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Notes: Plants that have evolved to survive on metal-rich soils—metallophytes—have key values that must drive research of their unique properties and ultimately their conservation. The ability of metallophytes to tolerate extreme metal concentrations commends them for revegetation of mines and metal-contaminated sites. Metallophytes can also be exploited in environmental technologies, for example, phytostabilization, phytoremediation, and phytomining. Actions towards conserving metallophyte species are imperative, as metallophytes are increasingly under threat of extinction from mining activity. Although many hundreds of papers describe both the biology and applications of metallophytes, few have investigated the urgent need to conserve these unique species. This paper identifies the current state of metallophyte research, and advocates future research needs for the conservation of metallophyte biodiversity and the sustainable uses of metallophyte species in restoration, rehabilitation, contaminated site remediation, and other nascent phytotechnologies. Six fundamental questions are addressed: (1) Is enough known about the global status of metallophytes to ensure their conservation? (2) Are metallophytes threatened by the activities of the minerals industry, and can their potential for the restoration or rehabilitation of mined and disturbed land be realized? (3) What problems exist in gaining prior informed consent to access metallophyte genetic resources and how can the benefits arising from their uses be equitably shared? (4) What potential do metallophytes offer as a resource base for phytotechnologies? (5) Can genetic modification be used to “design” metallophytes to use in the remediation of contaminated land? (6) Does the prospect of using metallophytes in site remediation and restoration raise ethical issues?

Notes: Thlaspi caerulescens J. & C. Presl is a distinctive metallophyte of central and western Europe that almost invariably hyperaccumulates Zn to〉 1.0% of shoot dry biomass in its natural habitats, and can hyperaccumulate Ni to〉 0.1% when growing on serpentine soils. Populations from the Ganges region of southern France also have a remarkable ability to accumulate Cd in their shoots to concentrations well in excess of 0.01% without apparent toxicity symptoms. Because hyperaccumulation of Cd appears to be highly variable in this species, the relationship between Cd tolerance and metal accumulation was investigated for seven contrasting populations of T. caerulescens grown under controlled conditions in solution culture. The populations varied considerably in average plant biomass (3.1-fold), shoot : root ratio (2.2-fold), Cd hyperaccumulation (3.5-fold), shoot : root Cd-concentration ratio (3.1-fold), and shoot Cd : Zn ratio (2.6-fold), but the degree of hyperaccumulation of Cd and Zn were strongly correlated. Two populations from the Ganges region were distinct in exhibiting high degrees of both Cd tolerance and hyperaccumulation (one requiring 3 µM Cd for optimal growth), whereas across the other five populations there was an inverse relationship between Cd tolerance and hyperaccumulation, as has been noted previously for Zn. Metal hyperaccumulation was negatively correlated with shoot : root ratio, which could account quantitatively for the differences between populations in shoot Zn (but not Cd) concentrations. On exposure to 30 µM Cd, the two Ganges populations showed marked reductions in shoot Zn and Fe concentrations, although Cd accumulation was not inhibited by elevated Zn; in the other five populations, 30 µM Cd had little or no effect on Zn or Fe accumulation but markedly reduced shoot Ca concentration. These results support a proposal that Cd is taken up predominantly via a high-affinity uptake system for Fe in the Ganges populations, but via a lower-affinity pathway for Ca in other populations. Total shoot Cd accumulated per plant was much more closely related to population Cd tolerance than Cd hyperaccumulation, indicating that metal tolerance may be the more important selection criterion in developing lines with greatest phytoremediation potential.

Notes: Abstract Water storage and nocturnal increases in osmotic pressure affect the water relations of the desert succulent Ferocactus acanthodes, which was studied using an electrical circuit analog based on the anatomy and morphology of a representative individual. Transpiration rates and osmotic pressures over a 24-h period were used as input variables. The model predicted water potential, turgor pressure and water flow for various tissues. Plant capacitances, storage resistances and nocturnal increases in osmotic pressure were varied to determine their role in the water relations of this dicotyledonous succulent. Water coming from storage tissues contributed about one-third of the water transpired at night: the majority of this water came from the nonphotosynthetic, water storage parenchyma of the stem. Time lags of 4 h were predicted between maximum transpiration and maximum water uptake from the soil. Varying the capacitance of the plant caused proportional changes in osmotically driven water movement but changes in storage resistance had only minor effects. Turgor pressure in the chlorenchyma depended on osmotic pressure, but was fairly insensitive to doubling or halving of the capacitance or storage resistance of the plant. Water uptake from the soil was only slightly affected by osmotic pressure changes in the chlorenchyma. For this stem succulent, the movement of water from the chlorenchyma to the xylem and the internal redistribution of water among stem tissues were dominated by nocturnal changes in chlorenchyma osmotic pressure, not by transpiration.

