Calcification in the Green Alga Halimeda: II. THE EXCHANGE OF CA2+ AND THE OCCURRENCE OF AGE GRADIENTS IN CALCIFICATION AND PHOTOSYNTHESIS

In Halimeda cylindracea and H. tuna segments, the concentrationof CaCO₃, MgCO₃, protein, and chlorophyll, as well as segmentvolume and wet and dry weight, increase with ‘age’i.e. from the apex of a branch downwards. Photosynthetic andcalcification rates decrease with age as does the degree oflight stimulation of calcification. Studies of the exchange of ⁴⁵Ca between the Halimeda thallusand the sea water under various conditions showed that mostof the Ca exchange is between the cell walls, the aragonitecrystals, and the intercellular space. The cell wall has twodistinguishable phases with half-times (t_0?5) of 200 and 35min while the CaCO₃ has a rapidly exchanging phase with a t_0?5of approximately 6 min. The t_0?5 of the exchange of Ca betweenthe intercellular space and the external medium is estimatedat about 6 min, on the basis of uptake studies. If the integrityof the barrier between the intercellular space and the externalsea water, created by the adpressed peripheral utricles is destroyedthe t_0?5 is smaller (<<3 min). These kinetic studies as well as comparative measurements ofcalcification rates by both isotopic and chemical methods showthat the ⁴⁵Ca method for measuring calcification rates overestimatesthe calcification rate, due to binding of ⁴⁵Ca in the cell wallsand retention of ⁴⁵Ca in the intercellular space. The ¹⁴C methodgives more accurate results and has the further advantage ofallowing simultaneous measurement of the photosynthetic andcalcification rate on the same segment.     

The Sensitivity of Net Photosynthesis in Several Plant Species to Short-term Fumigation with Sulphur Dioxide

The effects of exposure of up to 2 h with sulphur dioxide ona range of plant species was observed by measuring changes inthe rate of net photosynthesis under closely controlled environmentalconditions. Ryegrass, Lolium perenne ‘S23’ was thespecies most sensitive to SO₂; significant inhibition was detectedat 200 nl l^–1. Fumigations at 300 nl l^–1 also inhibitedphotosynthesis in field bean (Vicia faba cv. ‘Three FoldWhite’ and ‘Blaze’) and in barley (Hordeumvulgare cv. ‘Sonja’). No effect was detected inwheat (Triticum aestivum cv. ‘Virtue’) at concentrationsup to 600 nl l^–1 SO₂, or in oil-seed rape (Brassica napuscv. ‘Rafal’) except at 800 nl l^–1 SO₂). Recoverycommenced immediately after the fumigation was terminated andwas complete within 2 h when inhibition had not exceeded 20%during the SO₂ treatment. Key words: Sulphur dioxide, short-term fumigation, photosynthesis     

The Apparent Induction of Nitrate Uptake by Chara corallina Cells following Pretreatment with or without Nitrate and Chlorate

Nitrate provision has been found to regulate the capacity forChara corallina cells to take up nitrate. When nitrate was suppliedto N sufficient cells maximum nitrate uptake was reached after8 h. Prolonged treatment of the cells in the absence of N alsoresulted in the apparent ability of these cells to take up nitrate.Chlorate was found to substitute partially for nitrate in the‘induction’ step. The effects on nitrate reductionwere separated from those on nitrate uptake by experiments usingtungstate. Tungstate pretreatment had no effect on NO^–₃uptake ‘induced’ by N starvation, but inhibitedNO^–₃ uptake associated with NO^–₃ pretreatment. Chloridepretreatment similarly had no effect on NO^–₃ uptake ‘induced’by N deprivation, but inhibited NO^–₃ uptake followingNO^–₃ pretreatment. The data suggest that there are atleast two mechanisms responsible for the ‘induction’of nitrate uptake by Chara cells, one associated with NO^–₃reduction and ‘induced’ by CIO^–₃ or NO^–₃and one associated with N deprivation. Key words: Nitrate, Chlorate, Chara corallina, Induction     

