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     

CAM-Like carbon pathway in submerged aquatic plants

CAM-like photosynthesis was found in the isoetid aquatic plantsLittorella uniflora andIsoetes lacustris, but not in the isoetid speciesLobelia dortmanna or in the elodeidElodea canadensis. Of the taxa studied, the first three are known to utilize sediment-borne CO₂, whereasElodea is dependent on bicarbonate.     

Evidence of crussulacean acid metabolism in two North American isoetids

The internal acidity levels of four common North American isoetids, or rosette-form aquatic macrophytes, were monitored under field conditions to determine the presence of crassulacean acid metabolism (CAM). The leaves of Littorella uniflora var. americana (Fern.) Gl. and Isoetes macrospora Durieu were found to have a diurnal fluctuation in titratable acidity characteristic of CAM plants, 124 and 142 μeq g^?1 fresh wt., respectively. No variations were detected in the acidity of stems and leaves of Myriophyllum tenellum Bigel. and Lobelia dortmanna L. Litorella and Isoetes were then grown in the laboratory where the diurnal acid rhythm was shown to be due to the accumulation and disappearance of malic acid.Based on the magnitude of the diurnal acid rhythm and existing information on the productivity of these plants, it appears that the carbon assimilated via crassulacean acid metabolism may contribute substantially to their net annual productivity. This appears to be a case for which CAM has been selected directly as a response to carbon stress.     

Outstanding Lobelia dortmanna in iron armor

Lobelia dortmanna leads a group of small, highly-valued rosette species that grow on coarse, nutrient-poor soils in temperate soft-water lakes. They acquire most CO₂ for photosynthesis by root uptake and efficient gas transport in large air channels to the leaves. Lobelia is the only species that releases virtually all photosynthetic oxygen from the roots and generates profound day-night changes in oxygen and CO₂ in the sediment pore-water. While oxygen release from roots stimulates decomposition and supports VA-mycorrhiza fungi, the ready gas exchange presents a risk of insufficient oxygen supply to the distal root meristems as sediments accumulate organic matter from lake pollution. So the plant with the greatest oxygen release from roots is also the most sensitive to oxygen depletion in sediments and it dies or losses anchorage by shortening the roots from 10 to 2 cm at even modest contents (2.4%) of degradable organic matter. Coatings of oxidized iron on roots in organically enriched sediments reduce radial oxygen loss and, thereby, increase internal concentrations and supply of oxygen to root tips. Oxidized iron is also a redox buffer which may prevent the ingress of sulfides and other reduced toxic solutes during nights. Controlled experiments are under way to test if iron enrichment can help survival of rosette species threatened by lake pollution or whether removal of organic surface sediments is required.Key words: isoetids, Lobelia dortmanna, iron, ROL, sediment oxygen, iron plaques     

High resistance of oligotrophic isoetid plants to oxic and anoxic dark exposure

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Use of sediment CO2 by submersed rooted plants

Background and Aims

Submersed plants have different strategies to overcome inorganic carbon limitation. It is generally assumed that only small rosette species (isoetids) are able to utilize the high sediment CO₂ availability. The present study examined to what extent five species of submersed freshwater plants with different morphology and growth characteristics (Lobelia dortmanna, Lilaeopsis macloviana, Ludwigia repens, Vallisneria americana and Hydrocotyle verticillata) are able to support photosynthesis supplied by uptake of CO₂ from the sediment.

Methods

Gross photosynthesis was measured in two-compartment split chambers with low inorganic carbon availability in leaf compartments and variable CO₂ availability (0 to >8 mmol L⁻¹) in root compartments. Photosynthetic rates based on root-supplied CO₂ were compared with maximum rates obtained at saturating leaf CO₂ availability, and ¹⁴C experiments were conducted for two species to localize bottlenecks for utilization of sediment CO₂.

