Measurement of Yield Threshold and Cell Wal Extensibility of Intact Wheat Roots under Different Ionic, Osmotic and Temperature Treatments

Yield stress threshold (Y) and volumetric extensibility () arethe rheological properties that appear to control root growth.In this study they were measured in wheat roots by means ofparallel measurement of the growth rate (r) of intact wheatroots and of the turgor pressures (P) of individual cells withinthe expansion zone. Growth and turgor pressure were manipulatedby immersion in graded osmoticum (mannitol) solutions. Turgorwas measured with a pressure probe and growth rate by visualobservation. The influence of various growth conditions on Yand was investigated; (a) At 27 °C.In 0.5 mol m^–3 CaCl₂ r, P, Y and were20.7±4.6 µm min^–1, 0.77±0.05 MPa,0.07±0.03 MPa and 26±1.9 µm min^–1MPa^–1 (expressed as increase in length), respectively.Following 24 h growth in 10 mol m^–3 KC1 these parametersbecame 12.3±3.5 µm min^–1, 0.72±0.04MPa, 0.13±0.01 MPa and 21±0.7 µm min^–1MPa^–1. After 24 h osmotic adjustment in 150 mol m^–3mannitol/0.5 mol m^–3 CaCl₂ r= 19.6±4.2 µmmin^–1, P = 0.68±0.05 MPa and Y and were 0.07±0.04MPa and 30±0.2 µm min^–1 MPa^–01, respectively.After 24 h growth in 350 mol m^–3 mannitol/0.5 mol m^–3CaCl₂ r= 13.3±4.1 µm min^–1, P= 0.58±0.07MPa, Y=0.12±0.01 MPa and ø 32±0.2 tim min^–1MPa^–1. During osmotic adjustment in 200 mol m^–3mannitol/0.5 mol m^–3 CaCl₂, with or without KCl, the recoveryof growth rate corresponded to turgor pressure recovery (t1/2approximately 3 h). (b) At 15 °C. Lowered temperature dramatically influencedthe growth parameters which became r= 8.3±2.8 um min^–1,P=0.78 MPa, r=<0.2 MPa and =15±0.1 µm min^–1MPa^–1. Therefore, Y and are influenced by 10 mol m^–3 K⁺ ionsand low temperature. In each case the effective pressure forgrowth (P-Y) was large indicating that small fluctuations ofsoil water potential will not stop root elongation. Key words: Yield threshold, cell wall extensibility, wheat root growth, temperature, turgor pressur     

Defoliation in White Clover: Nodule Metabolism, Nodule Growth and Maintenance, and Nitrogenase Functioning During Growth and Regrowth

In two experiments, the functioning and metabolism of nodulesof white clover, following a defoliation which removed abouthalf the shoot tissue, were compared with those of undefoliatedplants. In one experiment, the specific respiration rates of nodulesfrom undefoliated plants varied between 1160 and 1830 µmolCO₂ g^–1h^–1, of which nodule ‘growth and maintenance’accounted for 22 ± 2 per cent, or 27 ± 3.6 percent, according to method of calculation. Defoliation reducedspecific nodule respiration and nodule ‘growth and maintenance’respiration by 60–70 per cent, and rate of N₂ fixationby a similar proportion. The original rate of nodule metabolismwas re-established after about 5 d of regrowth; during regrowthnodule respiration was quantitatively related to rate of N₂,fixation: 9.1 µmol CO₂ µmol^–1N₂. With the possible exception of nodules examined 24 h after defoliation,the efficiency of energy utilization in nitrogenase functioningin both experiments was the same in defoliated and undefoliatedplants: 2.0±0.1 µmol CO₂ µmol^–1 C₂H₄;similarly, there was no change in the efficiency of nitrogenasefunctioning as rate of N₂ fixation increased with plant growthfrom 1 to 22 µmol N₂ per plant h^–1. Exposure of nodulated white clover root systems to a 10 percent acetylene gas mixture resulted in a sharp peak in rateof ethylene production after 1.5–2.5 min; subsequently,rate of ethylene production declined rapidly before stabilisingafter 0.5–1 h at a rate about 50 per cent of that initiallyobserved. Regression of ‘peak’ rate of ethyleneproduction on rate of N₂ fixation indicated a value of 2.9 µmolC₂H₄ µmol^–1 N₂, for rates of N₂ fixation between1 and 22 µmol N₂ per plant h^–1. The relationshipsbetween nitrogenase respiration, acetylene reduction rates andN₂ fixation rates are discussed. Trifolium repens, white clover, defoliation, nodule respiration, N₂, fixation, nitrogenase     