Comparison of Biomass and Metal Uptake between Two Populations ofPhragmites australisGrown in Flooded and Dry Conditions

The biomass and metal concentrations of two populations ofPhragmitesaustraliswere studied by growth in a glasshouse in three amelioratedsubstrata [Mai Po (MP) sediment, fly ash (FA) and lead/zincmine tailings (TL)] under flooded and dry conditions for 90d. Plants were raised from seeds from ‘clean’ (MaiPo, Hong Kong) and metal-contaminated (Plombières, Belgium)sites. Seedling growth was best in fly ash, root dry weightsbeing higher in flooded than dry conditions, and growth poorestin tailings, in which shoot and root dry weights were higherunder dry conditions for both populations. However, in the MPsubstratum conditions did not significantly affect shoot androot dry weights of either population. In the fly ash and tailings,more metals were generally taken up in both roots and shootsin flooded than dry conditions, but there was little differencein the MP substratum. Metal uptake was mostly similar in bothpopulations in seedlings grown in the same substratum, therebeing no clear evidence of ecotypic differentiation.Copyright1998 Annals of Botany Company Biomass, flooded and dry conditions, metal uptake,Phragmites australis.     

Effects of Photoperiod and Maturity Genes on Plant Growth, Partitioning, Radiation Use Efficiency, and Yield in Soyabean [Glycine max(L.) Merrill] 'Clark'

Plants of four isolines of soyabean [Glycine max(L.) Merrill]‘Clark’, viz‘L71-920’ (maturity genecomplemente₁e₂e₃ ), ‘L80-5914’ (E₁e₂e₃), ‘Clark’(e₁E₂E₃), and ‘L65-3366’ (E₁E₂E₃), were grown inshort (12.25 h d ^{- 1}natural light) and long days (12.25 h d^{- 1}natural light supplemented with 2.75 h d ^{- 1}low-irradianceartificial light) from first flowering to maturity in a polythenetunnel maintained at 30/24°C (day/night). Whereas therewere few differences among the isolines grown in short days,in long days the dominant alleles increased crop duration, biomassand seed yield substantially. Increases in biological and economicyield were not solely a consequence of longer crop duration:the dominant alleles also increased crop growth rate and radiationuse efficiency in long days (from 1.3 g MJ ^{- 1}total radiationine₁e₂e₃ to 2.8 g MJ ^{- 1}inE₁E₂E₃ ). Greater radiation use efficiencyresulted from a relatively longer leaf area duration, betterdistribution and orientation of a larger mass of leaves withinthe canopy, and smaller partitioning of assimilates to reproductivestructures. The work reveals the substantial effects of thethree lociE₁ / e₁, E₂/ e₂and E₃/e₃ on the response of plantgrowth, as well as development, to environment. Their relevanceto crop adaptation is discussed. Copyright 2000 Annals of BotanyCompany Glycine max(L.) Merrill, soyabean, maturity genes, flowering, phenology, growth, yield     

Relative Sensitivity and Tolerance of some Gladiolus Cultivars to Sulphur Dioxide

Five Gladiolus cultivars, namely ‘Aldebaran’, ‘BrightEye’, ‘Illusion’, ‘Manisha’ and‘Manmohan’, were exposed to 1 and 2 µg l^–1sulphur dioxide to test their relative-sensitivity toleranceto the pollutant Plants were fumigated experimentally for 2h daily Foliar injury symptoms were observed first in ‘Manisha’followed by ‘Aldebaran’ and ‘Illusion’at the higher dose Photosynthetic pigments and leaf extractpH were significantly decreased, particularly in ‘Manisha’and ‘Illusion’ Overall disturbances in the plantmetabolism due to SO₂ treatment led to retarded growth of plants,as evident from decreased shoot length and phytomass valuesThe order of sensitivity of the five Gladiolus cultivars toSO₂ was as follows, with the greatest first Manisha, Illusion,Aldebaran, Bright Eye, Manmohan Cultivars, Gladiolus, sensitivity, sulphur dioxide, tolerance     

