Acid phosphatase role in chickpea/maize intercropping

• Background and aims Organic P comprises 30–80 %of the total P in most agricultural soils. It has been proventhat chickpea facilitates P uptake from an organic P sourceby intercropped wheat. In this study, acid phosphatase excretedfrom chickpea roots is quantified and the contribution of acidphosphatase to the facilitation of P uptake by intercroppedmaize receiving phytate is examined. • Methods For the first experiment using hydroponics, maize(Zea mays ‘Zhongdan No. 2’) and chickpea (Cicerarietinum ‘Sona’) were grown in either the sameor separate containers, and P was supplied as phytate, KH₂PO₄at 0·25 mmol P L^–1, or not at all. The second experimentinvolved soil culture with three types of root separation betweenthe two species: (1) plastic sheet, (2) nylon mesh, and (3)no barrier. Maize plants were grown in one compartment and chickpeain the other. Phosphorus was supplied as phytate, Ca(H₂PO₄)₂at 50 mg P kg^–1, or no P added. • Key results In the hydroponics study, the total P uptakeby intercropped maize supplied with phytate was 2·1-foldgreater than when it was grown as a monoculture. In the soilexperiment, when supplied with phytate, total P uptake by maizewith mesh barrier and without root barrier was 2·2 and1·5 times, respectively, as much as that with solid barrier.In both experiments, roots of both maize and chickpea suppliedwith phytate and no P secreted more acid phosphatase than thosewith KH₂PO₄ or Ca(H₂PO₄)₂. However, average acid phosphataseactivity of chickpea roots supplied with phytate was 2–3-foldas much as maize. Soil acid phosphatase activity in the rhizosphereof chickpea was also significantly higher than maize regardlessof P sources. • Conclusions Chickpea can mobilize organic P in both hydroponicand soil cultures, leading to an interspecific facilitationin utilization of organic P in maize/chickpea intercropping.     

Structure and Function of an Age Series of Eucalypt Plantations in Central Himalaya. I. Dry Matter Dynamics

The biomass and net primary productivity (NPP) of 2- to 8-year-oldplantations of Eucalyptus tereticornis Sm. (= E. hybrid) growingin the tarai (a level area of superabundant water) region ofCentral Himalaya were estimated. Allometric equations for allthe above-ground and below-ground components of trees and shrubswere developed for each stand. Understorey, forest floor biomassand litter fall were also estimated from stands. Shrubs appearedfirst at 5-year-old plantation. The biomass of vegetation, forestfloor littermass, tree litter fall and net primary productivity(NPP) of trees and shrubs increased with the increase in plantationage, whereas herb biomass and NPP significantly (P < 0·01)decreased with the increase in plantation age. The total plantationbiomass increased from 7·7 t ha^–1 in the 2-year-oldto 126·7 t ha^–1 in the 8-year-old plantation andNPP from 8·6 t ha^–1 year^–1 in the 2-year-oldto 23·4 t ha^–1 year^–1 in the 8-year-old plantation.The biomass accumulation ratio ranged from 0·81 to 5·93. Eucalyptus tereticornis Sm, plantation, biomass, forest floor, litter fall, net primary productivity, biomass accumulation ratio     

Twig and foliar nutrient concentrations in relation to nitrogen and phosphorus supply in a eucalypt (Eucalyptus diversicolor F. Muell.) and an understorey legume (Bossiaea laidlawiana Tovey and Morris)

The sensitivity of tissue nutrient concentrations to changes in plant age and the supply of P and N was compared between leaves and associated twigs in two forest species. In a young regrowth stand, tissues were sampled on three occasions from the mid-crown position of karri (Eucalyptus diversicolor F. Muell.) and Bossiaea laidlawiana Tovey and Morris, a major understorey legume. Leaves and twigs were also sampled from young plants of B. laidlawiana growing in a mature eucalypt stand to which P treatments had been applied.Nitrogen application increased N concentrations in twigs of karri and B. laidlawiana, but not in leaves. Phosphorus application increased P concentrations in both leaves and twigs of karri but the average increases were proportionally greater in twigs (65%) than in leaves (36%). Over the sampling period, P concentrations in leaves declined, while those in twigs were relatively stable. In B. laidlawiana, P supply also had a larger effect on P concentrations in twigs than in leaves. Addition of 200 kg P ha^–1 increased average P concentrations in twigs by 109% in the regrowth stand and by 215% in the mature stand while the corresponding increases in leaves were only 11% and 27%. Concentrations of other nutrients in both species were also affected by N and P application, the most notable being a decline in the concentrations of the minor nutrients, Zn and Cu, with increased P supply.The increased N concentrations in twigs of karri, and the increased P concentrations in tissues of both species, were associated with responses of karri to added N and P, and of B. laidlawiana to added P. This indicates that tissue concentrations of N and P were generally below critical concentrations where N and P were not applied. The results show that for these species twigs may be a better tissue than leaves for diagnosing deficiencies or predicting N and P requirements. The ratio of P concentrations in twigs to P concentrations in leaves also increased with added P. It is suggested that this ratio may be a useful index if it reduces the variability caused by sampling position within the crown or genetic differences between plants.     

