. Phosphate stress increased the levels of hexoses (glucose, fructose) in both varieties significantly. The sucrose levels remained higher for the Century variety even during phosphate stress. We conclude that the shoot of the Century variety supplies its root with an increased amount of energy in form of carbohydrates compared to the Woodworth variety. This may enable the plant to take up phosphate more efficiently. It is also remarkable that the Century variety reduces the pH of the nutrient medium more rapidly. This active proton deposition may create a high membrane potential which is known to be necessary for phosphate uptake.     

The influence of salinity on the kinetics of NH inf4 sup+ uptake in Spartina alterniflora

Summary The effects of short- and long-term exposure to a range in concentration of sea salts on the kinetics of NH _inf4 ^sup+ uptake by Spartina alterniflora were examined in a laboratory culture experiment. Long-term exposure to increasing salinity up to 50 g/L resulted in a progressive increase in the apparent K_m but did not significantly affect V_max (mean V_max=4.23±1.97 mole·g^–1·h^–1). The apparent K_m increased in a nonlinear fashion from a mean of 2.66±1.10 mole/L at a salinity of 5 g/L to a mean of 17.56±4.10 mole/L at a salinity of 50 g/L. These results suggest that the long-term effect of exposure to total salt concentrations within the range 5–50 g/L was a competitive inhibition of NH _inf4 ^sup+ uptake in S. alterniflora. No significant NH _inf4 ^sup+ uptake was observed in S. alterniflora exposed to 65 g/L sea salts. Short-term exposure to rapid changes in salinity significantly affected both V_max and K_m. Reduction of solution salinity from 35 to 5 g/L did not change V_max but reduced K_m by 71%. However, exposing plants grown at 5 g/L salinity to 35 resulted in an decrease in V_max of approximately 50%. Exposure of plants grown at 35 g/L to a total sea salt concentration of 50 g/L for 48h completely inhibited uptake of NH _inf4 ^sup+ . For both experiments, increasing salinity led to an increase in the apparent K_m similar to that found in response to long-term exposure. Our data are consistent with a conceptual model of changes in the productivity of S. alterniflora in the salt marsh as a function of environmental modification of NH _inf4 ^sup+ uptake kinetics.     

Simultaneous influx of ammonium and potassium into maize roots: kinetics and interactions

The interaction between ammonium and potassium during influx was examined in roots of dark-grown decapitated corn seedlings (Zea mays L., cv. Pioneer 3369A). Influx was measured during a 10-min exposure to either (¹⁵NH₄)₂SO₄ ranging from 10 to 200 M NH ₄ ⁺ with and without 200 M K(⁸⁶Rb)Cl or to K(⁸⁶Rb)Cl ranging from 10 to 200 M K⁺ with and without 200 M NH ₄ ⁺ as (¹⁵NH₄)₂SO₄. The simple Michaelis-Menten model described the data well only for potassium influx in the presence of ambient ammonium. For the other three instances, the data were improved by assuming that a second influx mechanism became operative as the low-concentration phase approached saturation. Two distinct mechanisms are thus indicated for both ammonium and potassium influx within the range of 10 to 200 M.The influx mechanism operating at low concentrations showed greater affinity for potassium than for ammonium, even though the capacity for ammonium transport was twice as large as that for potassium. It is suggested that this phase involved a common transport system for the two ions and that localized low acidity next to the internal surface, following H⁺ extrusion, favored ammonium deprotonation and dissociation from the transport system-ammonium complex. Parallel decreases in V _max and increases in K_m of the low-concentration saturable phase occurred for ammonium influx when ambient potassium was present and for potassium influx when ambient ammonium was present. The data support a mixed-type inhibition in each case. Simultaneous measurement of potassium and ammonium influx showed that they were highly negatively correlated at the lower concentrations, indicating that the extent to which influx of the inhibited ion was restricted was associated with influx of the inhibitor ion. Presence of ambient ammonium eliminated the second phase of potassium influx. In contrast, the presence of ambient potassium decreased the concentration at which the second phase of ammonium influx was initiated but did not restrict the rate.Paper no. 11131 of the Journal Series of the North Carolina Agricultural Research ServiceThe use of trade names in this publication does not imply endorsement by the North Carolina Agricultural Research Service of the products named, nor criticism of similar ones not mentioned     

Effects of oxygen partial pressure and combined nitrogen on N2-fixation (C2H2) associated withZea mays and other gramineous species