An Experimental Study of Calcium Acquisition and its Effects on the Calcifuge Moss Pleurozium schreberi

In a field experiment to investigate the sources and effectson growth of Ca in the calcifuge moss Pleurozium schreberi,significant quantities of Ca reached the growing shoot apicesfrom a CaCO₃ layer placed on the mineral soil surface Top applicationsof 0.5 and 5 mol m^–3 CaCl₂ raised the exchangeable andintracellular Ca concentrations and displaced natural exchangeableK and Mg The 5 mol m^–3 CaCl₂ treatment also caused a significantreduction in intracellular Mg indicating that Mg uptake is dependenton an initial exchange step No growth differences were notedbetween treatments, possibly because ionic changes had not reacheda detrimental level within the 28 weeks of the experiment ina second experiment, shoot apices of Pleurozium schreberi, Pseudoscleropodiumpurum and Calliergon cuspidatum were grown on nylon gauze underintermittent distilled-water mist At weekly intervals the shootswere saturated with CaCl₂ solutions providing factorial combinationsof Ca and pH Growth of C cuspidatum and P purum from chalk soilwas reduced at high (0.01) Ca concentration whereas Pleuroziumschreberi and Pseudoscleropodium purum from acidic clay wereunaffected The pH treatments did not significantly affect mossgrowth Initial tissue levels of K and Mg were lower in the mossesfrom chalk and it is suggested that the CaCl₂ treatments causednutrient deficiencies in these plants Mosses from acidic soilcontained less exchangeable Ca than the chalk plants and grewpoorly in the absence of CaCl₂, perhaps due to the developmentof Ca deficiency Bryophyte growth, calcium uptake, pH, mineral nutrition, Pleurozium schreberi, Pseudoscleropodium purum, Calliergon cuspidatum     

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.     

Leaf Abscission and Stem Lesions (Intumescences) on Poplar Clones after SO2 and O3 Fumigation: A Link with Ethylene Release?

Differences in premature leaf abscission and in visible steminjury in genetic lines of poplar followed continuous fumigationswith air pollutant levels of SO₂ (90–100 nl l^–1)and O₃ (70–80 nl l^–1) either separately or together.The clones used were: Populus deltoides var. missiouriensisMarsh., P. nigra cv. ‘italicd’ L., and the hybridsP. nigra cv. ‘italica’ xP. deltoides (He-X/3) andP. nigra cv.‘italica’ x P. nigra cv. ‘Serres’(He-K/7). While most leaf abscission occurred within 20 d fromthe start of fumigation, stem lesions (intumescences), appearedonly after 72 d. Their anatomical characteristics include theformation of lysigenous aerenchyma in the lower parts of theintumescence, the sloughing of superficial cells from the injuredarea, and the development of crystalline formations on the surfaceof the lesions. P. deltoides exhibited the least morphologicalresponse to the gases. Ethylene released from fumigated leaves was determined at thesame gas concentration of SO₂ (100 nl l^–1), O₃ (75 nll^–1) and their mixture. Leaves of P. deltoides consistentlyshowed the lowest ethylene production after the gas treatments.P. ‘italica’ production was higher but was littlealtered by the treatments. The two hybrids He-X/3 and He-K/7showed the greatest increases in ethylene evolution with time.With He-K/7 exposed to the gas mixture the production of ethylenedecreased after the initial sharp rise during days 1–2,and reflected the considerable leaf damage observed after day3. The results suggest that sensitivity to air pollution, (as shownby leaf abscission and the formation of stem intumescences)can be correlated with the level of pollutant-induced ethyleneevolution from leaves. Initially high levels could induce abscission,whilst prolonged production could be responsible for intumescenceinitiation. The discussion proposes a series of events fromSO₂ and/or O₃ entry into the leaf and the physiological reasonsfor the clonal differences. Key words: Sulphur dioxide, ozone, ethylene, poplar, leaf abscission, stem lesions     

