Changes in the Morphology of Roots and Leaves of Carob Seedlings Induced by Nitrogen Source and Atmospheric Carbon Dioxide

Carob seedlings were grown hydroponically for 9 weeks under360 and 800 µl l^-1CO₂. One of two nitrogen sources, nitrateor ammonium, was added to the nutrient medium at concentrationsof 3 mol m^-3. Root systems of the developing plants suppliedwith nitrate compared to those supplied with ammonium were characterizedby:(a)more biomass on the lower part of the root;(b)fewer lateralroots of first and second order;(c)longer roots;(d)higher specificroot length;(e)a smaller root diameter. The morphology of theroot systems of nitrate-fed plants changed in the presence ofelevated carbon dioxide concentrations, resembling, more closely,that of ammonium-fed plants. Total leaf area was higher in ammonium-than in nitrate-fed plants. Nitrate-fed plants had greater totalleaf area in the presence of high carbon dioxide than in normalCO₂, due to an increase in epidermal cell size that led to developmentof larger leaflets with lower stomatal frequency. The observedchanges in the morphology of roots and shoots agreed with theresults observed for total biomass production. Nitrate-fed plantsincreased their biomass production by 100% in the presence ofelevated CO₂compared to 15% in ammonium-fed plants, indicatingthat the response of carob to high CO₂concentrations is verydependent on the nitrogen source. Under elevated CO₂, nitrate-grownplants had a larger content of sucrose in both roots and shoots,while no significant difference was observed in the contentof sucrose in ammonium-grown plants, whether in ambient or enrichedcarbon dioxide. Hence, the differences in soluble carbohydratecontents can, at least partly, account for differences in rootand shoot morphology.Copyright 1997 Annals of Botany Company Ceratonia siliquaL.; carob; ammonium; carbohydrate; carbon dioxide; nitrate; morphology; sucrose     

The Influence of NO-3 and NH+4 Nutrition on the Gas Exchange Characteristics of the Roots of Wheat (Triticum aestivum) and Maize (Zea mays) Plants

Respiratory oxygen consumption by roots was 1·4- and1·6-fold larger in NH⁺₄-fed than in NO^-₃-fed wheat (Triticumaestivum L.) and maize (Zea mays L.) plants respectively. Higherroot oxygen consumption in NH⁺₄-fed plants than in NO^-₃-fedplants was associated with higher total nitrogen contents inNH⁺4-fed plants. Root oxygen consumption was, however, not correlatedwith growth rates or shoot:root ratios. Carbon dioxide releasewas 1·4- and 1·2-fold larger in NO⁺3-fed thanin NH⁺₄-fed wheat and maize plants respectively. Differencesin oxygen and carbon dioxide gas exchange rates resulted inthe gas exchange quotients of NH^-₄-fed plants (wheat, 0·5;maize, 0·6) being greatly reduced compared with thoseof NO^-₃-fed plants (wheat, 1·0; maize, 1·1). Measuredrates of HCO^-₃ assimilation by PEPc in roots were considerablylarger in 4 mM NH⁺₄-fed than in 4 NO^-₃ plants (wheat, 2·6-fold;maize, 8·3-fold). These differences were, however, insufficientto account for the observed differences in root carbon dioxideflux and it is probable that HCO^-₃ uptake is also importantin determining carbon dioxide fluxes. Thus reduced root extension in NH⁺₄-fed compared with NO^-₃-fedwheat plants could not be ascribed to differences in carbondioxide losses from roots.Copyright 1993, 1999 Academic Press Triticum aestivum, wheat, Zea mays, maize assimilation, ammonium assimilation, root respiration     

The Uptake and Flow of C, N and lons between Roots and Shoots in Ricinus communis L.

