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     

Estimating the Effects of Varying Temperature on the Rate of Development of Plants

Experiments on the effects of temperature on plant developmentusually yield measurements of the times at which recognizablestages of development are reached, and a record of the varyingtemperature patterns during development. Two complementary methodsfor analyzing such data to reveal the underlying relationshipsbetween rate of development and temperature are discussed. Inboth methods it is assumed that there exists a unique relationshipbetween current rate of plant development and current temperature. In the ‘discrete-rate’ method, the rate of developmentfunction, r(T), is approximated by a set of discrete valuesr₁, r₂...r_i...r_n, each applicable over a given temperature range.The fractional state of development of a plant, S, is writtenas the sum of the products of these rates, r_i, times the fractionof total time during development that temperature was withineach range, f_i, i.e. S = r_i. Systems of such equations can besolved for the r_i, values, and this enables an assessment ofthe rate-temperature relationship without preconceived notionson the form of the plant response. This assessment can then be used as a basis for selecting asuitable interpolation function to be fitted to the data usingthe ‘rate-function’ method. Here a mathematicalform for the function r(T) is selected at the outset. The temperaturescale is divided into discrete intervals and a set of equations,similar to the above, is developed. These equations are solvedfor best values of the parameters of the r(T) function. These methods are illustrated by application to controlled-environmentdata on times from sowing to flowering in soybeans and pigeonpeas,and on rates of leaf appearance in maize. Advantages claimed for the two methods are that they providea systematic approach for relating development to plant temperatureand that they are based upon an explicit physiological hypothesis. Phenology, Zea mays, Cajanus cajan, Glycine max     

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     

Whole Plant Studies Using Radioactive 13-Nitrogen: III. THE RELATIONSHIP BETWEEN NITRATE AND AMMONIUM ION UPTAKE AND WATER FLUX FOR ZEA MAYS

Nitrogen-13 labelled nitrate and ammonium ions were used tomeasure uptake and assimilation rates in non-destructive experimentsin which the water flux through hydroponically grown maize plantswas doubled by changing the ambient humidity. Although no immediateeffects on ion uptake were observed, longer term observationsshowed different patterns of coupling for the uptake parametersfor nitrate and for ammonium. The patterns were compared withpatterns predicted for different combinations of processes foruptake and for discharge to the xylem. Those which were compatiblewith the experimental patterns indicated that for nitrate theuptake into the root showed coupling to water flux whereas dischargeto the xylem did not. For ammonium there appeared to be couplingto the water flux of both uptake into the root and of dischargeto the xylem though it was less evident in the former when theexternal ammonium ion concentration was low. Key words: Ion-water coupling, Ammonium uptake, Nitrate uptake     

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     

Effective stomatal and boundary-layer resistances of heterogeneous surfaces

In nature surfaces are rarely uniform, so terms such as ‘surface’, ‘stomatal’ or ‘canopy’ resistance usually indicate some kind of average over a population of sub-areas, each with its own separate resistance. Questions then arise as to how gross measurements of these resistances should be interpreted in terms of the components, or how components should be aggregated into representative single values. Aggregation schemes have been published by Raupach (1991, Vegetatio 91, 105–120) and Lhomme (1992, Agriculture and Forest Meteorology 61, 11–21), but these are different for reasons that were not explained. This paper develops the idea that averaging schemes should be designed to serve particular purposes, and that they can be varied to suit these purposes. It is shown that the ‘effective’ resistances defined by Raupach and Lhomme preserve different quantities. A further averaging scheme is developed which preserves both correct transpiration rate and CO₂ flux when used in the Penman-Monteith equation and an equation describing assimilation. All of these schemes are fairly complex, so the work provides a warning against naive use of effective variables.