Studies with Radioactive Tracers in Plant Nutrition: III. THE EFFECT OF RADIATION FROM PHOSPHORUS-32 ON THE GROWTH AND PHOSPHATE UTILIZATION OF BARLEY SEEDLINGS

The effect of radiation from P³² on the growth and phosphateabsorption of barley seedlings was studied in water culture.Concentrations of carrier phosphate ranging from less than 001to 31 p.p.m. P were employed for varying periods. Between experiments wide variations occurred in the nature andextent of the effects of radiation. An increase in the proportionof the absorbed phosphate translocated to the shoots, and thereduction of root weight, were the most consistent symptoms.While severe injury was characterized by a reduction in bothgrowth and phosphate absorption, intermediate levels of P³²on occasion caused increases in dry weight, in nutrient absorption,and in the development of lateral meristems. The interrelation of the observed effects of radiation is discussed;the reduced metabolic activity of root meristems is consideredto be the primary effect in the syndrome of radiation injury.Although injury is due to the accumulation of P³² in meristematicregions, no close correlation can be shown between its extentand the level of radiation received by the meristems. The inaccuracyof present methods for estimating the levels of radiation towhich tissues are exposed has complicated the investigationsof this subject. It appears, however, that the injurious levelof radiation may vary from under 10 to over 40 rep. per daywhen the treatment period is 5 to 7 days. If active accumulationof phosphate occurs, the meristems of plant roots may receivethis dosage when treated with more than 5 µ C. of P³²per litre for periods of 6 days. The extent to which the effects of radiation may complicatethe interpretation of the results of plant physiological investigationsis considered.     

STUDIES OF THE TRIAD : II. Penetration of Tracers into the Junctional Gap

Ferritin and Imferon molecules were introduced as tracers inside "skinned" muscle fibers to test which part of the triadic junction gap is freely exchangeable with the sarcoplasm. At least 50% of the T-system surface is freely accessible from the sarcoplasm. Of the remainder, 30% of the total T-system surface is covered by the junctional feet, and 20% in the center of the junction may or may not be accessible. The possibility is discussed that the triadic junction may not function as an electrical coupling.     

Whole Plant Studies using Radioactive 13-Nitrogen: II. A COMPARTMENTAL MODEL FOR THE UPTAKE AND TRANSPORT OF NITRATE IONS BY Zea mays

Nitrate ion uptake by the roots of hydroponically grown maizeseedlings was measured using the short-lived isotope ¹³N. Itis shown to be described by a four compartment model, recognizablynitrogen in the root bathing solution, nitrogen which is readilyexchangeable from the root, nitrogen bound in the root, andnitrogen transported from the root. Some of the absorbed activity leaks back into the root bathingsolution with the efflux from the root, as a fraction of theinflux, increasing with concentration to be greater than 0–8at external nitrate ion concentrations above about 1.0 mol m^–3.The capacity of the exchangeable root pool increases with externalnitrate ion concentration, approaching the expected cytoplasmicnitrate ion content at the highest external nitrate ion concentrationsstudied (70 mol m^–3). The investigation has highlighted the problems of interpretinguptake profiles in experiments for which the 10 min half-lifeof ¹³N dictates experimental times that are comparable withthe times for saturation of root pools. Key words: Zea mays, ¹³N, Compartmental model, Nitrate uptake     

Physiological Studies in Plant Nutrition: XVI. The Mineral Nutrition of Bracken

Part I of this paper describes culture methods for the productionof large numbers of bracken sporelings of uniform size. Observationson the growth of the bracken plant and results of experimentson the mineral nutrition of the prothallial and sporophyte generationsare presented. The presentation of further growth data and afull analysis of the results is deferred to the second partof this paper.     