Are Mistletoes Shade Plants? CO2Assimilation and Chlorophyll Fluorescence of Temperate Mistletoes and their Hosts

Mistletoes usually have slower rates of photosynthesis thantheir hosts. This study examines CO₂assimilation, chlorophyllfluorescence and the chlorophyll content of temperate host–parasitepairs (nine hosts parasitized by Ileostylus micranthus and Carpodetusserratus parasitized by Tupeia antarctica). The hosts of I.micranthus had higher mean annual CO₂assimilation (3.59 ±0.41 µmol m^-2 s^-1) than I. micranthus(2.42 ± 0.20µmol m^-2 s^-1), and C. serratus(2.41 ± 0.43 µmolm^-2 s^-1) showed higher CO₂assimilation than T. antarctica(0.67± 0.64 µmol m^-2 s^-1). Hosts saturated at significantlyhigher electron transport rates (ETR) and light levels thanmistletoes. The positive relationship between CO₂assimilationand electron transport suggests that the lower CO₂assimilationrates in mistletoes are a consequence of lower electron transportrates. When photosynthetic rates, ETR and chlorophyll a /b ratioswere adjusted for photosynthetically active radiation, hostsdid not have significantly higher CO₂assimilation (3.21 ±0.37 µmol m^-2 s^-1) than mistletoes (2.54 ± 0.41µmol m^-2 s^-1), but still had significantly higher ETRand chlorophyll a / b ratios. The electron transport rates,saturating light and chlorophyll a / b ratios of sun leavesfrom mistletoes were similar to host shade leaves. These responsesindicate that in comparison with their hosts, mistletoe leaveshave the photosynthetic characteristics of the leaves of shadeplants. Copyright 2000 Annals of Botany Company CO₂assimilation, photosynthetic active radiation (PAR), chlorophyll fluorescence, electron transport rate (ETR), photochemical quenching (q_p), non-photochemical quenching (q_n), sun and shade leaves, chlorophyll content, Ileostylus micranthus, Tupeia antarctica, New Zealand     

Photosynthesis in the Tapinanthus-Diplorhynchus Mistletoe-Host Relationship

Hill reaction activity and gas exchange were studied in thesouthern African mistletoe Tapinanthus vittatus, its host treeDiplorhynchus condylocarpon and uninfected trees of the samespecies. Photosynthetic potential was assessed by measuringHill reaction activities in isolated thylakoids: reaction ratesin the parasite were less than half those of the host or uninfectedtrees. In addition, parasite thylakoids were more sensitiveto heat treatment than those of the trees. Mean maximum CO₂assimilation rates (±s.e.) were significantly lower (4·2±0·24µmol m^-1 s^-1) in the parasite, compared to both uninfectedtrees (7·6±0·22), and those of the infectedhost (5·2±0·27). Chlorophyll contents (±s.e.)were 800±40 mg m^-2 in the parasite, lower in uninfectedtrees (490±80) and lowest in infected trees (310±70).CO₂ assimilation rates calculated on a per unit chlorophyllbasis were similar in parasitized and non-parasitized trees,58±4·3 and 56±2·2 µmol mgchlorophyll^-1 h^-1 respectively, whereas the parasite's maximumCO₂ assimilation rate was considerably lower (20±1·2).Parasite transpiration rates were approximately double thoseof infected trees throughout the daylight period, and somewhathigher than uninfected trees from mid-morning onwards. Overall,parasite were generally less photosynthetically active thanhost plants; additionally, parasite infection was seen to reducehost carbon assimilation rates compared to those of uninfectedtrees.Copyright 1993, 1999 Academic Press Tapinanthus vittatus, Diplorhynchus condylocarpon, mistletoe, photosynthesis, Hill reaction, Zimbabwe     