Key Results

All species except Hydrocotyle were able to use sediment CO₂, however, with variable efficiency, and with the isoetid, Lobelia, as clearly the most effective and the elodeid, Ludwigia, as the least efficient. At a water column CO₂ concentration in equilibrium with air, Lobelia, Lilaeopsis and Vallisneria covered >75% of their CO₂ requirements by sediment uptake, and sediment CO₂ contributed substantially to photosynthesis at water CO₂ concentrations up to 1000 µmol L⁻¹. For all species except Ludwigia, the shoot to root ratio on an areal basis was the single factor best explaining variability in the importance of sediment CO₂. For Ludwigia, diffusion barriers limited uptake or transport from roots to stems and transport from stems to leaves.

Conclusions

Submersed plants other than isoetids can utilize sediment CO₂, and small and medium sized elodeids with high root to shoot area in particular may benefit substantially from uptake of sediment CO₂ in low alkaline lakes.Key words: Submersed rooted plants, CO₂ uptake, sediment CO₂, Lobelia dortmanna, Lilaeopsis macloviana, Ludwigia repens, Vallisneria americana, Hydrocotyle verticillata     

Organic enrichment of sediments reduces arbuscular mycorrhizal fungi in oligotrophic lake plants

1. Arbuscular mycorrhizal fungi (AMF) commonly colonise isoetid species inhabiting oxygenated sediments in oligotrophic lakes but are usually absent in other submerged plants. We hypothesised that organic enrichment of oligotrophic lake sediments reduces AMF colonisation and hyphal growth because of sediment O₂ depletion and low carbon supply from stressed host plants. 2. We added organic matter to sediments inhabited by isoetids and measured pore‐water chemistry (dissolved O₂, inorganic carbon, Fe²⁺ and ), colonisation intensity of roots and hyphal density after 135 days of exposure. 3. Addition of organic matter reduced AMF colonisation of roots of both Lobelia dortmanna and Littorella uniflora, and high additions stressed the plants. Even small additions of organic matter almost stopped AMF colonisation of initially un‐colonised L. uniflora, though without reducing plant growth. Mean hyphal density in sediments was high (6 and 15 m cm^?3) and comparable with that in terrestrial soils (2–40 m cm^?3). Hyphal density was low in the upper 1 cm of isoetid sediments, high in the main root zone between 1 and 8 cm and positively related to root density. Hyphal surface area exceeded root surface area by 1.7–3.2 times. 4. We conclude that AMF efficiently colonise isoetids in oligotrophic sediments and form extensive hyphal networks. Small additions of organic matter to sediments induce sediment anoxia and reduce AMF colonisation of roots but cause no apparent plant stress. High organic addition induces night‐time anoxia in both the sediment and the plant tissue. Tissue anoxia reduces root growth and AMF colonisation, probably because of restricted translocation of nutrient ions and organic solutes between roots and leaves. Isoetids should rely on AMF for P uptake on nutrient‐poor mineral sediments but are capable of growing without AMF on organic sediments.     

Rapid oxygen exchange across the leaves of Littorella uniflora provides tolerance to sediment anoxia

1. Littorella uniflora and Lobelia dortmanna are prominent small rosette species in nutrient‐poor, soft‐water lakes because of efficient root exchange of CO₂ and O₂. We hypothesise that higher gas exchange across the leaves of L. uniflora than of L. dortmanna ensures O₂ uptake from water and underlies its greater tolerance to sediment anoxia following organic enrichment. 2. We studied plant response to varying sediment O₂ demand and biogeochemistry by measuring photosynthesis, gas exchange across leaves and O₂ dynamics in plants during long‐term laboratory and field studies. Frequent non‐destructive sampling of sediment pore water was used to track changes in sediment biogeochemistry. 3. Addition of organic matter triggered O₂ depletion and accumulation of , Fe²⁺ and CO₂ in sediments. Gas exchange across leaf surfaces was 13–16 times higher for L. uniflora than for L. dortmanna. Oxygen in the leaf lacunae of L. uniflora remained above 10 kPa late at night on anoxic sediments despite organic enrichment. Leaf content of N and P of L. uniflora remained sufficient to keep up photosynthesis despite prolonged sediment anoxia, whereas nutrient content was too low for long‐term survival of L. dortmanna. 4. High gas exchange across L. uniflora leaves improves its performance and survival on anoxic sediments compared with L. dortmanna. Lobelia dortmanna uses the same gas‐tight leaves in air and water, which makes it highly susceptible to sediment anoxia but more cost‐effective in ultra‐oligotrophic environments because of slow leaf turnover.     