Nocturnal Accumulation of Acid in Leaves of Wall Pennywort (Umbilicus rupestris) Following Exposure to Water Stress

Physiological responses to water stress (drought) have beeninvestigated in Umbilicus rupestris (wall pennywort) by comparingcontrol (well-watered) and draughted plants with respect to(i) diurnal fluctuations in the acid content of the leaves,(ii) CO₂ exchange patterns and (iii) stomatal conductance. Controlplants show no diurnal fluctuations in acid content, whereasafter 6 d of drought a clear CAM-type pattern (nocturnal acidificationfollowed by deacidification in the light) is observed. In controlplants, the CO₂ exchange pattern over a 24 h period is of atypical C-3 ‘square-wave’ type, with extensive CO₂uptake in the light and CO₂ output in the dark. In droughtedplants the day-time CO₂ uptake is confined to a morning ‘burst’,whilst night-time CO₂ output is markedly reduced. There is howeverno net noctural uptake of CO₂. In control plants, stomatal conductanceis high during the day (especially in the first half of theday) falling to a low level at the onset of darkness, and thenrising slowly through the remainder of the night. In droughtedplants, stomatal conductance is very low, except that thereis morning ‘burst’ of high conductance and a periodduring the night when conductance is higher than in controlplants. These results are discussed in relation to the response of U.rupestris to drought both in laboratory and in field conditions. Umbilicus rupestris, wall pennywort, CO₂ exchange, Crassulacean acid metabolism, drought, stomatal conductance, water stress     

Sagebrush (Artemisia tridentata Nutt.) roots maintain nutrient uptake capacity under water stress

Phosphorus and nitrogen uptake capacities were assessed during36–58 d drying cycles to determine whether the abilityof sagebrush (Artemisia tridentata Nutt.) to absorb these nutrientschanged as the roots were subjected to increasing levels ofwater stress. Water was withheld from mature plants in large(6 I) containers and the uptake capacity of excised roots insolution was determined as soil water potentials decreased from–0.03 MPa to –5.0 MPa. Phosphorus uptake rates of excised roots at given substrateconcentrations increased as preharvest soil water potentialsdecreased to –5.0 MPa. V_max and K_m also increased as soilwater potentials declined. Declining soil water potentials depressednitrogen uptake at set substrate concentrations, but uptakecapacity, calculated as the sum V_max for both NH⁺₄+NO^–₃,did not change significantly with drying. The sum V_max correlatedwith root nitrogen concentration. Root uptake capacity for nitrogen and phosphorus was extremelystable under severe water stress in this aridland shrub. Maintenanceof uptake capacity, coupled with a previously demonstrated abilityto conduct hydraulic lift, may enable A. tridentata better tomaintain nitrogen and phosphorus uptake as soil water availabilitydeclines. These mechanisms may be important in the ability ofA. tridentata to maintain growth, complete reproduction, andgain an advantage against competitors late in the season whenthe soil layers with higher nutrient availability are dry. Key words: Kinetics, nitrogen, phosphorus, roots, water stress     

Increased Defoliation Frequency Depletes Remobilization of Nitrogen for Leaf Growth in Grasses

