Growth Responses of Tomato Plants in Non-aerated Water Culture to Foliar Sprays of Gibberellic Acid and Benzyladenine

Tomato plants were grown to the five-six leaf stage in aeratedwater culture and aeration then discontinued. Foliar spraysof gibberellic acid (G), N⁶ benzyladenine (B), and ammoniumnitrate (N) were applied for periods up to 7 days and the plantsharvested on the eighth day. A mixture of 2·5 ppm G,5·0 ppm B, and 280 ppm N increased growth of the wholeplant and this was attributable almost entirely to G and B.In the leaf, both increased water content (B>G) and dry weight.G, but not B, increased leaf area; B, but not G, increased freshweight leaf/unit area. In the stem, G increased dry weight andheight; B reduced height but increased diameter and water content.Inclusion of O·I ppm indol-3yl-acetic acid in the mixturewas ineffective. The percentage increase in growth of non-aerated plants withG+B+N was greater with non-acrated than with aerated plantsfor weight of whole plant and leaf area, but not for stem height.Growth of non-aerated plants could not be increased by increasingthe volume of the nutrient solution, by ‘aerating’with nitrogen, or by applying minerals in foliar sprays.     

Efficient rooting for establishment of papaya plantlets by micropropagation

A low cost micropropagation protocol to produce high quality root systems which are easy and economical to acclimatize is essential for large-scale micropropagation of papaya (Carica papaya L.). In this study, individual shoots (>0.5 cm) with 2_∼3 leaves from in vitro papaya multiple shoots were cultured on MS agar medium containing 2.5 μM IBA under dark conditions for 1 week for root induction. They were then transferred to agar or vermiculite media, containing half strength MS medium, under aerated or non-aerated conditions, for root development. Rooting percentage of shoots cultured for 2 weeks in aerated vermiculite was 94.5%, compared with 90.0% in non-aerated vermiculite, 71.1% in aerated agar, and 62.2% in non-aerated agar. Shoots with roots were acclimated in vermiculite under 100% RH for 1 week and then under ambient conditions for 2 weeks in a temperature-controlled growth chamber (28 °C). The survival rates of the plantlets were 94.5% from aerated vermiculite, 87.8% from non-aerated vermiculite, 42.2% from aerated agar, and 35.6% from non-aerated agar. Thus, root induction in low-concentration IBA agar medium followed by root development in vermiculite containing half strength MS medium under aerated conditions results in efficient rooting of in vitro papaya shoots. This revised version was published online in June 2006 with corrections to the Cover Date.     

The Influence of Nitrogen Supply on the Uptake and Remobilization of Stored N for the Seasonal Growth of Apple Trees

M26 apple rootstocks were grown in sand culture and suppliedwith three rates of nitrogen (N) with the irrigation: none,0·8 mol N m^–2 or 8·0 mol N m^–2. Allthe N supplied to the trees was labelled with ¹⁵N at 5·0atom percent enrichment. The effect of N supply on tree growth,N uptake and the remobilization of N from stems for the annualgrowth of the trees was measured. Increasing the N supply increasedleaf growth, but had no effect upon root mass and so alteredthe root/leaf dry matter ratio Plants receiving no fertilizer N had to rely entirely upon storedreserves of N for their seasonal growth. Initially this N wasused for leaf growth, which stopped after a few weeks. Thereafterthe N-deficient plants retranslocated some of the N from theirleaves to support root growth. Increasing the N supply had littleeffect upon the amount of N remobilized for growth, althoughwell-fertilized plants accumulated N in their leaves and didnot retranslocate any to support root growth. The partitioningof N between roots and shoots was, therefore, altered by increasingthe N supply. Amino acid analysis of stems showed that the majorforms of N remobilized during growth were protein rich in asparagineand arginine The results show the importance of internal N cycling for thegrowth of young apple trees, and are discussed in relation toother studies of N cycling in deciduous trees Malus domestica Borkh., nitrogen, remobilization, growth, partitioning, storage     

Apple dwarfing rootstocks and interstocks affect the type of growth units produced during the annual growth cycle: precocious transition to flowering affects the composition and vigour of annual shoots