Studies of Diurnal Acid Fluctuations in British Isoetid-type Submerged Aquatic Macrophytes

British isoetid species are examined for the presence of diurnalfluctuations in tritratable acidity (to pH 6·4), in plantscollected directly from a small lake and in plants grown inconstant conditions in the laboratory. Wide diurnal fluctuationsare present in Isoetes lacustris and in both submerged and terrestrialpopulations of Littorella uniflora. They are absent in Lobeliadortmanna, Subularia aquatica, Eriocaulon septangulare, Ranunculusflammula and Pilularia globulifera. The significance of submerged CAM is discussed in relation toother carbon accumulating mechanisms in isoetids and in considerationof their general ecology. Crassulacean acid metabolism, photosynthesis, isoetid, oligotrophic lakes     

Simultaneous CO2- and 16O2/18O2-gas exchange and fluorescence measurements indicate differences in light energy dissipation between the wild type and the phytochrome-deficient aurea mutant of tomato during water stress

The CO₂-, H₂O- and ¹⁶O₂/¹⁸O₂ isotopic-gas exchange and the fluorescencequenching by attached leaves of the wild-type and of the phytochrome-deficienttomato aurea mutant was compared in relation to water stressand the photon fluence rate. The chlorophyll content of aurealeaves was reduced and the ultra-structure of the chloroplastswas altered. Nevertheless, the maximum rate of net CO₂ uptakein air by the yellow-green leaves of the aurea mutant was similarto that by the dark-green wild-type leaves. However, less O₂was produced by the leaves of the aurea mutant than by leavesof the wild-type. This result indicates a reduced rate of photosyntheticelectron flux in aurea mutant leaves. No difference in bothphotochemical and non-photochemical fluorescence quenching wasfound between wild-type and aurea mutant leaves. Water stresswas correlated with a reversible decrease in the rates of bothnet CO₂ uptake and transpiration by wild-type and aurea mutantleaves. The rate of gross ¹⁶O₂ evolution by both wild-type andaurea mutant leaves was fairly unaffected by water stress. Thisresult shows that in both wild-type and aurea leaves, the photochemicalprocesses are highly resistant to water stress. The rate ofgross ¹⁸O₂ uptake by wild-type leaves increased during waterstress when the photon fluence rate was high. Under the sameconditions, the rate of gross ¹⁸O₂ uptake by ^aurea mutant leavesremained unchanged. The physiological significane of this differencewith respect to the (presumed) importance of oxygen reductionin photoprotection is discussed. Key words: Water stress, gas exchange, fluorescence quenching, Lycopersicon esculentum, mutant (tomato, aurea), energy dissipation     

Sources of Inorganic Carbon Acquired through CAM in Littorella uniflora (L.) Aschers

Madsen, T. V. 1987. Sources of inorganic carbon acquired throughCAM in Littorella uniflora (L.) Aschers.—J. exp. Bot.38: 367–377. The CO₂ dynamics of the lacunal air and the relative contributionof external and internal CO₂ sources to dark CO₂ assimilationwas examined in the submerged aquatic CAM species Littorellauniflora (L.) Aschers. Refixation of internal CO₂, released by dark respiration, constitutedabout 30–35% of the total dark CO₂ assimilation. At aCO₂ concentration of 0·2 mol m^–3 around the leavesthe external CO₂ uptake through the roots increased from 45%of the total CO₂ uptake at 0·7 mol m^–3 CO₂ to 100%at 1·6 mol m^–3 and 3·1 mol m^–3 CO₂around the roots. The negligible importance of leaf CO₂ uptakeat high CO₂ concentrations around the roots was the result ofa causative high CO₂ concentration in the leaf lacunae. The CO₂ permeability of Littorella leaves was high relativeto root permeability. This has at least two ecological implications:(1) it enhances the potential diffusive release of CO₂ fromthe sediment C0₂-pool via the lacunal system of the plants.This loss of CO₂, however, was found to be greatly reduced byCAM activity of the plants. (2) The high permeability of theleaf surface to CO₂ exchange allows the plants to assimilateCO₂ from the water surrounding the leaves when the concentrationis high, i.e. during extensive epiphyte dark respiration. Thus,CAM tends to facilitate retension of a high CO₂ pool in thesediment-plant system and at the same time allows the plantsto exploit the water column CO₂ source when it is abundant.This result is in accordance with the general idea that CAMin aquatics constitute a carbon conserving mechanism. Key words: Aquatic macrophytes, dark CO₂ assimilation, inorganic carbon sources     