Plants ofLolium perenneandFestuca rubrawere grown in sand culturereceiving all nutrients as a complete nutrient solution containing1.5 mMNH₄NO₃, and subjected to one of three defoliation treatments:undefoliated, defoliated on one occasion, or defoliated weekly.¹⁵Nlabelling was used to determine the rate of N uptake, allowingthe amount of N remobilized from storage for the growth of thetwo youngest leaves (subsequently referred to as ‘newleaves’) growing over a 14 d period after defoliationto be calculated. The total plant N uptake by both species wasreduced, compared with undefoliated plants, by both a singleand repeated defoliation, although neither caused complete inhibitionof uptake. Regularly defoliatedL. perennehad a greater reductionin root mass, concomitant with a greater increase in N uptakeper g root than did regularly defoliatedF. rubra. In both species,the amount of N derived from uptake recovered in the new leaveswas unaffected by the frequency of defoliation. BothL. perenneandF.rubramobilized nitrogen to the new leaves after a single defoliation,mobilization being sufficient to supply 50 and 41%, respectively,of the total nitrogen requirement. In regularly defoliated plants,no significant nitrogen was mobilized to the new leaves inL.perenne, and only a small amount was mobilized inF. rubra. Plantsachieved greater leaf regrowth when only defoliated once. Weconclude that increasing the frequency of defoliation of bothL.perenneandF. rubrahad little effect on the uptake of nitrogenby roots which was subsequently supplied to new leaves, butdepleted their capacity for nitrogen remobilization, resultingin a reduction in the rate of growth of new leaves. Lolium perenne; Festuca rubra; defoliation frequency; mobilization; root uptake; nitrogen     

Influence of Ni Supply on Growth and Nitrogen Metabolism ofBrassica napusL. Grown with NH4NO3or Urea as N Source

The significance of nickel (Ni), which is essential for ureaseactivity, for growth and nitrogen (N) metabolism ofBrassicanapusgrown in nutrient solution with either NH₄NO₃or urea assole N source was investigated. Although Ni contents were below25 µg kg^-1d. wt, growth of plants relying on NH₄NO₃wasnot affected by the Ni status. However, supplementing the growthmedium with 0.04 µMNi enhanced dry matter production ofurea-grown plants significantly. Urease activity was significantlyreduced in leaves and roots of plants grown without supplementaryNi irrespective of N source. Plants grown with urea withoutadditional Ni accumulated large amounts of urea and had loweramino acid contents indicating impaired usage of the N supplied,while those grown with NH₄NO₃under Ni-deprived conditions accumulatedendogenous urea in their older leaves. It is suggested thatNi may not be strictly essential for plants receiving mineralN, or that the critical level is well below 25 µg kg^-1d.wt. These results confirm that Ni is required for urease activityand thus for growth of plants on urea-based media, as well asfor recycling endogenous urea.Copyright 1999 Annals of BotanyCompany. Brassica napusvar.annua, amino acids, N nutrition, nickel, spring rape, urea, urease activity.     

Circulating Electric Currents Between Acid and Alkaline Zones Associated with Assimilation in Chara

Some charophyte species, like some other freshwater plants,have been shown to take up for photosynthetic fixation. Such plants show localized deposits of CaCO₃ associatedwith localized ‘OH^– excretion’. Chara corallinaproduces acid and alkaline regions in unbuffered external media,in the light, even when CaCO₃ is not being precipitated. Theseregions are associated with uptake and ‘OH^–efflux’ respectively. We have found that large currentscirculate, in the light, between acid and alkaline zones ofChara internodes, by (i) direct measurement of the short-circuitcurrent and (ii) measurement of potential differences in thesolution around the cell. Both methods suggest currents of 0.5–1µA per zone, with local current densities of about 75mA m^–2, or local univalent fluxes of 0.75 µmol m^–2s^–1. These circulating currents produce mean peak-to-peakpotential differences of 7 mV in the external medium. The fluxesare believed to be of and H⁺ or OH^–.While active transport of is strongly indicated, passive uniport of H⁺ or OH^– appears to be sufficient.Control systems for both fluxes would be required.     