Background and Aims: Precocious flowering in apple trees is often associated witha smaller tree size. The hypothesis was tested that floral evocationin axillary buds, induced by dwarfing rootstocks, reduces thevigour of annual shoots developing from these buds comparedwith shoots developing from vegetative buds. Methods: The experimental system provided a wide range of possible treevigour using ‘Royal Gala’ scions and M.9 (dwarfing)and MM.106 (non-dwarfing) as rootstocks and interstocks. Second-yearannual shoots were divided into growth units corresponding toperiods (flushes) of growth namely, vegetative spur, extensiongrowth unit, uninterrupted growth unit, floral growth unit (bourse)and extended bourse. The differences between the floral andvegetative shoots were quantified by the constituent growthunits produced. Key Results: The dwarfing influence was expressed, firstly, in reduced proportionsof shoots that contained at least one extension growth unitand secondly, in reduced proportions of bicyclic shoots (containingtwo extension growth units) and shoots with an uninterruptedgrowth unit. In treatments where floral shoots were present,they were markedly less vigorous than vegetative shoots withrespect to both measures. In treatments with M.9 rootstock,vegetative and floral shoots produced on average 0·52and 0·17 extension growth units, compared with 0·77extension growth units per shoot in the MM.106 rootstock treatment.Remarkably, the number of nodes per extension growth unit wasnot affected by the rootstock/interstock treatments. Conclusions: These results showed that rootstocks/interstocks affect thetype of growth units produced during the annual growth cycle,reducing the number of extension growth units, thus affectingthe composition and vigour of annual shoots. This effect isparticularly amplified by the transition to flowering inducedby dwarfing rootstocks. The division of annual shoot into growthunits will also be useful for measuring and modelling effectsof age on apple tree architecture.     

Kinetic model for growth of Phaeodactylum tricornutum in intensive culture photobioreactor

A kinetic model has been developed to estimate the specific growth rate of Phaeodactylum tricornutum in batch cultures. The cultures were carried out in a laboratory scale photobioreactor. Some factors like pH, temperature and irradiance were studied. In the first case, an optimum pH of 7.8 and a specific growth rate of 0.064 h⁻¹ were achieved for certain nitrate conditions and illumination. The temperature influence has been modelled by a modified Sinclair model. The optimum temperature was achieved at 20.4 °C in aerated cultures and at 22.3 °C in non-aerated cultures. Better adaptation to low temperatures than high ones has been obtained. The experiments carried out with different irradiances drive to a simple Monod's equation for the irradiance influence on growth, with semi-saturation irradiance of 10.2 μEinstein⁻² s⁻¹ in aerated cultures and of 6.8 μEinstein m⁻² s⁻¹ in non-aerated cultures.     

Studies in the Nutrition of Apple Rootstocks The Effect on Growth and Mineral Composition of Supplying Iron Through the Leaves

Apple rootstocks were grown with either 0.02 ppm Fe (Fe₀) or5 ppm (Fe₃), to give very chlorotic or dark-green plants. Toinvestigate whether iron can be supplied through leaves insteadof roots the shoots of half the plants in each treatment weredipped periodically in solutions of iron. This prevented chlorosisin Fe₀ plants and increased their growth, which did not, however,equal that of Fe₃ plants supplied with iron through the roots.Growth of Fe₃ plants was reduced by dipping. Iron was not translocated from leaves to roots, although theconcentration in leaves was greatly increased by dipping. Dippingreduced the amount of manganese in Fe₀ roots to one-quarterof that in roots of undipped Fe₀ plants. Effects of treatmentson nitrogen, potassium, calcium, magnesium, and copper levelsare also described.     