Carbon Loss from the Roots of Forage Rape (Brassica napus L.) Seedlings Following Pulse-labelling with 14CO2

The loss of organic material from the roots of forage rape (Brassicanapus L.,) was studied by pulse-labelling 25-d-old non-sterilesand-grown plants with ¹⁴CO₂. The distribution of ¹⁴C withinthe plant was measured at 0, 6 and 13 d after labelling whilst¹⁴ C accumulating in the root-zone was measured at more frequentintervals. The rates of ¹⁴C release into the rhizosphere, andloss of ¹⁴CO₂ from the rhizosphere were also determined. Thesedata were used to estimate the accumulative loss of ¹⁴C fromroots and loss respiratory ¹⁴CO₂ from both roots and associatedmicro-organisms. Approximately 17-19% of fixed ¹⁴CO₂ was translocatedto the roots over 2 weeks, of which 30-34% was released intothe rhizosphere, and 23-24% was respired by the roots as ¹⁴CO₂. Of the ¹⁴C released into the rhizosphere, between 35-51%was assimilated and respired by rhizosphere micro-organisms.Copyright1993, 1999 Academic Press Brassica napus L., carbon loss, carbon partitioning, microbial nutrition, microbial respiration, forage rape, pulse-labelling, rhizodeposition, root respiration, sand culture     

Impact of acidification and eutrophication on macrophyte communities in soft waters. II. Experimental studies

In The Netherlands, there has been a dramatic decline during the last 30 years in the number of stands belonging to the phytosociological alliance Littorellion. Generally, the communities classified within this alliance occur in poorly buffered, oligotrophic waters, with very low phosphate, nitrogen and carbon dioxide levels in the water layer and considerably higher nutrient levels in the sediment. The plant species dominating these communities are isoetids such as Litoorella uniflora (L.) Aschers., Lobelia dortmanna L. and Isoetes lacustris L., which show various adaptations to make successful growth possible under these conditions.Field observations showed that the water where Littorella uniflora had disappeared or strongly decreased could be divided into two groups. A major group (77%) was characterized by the presence of submerged Juncus bulbosus L. and/or Sphagnum species. These water appeared to be strongly acidified (pH < 4.5) and had increased nitrogen levels with ammonium as the dominant N-source. Within this group, the waters with luxuriant growth of Juncus bulbosus and/or Sphagnum spp. had strongly increased carbon dioxide levels in both sediment and water.Different types of experiments proved causal relationships between the observed changes in macrophytes and the changed physico-chemical parameters. Ecophysiological experiments showed that Juncus bulbosus lacks the typical adaptations of the isoetid plant species, i.e. it uses very low amounts of sediment-CO₂ and releases only a little oxygen from the roots. However, Juncus bulbosus is more able than Littorella uniflora to use CO₂ from the water layer. From the nutrient-uptake experiments, the decreased nitrate and increased ammonium levels seem to be favourable to Juncus bulbosus. The culture experiments clearly demonstrated that the biomass of Juncus bulbosus only increased strongly when the sediment was poorly buffered and the pH of water was low. When combining factors like CO₂ enrichment of the sediment, with and without phosphate, and/or ammonium enrichment of the water in the culture experiments, it is clearly shown that phosphate and/or ammonium enrichment without CO₂ enrichment do not lead to an increase in biomass of Juncus bulbosus. Therefore, it is obvious that the changes in the macrophyte community can be ascribed primarily to changes in the carbon budget as a result of acidification.A minor group of waters (23%) was characterized by the absence of submerged Juncus bulbosus and/or Sphagnum spp. In most of these waters, submerged plant species occurred, such as Myriophyllum alterniflorum DC or non-rooted species such as Riccia fluitans L. These waters were not acidified, and generally had an increased alkalinity and higher nitrogen and phosphate levels of sediment and/or water. Culture experiments showed that phosphate enrichment of the sediment alone leads to luxuriant growth of submerged macrophyte species such as Myriophyllum alterniflorum, whereas phosphate enrichment of both sediment and water leads to mass development of non-rooted plant species such as Riccia fluitans.     

