The Apoplastic Pathway of Transport to Salt Glands

The availability of the apoplastic route for ion transport tosalt glands was assessed by electron microscope examinationsand by the use of lanthanum as an electron-dense tracer to demarcatethe apoplast. The results show that this solute can reach thesalt glands via the apoplast and this observation is discussedin relation to current ideas concerning salt gland function.     

Long Distance Transport of Abscisic Acid in NaCI-Treated Intact Plants of Lupinus albus

Plants of Lupinus albus were grown for 51 d under control (1.1mol m^–3 NaCl) and saline (40 mol m^–3 NaCl) conditions.Plants were harvested and changes of carbon, nitrogen and abscisicacid (ABA) contents of individual organs were determined 41d and 51 d after germination. In the period between the twoharvests xylem and phloem saps were collected and respirationand photosynthesis of individual organs were measured. Usingflows of carbon, C/ABA ratios and increments of ABA flows ofABA in phloem and xylem and rates of biosynthesis and degradationof ABA were calculated. Both under control and saline conditionsnet biosynthesis occurred in the root, the basal strata of leavesand in the inflorescence. Metabolic degradation of ABA tookplace in the stem internodes and apical leaf strata. Salt stress increased xylem transport of ABA up to 10-fold andphloem transport to the root up to 5-fold relative to that ofthe controls. A considerable amount of ABA in the xylem saporiginated from biosynthesis in the roots, i.e. 55% in salt-treatedand smaller than 28% in control plants. The remaining part ofABA in the xylem sap originated from the shoot: it was translocatedin the phloem from fully differentiated leaves towards the rootand from there it was recirculated back to the aerial partsof the plant. The data suggest that ABA may serve as a hormonalstress signal from the root system. Key words: Lupinus albus, salt stress, abscisic acid, long distance transport     

Path of Downward Oxygen Transport from Aerial to Subterranean Parts in Intact Seedlings of Rice and Other Plants

There are two opposite opinions as regards the mechanism and the path of downward oxygen transport in rice and other higher plants. Van Raalte (1940), Yamada (1952), and others maintain that an oxygen pressure gradient of decreasing magnitude from the stem base down to the root tip exists in the intercellular air spaces, which are interconnected throughout the cortex, and the oxygen transported therein is in free molecular form and moves about by diffusion along its own gradient. Recent diffusion experiments in plants by Barber (1962), employing radioactive O15 as indicator, gave direct confirmation of this hypothesis. The opposite view is held by Brown (1947), Soldatenkov (1963) and others. They consider that the passive diffusion of oxygen along its own gradient is inadequate to account for the actual amount transported downwards. The fact that downward oxygen transport in roots comes almost to a standstill, once the aerial part is removed while the cut end of the short stump is still left in air, casts doubts as to the validity of the diffusion hypothesis; and is in favour of their claim that in addition to, or in placement of, diffusion, active participation of living tissues in shoot is necessary to drive enough oxygen to meet the demand of roots. The oxygen in active transport is no longer in free gaseous state but is in dissolved or combined form (as in peroxides) and moves presumably along the vascular bundles in a way which is hitherto unrevealed but is apparently dependent upon the physiological activity of the conducting tissue. In our previous report (Lou et al 1964), we gave data based on quantitative measurement of the amount of oxygen transported downwards from aerial to submerged parts in intact seedlings with the respiratory hydrometer specially designed for the purpose. In seedlings of marshy plants (e.g. rice), it amounts to about 50% of the total oxygen absorbed by the aerial part; in water cultured seedlings of ordinary land plants (e.g. pea), 20%–30%. By deliberately blocking the alternative paths of oxygen transport in seedlings, one at a time, and measuring the downward oxygen transport accordingly in the same way as before, we should be able to decide which one of the two paths is mainly responsible for the transport. The blocking can be conveniently carried out at the upper end of the radical in a pea (or broadbean) seedling by surgical treatment (see Fig.1); either by ringing off the peripheral cortex where most of the air spaces reside; or by piercing through the central cylinder, within which the vascular bundles are confined. The treated radical is then submerged in water and ready for measurement. Without recourse to surgical treatment and mechanical injury, the air space in the cortex can also be blocked by displacing its air content with water through vacuum infiltration. The present investigation has shown that when the intercellular spaces in the cortex of the radical are blocked either by ringing or by infiltration, the aerial part of the treated seedling absorbs much less oxygen than the control as though its radical were completely severed (Table 2); or, in other words, the downward oxygen transport is effectively stopped by such a means. On the other hand, interruption of vascular bundles in the central cylinder only reduces the amount of oxygen in transport to less than one half, which can be accounted for by the combined effect of the reduced root activity due to shortage of food supply and the unavoidable partial disruption of the peripheral cortex. Besides taking actual measurement, downward oxygen transport in intact pea (or broadbean) seedlings can also be detected by simply noticing the growth rate of its radical. As is shown in this investigation, the radical ceases growing in still water, if the oxygen supply from its aerial part is interrupted. As a result of oxygen deficiency, the radical tip deteriorates in a few days. These effects can be easily realized by ringing off the cortex or by infiltrating its air spaces with water. That the peripheral ringing of the radical does no harm to its growth process is revealed by the fact that if air is bubbled through the water culture steadily, normal growth ensues. The above results leave no doubt that in seedlings of rice, pea, and broadbean, downward oxygen transport mainly takes place in the intercellular spaces in the radical cortex, and seems to have no concern with the activities of vascular bundle and cortex. Although there are evidence that rice roots may actively secrete oxygen in the form of peroxides to its immediate neighborhood (the rhizosphere), the actual amount and the distance traversed in such an active transport however, is very much limited and is insignificant as compared with that taking place in the intercellular spaces.     