Impact of Plant Nutrition on Pratylenchus penetrans Infection of Prunus avium Rootstocks

A hypothesis that cherry rootstocks grown under optimal nutrient conditions are affected less by Pratylenchus penetrans infection than those grown under deficient nutrient conditions was tested by growing four Prunus avium L. rootstocks (''Mazzard'', ''Mahaleb'', ''GI148-1'', and ''GI148-8'') at a soil pH of 7.0 over a period of 3 months under greenhouse conditions (25 ñ 2 °C). Pratylenchus penetrans was inoculated at 0 (control) or 1,500 nematodes per g fresh root weight for a total of 3,600, 4,200, 10,500, and 11,400 per plant on Mazzard, Mahaleb, GI148-1, and GI148-8, respectively, with nutrients (commercial fertilizer) applied once at planting (deficient) or twice weekly (optimal). The experiment was repeated once. The optimum nutrient regime resulted in greater soil nutrient levels and plant growth; higher leaf concentrations of N, P, K, and Mg; and fewer P. penetrans than under the deficient nutrient regime. The addition of fertilizer either may increase nematode mortality in the soil or improve rootstock resistance to nematode infection. Increases in Ca in leaves from the nutrient-deficient and nematode-infected treatments suggested the plants were physiologically stressed. The Pf/Pi ratios indicated that these rootstocks may have had resistance to P. penetrans; however, because of the dominant role of nutrition in the experimental design, the question of resistance could not be properly addressed.     

Soil Solution Chemistry Controlling the Field Distribution of Melica ciliata L.

Germination, establishment and growth of Melica ciliata L.,an 'acidifuge' species of rocky habitats in Europe, were studiedexperimentally and related to chemical properties of the soilsolution, including pH, base cation composition, Al concentrationand speciation, and Mn concentration. M. ciliata failed to establishin acid leptite soil. However, it was able to grow in solutionat pH 3·6 and exhibited vigorous growth at pH 3·9,a typical soil solution pH of leptite sites, which Melica isunable to colonize. Higher concentrations of Mn than those measuredin any leptite soil solutions did not influence growth. Exposureto 0·037 mmol l^-1 (1 mg l^-1) of Al³⁺, a concentrationusually exceeded in the soil solution of leptite sites, severelyretarded root growth. Speciation technique applied to Al insoil solutions obtained by centrifugation demonstrated a closerelationship between H⁺ and Al concentrations and, in particular,between H⁺ and free ionic (quickly reacting) Al species. Soilsolution concentrations of free ionic Al proved to be < 0·002mmol l^-1 in sites lacking Melica , but often > 0·10mmol l^-1 in site lacking Metalica . It is concluded that theinability of M. ciliata to establish in acid soils is not primarilydue to the high H⁺ concentration but to the high soil solutionconcentrations of Al, especially free Al ionic species at lowsoil pH.Copyright 1993, 1999 Academic Press Melica ciliata, distribution, soil solution, pH, aluminium speciation, manganese, base cations, iron     

Effects of Humate, Agar-agar, and EDTA on the Development of Tomato Seedlings in Aerated and Non-aerated Water Cultures

The effect of sodium humate on the development of tomato seedlingsin aerated and non-aerated water cultures has been comparedwith the effect of colloidal solutions of agar-agar. Similarcomparisons have been made between the activities of humateand EDTA. The experiments have been conducted in fresh mediaas well as in nutrient solutions that had been formerly usedfor growing other tomato seedlings. Finally, a comparison betweeneffects of humate and EDTA on tomato seedlings growing in nutrientsolutions with various concentrations of calcium, magnesium,and iron salts in aerated and non-aerated water cultures hasbeen carried out. Media used for growth experiments have beentested microbiologically, and qualitative analyses of theirorganic compounds have been made. A distinct relationship between physiological activities ofhumate, EDTA, and the concentrations of calcium, magnesium,and iron cations in the medium has been found. It has also beenshown that in non-aerated media the plants mostly anifer thedeficiency of available iron; this is prevented by the additionof either humate or EDTA.     