Seedlings of Ricinus communis L. cultivated in quartz sand weresupplied with a nutrient solution containing either 1 mol m^–3NO^–₃ or 1 mol m^–3 NH⁺₄ as the nitrogen source. Duringthe period between 41 and 51 d after sowing, the flows of N,C and inorganic ions between root and shoot were modelled andexpressed on a fresh weight basis. Plant growth was clearlyinhibited in the presence of NH⁺₄. In the xylem sap the majornitrogenous solutes were nitrate (74%) or glutamine (78%) innitrate or ammonium-fed plants, respectively. The pattern ofamino acids was not markedly influenced by nitrogen nutrition;glutamine was the dominant compound in both cases. NH⁺₄ wasnot transported in significant amounts in both treatments. Inthe phloem, nitrogen was transported almost exclusively in organicform, glutamine being the dominant nitrogenous solute, but theN-source affected the amino acids transported. Uptake of nitrogenand carbon per unit fresh weight was only slightly decreasedby ammonium. The partitioning of nitrogen was independent ofthe form of N-nutrition, although the flow of nitrogen and carbonin the phloem was enhanced in ammonium-fed plants. Cation uptakerates were halved in the presence of ammonium and lower quantitiesof K⁺, Na⁺ and Ca²⁺ but not of Mg²⁺ were transported to theshoot. As NH⁺₄ was balanced by a 30-fold increase in chloride in thesolution, chloride uptake was increased 6-fold under ammoniumnutrition. We concluded that ammonium was predominantly assimilated inthe root. Nitrate reduction and assimilation occurred in bothshoot and root. The assimilation of ammonium in roots of ammonium-fedplants was associated with a higher respiration rate. Key words: Ricinus communis, nitrogen nutrition (nitrate/ammonium), phloem, xylem, transport, partitioning, nitrogen, carbon, potassium, sodium, magnesium, calcium, chloride     

The Influence of Nitrate and Ammonium Nutrition on the Growth of Wheat (Triticum aestivum) and Maize (Zea mays) Plants

The effects of NO^-₃ and NH⁺₄ nutrition on hydroponically grownwheat (Triticum aestivum L.) and maize (Zea mays L.) were assessedfrom measurements of growth, gas exchange and xylem sap nitrogencontents. Biomass accumulation and shoot moisture contents ofwheat and maize were lower with NH⁺₄ than with NO^-₃ nutrition.The shoot:root ratios of wheat plants were increased with NH⁺₄compared to NO^-₃ nutrition, while those of maize were unaffectedby the nitrogen source. Differences between NO^-₃ and NH⁺₄-fedplant biomasses were apparent soon after introduction of thenitrogen into the root medium of both wheat and maize, and thesedifferences were compounded during growth. Photosynthetic rates of 4 mM N-fed wheat were unaffected bythe form of nitrogen supplied whereas those of 12 mM NH⁺₄-fedwheat plants were reduced to 85% of those 12 mM NO^-₃-fed wheatplants. In maize supplied with 4 and 12 mM NH⁺₄ the photosyntheticrates were 87 and 82% respectively of those of NO^-₃-fed plants.Reduced photosynthetic rates of NH⁺₄ compared to NO^-₃-fed wheatand maize plants may thus partially explain reduced biomassaccumulation in plants supplied with NH⁺₄ compared to NO^-₃ nutrition.Differences in the partitioning of biomass between the shootsand roots of NO^-₃-and NH⁺₄-fed plants may also, however, arisefrom xylem translocation of carbon from the root to the shootin the form of amino compounds. The organic nitrogen contentof xylem sap was found to be considerably higher in NH⁺₄- thanin NO^-₃-fed plants. This may result in depletion of root carbohydrateresources through translocation of amino compounds to the shootin NH⁺₄-fed wheat plants. The concentration of carbon associatedwith organic nitrogen in the xylem sap of maize was considerablyhigher than that in wheat. This may indicate that the shootand root components of maize share a common carbon pool andthus differences induced by different forms of inorganic nitrogenare manifested as altered overall growth rather than changesin the shoot:root ratios.Copyright 1993, 1999 Academic Press Triticum aestivum, wheat, Zea mays, maize, nitrogen, growth, photosynthesis, amino acids, xylem     

A comparison of the impacts of various nitrogen sources on acid-base balance in C3 Triticum aestivum L. and C4 Zea mays L. plants