Quantitative Estimations of Downward Oxygen Transport from Aerial to Subterranean Parts in Intact Seedlings of Rice and Other Plants

Quantitative estimations of downward oxygen transport from aerial to subterranean parts in intact seedlings were carried out in the present investigation with the respiratory hydrometer specially designed by us for this purpose. The chief object of the investigation is rice, a crop which is notable for its marshy habitat and whose submerged roots are in particular demand of such transport. Some other common plants (wheat, pea, water cress, etc.), either of marshy or of mesophytic habitat, have also been included in the investigation for comparison. Although rice has long been known for its capability of downward oxygen transport, as has often been revealed by various qualitative demonstrations and indirect estimations; yet, data of direct quantitative measurement of the actual amount transported, so far as we are aware, have been very scanty. The few attempts of bringing about such quantitative measurement in an intact plant are made by enclosing its shoot and root in two adjoining compartments respectively, and gas analysis is made on samples taken from each compartment at intervals. The procedure is so elaborate and tedious that estimations on a large scale could not be readily carried out and the results have often been rendered unreliable by mishandling of the plant and air leakage between the compartments. Proposals to the path and mechanism of downward oxygen transport in higher plants have largely been based upon such scanty quantitative approximations and various qualitative observations, and the conclusions derived therefrom are contraversial and far from being convincing. The presentation in this communication of a simple yet accurate experimental method for the quantitative determination of this kind might be opportune and appropriate. The basic principle of the respiratory hydrometer employed in this investigation has been given previously (Lou et al., 1963). Seedlings raised in water culture are inserted into the vessel of the hydrometer (Fig. 1) with its aerial part in the air space above and roots in the water passage below. As the diffusion rate of oxygen in water is about 1/300,000 that in air, the submerged roots of an intact rice seedling practically have their immediate oxygen supply cut off and have to rely upon the oxygen transported from above. Downward oxygen transport in intact seedlings can be easily estimated through the following procedures and the results thus obtained are summarized below: 1. The difference between two consecutive determinations of the oxygen absorbed by the aerial parts of intact seedlings made before and after their roots are severed gives the amount of oxygen transported downwards to roots. For the marshy plant (rice, water cress), it is about 50% (range: 30%–70%) of the total amount absorbed; whereas for ordinary land plants raised in water culture (wheat, pea), it is 20%–30% of the total. 2. The above results are in good agreement with those obtained by determining the respiratory quotients of intact seedlings first in air (e.g.R.Q. ≌ 1 in case of rice seedling) and then with their roots submerged in water (R.Q. ≌ 0.5). The difference between the two consecutive determinations again gives the fraction of oxygen transported downwards. 3. Either by varying the oxygen supply to the aerial part (from 1/4 to twice the oxygen content in air) or by increasing the oxygen consumption of the root through temperature increase or DNP stimulation, the oxygen concentration gradient along the vertical axis of the plant can be steepened or lessened at will. When such experiment is carried out in rice seedlings, the amount of oxygen transported downwards increases with the gradient.     