Summary The objectives of this investigation were to determine the effects of oxygen partial pressure (pO₂) and combined nitrogen (NH ₄ ⁺ ) on rates of acetylene reduction (AR) associated with roots of intact corn, sorghum, and pearl millet plants. Soil-grown plants were carefully removed from soil and incubated hydroponically with the root system enclosed in a plastic cylinder; the tops were left exposed to ambient conditions. Oxygen concentrations around the root systems were controlled by sparging the nutrient solution with known quantities of O₂ in N₂. Ammonium nitrogen was added to the nutrient solution following establishment of AR rates to determine its effect on rates of N₂-fixation (AR). Substantial AR rates (0.1–1.5 mol C₂H₄ g dry wt^–1 h^–1) were associated with roots exposed to 0–2% O₂ (v/v) (0.0–2.02 kPa) in N₂ following at 12–24 h period of exposure to the reduced oxygen tension. Root systems exposed to air failed to demonstrate AR while those exposed to 100% N₂ showed lower activity than those at reduced pO₂ values. Addition of NH ₄ ⁺ (10–20 g N ml^–1 of nutrient solution) reduced AR by 75–90% within 24 h after addition. Oxygen uptake by roots exposed to low pO₂ was substantially reduced.     

High-affinity uptake of γ-aminobutyric acid in cultured glial and neuronal cells

Both glial and neuronal cells maintained in primary culture were found to accumulate [³H]GABA by an efficient high-affinity uptake system (apparentK _m=9 M,V _max=0.018 and 0.584 nmol/mg/min, respectively) which required sodium ions and was inhibited by 1 mM ouabain. Strychnine and parachloromercuriphenylsulfonate (pCS) (both at 1 mM) also strongly inhibited uptake of [³H]GABA, but metabolic inhibitors (2,4-dinitrophenol, potassium cyanide, and malonate) were without effect. Only three structural analogs of GABA (nipecotate, -alanine, and 2,4-diaminobutyrate) inhibited uptake of [³H]GABA, while several other compounds with structural similarities to GABA (e.g. glycine,l-proline, and taurine) did not interact with the system. The kinetic studies indicated presence of a second uptake (K _m=92 M,V _max=0.124 nmol/mg/min) in the primary cultures containing predominantly glioblasts. On the other hand, only one of the neuronal cell lines transformed by simian virus SV40 appeared to accumulate [³H]GABA against a concentration gradient. ApparentK _m of this uptake was relatively high (819 M), and it was only weakly inhibited by 1 mM ouabain and 1 mM pCS. The structural specificity also differed from that of the uptake observed in the primary cultures. Significantly, none of the nontransformed continuous cell lines of either tumoral (glioma, C₆; neuroblastoma, Ml; MINN) or normal (NN; I₆) origin actively accumulated [³H]GABA. It is suggested that for the neurochemical studies related to GABA and requiring homogeneous cell populations, the primary cultures offer a better experimental model than the continuous cell lines.     

M3 muscarinic receptors on murine HSDM1C1 cells: Further functional,regulatory, and receptor binding studies

In the present studies, the pharmacology and regulation of the functional muscarinic receptors on HSDM1C1 cells were probed using phosphoinositide (PI) turnover assays. In addition, the receptor binding of the putative M₃-selective radioligand, [³H]4-DAMP, to cell homogenates was characterized. Carbachol (EC₅₀=9 M), (+)muscarine (EC₅₀=4.5 M) and cis-dioxolane (EC₅=0.72 M) were full agonists which stimulated PI turnover by 13.3±1.0 fold above basal values. The potencies of numerous agonists in this assay system were relatively similar to their affinities in receptor binding assays. Exposure of HSDM1C1 cells to 10 nM–10 M muscarine during the last 24h of [³H]myo-inositol-labeling resulted in a concentration-dependent reduction in the cisdioxolane affinity and maximal PI response induced by subsequent treatment with cis-dioxolane. pertussis toxin (5–2000 ng/ml) caused a partial reduction in the cis-dioxolane-induced PI turnover. Likewise, exposure of the HSDM1C1 cells to an active phorbol ester (TPA) resulted in a partial inhibition of the cis-dioxolane-induced (100 M) PI turnover. The half-maximal effect of TPA was produced at 1.8±0.3 nM. [³H]4-DAMP binding to cell homogenates was of high affinity (K_d=0.19±0.04 nM) and moderate capacity (B_max=201±22 fmol/mg protein). The pharmacological specificity (4-DAMP>p-FHHSiD>dicyclomine>pirenzepine>methoctramine>AFDX-116 >gallamine) resembled that for [³H]NMS binding and correlated well with that observed for inhibition of PI turnover. These studies further support the identification of M₃ receptors on HSDM1C1 cells. These receptors have been shown to be influenced by pertussis toxin, an active phorbol ester and to exhibit desensitization.     