This work aimed to study the impacts of acquisition and assimilationof various nitrogen sources, i.e. NO₃^–, NH₄⁺ or NH₄NO₃,in combination with gaseous NH₃ on plant growth and acid-basebalance in higher plants. Plants of C₃ Triticum aestivum L.and C₄ Zea mays L. grown with shoots in ambient air in hydroponicculture solutions with 2 mol m^–3 of nitrogen source asNO₃^–, NH₄⁺ or NH₄NO₃ for 21 d and 18 d, respectively,had their shoots exposed either to 320 µg m^–3 NH₃or to ambient air for 7 d. Variations in plant growth (leaves,stubble and roots), and OH^– and H⁺ extrusions as wellas the relative increases in nitrogen, carbon and carboxylatewere determined. These data were computed as H⁺/N, H⁺/C, (C-A)/N,and (C-A)/C to analyse influences of different nitrogen sourceson acid-base balance in C₃ Triticum aestivum and C₄ Zea maysplants. Root growth in dry weight gain was significantly reduced bytreatment with 320 µg m^–3 NH₃ in Triticum aestivumand Zea mays growing with different N-forms, whereas leaf growthwas not significantly affected by NH₃. In comparison with C₃Triticum aestivum, non-fumigated C₄ Zea mays had low ratiosof OH^–/N in NO₃^–3-grown plants and of H⁺/N in NH₄⁺- and NH ₄NO₃-grown plants. Utilization of NH₃ from the atmospherereduced both the OH^–N ratios in NO₃^– -grown plantsand the H⁺/N ratio in NH₄⁺ - and NH₄NO₃ -grown plants of bothspecies. Furthermore, Zea mays had higher ratios of (C-A)/Nin NH₄⁺ - and NH₄NO₃-grown plants than Triticum aestivum. Thismeans that C₄ Zea mays had synthesized more organic anion perunit increase in organic N than C₃ Triticum aestivum plants.Within both species, different nitrogen sources altered theratios of (C-A)/N in the order: NH₄NO₃>NH₄⁺>NO₃^–.Fumigation with NH₃ increased organic acid synthesis in NO₃^–- and NH₄⁺ - grown plants of Triticum aestivum, whereas it decreasedorganic acid synthesis in Zea mays plants under the same conditions.Furthermore, these differences in acid-base regulation betweenC₃ Triticum aestivum and C₄ Zea mays plants growing with differentnitrogen sources are discussed. Key words: Acid-base balance, ammonia, ammonium, nitrate, ammonium nitrate, C₃ Triticum aestivum L., C₄ Zea mays L.     

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     

Uptake of 13C and 15N (ammonium, nitrate and urea) by Microcystis in Lake Kasumigaura

The uptake rate of carbon and nitrogen (ammonium, nitrate andurea) by the Microcystis predominating among phytoplankton wasinvestigated in the summer of 1984 in Takahamaira Bay of LakeKasumigaura. The V_max values of Microcystis for nitrate (0.025–0.046h^–1) and ammonium (0.15–0.17 h^–1) were considerablyhigher than other natural phytoplankton. The ammonium, nitrateand urea uptake by Microcystis was light dependent and was notinhibited with nigh light intensity. The K₁ values were farlower than the I_k values. The carbon uptake was not influencedby nitrogen enrichment. Microcystis accelerated the uptake rateby changing V_max/K _s value when nitrogen versus carbon contentin cells declined. Nitrate was scarcely existent in TakahamairiBay during the summer, when Microcystis usually used ammoniumas the nitrogen source. However, the standing stock of ammoniumin the water was far lower than the daily ammonium uptake rates.Therefore, the ammonium in this water had to be supplied becauseof its rapid turn-over time (–0.7–2.6 h).     

Nitrate and Ammonium Induced Photosynthetic Suppression in N-Limited Selenastrum minutum: II. Effects of NO(3) and NH(4) Addition to CO(2) Efflux in the Light

The effects of nitrate and ammonium addition on net and gross photosynthesis, CO₂ efflux and the dissolved inorganic carbon compensation point of nitrogen-limited Selenastrum minutum Naeg. Collins (Chlorophyta) were studied. Cultures pulsed with nitrate or ammonium exhibited a marked decrease in both net and gross photosynthetic carbon fixation. During this period of suppression the specific activity of exogenous dissolved inorganic carbon decreased rapidly in comparison to control cells indicating an increase in the rate of CO₂ efflux in the light. The nitrate and ammmonium induced rates of CO₂ efflux were 31.0 and 33.8 micromoles CO₂ per milligram chlorophyll per hour, respectively, and represented 49 and 48% of the rate of gross photosynthesis. Nitrate addition to cells at dissolved inorganic carbon compensation point caused an increase in compensation point while ammonium had no effect. In the presence of the tricarboxylic acid cycle inhibitor fluoroacetate, the nitrate-induced change in compensation point was greatly reduced suggesting the source of this CO₂ was the tricarboxylic acid cycle. These results are consistent with the mechanism of N-induced photosynthetic suppression outlined by Elrifi and Turpin (1986 Plant Physiol 81: 273-279).     