Phosphate Transport and Apoplastic Phosphate Homeostasis in Barley Leaves

Levels of apoplastic inorganic phosphate (P_i) in leaves andP_i-transport activities of mesophyll cells were measured insitu in control and P_i-deficient plants. When detached leaveswere fed a solution that contained 10 mM P_i, the apoplasticP_i levels, as measured by an infiltration method, remained almostconstant. When the leaves were immersed in pure water, the apoplasticP_i level gradually decreased. With 50 mM P_i in the feeding solution,the level increased dramatically. The apoplastic P_i levels inP_i-deficient leaves were somewhat, but not very much lower thanthose in controls. When the immersion medium was changed topure water 60 mm after feeding with 10 mM P_i, the apoplasticP_i levels started to decrease and then returned to the initiallevel. It is suggested that intracellular P_i may be transportedback to the apoplast to maintain the apoplastic P_i levels ata constant value. Changes in cytoplasmic pH were measured during feeding of P_ito the leaves by use of the pH-sensitive fluorescent dye, pyranineafter Yin et al. (l990a, b). On feeding of P_i the cytoplasmicpH decreased in P_i-deficient plants as a result of co-transportof P_i and protons in situ. After removal of P_i from the immersionmedium, the cytoplasmic pH returned to the original value. ³ Present address: Institute für Biochemische Pflanzenpathologie,GSF-München, D-8042 Neuherberg, Germany.     

High-Affinity Transport of Choline-O-Sulfate and Its Use as a Compatible Solute in Bacillus subtilis

We report here that the naturally occurring choline ester choline-O-sulfate serves as an effective compatible solute for Bacillus subtilis, and we have identified a high-affinity ATP-binding cassette (ABC) transport system responsible for its uptake. The osmoprotective effect of this trimethylammonium compound closely matches that of the potent and widely employed osmoprotectant glycine betaine. Growth experiments with a set of B. subtilis strains carrying defined mutations in the glycine betaine uptake systems OpuA, OpuC, and OpuD and in the high-affinity choline transporter OpuB revealed that choline-O-sulfate was specifically acquired from the environment via OpuC. Competition experiments demonstrated that choline-O-sulfate functioned as an effective competitive inhibitor for OpuC-mediated glycine betaine uptake, with a K_i of approximately 4 μM. Uptake studies with [1,2-dimethyl-¹⁴C]choline-O-sulfate showed that its transport was stimulated by high osmolality, and kinetic analysis revealed that OpuC has high affinity for choline-O-sulfate, with a K_m value of 4 ± 1 μM and a maximum rate of transport (V_max) of 54 ± 3 nmol/min · mg of protein in cells grown in minimal medium with 0.4 M NaCl. Growth studies utilizing a B. subtilis mutant defective in the choline to glycine betaine synthesis pathway and natural abundance ¹³C nuclear magnetic resonance spectroscopy of whole-cell extracts from the wild-type strain demonstrated that choline-O-sulfate was accumulated in the cytoplasm and was not hydrolyzed to choline by B. subtilis. In contrast, the osmoprotective effect of acetylcholine for B. subtilis is dependent on its biotransformation into glycine betaine. Choline-O-sulfate was not used as the sole carbon, nitrogen, or sulfur source, and our findings thus characterize this choline ester as an effective compatible solute and metabolically inert stress compound for B. subtilis. OpuC mediates the efficient transport not only of glycine betaine and choline-O-sulfate but also of carnitine, crotonobetaine, and γ-butyrobetaine (R. Kappes and E. Bremer, Microbiology 144:83–90, 1998). Thus, our data underscore its crucial role in the acquisition of a variety of osmoprotectants from the environment by B. subtilis.     

Alterations in Apoplastic and Total Solute Concentrations in Soybean Nodules Resulting from Treatments Known to Affect Gas Diffusion

Ureide concentration in the cortical apoplast of soybean (Glycinemax(L.) Merr.) nodules increases rapidly in response to noduleexcision. The objective here was to determine if changes inapoplastic ureide may be related to the control of resistanceto gas diffusion which is thought to be localized in the nodulecortex. Following decapitation of shoots, nitrogenase activity(acetylene reduction) and ureide concentration in total noduleextracts declined over a period of several hours. Apoplasticureide concentration relative to total nodule ureide was elevatedunder these conditions, but the treatment effect was small comparedto non-decapitated controls. Decapitation also caused a significantdecline in the concentrations of sucrose, glucose, and D-pinitolin nodules. However, the decline in carbohydrates was similarin the nodule cortex and the nodule as a whole, suggesting thatthe carbohydrate changes are not related to a cortex-localizedmechanism. Non-invasive treatments involving increases or decreasesin oxygen concentration supplied to nodulated roots caused rapiddecreases in respiration of nodulated roots and in ureide concentrationin total nodule extracts, but did not cause major changes inapoplastic ureide concentrations. The combined results indicatethat apoplastic ureide is probably not involved in the regulationof resistance to gas diffusion. The rapid decline in noduleureide concentrations in response to changing oxygen supplydocuments the sensitivity of ureide synthesis and/or transportto alterations in nodule respiration and/or nitrogenase activity Key words: Glycine max, Pisum sativum, ureide, carbohydrates     