Amino acid and monoamine transport in primary astroglial cultures from defined brain regions

The uptake ofl-[³H]glutamate,l-[³H]aspartate, -[³H]aminobutric acid (GABA), [³H]dopamine,dl-[³H]norepinephrine and [³H]5-hydroxytryptamine (5-HT) was studied in astrocytes cultured from the cerebral cortex, striatum and brain stem of newborn rat and grown for 2 weeks in primary cultures. The astrocytes exhibited a high-affinityl-glutamate uptake withK _m values ranging from 11 to 110 M.V _max values were 4.5 in cerebral cortex, 39.1 in striatum, and 0.4 in brain stem, nmol per mg cell protein per min. There was a less prominent high-affinity uptake ofl-aspartate withK _m values from 88 to 187 M.V _max values were 7.4 in cerebral cortex, 37.1 in striatum, and 3.1 in brain stem, nmol per mg cell protein per min. The high-affinity GABA uptake exhibitedK _m values ranging from 5 to 17 M andV _max values were 0.01 for cerebral cortex, 0.04 for striatum, and 0.1 for brain stem, nmol per mg cell protein per min. No high-affinity, high-capacity uptake was found for the monoamines. The results demonstrate a heterogeneity among the astroglial cells cultivated from the different brain regions concerning the uptake capacity of amino acid neurotransmitters. Furthermore, amino acid transmitters and monoamines are taken up by the cells in different ways.     

Release of carboxylic anions and protons by tomato roots in response to ammonium nitrate ratio and pH in nutrient solution

The exudation of certain organic anions and protons by roots which may affect solubility of metals and P and uptake by plants, is affected by nitrogen form and pH. The objective of this work was to study exudation of carboxylates and H⁺/OH^– by tomato plants in response to NH₄/NO₃ ratio and pH in nutrient solution. Four NH₄/(NH₄+NO₃) ratios (R= 0, 0.33, 0.67 and 1) and constant vs. variable solution pH treatments were investigated. The sum of the exudation rates of all carboxylates tended to decline with increasing R, particularly tri- and dicarboxylates. The molar fraction of the exuded tri- and dicarboxylates, averaged over all treatments and plant ages, increased in the order tartarate 2%), malate (6%), succinate (15%), citrate (26%) and fumarate (46%). At R=1 the solution pH dropped from 5.2 to 3 and at R=0 increased to 8. The R corresponding to the pH stat of tomato plant was 0.3. For the constant solution pH treatment, the effect of solution pH on carboxylate exudation rate was small as compared to the effect of R. The exudation of citrate and H⁺ efflux which were initiated when NO₃ and NH₄ uptake rates per plant exceeded certain threshold values, increased with plant age.     

Mechanisms of Na and Cl absorption across the distal colon epithelium of the pig

Summary Porcine distal colon epithelium was mounted in Ussing chambers and bathed in plasma-like Ringer solution. Tissue conductances ranged from 10 to 15 mS and the short-circuit current (I_sc) ranged from-15 to 220 A·cm^-2. Variations in basal I_sc resulted from differences in the amount of amiloride (10M mucosal addition)-sensitive Na⁺ absorption. Ion substitution and transepithelial flux experiments showed that 10 M amiloride produced a decrease in the mucosal-to-serosal (M-S) and net Na flux, and that this effect on I_sc was independent of Cl^- and HCO ₃ ^- replacement. When the concentration of mucosal amiloride was increased from 10 to 100 M, little change in I_sc was observed. However, increasing the concentration to 1 mM produced a further inhibition, which often reversed the polarity of the I_sc. The decrease in I_sc due to 1 mM amiloride was dependent on both Cl^- and HCO ₃ ^- , and was attributed to reductions in the M-S and net Na⁺ fluxes as well as the M-S unidirectional Cl^- flux. Ion replacement experiments demonstrated that Cl^- substitution reduced the M-S and net Na fluxes, while replacement of HCO ₃ ^- with HEPES abolished net Cl^- absorption by reducing the M-S unidirectional Cl^- flux. From these data it can be concluded that: (1) Na⁺ absorption is mediated by two distinct amiloride-sensitive transport pathways, and (2) Cl^- absorption is completely HCO ₃ ^- -dependent (presumably mediated by Cl^-/HCO ₃ ^- exchange) and occurs independently of Na⁺ absorption.Abbreviations G_t tissue conductance - HEPES tris (hydroxymethyl) aminomethane - (tris) N-2-hydroxyethylpiperazine-N-2-ethanesulfonic acid - I_sc short-circuit current - J_r residual flux - M-S mucosal-to-scrosal - S-M serosal-to-mucosal - TTX tetrodotoxin     