Physiology and Growth of Wheat Across a Subambient Carbon Dioxide Gradient

Two cultivars of wheat (Triticum aestivum L.), 'Yaqui 54' and'Seri M82', were grown along a gradient of daytime carbon dioxideconcentrations ([CO₂]) from near 350-200 µmol CO₂ mol^-1air in a 38 m long controlled environment chamber. Carbon dioxidefluxes and evapotranspiration were measured for stands (plantsand soil) in five consecutive 7·6-m lengths of the chamberto determined potential effects of the glacial/interglacialincrease in atmospheric [CO₂] on C₃ plants. Growth rates andleaf areas of individual plants and net assimilation per unitleaf area and daily (24-h) net CO₂ accumulation of wheat standsrose with increasing [CO₂]. Daytime net assimilation (P_D, mmolCO₂ m^-2 soil surface area) and water use efficiency of wheatstands increased and the daily total of photosynthetic photonflux density required by stands for positive CO₂ accumulation(light compensation point) declined at higher [CO₂]. Nighttimerespiration (R_N, mmol CO₂ m^-2 soil surface) of wheat, measuredat 369-397 µmol mol^-1 CO₂, apparently was not alteredby growth at different daytime [CO₂], but R_N /P_D of stands declinedlinearly as daytime [CO₂] and P_D increased. The responses ofwheat to [CO₂], if representative of other C₃ species, suggestthat the 75-100% increase in [CO₂] since glaciation and the30% increase since 1800 reduced the minimum light and waterrequirements for growth and increased the productivity of C₃plants.Copyright 1993, 1999 Academic Press Atmospheric carbon dioxide, carbon accumulation, evapotranspiration, light compensation point, net assimilation, respiration, Triticum aestivum, water use efficiency, wheat     

Ammonium Tolerance and Carbohydrate Status in Maize Cultivars

Four maize (Zea mays L.) hybrids were grown hydroponically for4 weeks with 20 mM ammonium or nitrate as the sole nitrogensource. Dry matter production was strongly depressed by ammoniumnutrition in the hybrid Helga relative to plants grown on nitrate,and moderately decreased in the hybrid Melina. Ammonium hadno inhibitory effect on total yield in the other two hybrids(Ramses and DK 261). The relative growth rate (RGR) of rootsand shoots of the sensitive hybrid Helga decreased significantlyunder ammonium nutrition during the first 2 weeks of the experiment,while at the end of the experiment nitrogen form had no effecton the RGR in any of the four hybrids. The strong reductionin RGR of Helga in the early seedling stage was correlated withthe accumulation of twice the concentration of free ammoniumin the shoot tissue relative to the other hybrids. Helga wastherefore unable to sufficiently detoxify ammonia in the roots.Root concentrations of water soluble carbohydrates (WSC) inHelga and Melina in the early seedling stage did not differunder ammonium and nitrate nutrition. In contrast, Ramses andDK 261 both had elevated WSC concentrations in ammonium-fedroots. It is hypothesized that a sufficient supply of carbonskeletons for ammonium assimilation in the roots is requiredfor maximum growth under high ammonium concentrations, and thatthere is genotypic variability in this physiological trait. Ammonium; carbohydrates; growth rate; maize; nitrate; roots; Zea mays L     

Effects of Root Temperature and Form of Nitrogen Nutrition on Nitrate Reductase Activity in Oilseed Rape (Brassica napus L.)