Transport of Sodium in Intact Peanut Seedling

Peanut (Arachis hypogea L.) seedlings readily transported Nato the shoot. The amount of Na transported was linearly relatedto the absorption period (which ranged from 1 to 8 h) and alsoto the external Na level which varied from 0.05 to 1 mM.     

Apoplastic pH and Fe3+ Reduction in Intact Sunflower Leaves

It has been hypothesized that under NO₃⁻ nutrition a high apoplastic pH in leaves depresses Fe³⁺ reductase activity and thus the subsequent Fe²⁺ transport across the plasmalemma, inducing Fe chlorosis. The apoplastic pH in young green leaves of sunflower (Helianthus annuus L.) was measured by fluorescence ratio after xylem sap infiltration. It was shown that NO₃⁻ nutrition significantly increased apoplastic pH at distinct interveinal sites (pH ≥ 6.3) and was confined to about 10% of the whole interveinal leaf apoplast. These apoplastic pH increases presumably derive from NO₃⁻/proton cotransport and are supposed to be related to growing cells of a young leaf; they were not found in the case of sole NH₄⁺ or NH₄NO₃ nutrition. Complementary to pH measurements, the formation of Fe²⁺-ferrozine from Fe³⁺-citrate was monitored in the xylem apoplast of intact leaves in the presence of buffers at different xylem apoplastic pH by means of image analysis. This analysis revealed that Fe³⁺ reduction increased with decreasing apoplastic pH, with the highest rates at around pH 5.0. In analogy to the monitoring of Fe³⁺ reduction in the leaf xylem, we suggest that under alkaline nutritional conditions at interveinal microsites of increased apoplastic pH, Fe³⁺ reduction is depressed, inducing leaf chlorosis. The apoplastic pH in the xylem vessels remained low in the still-green veins of leaves with intercostal chlorosis.     

Solute Transport Process in Intestinal Epithelial Cells

In rat small intestine, the active transport of organic solutes results in significant depolarization of the membrane potential measured in an epithelial cell with respect to a grounded mucosal solution and in an increase in the transepithelial potential difference. According to the analysis with an equivalent circuit model for the epithelium, the changes in emf's of mucosal and serosal membranes induced by active solute transport were calculated using the measured conductive parameters. The result indicates that the mucosal cell membrane depolarizes while the serosal cell membrane remarkably hyperpolarizes on the active solute transport. Corresponding results are derived from the calculations of emf's in a variety of intestines, using the data that have hitherto been reported. The hyperpolarization of serosal membrane induced by the active solute transport might be ascribed to activation of the serosal electrogenic sodium pump. In an attempt to determine the causative factors in mucosal membrane depolarization during active solute transport, cell water contents and ion concentrations were measured. The cell water content remarkably increased and, at the same time, intracellular monovalent ion concentrations significantly decreased with glucose transport. Net gain of glucose within the cell was estimated from the restraint of osmotic balance between intracellular and extracellular fluids. In contrast to the apparent decreases in intracellular Na⁺ and K⁺ concentrations, significant gains of Na⁺ and K⁺ occurred with glucose transport. The quantitative relationships among net gains of Na⁺, K⁺ and glucose during active glucose transport suggest that the coupling ratio between glucose and Na⁺ entry by the carrier mechanism on the mucosal membrane is approximately 1:1 and the coupling ratio between Na⁺-efflux and K⁺-influx of the serosal electrogenic sodium pump is approximately 4:3 in rat small intestine. In addition to the electrogenic ternary complex inflow across the mucosal cell membrane, the decreases in intracellular monovalent ion concentrations, the temporary formation of an osmotic pressure gradient across the cell membrane and the streaming potential induced by water inflow through negatively charged pores of the cell membrane in the course of an active solute transport in intestinal epithelial cells are apparently all possible causes of mucosal membrane depolarization.     