Subcellular Distribution of Inorganic Phosphate, and Levels of Nucleoside Triphosphate, in Mature Maize Roots at Low External Phosphate Concentrations: Measurements with 31P-NMR

Maize plants were grown in nutrient solution without phosphate,or in which inorganic phosphate (Pi) was maintained at nearlyconstant concentrations of 1 µM, 10µM or 0·5mM. In vivo ³¹P-NMR measurements showed that there was no discernibledifference in the cytoplasmic Pi content (µmol cm^–3root volume) of the mature roots of plants exposed to 1 µM,10µM or 0·5 mM external phosphate for up to 12d. However, the vacuolar Pi content of the mature roots variedabout 10-fold between these three groups. The cytoplasmic Pi content of roots receiving no external phosphatedecreased significantly after about 7 d total growth, and atabout this time the vacuolar pool of Pi became too small foraccurate measurement. The presence of 1 µM Pi in the nutrientsolution completely prevented this decline in cytoplasmic Pi,and there was some evidence that it also raised the Pi contentof the root vacuoles above the almost undetectable level foundin the totally P-starved roots. During the first 7–9 d of growth, the nucleoside triphosphatecontent of the mature roots was unaffected by the concentrationof phosphate in the nutrient solution. The results highlight the close control of cytoplasmic concentrationsof certain important phosphorus metabolites in roots growingin soil of normal agricultural fertility. Key words: Vacuole, cytoplasm, intracellular compartmentation, NTP, P-nutrition     

Relationship between Coleoptile Elongation and Alcoholic Fermentation in Rice Exposed to Anoxia. II. Cultivar Differences

The relationship between coleoptile elongation and survivalvs. alcoholic fermentation of rice under anoxia is examinedusing eight cultivars differing in submergence tolerance. Anoxiawas imposed on either 1 or 4 d aerated seeds either by N₂ flushingsubmerged tissues or by incubating tissues in stagnant deoxygenatedagar at 0·1% w/v; the latter simulated the stagnant conditionsof waterlogged soil. Two cultivars that were most tolerant tosubmergence also had the greatest tolerance to anoxia, whilea submergence intolerant cultivar was also intolerant to anoxia. Coleoptile growth under anoxia was related to rates of ethanolsynthesis (R_E), however differences between growth during anoxiaand survival after anoxia indicated that post-anoxic injurymay also be important in rice seeds exposed to anoxia. The correlationbetween coleoptile growth and R_E measured on a tissue basisusing intact seeds was r² = 0·67 among six varietiesover 0-3 d anoxia. This correlation improved to about r² = 0·85using R_E of (embryos plus coleoptiles) over 0-3 d, or coleoptilesat 3 d after anoxia. Coleoptile growth of individual seeds wasusually poorly correlated to R_E in these cultivars at 2-3 dafter anoxia. When coleoptiles of similar lengths were obtainedfrom different cultivars using 4 d aerated seeds, there weredifferences in R_E and coleoptile growth which were related tocoleoptile growth during 3 or 5 d anoxia, either on a tissue(r² = 0·85) or a fresh weight basis (r² = 0·70-0·97respectively). Results are discussed in relation to factorswhich may limit ethanol synthesis in rice exposed to anoxiaand the importance of growth to the survival of seeds and matureplants during submergence in the natural environment.Copyright1994, 1999 Academic Press Anoxia, ethanol, alcoholic fermentation, Oryza sativa L., rice, submergence     

Agrobacterium rhizogenes Mediated Transformation of the Forage Legumes Medicago sativa and Onobrychis viciifolia

Three cultivars of M. sativa and one cultivar of O. viciifoliawere evaluated for their response to inoculation with A. rhizogenesstrain A4T (containing pRiA4b). A cultivar-dependent responsewas observed in M. sativa with 94%, 25%, and 4% of infectedstem explants producing transformed roots in the cultivars Vertus,Regen-S, and Rangelander, respectively. In O. viciifolia cv.Hampshire Giant, an explant-dependent response was observedwith 78% and 50% of seedling cotyledon and hypocotyl explantsresponding, respectively. Leaf explants failed to produce transformedroots. Transformed roots showed plagiotropic and negativelygeotropic growth on hormone-free agar MS medium. Productionof transgenic shoots from O. viciifolia root cultures occurredspontaneously. Recovery of transgenic plants from M. salivacv. Rangelander was achieved by transfer of callus (inducedon UM medium containing 2·0mg dm^–3 2,4-D and 0·25mg dm^–3 kinetin) to MS medium containing 0·5 ingdm^–3 BAP and 0·05 mg dm^–3 NAA. Cultured rootsof both species synthesized opines (agropine and mannopine).Extensive morphological variation was observed in plants ofM. sativa (clone Al) and O. viciifolia (clone A4Tl) establishedin the glasshouse. DNA sequences homologous to TL-DNA and TR-DNAwere present in root clones and regenerated plants. Key words: Agrobacterium rhizogenes, Medicago sativa, Onobrychis viciifolia, transformed roots, transgenic plants     