Improvement of dioscorealide B production by elicitation in shoot cultures of Dioscorea membranacea Pierre ex Prain & Burkill

Dioscorealide B is an important secondary metabolite isolated from Dioscorea membranacea Pierre ex Prain & Burkill. The effect on secondary metabolite content of different concentrations of two elicitors [jasmonic acid (JA) and salicylic acid (SA)], and of medium status and JA exposure period were investigated. In the JA and SA concentration experiment, 6-week-old shoots were cultured on MS medium supplemented with 8.87 µM BA (6-benzyladenine) in combination with 100–500 µM JA or 50–200 µM SA for 3 weeks. MS medium supplemented only with 8.87 µM BA was used as a control. The highest dioscorealide B content was recorded in the 100 µM JA shoots. To determine the optimal medium status and JA exposure period, shoots were cultured on solid and in liquid MS media supplemented with 8.87 µM BA and 100 µM JA for 2, 3, 4 and 5 weeks. No interaction was found between the medium status and the elicitor exposure period in the dioscorealide B production. Shoots cultured on the solid MS medium supplemented with 100 µM JA had a higher dioscorealide B content (0.57 ± 0.35% w/w) than those cultured in liquid medium (0.36 ± 0.40% w/w) and 5-week JA exposure produced the highest dioscorealide B content of 1.05 ± 0.15% (w/w).     

Accurate and Simple Measurement of Phytic Acid Contents in Cereal Grains

We developed an accurate and simple method for measuring thephytic acid contents in cereal grains on the basis of closeexamination of various factors affecting the accuracy and reproducibilityof the measurement. Our conclusions were: (1) As extracting medium for phytic acid,HCl was better than trichloroacetic acid or H₂SO₄. (2) The suitablepH range for extracting phytic acid was 0.3 to 1.0. (3) Completeformation of Fe-phytate required four to nine times as muchFe^3$ as phytic acid. (4) No definite effect of Na₂SO₄ on therecovery of phytic acid was observed, though it increased theextracting efficiency of trichloroacetic acid. (5) The determinationlimit of phytic acid by the iron precipitation method was 3.3µmol per 20 ml. Our new method gave reliable results, and a linear relationshipof y=0.3163x$0.0597 was obtained up to 0.025 µmol µ^–1between the concentration of phytic acid and the zone lengthin isotachophoresis. The phytic acid contents of two kinds ofrice, two kinds of rye, and one kind of wheat and of barleywere determined with this method. (Received March 19, 1985; Accepted August 2, 1985)     

Cytotoxic Effect of Prodigiosin,Natural Red Pigment,Isolated from Serratia marcescens UFPEDA 398

Prodigiosin is a secondary metabolite, with red pigmentation, produced by Serratia marcescens. Red pigment is a natural alkaloid whose chemical structure has three pyrrole rings. Prodigiosin has been described for several biological activities, including antitumor, inducing apotosis in T and B lymphocytes. This work aimed to evaluate the cytotoxic activity of prodigiosin in NCHI-292, HEp-2, MCF-7 and HL-60 tumor cell lines. The red pigment was isolated from Serratia marcescens UFPEDA 398 biomass whose fractions were previously separated by column chromatography, purified, identified and further characterized by GC–MS and compared with the computerized library of m/z values. The pigment corresponded to prodigiosin with maximum absorption at 534 nm, molecular weight 323 and structural formula C20H25N3O. During the prodigiosin purification process a purple absorbance fraction at 272.65 nm was also observed. Significant cytotoxic effects of prodigiosin were evidenced for NCHI-292, Hep-2, MCF-7 and HL-60 tumor cell lines. The isolated purple fraction had no cytotoxic effect (IC50 11.3 µg/mL) when compared to prodigiosin (IC50 3.4 µg/mL) for the tumor cell lines studied. The MCF-7 strain was slightly more pigment resistant (IC50 5.1 µg/mL). Therefore, further studies will be needed to elucidate the antitumor mechanisms of prodigiosin action against tumor strains from flow cytometry tests. However, although these data are preliminary, it was evidenced that prodigiosin showed cytotoxic activity in tumor cell lines suggesting promising antitumor properties. In this sense, future studies on the cytotoxic and genotoxic effects of prodigiosin produced by S. marcecsens UFPEDA 398 are suggested.     