The effect of root temperature and form of inorganic nitrogensupply on in vitro nitrate reductase activity (NRA) was studiedin oilseed rape (Brassica napus L. cv. bien venu). Plants weregrown initially in flowing nutrient solution containing 10 µMNH₄NO₃ and then supplied with either nitrate or ammonium for15 d at root temperatures of 3, 7, 11 or 17 °C. Shoot temperatureregime was similar for all plants; 20/15 °C, day/night.Root NRA was highest when roots were grown at 3 and 7 °C.In laminae and petioles NRA was highest when roots were 11 or17 °C. The plants supplied with ammonium had much lowerlevels of NRA in roots after 5 d than the plants supplied onlywith nitrate. NRA in the laminae of plants supplied with ammoniumwas low relative to that in plants supplied with nitrate onlywhen root temperature was 11 or 17 °C. Values of the apparent activation energy (E_a) of NR, calculatedfrom the Arrhenius equation, in laminae and petioles were differentfrom roots suggesting difference in enzyme conformation. Evidencethat the temperature at which roots were growing affected E_awas equivocal. Oilseed rape, Brassica napus L., activation energy, ammonium, Arrhenius equation, nitrate, root temperature, nitrate reductase     

Changes in Crassulacean Acid Metabolism of Kalanchoe blossfeldiana by Different Nitrogen Sources

Kalanchoe blossfeldiana Poelln. cv. Hikan (a Crassulacean acidmetabolism (CAM) plant) was grown in pots containing soil for6 months and then cultured in nutrient solution containing 10mM nitrate or ammonium as a sole nitrogen source for 2 or 3months, under a long-day (16 h) condition. Plant growth was better in the nitrate medium. Leaves of thenitrate-grown plants showed greater diurnal fluctuations intitratable acidity and malate content than those of the ammonium-grownplants. The diurnal patterns in CO₂ exchange of nitrate-grownplants were basically similar for both groups, but the amountof net CO₂ uptake at night was twice as large in the nitrate-grownplants. The leaves of the nitrate-grown plants had 1.3 to 2.5times higher activities of phosphoenolpyruvate carboxylase (PEPC),phosphofructokinase (PFK) and NAD glycelaldehyde-3-phosphatedehydrogenase (G3PDH). These results indicate that K. blossfeldianagrown in nitrate medium showed more CAM activity than thosein ammonium medium. (Received August 13, 1987; Accepted February 22, 1988)     

Boron in Relation to Membrane Function in Higher Plants

The capacity for absorption of phosphate was shown to be reducedin Zea mays and Vicia faba suffering from boron deficiency;addition of 10^–5 M boron rapidly restored this capacity.Root tips of normal plants also responded to the addition ofboron during a short pretreatment period prior to the determinationof phosphate absorption. Comparable effects of boron on theabsorption of chloride and rubidium ions were also demonstratedin Zea mays. Specific inhibition of ion uptake by auxins maybe relevant to the mechanism of the impaired ion transport seenduring boron deficiency. The KCl-stimulated adenosine triphosphatase (ATPase) of theroots of Zea mays was also studied, with a view to examiningthe effect of boron deficiency on the activity of the enzyme.Boron-deficient roots had a reduced ATPase activity which couldbe rapidly restored by the addition of 10^–4 M H₃BO₃ 1h before extraction of the enzyme. The results suggest that the activity of specific membrane componentscan be directly influenced by boron. Possible mechanisms wherebythis control is exercised include direct interaction of boronwith polyhydroxy components of the membrane and the elevationof endogenous levels of auxins. The evidence presented stronglysupports the view that boron plays an essential role in theregulation of the functions of higher plant membranes and thatthe ATPase is a possible component of the transplant process.     

Short- and Long-Term Inhibition of Respiratory Carbon Dioxide Efflux by Elevated Carbon Dioxide

Dark carbon dioxide efflux rates of recently fully expandedleaves and whole plants of Amaranthus hypochondriacus L., Glycinemax (L.) Merr., and Lycopersicon esculentum Mill. grown in controlledenvironments at 35 and 70 Pa carbon dioxide pressure were measuredat 35 and 70 Pa carbon dioxide pressure. Harvest data and whole-plant24-h carbon dioxide exchange were used to determine relativegrowth rates, net assimilation rates, leaf area ratios, andthe ratio of respiration to photosynthesis under the growthconditions. Biomass at a given time after planting was greaterat the higher carbon dioxide pressure in G. max and L. esculentum,but not the C₄ species, A. hypochondriacus. Relative growthrates for the same range of masses were not different betweencarbon dioxide treatments in the two C₃ species, because highernet assimilation rates at the higher carbon dioxide pressurewere offset by lower leaf area ratios. Whole plant carbon dioxideefflux rates per unit of mass were lower in plants grown andmeasured at the higher carbon dioxide pressure in both G. maxand L. esculentum, and were also smaller in relation to daytimenet carbon dioxide influx. Short-term responses of respirationrate to carbon dioxide pressure were found in all species, withcarbon dioxide efflux rates of leaves and whole plants lowerwhen measured at higher carbon dioxide pressure in almost allcases. Amaranthus hypochondriacus L., Glycine max L. Merr., Lycopersicon esculentum Mill., soybean, tomato, carbon dioxide, respiration, growth     