The Effect of Moisture Stress on Infection of Trifolium subterraneum L. by Rhizobium trifolii Dang

Moisture stress and method of inoculation greatly affected thenumber and distribution of infected root hairs and nodules ofyoung seedlings of Trifolium subterraneum. A reduction of soilmoisture from 5·5 to 3·5% (–0·36to –3·6 x 10⁵ Pa) significantly decreased the numberof infection threads and completely inhibited nodulation, althoughthe number of rhizobia in the rhizosphere was unaffected. Atlow soil moisture levels the root hairs were abnormally shortand swollen. Infection and nodulation were little affected between5·5 and 9·5% moisture (–0·36 to –0·089x 10⁵ Pa). Distribution of infected root hairs depended on the initialplacement of the inoculum; with the inoculum mixed evenly throughthe soil, infection threads occurred at discrete foci alongthe root. With seedlings inoculated at planting, infection threadswere restricted to the top 1–2 cm of root, even at thehighest soil moisture tested. Watering increased the number of infections in plants grownat 3·5% moisture; nodules were formed at a rate equivalentto non-stressed plants. Watering also enabled movement of theseedling-borne inocula; new infections were formed along theroot surface bearing mature root hairs.     

Studies in the Nutrition of Apple Rootstocks: IV. The Effect on Growth and Mineral Coomposition of Supplying Magnesium through the Leaves

One-year rooted shoots of M.VII apple rootstock were grown fora single season by spraying their roots continuously with nutrientsolutions containing either < 3 ppmMg(Mg₍₀₎) or 45 ppm(Mg₍₀₎)to give, respectively, potentially very deficient or healthyplants. The new shoots of half the plants in each of these treatmentswere dipped periodically in a 2 per cent solution of MgSO₄.₇H₂O plus ‘wetter’. Mg₍₀₎ undipped plants developed severe symptoms of Mg deficiency,growth was poor, and the shoot/root dry-weight increment ratiowas high; none of these characteristics was found in Mg₍₀₎ dippedplants, whose growth was not appreciably less than that of Mg₍₂₎undipped controls. There was little translocation of Mg from leaves to roots: theconcentration of Mg in roots of Mg(0) dipped plants was as lowas that of the undipped. The large accumulation of Mn and, toa less extent, of Fe in Mg₍₀₎ dipped roots was not apparentlydetrimental to growth. Growth of Mg⁽²⁾ dipped plants was similar to that of the undipped. Dipping had little effect on the chemical composition of leaves,except to raise the concentration of Mg.     

The Effect of Artificial Wind on the Growth-rate of Plants in Water Culture: With Plate and three Figures in the Text

Young plants of Brassica napus (rape), Hordeum vulgare, andPisum sativum growing in water culture have been exposed tofour continuous wind speeds for from 4 to 5 days. The plantswere exposed to natural daylight, humidity, and temperaturein a wind tunnel in which only air movement was controlled.The wind speeds found among the plants in the four sectionsof this tunnel were approximately 0·3, 0·7, 1·7,and 4·0 m./sec. The results showed no significant changein relative growth-rate or net assinilation rate with wind speed. Previously published results, obtained with plants in soil orsand culture, have differed from those in the present experimentsin showing a fall in the amount of growth as wind speed increased.It is suggested that the reduction in growth found in theseprevious experiments was caused by partial drying out of theplants, whilst in the present experiments water-supply to theroots was abundant and this effect was considerably reduced.     