Effect of elevated CO2 on the utilization of light energy in Nothofagus fusca and Pinus radiata

Red beech (Nothofagus fusca (Hook. F.) Oerst.; Fagaceae) andradiata pine (Pinus radiata D. Don; Pinaceae) were grown for16 months in large open-top chambers at ambient (37 Pa) andelevated (66 Pa) atmospheric partial pressure of CO₂, and incontrol plots (no chamber). Summer-time measurements showedthat photosynthetic capacity was similar at elevated CO₂ (lightand CO₂-saturated value of 17.2 µmol m^–2 s^–1for beech, 13.5 µmol m^–2 s^–1 for pine), plantsgrown at ambient CO₂ (beech 21.0 µmol^–2 s^–1,pine 14.9 µmol m^–2s^–1) or control plants grownwithout chambers (beech 23.2 µmol m^–2 s^–1,pine 12.9 µmol m^–2 s^–1). However, the higherCO₂ partial pressure had a direct effect on photosynthetic rate,such that under their respective growth conditions, photosynthesisfor the elevated CO₂ treatment (measured at 70 Pa CO₂ partialpressure: beech 14.1 µmol m^–2 s^–1 pine 10.3)was greater than in ambient (measured at 35 Pa CO₂: beech 9.7µmol m^–2 s^–1, pine 7.0 µmol m^–2s^–1) or control plants (beech 10.8 µmol m^–2s^–1, pine 7.2 µmol m^–2 s^–1). Measurementsof chlorophyll fluorescence revealed no evidence of photodamagein any treatment for either species. The quantity of the photoprotectivexanthophyll cycle pigments and their degree of de-epoxidationat midday did not differ among treatments for either species.The photochemical efficiency of photosystem II (yield) was lowerin control plants than in chamber-grown plants, and was higherin chamber plants at ambient than at elevated CO₂. These resultssuggest that at lower (ambient) CO₂ partial pressure, beechplants may have dissipated excess energy by a mechanism thatdoes not involve the xanthophyll cycle pigments. Key words: Carotenoids, chlorophyll fluorescence, photosynthesis, photoinhibition, photoprotection, xanthophyll cycle     

The Interaction Between Silicon and Aluminium in Sorghum bicolor (L.) Moench: Growth Analysis and X-ray Microanalysis

Seeds of Sorghum bicolor (L.) Moench. were germinated on moistfilter paper for 6 d, before the seedlings were transferredto pots containing 500 µmol l^-1 Ca(NO₃)₂ for 2 d. Theseedlings were then treated with 0 or 100 µmol l^-1 Alin factorial combination with 0, 1400 or 2800 µmol l^-1Si for 8 d. The background solution used throughout was 500µmol l^-1 Ca(NO₃)₂. Aluminium treatment reduced root growthand caused a significant increase in shoot/root ratio. The presenceof silica in the solution significantly ameliorated the effectsof aluminium on root growth. Three treatment were selected for a microanalytical investigationof the basal region of the root: 2800 µmol l^-1 Si only;100 µmol l^-1 Al only; and a combination of the two. Inthe 2800 µmol l^-1 treatment silica was deposited in theendodermis, with the greatest accumulation being in the innertangential wall (ITW). When plants were treated with 100 µmoll^-1 Al only, aluminium concentration was highest in the outertangential wall (OTW) of the epidermis. The element was presentin the hypodermal walls and OTW of the endodermis and was notdetectable in the stele. With both 2800 µmol l^-1 Si and100 µmol l^-1 Al in the nutrient solution the two biomineralizationsites were the ITW of the endodermis, where silicon was themajor element deposited, and atypically in the OTW of the epidermiswhere both aluminium and silicon were present. The sequestrationof aluminium in the Al-Si deposit in the OTW of the epidermismay represent the mechanism that allows greater root growthin this treatment.Copyright 1993, 1999 Academic Press Sorghum bicolor (L.) Moench., aluminium, silicon, calcium, root, toxicity, biomineralization, X-ray microanalysis, freeze substitution     