Soil Water Extraction, Measured by Computer-Assisted Tomography, in Seedling Lupinus angustifolius cv. Yandee when Healthy and Infected with Phytophthora cinnamomi

The water-mould fungus Phytophthora cinnamomi Rands causes drought-likesymptoms on many hosts, and yet the mechanisms by which infectionleads to wilting are not fully understood. This is the firststudy to describe in detail changes in soil water around theroot with infection. Computer-assisted tomography (CAT) wasused with Lupinus angustifolius L. cv. Yandee to examine drawdowns(removal of soil water) around a central root infected by P.cinnamomi in a white sand. No growth differences in roots or shoots were found betweenhealthy and diseased plants during the 8 d of the experiment.However,drawdowns failed at high levels of inoculum (8–16 /Pc-infectedmillet seeds/plant) by 8 d. Water contents in pots with uninfectedplants were in the range 0·09–0·12 cm³ watercm^–3 soil in the centre of the pot, while water contentsin pots with infected plants at 16 millet seeds applied werein the range of 0·16–0·19 cm³ water cm^–3soil in the centre of the pot. A higher transpirational demand produced lower soil water contentsnear the root but this effect was confounded with infection:disease was more pronounced with higher transpirational demand,and disease led to an increase in water content. Key words: Root disease, Phytophthora cinnamomi, water uptake, soil-root interface, computer-assisted tomography     

Plant Growth in Relation to the Supply and Uptake of NO3-: A Comparison Between Relative Addition Rate and External Concentration as Driving Variables

Two approaches to quantifying relationships between nutrientsupply and plant growth were compared with respect to growth,partitioning, uptake and assimilation of NO₃^– by non-nodulatedpea (Pisum sativum L. cv. Marma). Plants grown in flowing solutionculture were supplied with NO₃^– at relative addition rates(RAR) of 0·03, 0·06, 0·12, and 0·18d^–1, or constant external concentrations ([NO₃^–)of 3, 10, 20, and 100 mmol m^–3 over 19 d. Following acclimation,relative growth rates (RGR)approached the corresponding RARbetween 0·03–0.12 d^-1, although growth was notlimited by N supply at RAR =0.18 d^-1. Growth rates showed littlechange with [NO₃–] between 10–100 mmol m^–3(RGR=0·15 –0·16 d^-1). The absence of growthlimitation over this range was suggested by high unit absorptionrates of NO₃^–, accumulation of NO₃^– in tissues andprogressive increases in shoot: root ratio. Rates of net uptakeof NO₃^– from 1 mol m^–3 solutions were assessed relativeto the growth-related requirement for NO₃^–, showing thatthe relative uptake capacity increased with RGR between 0·03–0·06d^–1 , but decreased thereafter to a theoretical minimumvalue at RGR     

Structure and Function of Oak Forests in Central Himalaya. II. Nutrient Dynamics

This paper elucidates nutrient dynamics in oak forests previouslyinvestigated for dry matter dynamics. The nutrient concentrationsin different life forms were of the order: herb > shrub >tree, whereas the standing state of nutrients were of the order:tree > shrub > herb. Soil, litter and vegetation, respectively,accounted for 32·4–98·0 %; 0·3–3·5%, and 10·2–66·6 % of the total nutrientsin the system. Considerable reductions (8·5–41·7%)in concentrations of nutrients in leaves occurred during senescence.The uptake of nutrients by vegetation, and also by differentcomponents with and without adjustment for internal recycling,has been calculated separately. Annual transfer of litter (above+ below ground) to the soil by vegetation was 115·9–187N, 7·5–15·6 P, 122·7–195·1Ca, 36·1–48·8 K and 2·88–5·16Na kg ha^–1 yr^–1. Turnover rate and turnover timefor different nutrients ranged between 0·66–0·84yr^–1 and 1·19–1·56 yr^–1, respectively.Compartment models for nutrient dynamics have been developedto represent the distribution of nutrient contents and net annualfluxes within the system. Quercus leucotrichophora forest, Q.floribunda forest, Q. lanuginosa forest, Nutrient concentration, standing state, uptake, internal cycling, turnover     