Influence of sediment organic enrichment and water alkalinity on growth of aquatic isoetid and elodeid plants

1. Lake eutrophication has increased phytoplankton blooms and sediment organic matter. Among higher plants, small, oligotrophic rosette species (isoetids) have disappeared, while a few tall, eutrophic species (elodeids) may have persisted. Despite recent reduction of nutrient loading in restored lakes, the vegetation has rarely regained its former composition and coverage. Patterns of recovery may depend on local alkalinity because HCO₃^? stimulates photosynthesis of elodeids and not of isoetids. In laboratory growth experiments with two isoetids (Lobelia dortmanna and Littorella uniflora) and two elodeids (Potamogeton crispus and P. perfoliatus), we test whether organic enrichment of lake sediments has a long‐lasting influence by: (i) reducing plant growth because of oxygen stress on plant roots and (ii) inhibiting growth more for isoetids than elodeids. We also test whether (iii) increasing alkalinity (from 0.17 to 3.20 meq. L^?1) enhances growth and reduces inhibition of organic sediment enrichment for elodeids but not for isoetids. 2. In low organic sediments, higher oxygen release from roots of isoetids than elodeids generated oxic conditions to greater sediment depth for Lobelia (4.3 cm) and Littorella (3.0 cm) than for Potamogeton species (1.6–2.2 cm). Sediment oxygen penetration depth fell rapidly to 0.4–1.0 cm for all four species at even modest organic enrichment and oxygen consumption in the sediments. Roots became shorter and isoetid roots became thicker to better supply oxygen to apical meristems. 3. Growth of elodeids was strongly inhibited across all levels of organic enrichment of sediments being eight‐fold lower at the highest enrichment compared to the unenriched control. Leaf biomass of isoetids increased three‐fold by moderate organic enrichment presumably because of greater CO₂ supply from sediments being their main CO₂ source. At higher organic enrichment, isoetid biomass was reduced, leaf chlorophyll declined up to 10‐fold, root length declined from 7 to <2 cm and mortality rose (up to 50%) signalling high plant stress. 4. Lobelia was not affected by HCO₃^? addition in accordance with its use of sediment CO₂. Biomass of elodeids increased severalfold by rising alkalinity from 0.17 to 3.20 meq. L^?1 in accordance with their use of HCO₃^? for photosynthesis, while the negative impact of organically enriched sediments remained. 5. Overall, root development of all four species was so strongly restricted in sediments enriched with labile organic matter that plants if growing in situ may lose root anchorage. Other experiments demonstrate that this risk is enhanced by greater water content and reduced consolidation in organically rich sediments. Therefore, formation of more muddy and oxygen‐demanding sediments during eutrophication will impede plant recovery in restored lakes while high local alkalinity will help elodeid recovery.     

Impact of acidification and eutrophication on macrophyte communities in soft waters in The Netherlands I. Field observations

During the last decades a strong decline has been noticed in the number of waters dominated by “Littorellion” species, mostly isoetids such as Lobelia dortmanna L., Isoetes lacustris L. and Littorella uniflora (L.) Aschers. Sixty-eight waters, which were known to be dominated by L. uniflora after 1950 were investigated. In 1980, L. uniflora appeared to be absent or to have strongly decreased in 53 (78%) of the waters. In 41 of them, Littorella had been replaced by submerged Juncus bulbosus L. and/or Sphagnum spp. These changes seem to have been caused by changed inorganic carbon budgets as a consequence of acidification.In the remaining 12 waters, eutrophication of the water and/or sediment seems to be responsible for the changes in the plant communities. Enrichment with phosphate of the mineral sediment alone, leads to luxurious growth of submerged, rooted macrophyte species such as Myriophyllum alterniflorum DC and Ranunculus peltatus Schrank, whereas phosphate-enrichment of both sediment and water leads to luxurious growth of pleustophytes such as Riccia fluitans L. and Lemna minor L. in small, shallow waters, and to plankton bloom and luxurious growth of epiphytes in larger, deeper waters.In these cases light limitation seems to be responsible for the disappearance or decline of the “Littorellion” species.     