ROLE OF SETTLEMENT DENSITY ON GAMETOPHYTE GROWTH AND REPRODUCTION IN THE KELPS PTERYGOPHORA CALIFORNICA AND MACROCYSTIS PYRIFERA (PHAEOPHYCEAE)1

Laboratory studies were used to examine how variation in the density of spore settlement influences gametophyte growth, reproduction, and subsequent sporophyte production in the kelps Pterygophora californica Ruprecht and Macrocystis pyrifera (L.) C. Ag. In still (non-aerated) cultures, egg maturation in both species was delayed when spores were seeded at densities 300 · mm^?2. Although the density at which this inhibition was first observed was similar for both species, the age at which their eggs matured was not. P. californica females reached sexual maturity an average of 4 days (or ～ 30%) sooner than did M, pyrifera. As observed previously in field experiments, per capita sporophyte production was negatively density dependent for both species when seeded at spore densities of 10 · mm^?2. Total sporophyte production (i.e. number · cm^?2) for both species, however, was greatest at intermediate densities of spore settlement (～ 50 spores · mm^?2). In contrast, total sporophyte production by P. californica steadily increased with increasing spore density in aerated cultures; highest sporophyte density was observed on slides seeded at a density of 1000 spores · mm^?2. Preliminary experiments with P. californica involving manipulation of aeration and nutrients indicate that inhibition of gametophyte growth and reproduction at higher densities of spore settlement in non-aerated cultures was probably caused by nutrient limitation.     

Effects of lanthanum and cerium on the growth and mineral nutrition of corn and mungbean

Background and Aims: Plant growth responses to the rare earth elements lanthanum(La) and cerium (Ce) have been reported, but little is knownabout the effects of these two elements on plant mineral nutrition. Methods: Corn (Zea mays ‘Hycorn 82’) and mungbean (Vignaradiata ‘Berken’) were grown in continuous flowingnutrient solutions containing 0, 0·2, 1·0 and5·0 µM La or Ce. At harvest plants were dividedinto roots and shoots, dried, weighed and analysed for macro-and micronutrients, as well as for La and Ce. Key Results: La and Ce did not increase the growth of corn or mungbean. Thedry weight of corn shoots was decreased by 32 % in the presenceof 5·0 µM Ce; the other La and Ce concentrationshad no effect. La and Ce concentrations of 0·9 and 5·0µM decreased the shoot dry weight of mungbean by 75 or95 %, the two elements having closely similar effects. Decreasesin the uptake of Ca, Na, Zn and Mn by corn were observed withincreases in solution La and Ce. For mungbean, the uptake ratesof all measured elements decreased with increases in solutionLa and Ce. The concentrations of La and Ce in the roots of bothspecies were higher than in the shoots and increased stronglywith increasing concentrations of La or Ce in solution. TheLa and Ce concentrations in mungbean shoots were always higherthan in corn shoots. Conclusions: La and Ce did not enhance the growth of corn or mungbean, butdecreased the growth, root function and consequently the nutritionalstatus of mungbean at concentrations >0·2 µMin solution. It is concluded that if La or Ce have positiveeffects on corn and mungbean growth, they can only occur atsolution concentrations below 0·2 µM.     

Sprouting and Growth of Purple Nutsedge, Cyperus rotundus, in Relation to pH and Aeration

The results of experiments on the sprouting and growth of purple nutsedge (Cyperus rotundus L.) in relation to pH and aeration are described. All pH levels between 2.2 to 9.0 allowed high sprouting percentage but the sprouted tubers did not survive at pH below 3 and survival beyond pH 7 was very poor. Post-sprouting growth of nutsedge was optimal at pH 4. Pre-sprouting aeration of tubers stimulated early sprouting and produced vigorous shoots, whereas submersion of the tubers was inhibitory. Continuous aeration of the submerged tubers substantially removed the inhibitory influence on sprouting but the shoots remained weak. Seedlings grown in constantly aerated nutrient solution showed significantly better growth than in a non-aerated one.     