Estimation of the Annual Cost of Kiwifruit Vine Growth and Maintenance

Elemental analysis (for carbon, hydrogen, nitrogen and sulphur)and ash data for kiwifruit [Actinidia deliciosa (A. Chev.) C.F. Liang et A. R. Ferguson var. deliciosa cv. Hayward] stems,leaves and fine roots were used to calculate the specific costs(kg carbohydrate kg^-1 dry matter) of organ synthesis with ammoniacalnitrogen supply. Those costs ranged between 1·19 and1·35 for stems and 1·19 and 1·27 for leaves.The mean annual specific cost for fine roots was 1·17.Seasonal vine growth costs were calculated by multiplying thespecific costs by biomass data for a typical vine. Total costof synthesis was 57·2 kg carbohydrate per vine year^-1,taking fine root turnover as three times per season. Nitratenitrogen supply increased that cost by 6·6% to 61·0kg carbohydrate per vine year^-1. Fruit growth accounted forthe largest proportion of synthetic costs. Vine growth respiration(expressed in terms of carbohydrate equivalents) accounted forapproximately 11·5% of the total cost of synthesis. Maintenancerespiration was estimated to be 5·28, 8·44, 1·90,8·62 and 13·3 kg carbohydrate per organ year^-1for stems, leaves, fruit, above-ground perennial componentsand roots, respectively. Total annual cost of growth and maintenancefor a mature vine was 94·7 and 98·5 kg carbohydrateper vine year^-1 with ammoniacal and nitrate nitrogen supply,respectively. Both values are similar to an estimate of vinephotosynthesis. Maintenance respiration accounted for approximately40% of the total annual cost of vine growth, regardless of theform of nitrogen supplied. Peak carbohydrate demand was duringthe period from 60 to 160 d after budbreak.Copyright 1995, 1999Academic Press Actinidia deliciosa, kiwifruit, carbon economy, growth respiration, maintenance respiration     

Structure and Function of Oak Forests in Central Himalaya. I. Dry Matter Dynamics

The present study deals with structure and functioning of threeareas of Himalayan oak forest. Low- and mid-altitude oaks, namelyQuercus leucotrichophora, and Quercus floribunda, form predominantevergreen forests in Central and Western Himalaya. The totaltree basal cover ranged between 33·89 m² ha^–1 (Q.floribunda site) to 36·83 m² ha^–1 (Q. leucotrichophorasite). The density ranged between 570 and 760 individuals ha^–1.Allometric equations relating biomass of different tree componentsto GBH (girth at breast height) were significant with the exceptionof leaf biomass in Q. leucotrichophora and Rhododendron arboreum.Total vegetation biomass (29·40–467·0 tha^–1) was distributed as 377·1 t ha^–1 intrees, 5·40 t ha^–1 in shrubs and 1·23 tha^–1 in herbs. Total forest floor biomass ranged between4·6 and 6·2 t ha^–1. Of the total annuallitter fall (4·7–4·8 t ha^–1), 77·5% was contributed by leaf litter, 17·8 % by wood litterand 4·7 % by miscellaneous litter. Turnover rate of treelitter varied from 0·66 to 0·70. Net primary productionof total vegetation ranged between 15·9 and 20·6t ha^–1 yr^–1, of which the contribution of trees,shrubs and herbs was 81·2 %, 8·6 % and 10·2%, respectively. A compartment model of dry matter on the basisof mean data across sites was developed to show dry matter storageand flow of dry matter within the system. Quercus leucotrichophora forest, Q. floribunda forest, Q. lanuginosa forest, biomass, litter fall, net primary production, compartmental transfer     

Quantitative Aspects of Leaf Acid Phosphatase Activity and the Phosphorus Status of Tomato Plants