Role of Coccoliths in the Utilization of Inorganic Carbon by a Marine Unicellular Coccolithophorid, Emiliania huxleyi: a Survey Using Intact Cells and Protoplasts

The utilization of inorganic carbon and role of the coccolithswere investigated in intact cells and protoplasts of a marineunicellular calcareous alga, Emiliania huxleyi. Protoplastswith high photosynthetic activity were obtained by artificialdecalcification with 50 mM MES-NaOH (pH5.5). (1) The kineticsof the photosynthetic evolution of O₂ at various concentrationsof externally added NaHCO₃ were the same for intact cells andprotoplasts, indicating that the kinetic properties with respectto dissolved inorganic carbon (DIC) were not affected by thepresence or absence of the coccoliths on the cell surface. Double-reciprocalplots and plots of the concentration of substrate divided byvelocity (s/v) against the concentration of substrate (s) werebiphasic in the case of both intact cells and protoplasts. TheCO₂-utilization reaction was, therefore, considered to involvetwo processes with different values of K_m and V_max. From thekinetic analyses, K_m and V_max [µmoles O₂ (ml PCV)^–1h^–1] were deduced to be 92 µM and 76.3 for a "low-K_m"reaction and 4.1 mM and 252 for a "high-K_m" reaction, respectively.(2) In short-term (40-min) experiments, time courses of thetotal uptake of ¹⁴C-DIC and the incorporation of ¹⁴C into acid-stableproducts of photosynthesis and the internal pool of DIC, determinedas acid-labile compounds, under CO₂-limiting conditions (80µM) were very similar for intact cells and protoplasts.However, incorporation of ¹⁴C into CaCO₃ apparently occurredmore slowly in protoplasts than in intact cells. (3) In longterm (24-h) experiments, patterns of incorporation of ¹⁴C werealmost same for intact cells and protoplasts, with the exceptionthat the amount of ¹⁴C incorporated into CaCO₃ was much smallerin the former than the latter. The production of Ca¹⁴CO₃ increasedduring the course of 10 h after a 4-h lag. However, after 10h the level of Ca¹⁴CCO₃ started to decrease. The decrease wasaccompanied by an increase in ¹⁴C in the products of photosynthesis,suggesting that CaCO₃ was reutilized for the photosyntheticfixation of CO₂ and, therefore, that the coccoliths functionas sites of storage of DIC. However, the internal level of DICremained at the same level even after the supply of externalDIC has been almost completely depleted. (Received July 25, 1995; Accepted December 11, 1995)     

Evidence for the Involvement of Mitochondrial Respiration in Calcification in a Marine Coccolithophorid, Emiliania huxleyi

In the marine coccolithophorid, Emiliania huxleyi, CaCO₃ productionunder illumination showed a lag phase for about 3 h and thenincreased greatly. During the lag phase the rate of CaCO₃ productionin the light was similar to that in the dark. The productionof CaCO₃ in the dark was inhibited by the addition of 170 µMCCCP, 1 mM KCN and 1 mM SHAM. These results suggest that a littleproduction of CaCO₃ is supported by energy from mitochondrialrespiration, but that large amount of CaCO₃ production requiresphotosynthesis. ¹Present address: SDS Biotech K.K., Tsukuba Technology Center,Midorigahara 2-1, Tsukuba, Ibaraki, 300-26 Japan     

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     

Utilization of Sediment CO2 by Selected North American Isoetids

The photosynthetic uptake of root-zone CO₂ was determined forEriocaulon septangulare, Gratiola aurea, Isoetes macrospora,Littorella uniflora var. americana and Lobelia dortmanna aspart of a study of the photosynthetic carbon economy of submergedaquatic isoetids. The pH and dissolved inorganic carbon (DIC)of the sediment interstitial water in four Wisconsin lakes reflectedthe water column character, where the DIC increased with depthin the sediment to concentrations five to ten times those ofthe water column. Sediment free CO₂ concentrations were 5–50times those in the water column and were similar at all sites(about 05–1.0mM CO₂ in the root-zone). In ‘pH-drift’studies these plants were unable to take up HCO₂^–. Laboratory determinations of the carbon uptake from the rootand shoot-zones were made for all five species. These experimentsshowed that CO₂ in the root-zone accounted for 65–95 percent of external carbon uptake for the five species. For G.aurea and E. septangulare, root-zone CO₂ was > 85 per centof carbon uptake. Carbon, CO₂, photosynthesis, sediment, isoetid, Eriocaulon septangulare, Gratiola aurea, Isoetes macrospora, Littorella uniflora, Lobelia dortmanna     

