The role of CO₂uptake by roots and CAM in acquisition of inorganic C by plants of the isoetid life‐form: a review, with new data onEriocaulon decangulareL.

Abstract

SUMMARY The isoetid life‐form was originally defined on morphological grounds; subsequent physiological investigations showed that all of the isoetids examined took up a large fraction of the inorganic C fixed in their leaves from the root medium under natural conditions, and that some of them carried out much of their assimilation of inorganic C via a CAM‐like mechanism. Root‐dominated uptake of inorganic C appeared to be unique to, and ubiquitous in, the isoetids. I However, a large capacity for CAM‐like metabolism in submerged vascular plants is not universal in isoetids, nor is it restricted to this life‐form, being also found in Crassulaa aquatica. The work described here shows that submerged specimens of the North American Eriocaulon decangulare have a high fraction of their dry weight in the root system, a trait characteristic of isoetids but uncommon in other submerged vascular plants. E. decangulare has vesicular‐arbuscular mycorrhizas, as do other flowering plant isoetids hut not, generally, submerged Isoetes spp. Under conditions of natural supply of inorganic C, E. decangulare, like other isoetids, takes up most of its inorganic C through its roots. Uptake of inorganic C by both roots and shoots involves CO2 rather than HCO3: photosynthesis at high external pH values does not exceed the rate of uncatalysed HCO3‐ to CO2 conversion in the medium and there is no detectable extracellular carbonic anhydrase activity. Measurements of titratable acidity and of malate content of leaves sampled at dawn and at dusk showed that E. decangulare, growing and tested under either emersed or submersed conditions, did not exhibit CAM‐like behaviour. CAM was also absent from three non‐isoetid aquatic macrophytes (Amphibolic antarctica, Eeklonia radiata and Vallisneria spiralis) which were examined. E. decangulare thus resembles all other isoetids tested in acquiring much of its inorganic C via the root system. E. decangulare also resembles most of the isoetids which are not members of the Isoetaceae (e.g.) E. septangulare, Lobelia dortmanna and Subularia aquatica) but differs from submerged Isoetaceae and Littorella uniflora in lacking CAM. The ecological significance of uptake of CO2 via the roots and, where it occurs, of CAM in isoetids may be related to either inorganic C or, via improved N use efficiency, inorganic C as a limiting resource. The isoetid life‐forms has evolved independently in at last five different families of vascular plants; it probably derived fairly immediately from terrestrial or amphibious ancestors with a similar rosette form. Emergent Isoetaceae with acquisition of CO2 via roots and CAM probably evolved from submerged isoetids.

Adaptations for an amphibious life: changes in leaf morphology, growth rate, carbon and nitrogen investment, and reproduction during adjustment to emersion by the freshwater macrophyte Littorella uniflora

“…Aquatic L. uniflora possessed the large root system (reaching a depth of 30 cm in the sediments and comprising 30 % of fresh and dry weight ; Table 2) typical of this species (see Raven et al, 1988). The size of the root system appeared to remain relatively constant following emersion, although it became difficult to extract intact roots systems from the drying sediments.…”

“…Littorella uniflora L. Ascherson (Shoreweed) is a common species at the margins of tarns, lakes and reservoirs throughout the UK, northern Europe and North America (see Fig. 1, and Clapham, Tutin & Warburg, 1981 ;Raven et al, 1988 ;Robe & Griffiths, 1992). This small perennial plant (4-6 cm high) has a rosette of cylindrical evergreen leaves, a short stem and a large root system.…”

“…CO # supply to the leaves is predominantly by diffusion within the lacunal system from the CO # -enriched sediments, where concentrations are in the range 0n3-3n0 mol m −$ (Roelofs, 1983 ;Boston, Adams & Pienkowski, 1987 ;Robe & Griffiths, 1990. Daytime CO # supply within the leaves is boosted by recycling of respiratory CO # and Crassulacean Acid Metabolism (CAM), and lacunal concentrations are typically 0n45-2n70 mol m −$ (1-6 %) and saturating for photosynthesis in vitro (Madsen, 1987 a, b ;Raven et al, 1988 ;Robe & Griffiths, 1988, 1990. Photosynthetically generated O # diffuses from the leaf surfaces and via the lacunae into the roots and rhizosphere, facilitating nutrient supply and uptake.…”

Seasonal variation in the ecophysiology of Littorella uniflora (L.) Ascherson in acidic and eutrophic habitats

“…At Esthwaite the higher sediment COĉ oncentrations may have resulted in higher rates of exogenous carbon fixation. J.. unifiora utilizes only CO.^ (Boston, Adams & Pienkowski, 1987;Raven et al, 1988) the majority of which is taken up from the COj-enriched sediments (Robe & Griffiths, 1990). CO2 probably diffuses into roots and, via lacunae, into the leaves where light and dark fixation takes place in the cells lining the gas channels (Sondergaard & Sand-Jensen, 1979;Raven et al., 1988).…”

“…relatively large root area and an extensive system of 1N T R O D ir C T10 N , ,1 i • 1 *i, intercellular air spaces, or lacunae, running the Littorella unifiora (L.) Ascherson is a submersed length of (and continuous between) shoots and roots aquatic macrophyte of isoetid life form which grows (Raven et al, 1988). The cells surrounding the leaf rooted in the sediments close to the margins of lacunae are lined with chloroplasts.…”