The relation between tomato leaf acid phosphatase activity andleaf tissue P content has been examined, and a study made ofthe effects of leaf development, variation in nitrogen supply,and variation in the growing medium on this relationship. Tomatoplants were grown in sand and given various concentrations ofphosphate. Plants were also grown for an initial period in peatcontaining an adequate level of phosphate, then transferredto peat to which was added 0 or 2.3 kg superphosphate m^–3and supplied with either 50 of 300 µg N ml^–1. Expressed on a unit tissue f. wt basis, acid phosphatase activityin the control plants in sand (given 41 µg P ml^minus;1)was highest in extracts from the expanding leaves and decreasedwith leaf maturity. However, when given a reduced supply ofphosphate, the enzyme activity in the more mature leaves wasequal to, or greater than, that in the expanding leaves. Thephosphatase activity increased first in the young, fully-expandedleaves and in the mature leaves (with 4.1 µg P ml^–1),but did not increase in the expanding leaves until the supplywas restricted to 2.1 µg P ml^–1. On closer examination,the increase in enzyme activity appeared to be associated withthe P level in the leaf tissues, the activity increasing whenthe level fell below about 0.25 per cent (g P per 100 g drywt tissue). The same relation was found with the plants grownin peat, and was independent of the concentration of nitrogensupplied to the plants. The fully expanded leaves showed the best enzyme response whenthe phosphate supply was restricted and the activity reflectedclosely the local levels of tissue P. The assay of the enzymein unpurified leaf extracts is simple and rapid, and could beused in a test to detect P-deficiency in tomato plants. Lycopersicon esculentum L, tomato, acid phosphatase activity, phosphorus status     

The Effects of Elevated Atmospheric Carbon Dioxide and Water Stress on Ligth Interception, Dry Matter Production and Yield in Stands of Groundnut (Arachis hypogaea L.

Stands of groundnut (Arachis hypogaea L.), a C₃ legume, weregrown in controlled-environment glasshouses at 28 °C (±5°C)under two levels of atmospheric CO₂ (350 ppmv or 700 ppmv) andtwo levels of soil moisture (irrigated weekly or no water from35 d after sowing). Elevated CO₂ increased the maximum rate of net photosynthesisby up to 40%, with an increase in conversion coefficient forintercepted radiation of 30% (from 1–66 to 2–16g MJ^–1) in well-irrigated conditions, and 94% (from 0–64to 1·24 g MJ^–1) on a drying soil profile. In plantswell supplied with water, elevated CO₂ increased dry matteraccumulation by 16% (from 13·79 to 16·03 t ^–1) and pod yield by 25% (from 2·7 to 3·4t ha^–1).However, the harvest index (total poddry weight/above-grounddry weight) was unaffected by CO₂ treatment. The beneficial effects of elevated CO₂ were enhanced under severewater stress, dry matter production increased by 112% (from4·13 to 8·87 t ha^–1) and a pod yield of1·34t ha^–1 was obtained in elevated CO₂, whereascomparable plotsat 350 ppmv CO₂ only yielded 0·22 t ha^-1.There was a corresponding decrease in harvest index from 0·15to 0·05. Following the withholding of irrigation, plants growing on astored soil water profile in elevated CO₂ could maintain significantlyless negative leaf water potentials (P<0·01) for theremainder of the season than comparable plants grown in ambientCO₂, allowing prolonged plant activity during drought. In plants which were well supplied with water, allocation ofdry matter between leaves, stems, roots, and pods was similarin both CO₂ treatments. On a drying soil profile, allocationin plants grown in 350 ppmv CO₂ changed in favour of root developmentfar earlier in the season than plants grown at 700 ppmv CO₂,indicating that severe waterstress was reached earlier at 350ppmv CO₂. The primary effects of elevated CO₂ on growth and yield of groundnutstands weremediated by an increase in the conversion coefficientfor intercepted radiation and the prolonged maintenance of higherleaf water potentials during increasing drought stress. Key words: Arachis hypogaea, elevated CO₂, water stress, dry matter production     

Vanadate Sensitive ATPase and Phosphatase Activity in Guard Cell Protoplasts of Commelina

Flicker, M. D. and Willmer, C. M. 1986. Vanadate sensitive ATPaseand phosphatase activity in guard cell protoplasts of Commelina.—J.exp. Bot. 38: 642–648. Phosphatase activity was measured in extracts of guard cellprotoplasts of Commelina communis L. using the artificial substratep-nitrophenylphosphate. A pH optimum of 5.8 to 6.3 was determined.Ammonium molybdate (Ol mol m^–3) and sodium vanadate (1–0mol m^–3) gave almost complete inhibition of phosphataseactivity at pH 60. ATPase assays were, therefore, conductedin the presence of 0–2 mol m ^–3 molybdate and vanadatewas used as a specific inhibitor of plasmamembrane ATPase activity.Vanadate sensitive ATPase activity showed a pH optimum of 6.6and activity was stimulated by KC1. These properties are characteristicof plasmamembrane proton pumping ATPases in other systems andsuggest that proton extrusion in guard cells could be mediatedby a similar enzyme. The maximum ATPase activity is sufficientto account for all the proton flux observed during the stomatalopening response. Key words: ATPase, Commelina, guard cell protoplasts, phosphatase, vanadate     