Oxygen and carbon dioxide fluxes from barley shoots depend on nitrate assimilation

A custom oxygen analyzer in conjunction with an infrared carbon dioxide analyzer and humidity sensors permitted simultaneous measurements of oxygen, carbon dioxide, and water vapor fluxes from the shoots of intact barley plants (Hordeum vulgare L. cv Steptoe). The oxygen analyzer is based on a calciazirconium sensor and can resolve concentration differences to within 2 microliters per liter against the normal background of 210,000 microliters per liter. In wild-type plants receiving ammonium as their sole nitrogen source or in nitrate reductase-deficient mutants, photosynthetic and respiratory fluxes of oxygen equaled those of carbon dioxide. By contrast, wild-type plants exposed to nitrate had unequal oxygen and carbon dioxide fluxes: oxygen evolution at high light exceeded carbon dioxide consumption by 26% and carbon dioxide evolution in the dark exceeded oxygen consumption by 25%. These results indicate that a substantial portion of photosynthetic electron transport or respiration generates reductant for nitrate assimilation rather than for carbon fixation or mitochondrial electron transport.     

Nitrogen and Nitrate Accumulation in Species Having Different Relationships Between Nitrate Uptake and Reduction

Species differ in the relationship of nitrate reductase activityto nitrate uptake. In Capsicum annuum different diurnal patternsof leaf nitrate reductase activity and nitrate uptake have beenreported. As a consequence, the relationship of free nitratein the plant to nitrate supplied has a higher level of significancethan has reduced nitrogen to nitrate supplied. In Zea mays ithas been reported that leaf nitrate reductase activity respondsdirectly to nitrate translocation to the leaf and in this speciesthe relationship of greatest significance is reduced nitrogencontent to nitrate supplied. In both species, and also in Cucumis melo, the proportion oftotal plant free nitrate and reduced nitrogen in the roots decreases,and in the stem increases, with increasing nitrate supplied. The accumulation of free nitrate in leaves is accompanied bya quantitatively different relationship between reduced nitrogenand dry weight compared to leaves not accumulating nitrate. Capsicum annuum. L., Cucumis melo L., melon, Zea mays L., maize, sweet corn, nitrate reductase, nitrate uptake     

Maximum Quantum Yields of O2 Evolution in C4 Plants under High CO2

The quantum yields of photosynthetic O₂ evolution were measuredin 15 species of C₄ plants belonging to three different decarboxylationtypes (NADP-ME type, NAD-ME type and PEP-CK type) and 5 speciesof C₃ plants and evaluated relative to the maximum theoreticalvalue of 0.125 mol oxygen quanta^-1. At 25°C and 1% CO₂,the quantum yield in C₄ plants averaged 0.079 (differences betweensubgroups not significant) which was significantly lower thanthe quantum yield in C₃ plants (average of 0.105 for 5 species).This lower quantum yield in C₄ plants is thought to reflectthe requirement of energy in the C₄ cycle. For the C₄ NADP-MEtype plant Z. mays and NAD-ME type plant P. miliaceum, quantumyields were also measured over a range of CO₂ levels between1 and 20%. In both species maximum quantum yields were obtainedunder 10% CO₂ (0.105 O₂ quanta_-1 in Z. mays and 0.097 O₂ quanta_-1in P. miliaceum) indicating that at this CO₂ concentration thequantum yields are similar to those obtained in C₃ plants underCO₂ saturation. The high quantum yield values in C₄ plants undervery high CO₂ may be accomplished by direct diffusion of atmosphericCO₂ to bundle sheath cells, its fixation in the C₃ pathway,and feedback inhibition of the C₄ cycle by inorganic carbon. (Received June 6, 1995; Accepted August 15, 1995)     

Identification and Semi-Quantification of Gibberellins from the Pollen and Anthers of Zea mays by Immunoassay and GC/MS