Measuring Fluxes of Mineral Nutrients and Toxicants in Plants with Radioactive Tracers

Unidirectional influx and efflux of nutrients and toxicants, and their resultant net fluxes, are central to the nutrition and toxicology of plants. Radioisotope tracing is a major technique used to measure such fluxes, both within plants, and between plants and their environments. Flux data obtained with radiotracer protocols can help elucidate the capacity, mechanism, regulation, and energetics of transport systems for specific mineral nutrients or toxicants, and can provide insight into compartmentation and turnover rates of subcellular mineral and metabolite pools. Here, we describe two major radioisotope protocols used in plant biology: direct influx (DI) and compartmental analysis by tracer efflux (CATE). We focus on flux measurement of potassium (K⁺) as a nutrient, and ammonia/ammonium (NH₃/NH₄⁺) as a toxicant, in intact seedlings of the model species barley (Hordeum vulgare L.). These protocols can be readily adapted to other experimental systems (e.g., different species, excised plant material, and other nutrients/toxicants). Advantages and limitations of these protocols are discussed.     

Whole Plant Studies using Radioactive 13-Nitrogen: IV. A COMPARTMENTAL MODEL FOR THE UPTAKE AND TRANSPORT OF AMMONIUM IONS BY ZEA MAYS

Presland, M. R. and McNaughton, G. S. 1986. Whole plant studiesusing radioactive 13-nitrogen IV. A compartmental model forthe uptake and transport of ammonium ions by Zea mays.—Jexp. Bot. 37: 1619–1632 Ammonium ion uptake by roots of hydroponically grown maize seedlingsand the transport of ammonium-sourced nitrogen to the shootwere measured using the short-lived isotope 13-nitrogen. Theyare shown to be described by a five compartment model—ammoniumin the root bathing solution, ammonium entering the root, nitrogenbound in the root, nitrogen outside the load region but stillwithin the root, and nitrogen in the shoot. Once taken intothe root symplasm, ammonium-sourced nitrogen was not exchangedwith the external solution. Activity-time profiles for nitrogentransported both basally and apically from the load region ofthe root implied the existence of large well-mixed pools inthe transport path, though the capacity of the pools inferredfrom modelling the profiles was greater than found by dissectingroots following loading with labelled ammonium. Key words: Ammonium uptake, Zea mays, compartmental model, 13-nitrogen     

Studies in Stomatal Behaviour: II. THE ROLE OF STARCH IN THE LIGHT RESPONSE OF STOMATA

The relationship between starch-content and aperture in thestomata of Pelargonium has been investigated by a quantitativetechnique. Heath's suggestion of an inherent diurnal rhythmin starch-content is confirmed, and the light effect which hasbeen the subject of previous contradictory reports is foundto be dependent on external humidity. When humidity is high,light (which in these experiments is confounded with reducedC0₂-content) causes a striking reduction in stomatal starch;when humidity is low, light has no effect on starch, but itseffect on aperture is unchanged. No evidence for any dependenceof aperture on carbohydrate status was obtained, and it is suggestedthat the function of carbohydrate changes in stomata is, asearlier suggested by Kisselew, the amplifying and stabilizingof changes primarily controlled by other factors.     

Studies on Plant Growth Hormones: II. FURTHER BIOLOGICAL PROPERTIES OF 3-INDOLYLACETONITRILE

1. 3-Indolylacetonitrile is more active than 3-indolylacetic acidin the Avena straight-growth test, but less active in the Avenacurvature test at comparable concentrations. Reasons for thisare discussed, and results of previous work on plant extractsusing the curvature test as a means of assay are considered.
2. Transport of both the acid and the nitrile is polar, fromapexto base of the coleoptile. The nitrile can reach the growingcells as easily, and possibly more easily, than the acid. Thesignificance of these findings for a theory on the mechanismof action of the nitrile is discussed.
3. The nitrile is inactivein the pea curvature test and straight-growthof pea stem sectionsexcept at high concentrations. It is alsoinactive or only slightlyactive in lateral bud inhibition,root initiation, and petioleabscission at the concentrationstested.
4. It is less activethan the acid in root inhibition in cress,but approximatelyas active in Avena. It is approximately asactive as the acidin parthenocarpic fruit development, andinitiation of cambialactivity.
5. The significance of these results is discussed.