Soil O2 and CO2 Effects on Apparent Cell Viability for Roots of Desert Succulents

Roots of desert succulents occupy the upper layers of porous,well-aerated soils. However, roots of Agave deserti, Ferocactusacanthodes, and Opuntia ficus-indica all tolerated many daysof soil anoxia; 0% O₂ in the soil gas phase for 30 d reducedthe fraction of cells taking up the vital stain neutral red,an average of only 18% for the cortex and 6% for parenchymacells within the stele of perennial established roots. Ephemeralrain roots, induced by watering as branches on the establishedroots, were more susceptible to 0% O₂ in the soil gas phase;19 d abolished stain uptake for cortical cells and 32 d forstelar parenchyma cells. Soil CO₂ levels above the 0.1% observedin the root zone in the field rapidly reduced uptake of neutralred; the fraction of cortical cells taking up the stain decreased30% in 10 h at 0.5% CO₂ and was abolished in 9 h at 2% and 7h at 10% CO₂ averaged for the three species. Rain roots weresomewhat more susceptible than established roots to elevatedsoil CO₂ levels, and stelar parenchyma cells were much lesssusceptible than were cortical cells. When uptake of the vitalstain was abolished by elevated soil CO₂, no anatomical evidenceof cellular damage was observed. For A. deserti exposed to 2%CO₂, the pH of macerated root tissue decreased about 0.35 pHunit over 10 h; CO₂ apparently entered the cells, lowered theintracellular and/or cell wall pH, and prevented the accumulationof neutral red. Elevated soil CO₂ also inhibits root respirationfor the three desert succulents considered. Hence, the restrictionof such species to porous soils may reflect the relatively rapidinhibiting effects of elevated soil CO₂ levels rather than arequirement for high soil O₂ levels, consistent with the observationthat desert soils tend to have low gas-phase CO₂ levels near0.1% compared with 1% or more in the root zone of non-desertspecies. Key words: Agave deserti, Ferocactus acanthodes, neutral red, Opuntia ficus-indica, pH     

Photosynthetic Uptake of Free CO2, by the Roots of Lobelia dortmanna

Lobelia dortmanna L. is probably unable to utilize HCO₃^?and uses only free CO₂ for photosynthesis as shown by the Winkler method. A ¹⁴C technique was used to show that if CO₂ is added to the water around roots, photosynthesis increases 3–5 times more than when the corresponding amount of CO₂ is added to the water surroundings the leaves. As the CO₂ content in lakes where Lobelia grows is very limited, Lobelia must absorb CO₂ from the sediment, and the carbon dioxide will have to diffuse from the roots up into the leaves through the intercellular system of the plant. In conjunction with Lobelia's CO₂ uptake from the sediment O₂ is liberated. The plant, therefore, acts as an oxygen pump, which oxidizes the sediment down to a depth of 20 cm.     

Root Water Uptake, Leaf Water Storage and Gas Exchange of a Desert Succulent: Implications for Root System Redundancy