Apoplastic Solute Concentrations of Organic Acids and Mineral Nutrients in the Leaves of Several Fagaceae

Ion chromatographic methods determined organic acids and mainnutrient minerals in the apoplastic solution from leaves ofseveral Fagaceae (Quercus ilex L., Quercus cerris L., Quercusvirgiliana (Ten.) Ten, and Fagus sylvatica L.). The anions oforganic acids found in high amounts (250 to 650 µM) werequinate, malate, and oxalate. Lactate, pyruvate, formate andacetate were detected in relatively low amounts with concentrationsbetween 20 and 200 µM. The total concentration of organicacids in the apoplastic sap ranged between 1.5 and 2 mM. Thetotal concentration of inorganic cations (K⁺, Mg²⁺, NH₄⁺, Ca²⁺,Na⁺) and anions (C1^–, NO₃^–, SO^2–₄ and PO^3–₄)in the apoplastic sap varied between 5 and 10 mM, and 0.35 and1.8 mM, respectively. We conclude that the concentration oforganic acid ions in the leaf apoplast depends mainly on theexchange with the leaf cells and is influenced by the electrochemicalgradient between the symplast and the apoplast in relation tothe water potential of the leaf. The determination of formateand acetate in the apoplastic compartment of leaves lend weightto the argument that the production of these acids by treesis a important emission source to the atmosphere. (Received June 9, 1998; Accepted April 8, 1999)     

Rhizosphere Acidification by Iron Deficient Bean Plants: The Role of Trace Amounts of Divalent Metal Ions: A Study on Roots of Intact Plants with the Use of C- and P-NMR

Rhizosphere acidification by Fe-deficient bean (Phaseolus vulgaris L.) plants was induced by trace amounts of divalent metal ions (Zn, Mn). The induction of this Fe-efficiency reaction was studied by ¹⁴CO₂ and ¹¹CO₂ fixation experiments, and with ³¹P-NMR on roots of whole plants. The starting and ending of an acidification cycle was closely coupled to parallel changes in CO₂ fixation, within the maximal resolution capacity of 20 min. ³¹P-NMR experiments on intact root systems showed one peak which was ascribed to vacuolar free phosphate. At the onset of proton extrusion this peak shifted, indicating increase of pH in the cells. Proton extrusion was inhibited, with a lag period of 2 hours, by the protein synthesis inhibitors cycloheximide and hygromycin. It is assumed that Zn and Mn induce proton extrusion in Fe-deficient bean roots by activating the synthesis of a short-living polypeptide; the NMR data suggest a role for this peptide in the functioning of a proton pumping ATPase in the plasma membrane.     

A Model of Solute Transport through Stratum Corneum Using Solute Capture and Release

A one-dimensional model of solute transport through the stratum corneum is presented. Solute is assumed to diffuse through lipid bi-layers surrounding impermeable corneocytes. Transverse diffusion (perpendicular to the skin surface) through lipids separating adjacent corneocytes, is modeled in the usual way. Longitudinal diffusion (parallel to the skin surface) through lipids between corneocyte layers, is modeled as temporary trapping of solute, with subsequent release in the transverse direction. This leads to a linear equation for one-dimensional transport in the transverse direction. The model involves an arbitrary function whose precise form is uncertain. For a specific choice of this function, closed form expressions for the Laplace transform of solute out-flux at the inner boundary, and for the time lag are obtained in the case that a constant solute concentration is maintained at the outer skin surface, with the inner boundary of the stratum corneum kept at zero concentration, and with the stratum corneum initially free of solute.     

植物钾营养高效与膜运系统的关系

HKT1和HAK1等转运子介导钾离子的高亲和吸收以及K^ /Na^ 共运转,从而可能增强Na^ 替代K^ 能力,KAT1和KST1等离子通道介导钾离子的累积和转运,从而调节气孔细胞的渗透压,控制气孔运动,阐述了植物生物膜上离子转运机制和钾营养高效机理的某种可能的关系,这些转运子和通道的高效表达可能与植物钾营养高效有很大的相关性。     

植物钾营养高效与膜转运系统的关系(英)

HKT1和HAK1等转运子介导钾离子的高亲和吸收以及K+/Na+共运转,从而可能增强Na+替代K+的能力;KAT1和KST1等离子通道介导钾离子的累积和转运,从而调节气孔细胞的渗透压,控制气孔运动。阐述了植物生物膜上离子转运机制和钾营养高效机理的某种可能的关系。这些转运子和通道的高效表达可能与植物钾营养高效有很大的相关性。     