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     

Nutrient Cycling in a New South Wales Subtropical Rainforest: Organic Matter and Phosphorus

Biomass and phosphorus distribution and accumulation rates wereestimated for an undisturbed subtropical rainforest in northernNew South Wales. The accumulation rates were estimated overa 16-year period. It is estimated that the steady-state above-groundbiomass for this forest is 35.0 tonne ha^–1. Most of theannual biomass production was replacing litterfall and mortality.The above-ground forest contained 52 kg P ha^–1 with agross annual accumulation of about 0.4 kg P ha^–1/yr^–1.The forest understorey and forest floor contained 4.7 kg P ha^–1and 7.9 kg P ha^–1, respectively. The annual uptake wasapprox. 4 kg P ha^–1 yr^–1. The phosphorus utilizationof this stand was compared with that of a Eucalyptus grandisplantation Sub-tropical rainforest, biomass accumulation, phosphorus cycling     

The Water Relations of Young Olive Trees in a Mediterranean Winter: Measurements of Evaporation from Leaves and Water Conduction in Wood

We report measurements of evaporation rate, leaf resistanceto evaporation and water conduction in the stems of young olivetrees (Olea europea L.) growing in Messina, Italy, during thewinter and early spring. We have measured what Zimmermann calls‘leaf specific conductivity’ (LSC) of stem segmentsexcised from olive trees. The LSC is a measure of the specifichydraulic conductivity of stem segments normalized per unitarea of leaves supplied by the stem segment rather than perunit area of sapwood cross-sectional area. We find that theLSC's of primary stems were the largest followed in magnitudeby the LSC's of secondary stems and tertiary stems. Under winterand early spring conditions the maximum evaporative flux fromCoratina and Nocellara varieties of olive trees is about 2.6x 10^–5 kg 8^–1 m^–2. From this and the LSC measurementswe calculate that the pressure gradients needed to maintainthis rate of evaporation in the steady state is 65 kPa m^–1in primary stems, 170 kPa m^–1 in secondary stems and 560kPa m^–1 in tertiary stems. Olive, Olea europea L, evaporation, leaf specific conductivity, hydraulic conductivity, leaf resistance     

Cyclic Nucleotides and In Vitro Plant Cultures: I. INDUCTION OF ORGANOGENESIS IN TOBACCO (NICOTIANA TABACUM CALLUS CULTURES

Mangat, B. S. and Janjua, S. 1987. Cyclic nucleotides and invitro plant cultures. I. Induction of organogenesis in tobacco(Nicotiana tabacum) callus cultures.—J. exp. Bot. 38:2059–2067. The possibility that cyclic nucleotides have a mediatory rolesimilar to cytokinins in plant tissue cultures was examined.Calli obtained from tobacco pith tissue were incubated on growthmedia supplemented with either cyclic AMP, cyclic GMP, adenosineor guanosine, in concentrations ranging from (mg dm^–3)0 to 2·0 together with 2·0 mg dm^–3 of IAA.Results were compared with identical calli grown on media containingcomparable amounts of kinetin and IAA. Increase in callus growthwas observed on all media containing cyclic AMP, cyclic GMP,adenosine, guanosine or kinetin. Adenosine or guanosine didnot promote organogenesis. Low concentrations (0·02 and0·05 mg dm^–3) of kinetin stimulated extensive rootdevelopment. Some root formation was also elicited with higheramounts of cyclic AMP (0·1 and 0·2 mg dm^–3)or cyclic GMP (0·2 and 0·5 mg dm^–3). Bothkinetin and cyclic GMP promoted shoot differentiation. However,in contrast to kinetin, cyclic GMP induced organogenesis atlower concentrations (0·02 and 0·1 mg dm^–3).The addition of 2·0 mg dm ^–3 of cyclic AM P toIAA-free growth media elicited shoot differentiation. This wasalso the case with a similar concentration of kinetin or cyclicGMP. Results suggest cytokinin activity for the two cyclic nucleotides. Key words: Tobacco, Nicotiana tabacum, tissue culture, cyclic nucleotides, cyclic AMP, cyclic GMP organogenesis     