Cyclopiazonic acid-induced changes in the contraction and Ca2+ transient of frog fast-twitch skeletal muscle

The effects ofcyclopiazonic acid (CPA) were investigated on isolated skeletal musclefibers of frog semitendinosus muscle. CPA (0.5-10 µM) enhancedisometric twitch but produced little change in resting tension. Athigher concentrations (10-50 µM), CPA depressed twitch andinduced sustained contracture without affecting resting and actionpotentials. In Triton-skinned fibers, CPA had no significant effect onmyofibrillar Ca²⁺ sensitivity butdecreased maximal activated force at concentrations >5 µM. Inintact cells loaded with the Ca²⁺fluorescence indicator indo 1, CPA (2 µM) induced an increase inCa²⁺-transient amplitude (10 ± 2.5%), which was associated with an increase in time to peak and inthe time constant of decay. Consequently, peak force was increased by35 ± 4%, and both time to peak and the time constant of relaxationwere prolonged. It is concluded that CPA effects, at a concentration ofup to 2 µM, were associated with specific inhibition of sarcoplasmicreticulumCa²⁺-adenosinetriphosphatase inintact skeletal muscle and that inhibition of the pump directlyaffected the handling of intracellularCa²⁺ and force production.

     

Regeneration and uptake of ammonium by plankton in an Amazon floodplain lake

Planktonic regeneration and uptake of ammonium was measuredin Lake Calado on the floodplain of the Amazon River (Brazil).In the epilimnion of the lake, regeneration and uptake averaged0.86 ± 0.15 and 1.9 ± 0.4 µmol NH₄1^–1h^–1 respectively. Uptake exceeded regeneration under theammonium-enriched conditions of our experimental incubations,but uptake and regeneration are in near-balance at in situ ammoniumconcentrations of <1 µM. In experiments that simulatedlake overturn, samples from the meta- and hypolimnion had higherrates of regeneration (1.4 ± 0.6) and lower rates ofuptake (0.89 ± 0.53) than epilimnetic samples. Duringinflow of water from the Amazon River and a local stream, ammoniumregeneration rates were similar to those measured in the epilimnion(0.84 ± 0.22), but the uptake rates were lower (0.49± 0.13). Although regeneration always exceeded uptakein the overturn and inflow experiments, ammonium did not accumulate.Rapid isotope dilution of the ammonium pool occurred in theincubations, and the effect of not accounting for isotope dilutioncould result in underestimating uptake rates by a factor of2–5 in the epilimnion of this tropical lake.     

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     

Response of young barley plants to CO2 enrichment

Barley (Hordeum vulgare L. cv. Digger) was grown for 22 d inenclosed chambers with a CO₂ enrichment of 35, 155, 400 or 675µmol CO₂ mol1. CO₂ enrichment increased photosyntheticcapacity in the plants grown at either of the two highest levelsof pCO₂. A CO₂ enrichment of 675µmol CO₂ caused a significantincrement of shoot dry weight, whereas no changes were observedin fresh weight, chlorophyll or protein levels. At a light intensityof 860µmol m^–2s^–1 CO₂ enrichment caused photosyntheticcapacity to increase by 250%, whereas no effect was observedat 80 µmol m^–2 s^–1. Over time, photosynthesisdecreased by 70% independent of CO₂. A time-dependent increasein the level of extractable fructose was observed whereas totalextractable carbohydrate only changed slightly. Key words: Carbohydrates, CO₂ enrichment, Hordeum vulgare, photosynthesis, respiration     