Apoplastic Transport of 14C-Photosynthates Measured under Drought and Nitrogen Supply

Using water infiltration of the plant and individual shoots with the subsequent intercellular liquid extraction by the pressure chamber, dynamics of the movement ¹⁴C-photosynthates from cell to apoplast, and ¹⁴C distribution among photosynthetic products in mesophyll cells and apoplast were studied. The relative quantity of ¹⁴C-photosynthetes in leaf apoplast depended on growing conditions; drought increased, and nitrate supply decreased it. When the middle leaves absorbed ¹⁴CO₂, photosynthates moving down in stem phloem appeared in intercellular space, where they were transported up by transpiration stream. ¹⁴C-photosynthates entering to the apex and young leaves were utilized a accumulated, and photosynthates transported to the mature leaves were reloaded into the phloem and reexported. Thus, photosynthates circulated through the plant and were redistributed to the plant organs according to their transpiration. In leaf apoplast photosynthetic sucrose was partly hydrolyzed to glucose and fructose. This increased under high nitrogen supply. The result indicate that apoplast sucrose hydrolysis is the basic cause of the reduction of photosynthate flux from leaves when the nitrate concentration in soil increases.     

Energized Uptake of Ascorbate and Dehydroascorbate From the Apoplast of Intact Leaves in Relation to Apoplastic Steady State Concentrations of Ascorbate

Abstract: Transport of ascorbate (AA) and dehydroascorbate (DHA) through the petiole into detached leaves of Lepidium sativum and other plant species via the transpiration stream, and energized uptake into leaf tissue, were measured indirectly by recording changes in membrane potential and apoplastic pH simultaneously with substrate‐stimulated respiration and transpiratory water loss. When 25 mM AA or DHA was fed to the leaves, steady state respiration at 25 °C was transiently increased by more than 50 % with AA and 70 % with DHA. Stimulation of respiration was accompanied by a transient breakdown of membrane potential followed by alkalinization of the leaf apoplast suggesting energized uptake at the expense of the transmembrane proton motive force. The average CO₂/AA ratio calculated from stimulated respiration during ascorbate uptake was 0.76 ± 0.26 (n = 17). The corresponding ratio for DHA was 1.38 ± 0.28 (n = 11). Far lower CO₂/substrate ratios were observed when NaCl or KCl were fed to leaves. The differences indicate either partial metabolism of AA and DHA in addition to energized transport, or less likely, higher energy requirement for transport of AA and DHA than for the inorganic salts. Maximum rates of energized AA transport into leaf tissue (deduced from maxima of extra respiration and calculated on the basis of CO₂/AA = 0.76) were close to 650 nmol m^‐2 leaf area s^‐1, i.e. far higher than most previously reported rates of transport. When the apoplastic concentration of AA was decreased below steady state levels during infiltration/centrifugation experiments, AA was released from leaf cells into the apoplast. This suggests that AA oxidation to DHA in the apoplast (as occurs during extracellular ozone detoxification) triggers energized transport of the DHA into the symplast and simultaneously AA release from the symplast into the apoplast, perhaps together with protons in a reversal of the energized uptake process.     

Nitrogenase Activity Measurements in Intact Plants of Alnus incana

A technique for C₂H₂-reduction assay on intact plants of Alnus incana (L.) Moench was evaluated. Cloned plants were grown, in pots, on fine gravel. During assay only the pot was inserted into a Perspex incubation chamber of simple construction. The incubation volume was rather small, plants with various shoot heights could be used, and the shoot was not exposed to the C₂H₄ produced. Intact plants showed high and constant C₂H₂-reduction rates during several hours of incubation. In comparison, excised nodulated roots conventionally incubated in test tubes showed low and decreasing rates, due to removal of the photo-synthesizing shoot and injury to the root nodules when drawn from the pot. Repeated nitrogenase activity assays on the same intact individual plants did not affect growth. The technique thus proved useful in studies. where repeated nitrogenase activity measurements are important.     

Mechanism of Zinc Absorption by Intact Bean Plants

The time course of absorption of ⁶²Zn and ³²P by roots of intactbean plants (Phaseolus vulgaris L.), and the effects of azideand amytal have been studied. The evidence indicates that zincuptake occurs mainly by a non-metabolic process.