Comparison of the Effects of Ammonia and Methylamine on Chloride Transport and Intracellular pH in Chara corallina

Ammonium and methylammonium ions greatly increase the rate ofCl^– transport in Chara corallian. This effect is dependenton the pH of the bathing solution. The amine-stimulated Cl^–influx is small at pH 5·5, increases to a maximum atpH 6·5–7·5, and decreases again as the pHis raised to 8·5. Increased Cl^– influx is accompaniedby an increase in cytoplasmic pH, as calculated from the distributionof DMO. When the external pH lies between 5·5 and 7·3,cytoplasmic pH in the absence of amine is 7·65–7·70,with an increase of 0·15–0·25 in the presenceof amine. As external pH is increased above 7·3, cytoplasmicpH also increases, with progessively less effect of amine. Although the relationship between Cl^– influx and cytoplasmicpH is not simple, the results provide evidence in accord withthe hypothesis that Cl^– transport in Chara involves H⁺—Cl^–symport, or the equivalent OH^–—Cl^– antiport.The possible role of cytoplasmic pH as a factor involved inthe regulation of membrane transport in Chara is discussed.     

Root Aeration and Respiration in Young Mangrove Plants (Avicennia marina (Forsk.) Vierh.)

The roots of young plants of Avicennia marina (Forsk.) Vierh.grown under simulated tidal conditions were harvested so asto obtain the entire root system. The roots were subdividedand weighed and subsamples taken for manometric determinationof respiration rates at different temperatures. The supply capacityof the above-ground portion of the root system was determinedand the results compared in terms of supply and demand. Theoxygen consumption rate of the roots at 15°C was found tobe 1·69±0·07 µmol kg^–1 s^–1for cable roots and 3·27±0·12 µmolkg^–1 s^–1 for fine roots. The Q₁₀ for respirationwas 2·55 for oxygen consumption in both fine and cableroots, and for carbon dioxide production was 2·66 forfine roots and 3·04 for cable roots. The respiratoryquotient varied with temperature but was less than unity. Concentrationdifferences of between 1·8 mol m^–3 and 3·4mol m^–3 between the inside of root and the air were sufficientto permit aeration of the root system by diffusion alone, andthe aerenchyma contained sufficient oxygen to maintain aerobicconditions while the roots were covered with water. The effectof tide and seasonal temperature change on gas exchange, togetherwith the possibility of some form of carbon dioxide fixationwithin the root, are examined and the implications of theseeffects on growth and development are discussed. Key words: Mangrove, root aeration, respiration, aerenchyma     

Measurement of the Apoplastic Activity of K+ and Cl- in the Leaf Epidermis of Commelina communis in Relation to Stomatal Activity

Bowling, D. J. F. 1987. Measurement of the apoplastic activityof K⁺ and Cl^– in the leaf epidermis of Commelina communisin relation to stomatal activity.–J. exp. Bot. 38: 1351–1355. Ionic activity of K⁺ and Cl^– in the apoplast of the lowerepidermis of the leaf of Commelina communis was measured usingion selective micro-electrodes. Large gradients across the stomatalcomplex were observed which were related to stomatal aperture.On stomatal closure the activity of K⁺ and Cl^– in theapoplast of the guard cell rose from 3·0 mol m^–3to 100 mol m^–3 and 33 mol m^–3 respectively. It wasconcluded that the apoplast is an important pathway for iontransport between the cells. Key words: Stomata, ionic activity, leaves, apoplast     

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.     

Observations on Net Assimilation Rates in Arctic Environments: With two Figures in the Text

Examination of the net assimilation rate (E) during the growingseason in arctic regions by a detached-leaf method revealedno differences between species or with soil richness, but showeda reduction of E with exposure to wind–probably resultingfrom cooling–and a tendency for E to fall towards thelater part of the growing season. E generally lay in the range0·5 to o·8 g./dm.²/week. E for detached leaves ignores respiratory losses in other partsof the plant and is not comparable with E for whole plants;failure to appreciate this confused a previous comparison ofE under arctic and temperate environments. E for detached leavesin temperate summer conditions is normally around 1·1to 1·5 g./dm.²/week. Thus E is reduced in arctic environmentsto about half the value in temperate conditions. This reductionis due mainly to the cold climate.