Brain natriuretic peptide inhibits hypoxic pulmonary hypertension in rats

Brainnatriuretic peptide (BNP) is a pulmonary vasodilator that is elevatedin the right heart and plasma of hypoxia-adapted rats. To test thehypothesis that BNP protects against hypoxic pulmonary hypertension, wemeasured right ventricular systolic pressure (RVSP), right ventricle(RV) weight-to-body weight (BW) ratio (RV/BW), and percentmuscularization of peripheral pulmonary vessels (%MPPV) in rats givenan intravenous infusion of BNP, atrial natriuretic peptide (ANP), orsaline alone after 2 wk of normoxia or hypobaric hypoxia (0.5 atm).Hypoxia-adapted rats had higher hematocrits, RVSP, RV/BW, and %MPPVthan did normoxic controls. Under normoxic conditions, BNP infusion(0.2 and 1.4 µg/h) increased plasma BNP but had no effect on RVSP,RV/BW, or %MPPV. Under hypoxic conditions, low-rate BNP infusion (0.2 µg/h) had no effect on plasma BNP or on severity of pulmonaryhypertension. However, high-rate BNP infusion (1.4 µg/h) increasedplasma BNP (69 ± 8 vs. 35 ± 4 pg/ml, P < 0.05),lowered RV/BW (0.87 ± 0.05 vs. 1.02 ± 0.04, P < 0.05), and decreased %MPPV (60 vs. 74%,P < 0.05). There was also a trend towardlower RVSP (55 ± 3 vs. 64 ± 2, P = not significant).Infusion of ANP at 1.4 µg/h increased plasma ANP in hypoxic rats (759 ± 153 vs. 393 ± 54 pg/ml, P < 0.05) but had noeffect on RVSP, RV/BW, or %MPPV. We conclude that BNP may regulatepulmonary vascular responses to hypoxia and, at the doses used in thisstudy, is more effective than ANP at blunting pulmonary hypertensionduring the first 2 wk of hypoxia.

     

Photosynthetic Properties of Guard Cell Protoplasts from Vicia faba L.

Guard cell protoplasts were isolated enzymatically from theepidermis of Vicia faba L. and their photosynthetic activitieswere investigated. Time courses of light-induced changes inthe chlorophyll a fluorescence intensity of these protoplastsshowed essentially the same induction kinetics as found formesophyll protoplasts of Vicia. The transient change in thefluorescence intensity was affected by DCMU, an inhibitor ofphotosystem II; by phenylmercuric acetate, an inhibitor of ferredoxinand ferredoxin NADP reductase; and by methyl viologen, an acceptorof photosystem I. Low temperature (77 K) emission spectra ofthe protoplasts had peaks at 684 and 735 nm and a shoulder near695 nm. A high O₂ uptake (175 µmol mg^–1 Chl hr^–1)was observed in guard cell protoplasts kept in darkness, whichwas inhibited by 2 mM KCN or NaN₃ by about 60%. On illumination,this O₂ uptake was partially or completely suppressed, but itssuppression was removed by DCMU, which indicates that oxygenwas evolved (150 µmol mg^–1 Chl hr^–1) photosynthetically.We concluded that both photosystems I and II function in guardcell chloroplasts and that these protoplasts have high respiratoryactivity. (Received January 30, 1982; Accepted May 15, 1982)     

Occurrence of Inducible Crassulacean Acid Metabolism in Leaves of Talimum triangulare (Portulacaceae)