Photosynthetic apparatus in primary leaves of barley seedlings grown under blue or red light of very low photon flux densities

Chlorophyll (Chl) a and Chl b contents, rate of CO₂ gas exchange, quenching coefficients of chlorophyll fluorescence, and endogenous phytohormones have been studied in primary leaves of barley seedlings cultivated under blue (BL) or red (RL) light. Photon flux densities (PFD) were between 0.3 and 12 mol m^-2 s^-1. Plants grown at PFD of 0.3 mol m^-2 s^-1 demonstrated in BL tenfold and in RL threefold decreased Chl content compared to plants grown at 12 mol m^-2 s^-1. Chl a/b ratio increased from 2.3–2.5 to 4.4–4.5 in BL, not in RL, following the decrease in PFD at plant cultivation from 12 to 0.3 mol m^-2 s^-1. Plants cultivated at weak BL demonstrated severalfold decreased rate of photosynthetic CO₂ uptake, whereas decrease in PFD of RL from 12 to 0.3 mol m^-2 s^-1 caused only 20% de cline in the rate of photosynthesis. Decrease in PFD during a plant cultivation reduced the maximum quantum yield of photosynthesis in BL, not in RL leaves. Light response curves of non-photochemical and photochemical quenching of chlorophyll fluorescence calculated on the basis of absorbed quanta were not affected by PFD of RL during plant cultivation. On the contrary, both non-photochemical quenching and accumulation of Q_A ^-, reduced primary acceptor of Photosystem II, occurred at lower amounts of absorbed quanta in leaves of BL plants grown at 0.3 than at 12 mol m^-2 s^-1. Two photoregulatory reactions were suggested to exert the light control of the development of photosynthetic apparatus in the range of low PFDs. The photoregulatory reaction saturating by very low PFDs of RL was supposed to be mediated by phytochrome. Phytochrome was proposed to enhance (as related to other pigment-protein complexes of thylakoids) the accu mulation of chlorophyll- b-binding light-harvesting complex of Photosystem II (LHC II). It acts independently of the pigment mediating the second photoregulatory reaction, as evidenced by the results of experiments with plant growth under mixed blue plus red light. The contents of cytokinins and indole-3-acetic acid in a leaf were not significantly affected by either light quality and PFD thus indicating those phytohormones not to be involved into photoregulatory processes.     

Interactive effects of soil nitrogen and atmospheric carbon dioxide on root/rhizosphere carbon dioxide efflux from loblolly and ponderosa pine seedlings

We measured CO₂ efflux from intact root/rhizosphere systems of 155 day old loblolly (Pinus taeda L.) and ponderosa (Pinus ponderosa Dougl. ex Laws.) pine seedlings in order to study the effects of elevated atmospheric CO₂ on the below-ground carbon balance of coniferous tree seedlings. Seedlings were grown in sterilized sand culture, watered daily with either 1, 3.5 or 7 mt M NH ₄ ⁺ , and maintained in an atmosphere of either 35 or 70 Pa CO₂. Carbon dioxide efflux (mol CO₂ plant^–1 s^–1) from the root/rhizosphere system of both species significantly increased when seedlings were grown in elevated CO₂, primarily due to large increases in root mass. Specific CO₂ efflux (mol CO₂ g root^–1 s^–1) responded to CO₂ only under conditions of adequate soil nitrogen availability (3.5 mt M). Under these conditions, CO₂ efflux rates from loblolly pine increased 70% from 0.0089 to 0.0151 mol g^–1 s^–1 with elevated CO₂ while ponderosa pine responded with a 59% decrease, from 0.0187 to 0.0077 mol g^–1 s^–1. Although below ground CO₂ efflux from seedlings grown in either sub-optimal (1 mt M) or supra-optimal (7 mt M) nitrogen availability did not respond to CO₂, there was a significant nitrogen treatment effect. Seedlings grown in supra-optimal soil nitrogen had significantly increased specific CO₂ efflux rates, and significantly lower total biomass compared to either of the other two nitrogen treatments. These results indicate that carbon losses from the root/rhizosphere systems are responsive to environmental resource availability, that the magnitude and direction of these responses are species dependent, and may lead to significantly different effects on whole plant carbon balance of these two forest tree species.