Radioimmunoassays and enzyme-linked immunosorbent assays formethyl esters of gibberellins A₁, A₃, A₄, and A₇ were establishedusing an antiserum specific for GA₁-Me. The antiserum was characterizedby high titer and specificity for such C₁₉-GAs with 3ß-hydroxylgroup as GA₁, GA₃, GA₄ and GA₇. Combination of this antiserumand HPLC enabled us to identify and quantify GA, and GA₄ fromthe pollen of Zea mays with a high degree of reliability. Similarly,identification and quantification of GA₉ and GA₂₀ were alsomade possible by use of an antiserum specific for GA₂₀-Me. Combineduse of immunoassays and GC/MS enabled us to identify nine GAsfrom the pollen and four from the anthers of Zea mays. The identificationof non-13-hydroxylated GAs, such as GA₄ and GA₉, in additionto 13-hydroxylated GAs from the pollen and the anthers suggeststhat the early-non-hydroxylation pathway, as well as the early-13-hydrox-ylationpathway, operates in the male reproductive organs of Zea mays,and that the organ-specific biosynthesis and/or localizationof GAs in Zea mays is similar to that in Oryza saliva. (Received May 7, 1990; Accepted August 20, 1990)     

Nitrogen-13 Studies of Nitrate Fluxes in Barley Roots: II. EFFECT OF PLANT N-STATUS ON THE KINETIC PARAMETERS OF NITRATE INFLUX

Influx of nitrate into the roots of intact barley plants wasfollowed over periods of 1–15 min using nitrogen-13 asa tracer. Based on measurements taken over 15 min from a rangeof external nitrate concentrations (0·2–250 mmolm^–3), the kinetic parameters of influx, I_max and K_m, werecalculated. Compared with plants grown in the presence of nitrate throughout,plants that had been starved of N for 3 d showed a significantlygreater value ofI_max for ¹³N-nitrate influx (by a factor of1·4–1·8), but a similar value of K_m (12–14mmol m^–3). Pre-treating N-starved plants with nitratefor about 5 h further increased the subsequent rate of ¹³N-nitrateinflux, but had little effect in the unstarved controls. Allowingfor this induction of additional nitrate transport, the differencein rates of nitrate influx in control and N-starved plants wassufficient to account for the previously-observed differencein net uptake by the two groups of plants. In barley plants grown without any exposure to nitrate, butwith ammonium as N-source, both I_max and K_m for subsequent ¹³N-nitrateinflux were significantly decreased (by about one-half) comparedwith the corresponding nitrate-grown controls. The importance of changes in the rate of influx in the regulationof net uptake of nitrate is discussed. Key words: Ion transport, nitrate, influx, kinetic parameters, N-deficiency     

Root Morphology and Nitrogen Uptake of Maize Simultaneously Supplied with Ammonium and Nitrate in a Split-root System

We studied the response of maize (Zea mays L. cv. Anjou 256)to a simultaneous, but separated supply of ammonium and nitrate(localized supply, LS). A split-root system was used to supplyhalf of the roots with ammonium and the other half with nitrate.A homogeneously distributed supply of both nitrogen forms (HS)was the control treatment. Seedlings were grown for 12 d fromthe two-leaf to the three-leaf stage in hydroponics at threepH levels (4, 5·5 and 7). The total N concentration was3 mol m^-3. The split-root system was established by removingthe seminal root system and using only four nodal roots perplant. Total root length and root surface area were recordedautomatically with a modified Delta- T area meter. Other morphologicalroot traits (such as main axis length and diameter, number,density, and length of laterals) were recorded manually. Uptakeof ammonium and nitrate was measured by the depletion of thenutrient solution. As compared with LS, HS was superior in shootand root DM, total root length and root surface area, ammoniumand nitrate uptake and shoot nitrogen concentration, irrespectiveof pH level. This indicates that, also under field conditions,mixed ammonium and nitrate fertilization is only beneficialto plant growth if both N forms are evenly distributed in thesoil. At both HS and LS, ascending pH increased the ammonium:nitrateuptake ratio. At LS, declining pH induced a considerable shiftin the distribution of root DM, root length, and root surfacearea the nitrate-fed compartment.Copyright 1993, 1999 AcademicPress Maize, Zea may L., ammonium, nitrate, pH, root morphology, split-root