In this paper we report for the first time the occurrence ofan inducible weak CAM in leaves of Talinwn triangulare (Jacq.)Willd. This plant is a terrestrial perennial deciduous herbwith woody stems and succulent leaves which grows under fullexposure and in the shade in northern Venezuela. Plants grownin a greenhouse (‘sun’ plants) and a growth cabinet(‘shade’ plants) with daily irrigation showed CO₂uptake only during the daytime (maximum rate, 4?0 µmolm^–2 s^–1) and a small acid accumulation during thenight (6?0 µmol H⁺g^–1 FW). Twenty-four hours aftercessation of irrigation, no CO₂ exchange was observed duringpart of the night. Dark fixation reached a maximum (1?0 µmolCO₂ m^–2 s^–1, 100 µmol H⁺ g^–1 FW) onday 9 of drought. By day 30 almost no gas exchange was observed,while acid accumulation was still 10 µmol H⁺ g^–1FW. Rewatering reverted the pattern of CO₂ exchange to thatof a C₃ plant within 24 h. Daytime and night-time phosphoenolpyruvatecarboxylase activity increased up to 100% (shade) and 62% (sun)of control values after 10 and 15 d of drought, respectively.Light compensation point and saturating irradiance were similarin well-watered sun and shade plants, values being characteristicof sun plants. CAM seems to be important for the tolerance ofplants of this species to moderately prolonged (up to 2 months)periods of drought in conditions of full exposure as well asshade, and also for regaining high photosynthetic rates shortlyafter irrigation. Key words: Talinum triwigulare, inducible CAM, PEP-C activity, recycling     

The Metabolism of Abscisic Acid

The light-catalysed isomerization of (+)-abscisic acid (ABA)to its trans isomer during isolation from leaves was monitoredby the addition of (±)-[2-¹⁴C]ABA to the extraction medium.(+)Trans-abscisic acid (t-ABA) was found to occur naturallyin rose (Rosa arvensis) leaves at 20µg/kg fresh weight;(+)-ABA was present at 594µg/kg. (±)-[2-¹⁴D]Trans-abscisicacid was not isomerized enzymically to ABA in tomato shoots. (±)-Abscisic acid was converted by tomato shoots to awater-soluble neutral product, ‘Metabolite B’, whichwas identified as abscisyl-ß-D-glucopyranoside. When(±)-[2-¹⁴C]trans-abscisic acid in an equimolar mixturewith (±)-[2-¹⁴C}ABA was fed to tomato shoots it was convertedto its glucose ester 10 times faster than was ABA. Trans-abscisyl-ß-D-glucopyrano8ide only was formedfrom (±)-[2-¹⁴C]t-ABA in experiments lasting up to 30h. Glucosyl abscisate was formed slowly from ABA and the freeacid fraction contained an excess of the unnatural (–).ABAas did the ABA released from abscisyl-ß-D-glucopyranosideby alkaline hydrolysis. The (+).ABA appeared to be the solesource of the acidic ‘Metabolite C" previously noted. The concentrations of endogenous (+)-, (+)-[2-¹⁴C]-, and (–)-[2-¹⁴C]ABAremaining as free acid, and also in the hydrolysate of abscisyl-ß-D-glucopyranoside,were measured by the ORD, UV absorption, and scintillation spectrometryof highly purified extracts of ABA from tomato shoots whichhad been supplied with racemic [2-^l4C]ABA.     

EFFECTS OF HEAVY METALS ON SURVIVAL AND GROWTH OF BIOMPHALARIA GLABRATA SAY (GASTROPODA: PULMONATA) AND INTERACTION WITH SCHISTOSOME INFECTION

The effects of dissolved Pb, Cd and Hg on growth and survivalof adult Biomphalaria glabrata Say. uninfected and infectedwith Schistosoma mansoni Sambon, were examined. Pb at concentrationsfrom 0.25–100 µM, Cd from 0.075–0.25 µMand Hg from 0.25–1 µM significantly reduced growthand survival of normal snails. With each metal the effects increasedwith increasing concentration. The LC 25 at 2 wks exposure was82 ± 19, 0.22 ± 0.04 and 0.94 ± 0.13 µM(x ± S.E.) for Pb, Cd and Hg, respectively. Snails exposedto heavy metals continued to be reproductively active, but theegg production was highly variable and no significant effectof heavy metal exposure was demonstrated. Infection also decreasedsurvival and growth of snail hosts and infected individualsexposed to heavy metals displayed the greatest mortality. TheLC 25 for infected snails at 2 wks exposure was 8 ± 3,0.9 ± 0.02 and 0.29 ± 0.08 for Pb, Cd and Hg respectively.A significant interaction between heavy metal exposure and infectionwas apparent. Infected snails were not reproductively active.Cercarial shedding by infected snails was significantly reducedin the presence of heavy metals and by 6 wks shedding had ceasedat the highest metal concentrations. (Received 20 May 1996; accepted 31 July 1996)     