     

The Role of Boron in Plant Growth: II. THE EFFECT ON GROWTH OF THE RADICLE

Estimates of cell number, cell volume, respiratory rate, nitrogen,sugar, and nucleic acid content were made on 1 mm. sectionsof the radicles of field bean at frequent intervals during thefirst 96 hours of growth in nutrient solutions with and withoutboron. The primary effects of deficiency were cessation of cell divisionand enlargement of the apical cells. The increased volume ofthe apical cells may have been due to either a longer periodbeing available for development as the rate of mitosis decreased,or to an unusually rapid rate of cell extension. The resultsindicated that cell division did not cease for lack of availablesugar nor as a result of failure to synthesize protein or nucleicacids. It is suggested that in the absence of boron divisionceases because abnormalities in the formation of the cell wallprevent the cell from becoming organized for mitosis. In particularthe hypothesis that boron is concerned with the formation of‘pectin’ from uridinediphosphate-D-glucose is examinedin the light of published evidence.     

Phosphorylated Compounds in Plants: II. THE ESTIMATION OF HEXOSE PHOSPHATES AND ADENOSINE PYROPHOSPHATES IN PLANT TISSUE 3

Modifications of the method of Slater for the estimation ofhexose phosphates and adenosine pyrophosphates in plant tissueare described. Triphosphatea of nucleosides other than adenosineinterfere with the estimation of adenosine pyrophosphates inplant tissue, but other interference is not serious if the concentrationof hexose phosphates or adenosine pyrophosphates is not lowerthan about 0.15 µmoles per ml. of tissue extract.     

Whole Plant Studies using Radioactive 13-Nitrogen: I. TECHNIQUES FOR MEASURING THE UPTAKE AND TRANSPORT OF NITRATE AND AMMONIUM IONS INHYDROPONICALLY GROWN ZEA MAYS

Methods are described for studying the uptake, by hydroponicallygrown Zea mays seedlings, of ammonium and nitrate ions labelledwith radioactive nitrogen-13, which has a half-life often minutes.For nitrate only, some of the activity absorbed by the rootexchanges back out again into the root bathing solution. Theamount of this activity is about five times too large to beattributable to exchange with ions in the root cortical apoplasm.Much of it must be transferred from the root symplasm with ahalf-time of exchange of 2–5 min. After exposing the rootto the labelled solution, equilibrium rates of transport to,and distribution in the shoot were attained within 2 min, fornitrate, or 5 min, for ammonium. The pools within the root,and the transport pathway through which the label passes musttherefore rapidly attain the specific activity of the nutrientsolution. Distribution patterns through the plant are reasonablyconsistent with earlier work on nitrogen assimilation and transport. Key words: Zea mays, Nitrate uptake, Amonium uptake, ¹³N tracer     

Studies in the Physiology of the Onion Plant: IV. THE INFLUENCE OF DAY-LENGTH AND TEMPERATURE ON THE FLOWERING OF THE ONION PLANT

At temperatures above about 17° C. inflorescence initiationin growing onion plants, as in stored sets, is suppressed whetherthe plants are kept in long or short days. Independently ofcurrent day-length and of previous day-length treatment, ifthe plants are sufficiently large initiation begins very shortlyafter the temperature falls below c. 15° C. Emerged infiorescencesappear some ten or so weeks later. Small plants are unable toinitiate inflores cences under any of the conditions tested,and actual size (perhaps leaf area) rather than leaf or nodenumber seems to be the important factor. Inflorescence emergenceis suppressed at high temperatures in short days or long days;in long days bulb formation also suppresses emergence at lowertemperatures. In long days at temperatures sufficiently lowfor bulbing to be delayed, however, emergence is accelerated.Plants which have produced bulbs in long days in the summershow a delay of inflorescence emergence in the following winter.     

Studies on Isolated Protoplasts and Vacuoles: II. THE ACTION OF GROWTH SUBSTANCES

The response of isolated protoplasts to indol-3-yl acetic acidwas investigated, and they were found to undergo a rapid water-uptakewith ultimate rupture of the plasmalemma and release of thelarge central vacuole. The use of a photomicrographic methodshowed that this response was optimal at 10^-5 M indol-3-yl aceticacid. No such response could be detected for isolated vacuoles.¹⁴C-labelled indol-3-yl acetic acid was used to obtain furtherinsight into the site of action of this growth substance. Evidenceis presented which suggests that the site of action of indol-3-ylacetic acid, for this response, is the plasmalemma, where itfacilitates an increased uptake of solutes which is followedby an osmotic water uptake.