A technique used for hydroponics was adapted to measure instantaneousroot water uptake from the soil for a leaf succulent CAM species,Agave deserti. Comparisons were made to previously modelledwater fluxes for A. deserti and to Encelia farinosa, a non-succulentC₃species. Net CO₂uptake and transpiration forA. deserti underwell-watered conditions occurred primarily at night whereasroot water uptake was relatively constant over 24 h. Leaf thicknessdecreased when transpiration commenced and then increased whenrecharge from the stem and soil occurred, consistent with previousmodels. A drought of 90 d eliminated net CO₂uptake and transpirationand reduced the water content of leaves by 62%. Rewetting theentire root system for 7 d led to a full recovery of leaf waterstorage but only 56% of maximal net CO₂uptake. Root water uptakewas maximal immediately after rewetting, which replenished rootwater content, and decreased to a steady rate by 14 d. Whenonly the distal 50% of the root system was rewetted, the timefor net CO₂uptake and leaf water storage to recover increased,but by 30 d gas exchange and leaf water storage were similarto 100% rewetting. Rewetting 10 or 20% of the root system resultedin much less water uptake; these plants did not recover leafwater storage or gas exchange by 30 d after rewetting. A redundancyin the root system of A. deserti apparently exists for dailywater uptake requirements under wet conditions but the entireroot system is required for rapid recovery from drought.Copyright1999 Annals of Botany Company Agave deserti Engelm., desert, drought, gas exchange, rewetting, roots, succulent, water uptake.     

Inorganic carbon assimilation in the Isoetids,Isoetes lacustris L. and Lobelia dortmanna L.

Summary The inorganic carbon fixation patterns of Isoetes lacustris and Lobelia dortmanna from an oligotrophic Scottish loch have been examined by following titratable acidity changes in plant sap and light/dark ¹⁴CO₂ incorporation by roots and shoots. The diurnal pattern of titratable acidity changes in I. lacustris suggests crassulacean acid metabolism (CAM) while the lack of any change in titratable acidity in L. dortmanna suggests C₃ metabolism. Of the carbon fixed by L. dortmanna, 99.9% was taken up through the roots and fixation occurred primarily during the day. In Isoetes, CO₂ was taken up by both roots and shoots and during both day and night. Regardless of the site of CO₂ uptake, fixation occurred only in the shoots of both plants. Analysis of carbon isotope ratios of plant organic material was used to further investigate the photosynthetic mechanisms of these Isoetids. Considering the absence of a nighttime peak in titratable acidity in L. dortmanna, the ¹³C (=¹³C plant-¹³C source) value of the shoots of L. dortmanna (-14.2) is indicative of C₃ photosynthesis limited by the rate of CO₂ diffusion. The less negative of I. lacustris (-6.0) is consistent with both dark acidification of CAM and CO₂ limited C₃ photosynthesis. This is in contrast to the terrestrial Isoetes durieui which is shown to have a value which is similar to a terrestrial C₃ plant. The carbon fixation patterns of these Isoetids suggest that the CO₂ concentration in the loch may be growth limiting, and that root uptake and/or dark acidification are means of optimising CO₂ supply. However, in view of the relatively high levels of CO₂ in sediment and bulk water, it is suggested that low levels of nutrients may also limit growth in these plants.     

A community of submerged aquatic CAM plants in Lake Kalgaard,Denmark

The CO₂ exchange pattern of leaves of dominant Isoetes lacustris L., Littorella uniflora (L.) Aschers. and Lobelia dortmanna L. from oligotrophic, carbon-poor Lake Kalgaard, Denmark was examined by gas-exchange experiments and by following the diurnal acidity rhythm. Both variables suggest Crassulacean Acid Metabolism (CAM) in Littorella and Isoetes. During the dark, both species had a continuous, but declining CO₂ uptake. The maximum uptake in Littorella was 28 μmol CO₂ g^?1 DW h^?1 or about 36% of the light CO₂ assimilation rate. In Isoetes, corresponding figures were 19 μmol CO₂ g^?1 DW h^?1 and 22%.A peak of 49.5 μEq g^?1 FW titratable acidity (Littorella) and 41.9 μEq g^?1 FW (Isoetes) was found at the end of the dark period, reflecting an increase of 11.6 μEq g^?1 FW (9 h)^?1 (Littorella) and 16.3 μEq g^?1 FW (9 h)^?1 (Isoetes). These results agreed with the cumulative CO₂ assimilation during the dark.The assimilation of CO₂ in the dark is important, apparently, for growth of Littorella and Isoetes in Lake Kalgaard. Lobelia, which exhibits little dark CO₂ assimilation, is also the least abundant species, at less than 1% of the total cover.