Seasonal patterns of growth, dehydrins and water-soluble carbohydrates in genotypes of Dactylis glomerata varying in summer dormancy

• Background and Aims Summer dormancy in perennial grasseshas been studied inadequately, despite its potential to enhanceplant survival and persistence in Mediterranean areas. The aimof the present work was to characterize summer dormancy anddehydration tolerance in two cultivars of Dactylis glomerata(dormant ‘Kasbah’, non-dormant ‘Oasis’)and their hybrid using physiological indicators associated withthese traits. • Methods Dehydration tolerance was assessed in a glasshouseexperiment, while seasonal metabolic changes which produce putativeprotectants for drought, such as carbohydrates and dehydrinsthat might be associated with summer dormancy, were analysedin the field. • Key Results The genotypes differed in their ability tosurvive increasing soil water deficit: lethal soil water potential(_s) was –3·4 MPa for ‘Kasbah’ (althoughnon-dormant), –1·3 MPa for ‘Oasis’,and –1·6 MPa for their hybrid. In contrast, lethalwater content of apices was similar for all genotypes (approx.0·45 g H₂O g d. wt^–1), and hence the greater survivalof ‘Kasbah’ can be ascribed to better drought avoidancerather than dehydration tolerance. In autumn-sown plants, ‘Kasbah’had greatest dormancy, the hybrid was intermediate and ‘Oasis’had none. The more dormant the genotype, the lower the metabolicactivity during summer, and the earlier the activity declinedin spring. Decreased monosaccharide content was an early indicatorof dormancy induction. Accumulation of dehydrins did not correlatewith stress tolerance, but dehydrin content was a function ofthe water status of the tissues, irrespective of the soil moisture.A protein of approx. 55 kDa occurred in leaf bases of the mostdormant cultivar even in winter. • Conclusions Drought avoidance and summer dormancy arecorrelated but can be independently expressed. These traitsare heritable, allowing selection in breeding programmes.     

Salinity Reduces Water Use and Nitrate-N-use Efficiency of Citrus

Five-month-old Cleopatra mandarin (Citrus reticulata Blanco)(CM) and Volkamer lemon (Citrus volkameriana Ten. and Pasq.)(VL) seedlings were grown in a glasshouse in 2·3-1 potsof Candler fine sand. Plants were irrigated with either non-saline(EC_e = 0·23 dS m^-1) or saline (6·13 dS m^-1) waterusing 3:1 NaCl:CaCl₂ solution over a 4-week period. A singleapplication of K¹⁵NO₃ (19·64 atom % excess ¹⁵N) at 212mg N1^-1, was substituted for a normal weekly fertilization after3 weeks and plants were harvested 7 d later. The transpirationrate, uptake of nitrogen, growth and nitrogen-use efficiency(NUE) on a dry weight basis (mg d. wt mg^-1 N) of both specieswas reduced by salinity. Based on growth, water-use and chloride(Cl) accumulation in leaves, VL was more salt-sensitive thanCM, but ¹⁵N uptake was equally reduced by salinity in both species.Salinity reduced ¹⁵N uptake relatively more than shoot growthover the 7-d period, such that the ¹⁵NUE (mg d. wt µg^-115N) of new shoot growth of both species increased. There wasno evidence of Cl antagonism of nitrate (NO₃) uptake but totalplant ¹⁵NO₃ uptake was positively correlated with whole planttranspiration in both species. Thus, it appears that reductionsin NO₃ uptake are more strongly related to reduced water usethan to Cl antagonism from salt stress.Copyright 1993, 1999Academic Press Sodium, chloride, salinity, calcium, nitrate, ¹⁵NO₃ uptake, nitrogen allocation, nitrogen-use efficiency, water use, Citrus reticulata, Citrus volkameriana