Nitrogen Utilization in N-limited Barley During Vegetative and Generative Growth: III. POST-ANTHESIS KINETICS OF NET NITRATE UPTAKE AND THE ROLE OF THE RELATIVE ROOT SIZE IN DETERMINING THE CAPACITY FOR NITRATE ACQUISITION

Barley (Hordeum vulgare L., cvs Golf, Mette, and Laevigatum)was grown under nitrogen limitation in solution culture untilnear maturity. Three different nitrogen addition regimes wereused: in the ‘HN’ culture the relative rate of nitrate-Naddition (RA) was 0·08 d^–1 until day 48 and thendecreased stepwise to, finally, 0·005 d^–1 duringgrain-filling; the ‘LN’ culture received 45% ofthe nitrogen added in HN; the ‘CN’ culture was maintainedat RA 0·0375 d^–1 throughout. Kinetics of net nitrateuptake were measured during ontogeny at 30 to 150 mmol m^–3external nitrate. V_max (which is argued to reflect the maximuminflux rate in these plants) declined with age in both HN andLN cultures. A pronounced transient drop was observed just beforeanthesis, which correlated in time with a peak in root nitrateconcentration. Similar, but less pronounced, trends were observedin CN. The relative V_max (unit nitrogen taken up per unit nitrogenin plants and day) in all three cultures declined from 1·3–2·3d^–1 during vegetative growth to 0·1–0·7d^–1 during generative growth. These values are in HN andLN cultures 15- to more than 100-fold in excess of the demandset by growth rates throughout ontogeny. Predicted balancingnitrate concentrations (defined as the nitrate concentrationrequired to support the observed rate of growth) were below6·0 mmol m^–3 in HN and LN cultures before anthesisand then decreased during ontogeny. In CN cultures the balancingnitrate concentration increased during grain-filling. Apartfrom the transient decline during anthesis, most of the effectof ageing on relative V_max can be explained in terms of reducedcontribution of roots to total biomass (R:T). The loss in uptakeper unit root weight is largely compensated for by the declinewith time in average tissue nitrogen concentrations. The quantitativerelationships between relative V_max and R:T in ageing plantsare similar to those observed for vegetative plants culturedat different RAs. The data support the contention that the capacity for nitrateacquisition in N-limited plants is under general growth control,rather than controlled by specific regulation of the biochemicalpathway of nitrate assimilation. Key words: Barley, nitrogen concentration, root: total plant biomass ratio, V_max     

Nitrate-Ammonium Ratio Required for pH Homeostasis in Hydroponically Grown Soybean

Imsande, J. 1986. Nitrate-ammonium ratio required for pH homeostasisin hydroponically grown soybean.—J. exp. Bot. 37: 341–347. Plant acid-base homeostasis is achieved when the mmoles of hydroxylions produced in the plant equal the mmoles of protons. Reductionof nitrate to ammonia is the major source of hydroxyl ions whereasammonium uptake-assimilation and the metabolism of neutral sugarsto organic acids are the primary sources of protons. Soybean[Glycine max (L.) Merr plants were grown hydroponically on mediumsupplemented with 3·0 mol m^–3 nitrogen providedas various combinations of KNO₃ and NH₄NO₃ Plant growth consumedessentially all available nitrogen in each case; however, onlyin flasks supplemented with approximately 1·8 minolesof KNO₃ plus 0·6 mmole of NH₄NO₃ was the pH of the mediumunchanged. Thus, for every mmole of nitrogen assimilated, approximately0·6 mmole of dissociable protons must have been producedby the conversion of neutral sugars to carboxylic acids. Also,it was shown that a plant obtaining all of its nitrogen fromnitrate must neutralize or excrete approximately 0·5mmole of hydroxyl ion d^–1. Conversely, the plant derivingall of its nitrogen from dinitrogen must excrete or neutralizeat least 0·8 mmole of hydrogen ion d^–1 whereasthe plant deriving all of its nitrogen from aminonium must excreteor neutralize approximately 2·1 mmoles of hydrogen iond^–1. Nevertheless, plants grown on medium supplementedwith 2·4 mol m^–1 nitrate plus 0·6 mol m^–3ammonium did not achieve a higher growth rate than plants grownon 3·0 mol m^–3 nitrate. Key words: Glycine max, nitrogen fixation, nitrate utilization