The Regulation of Citric Acid Accumulation and Carbon Recycling During CAM in Ananas comosus

The carbon balance of shade-grown Ananas comosus was investigatedwith regard to nitrogen supply and responses to high PAR. Netdark CO₂ uptake was reduced from 61.2 to 38.5 mmol CO₂ m^–2in N limited (–N) plants grown under low PAR (60 µmolm^–2 s^–1) and apparent photon yield declined from0.066 to 0.034 (mol 0².mol^–1 photon), although photosyntheticcapacities (measured under 5% CO₂) were similar. Following transferfor 7 d to high PAR (600. µmol m^–2 s^–1), netCO² uptake at night increased by 14% in +N plants, and daytimephotosynthetic capacity was higher, with a maximum value of7.8 µmol m^–2 s^–1. The magnitude of dark CO₂ fixation during CAM was measured asdawn—dusk variations in leaf-sap titratable acidity (H+)and as the proportion of malic and citric acids. The contributionfrom re-fixation of respiratory CO₂ recycling (measured as thedifference between net CO₂ uptake and malic acid accumulation)varied with growth conditions, although it was generally lower(30%) than reported for other bromeliads. Assuming a stoichiometryof 2H+: malate and 3H+: citrate, there was a good agreementbetween titratable protons and enzymatically determined organicacids. The accumulation of citric acid was related to nitrogensupply and PAR regime, increasing from 7.0 mol m–3 (+Nplants) to 18 mol m^–3 (–N plants) when plants weretransferred to high PAR; malate: citrate ratios decreased from13.1 to 2.5 under these conditions. Under the low PAR regime, leaf-sap osmotic pressure increasedat night in proportion to malic acid accumulation. However,following the transfer to high PAR for 7 d, there was a muchgreater depletion of soluble sugars at night which correspondedto a decrease in leaf-sap osmotic pressure. Although a rolefor citric acid in CAM has not been properly defined, it appearsthat the accepted stoichiometry for CAM in terms of gas exchange,titratable acidity, malic acid and osmotic pressure may nothold for plants which accumulate citric acid. Key words: Ananas comosus, CAM, citric acid accumulation, carbon recycling     

The Influence of Seedling Size and Carbohydrate Status and of Photon Flux Density During Vernalization on Inflorescence Initiation in Onion (AIlium cepa L.)

Experiments were done in controlled conditions to investigatethe relationship of plant weight, leaf number and raising conditionsto inflorescence initiation in onion seedlings. Above a shootdry weight of 0.06 g the spring-sown cv. Rijnsburger could initiateinflorescences, whereas the autumn-sown cv. Senshyu semi-globeYellow needed to be heavier than 0.45 g. Plants raised at aphoton flux density of 200 µmol m^–2S^–1 anda temperature of 25 C required longer to initiate inflorescencesthan plants raised at 600 µmol m^–2S^–1 and17 C which had higher reserve carbohydrate content. The minimumleaf number for inflorescence initiation was larger for plantswith low reserve carbohydrate content. Photon flux densitiesof 50, 100, 200 and 400 µmol m^–2S^–1 duringvernalization at 9 C caused no differences in inflorescenceinitiation in plants previously raised at l7 C and 600 µmolm^–2S^–1 but the lowest photon flux density duringvernalization reduced initiation in plants previously raisedat 25 C and 200 µmol m^–2S^–1.