Notes: The aim of the present study was to develop and test an extended theory of the pressure-clamp technique that would allow the estimation of additional cell water-relation parameters using the pressure probe. It was assumed that intercellular water transport (vacuole → vacuole) occurs via a composite ‘membrane’ comprising the tonoplast, cytoplasm, plasma membrane, plasmodesmata and cell wall and that solute transport across the micropipette tip of the probe is dominated by convection. The extended theory allows the simultaneous estimation of cell volume (V), effective cell-sap osmotic pressure (σΠ0), composite-membrane hydraulic conductance (ALP) and the ‘instantaneous’ volume change (v0) at the start of the pressure clamp. With an estimate of cell-sap osmotic pressure (Π0), the weighted-average reflection coefficient (σ) of the composite membrane may also be determined from an endosmotic pressure-clamp experiment. In principle, the cell volume before the clamp (V0) can be estimated as V0 = V + v0, and the cell volumetric elastic modulus (ɛ) can be estimated as ɛ = –V0(ΔP/v0), where ΔP (〈 0) is the change in turgor pressure at t = 0. In practice it may be necessary to correct v0 for transmembrane water flow when estimating V0 and ɛ. To test this theory, a previously described pressure-probe system was upgraded by incorporating a data-acquisition system to record turgor pressure, meniscus position, the penetration depth of the pressure-probe micropipette and the micropipette profile. An air bubble in the system allowed the turgor pressure to be clamped at a near-constant value. For mesophyll cells in leaf discs of Kalanchoë daigremontiana, the estimated mean value of V0[(2·46 ± 0·24) × 10–13 m3] was in reasonable agreement with a value obtained from microscopy [(1·91 ± 0·10) × 10–13 m3]. When the positive correlation between ɛ and |ΔP| was taken into account, the parameter estimates obtained from pressure-clamp experiments were found to be consistent with those derived from pressure-relaxation experiments. The mean values of ɛ (1·4 ± 0·2 MPa) and LP[(5·76 ± 0·83) × 10–12 m s–1 Pa–1] were significantly different from previous estimates obtained with older leaves. Furthermore, in contrast to the estimated reflection coefficient of the plasma membrane/tonoplast (≈ 1), the mean value of σ obtained in the present study (0·62 ± 0·05) was significantly less than unity. This is attributed to a high hydraulic conductance and a low reflection coefficient of a symplastic pathway for intercellular water transport. The qualitative behaviour of LP and σ was consistent with a quasi-steady-state model of intercellular water transport that included volume flow through plasmodesmata. A quantitative description was obtained by fitting a simplified version of this model to the data. This led to the conclusion that significant volume flow can occur through plasmodesmata during pressure-clamp experiments, especially for small A and |ΔP|.

Notes: Abstract An investigation was carried out into the water relations of CAM and C3 bromeliads in their natural habitat during the dry season in Trinidad. Measurements were made of xylem tension with the pressure chamber and of cell-sap osmotic pressure and titratable acidity on crushed leaf samples. A steady-state CO2 and H2O-vapour porometer was also used so that changes in leaf water relations during individual day-night cycles could be directly related to gas-exchange patterns in situ.Xylem tension changed in parallel with transpiration rate and in general reached its maximum value in CAM bromeliads at night and in C3 bromeliads during the day. In addition, large nocturnal increases in cell-sap osmotic pressure and titratable acidity (ΔH+) typically occurred in the CAM bromeliads. The C3-CAM intermediate Guzmania monostachia showed slight nocturnal acidification, but had higher values of xylem tension during the day. Very high values of AH+ were observed in the CAM species when the tanks of the epiphytic bromeliads contained water: Aechmea nudicaulis showed a mean maximum ΔH+ of 474 mol m−3, the highest value so far observed for CAM plants. On some nights dew formed on the leaf surfaces of the epiphytes, partially curtailing gas exchange and leading to a marked decrease in xylem tension in both C3 and CAM species.Between-site comparisons were also made for a wide range of habitats from arid coastal scrub to montane rain forest. Compared with values characteristic of other life-forms, xylem tension and cell-sap osmotic pressure were low for all bromeliads, and did not differ significantly in co-occurring CAM and C3 bromeliads. Mean maximum xylem tension (10 species in total) ranged from 0.29 M Pa at the montane sites to 0.67 MPa at the most arid site, and mean minimum osmotic pressure (17 species) from 0.51 to 0.97 MPa. At the arid sites the bromeliads were exclusively CAM species, two of which (Aechmea aquilega and Bromelia plumieri) grew terrestrially in the undergrowth of the coastal scrub. Xylem tension in these species was low enough to indicate that they must be functionally independent of the substratum during the dry season. In the wetter part of Trinidad, no between-site differences in leaf water relations were found along an altitudinal gradient in the Northern Mountain Range; seasonal differences in this area were also small. Overall, leaf water relations and gas exchange in the bromeliads were strongly affected both by short-term changes in water availability and by longer-term climatic differences in the various regions of the island.

Notes: Abstract Pitcairnia integrifolia is endemic to northern Trinidad and the Paria peninsula of Venezuela and is the only member of the bromeliad-subfamily Pitcairnioideae in Trinidad. It is terrestrial with roots fully functional in water and solute uptake, grows on exposed steep rocky cliffs and can occur just above the spill zone of sea waves under continuous sea spray. Thus, it can be subject both to water stress, particularly during the dry season, and to salt stress.Gas exchange of P. integrifolia leaves was measured on a clear day in Trinidad. Uptake of CO2 and leaf conductance to diffusion of water vapour had two peaks during the light period, a larger one in the early morning and a smaller one in the late afternoon, which were separated by an extended midday depression of gas exchange. CO2 partial pressure in the leaf air-spaces increased during the midday depression. Leaf temperatures reached a maximum of 51.6°C and leaf-air water-vapour-concentration differences were also very high during the midday depression, when quantum fluxes were up to 2 mmol m−2 s−1 and higher. The midday depression is considered as a functional adaptation to temporary water stress.Although P. integrifolia is subject to sea-spray, an internal osmotic pressure of only 0.91 MPa indicated that NaCl is not accumulated. Leaf epidermis cells are thick-walled and have a prominent cuticle. The abaxial leaf surface with the trichomes is not wettable, and the trichomes apparently do not function in water and solute uptake. They cover the stomata densely and may create a favourable microenvironment around them. They also have a high reflectance. This does not prevent overheating of the leaves, but does reduce the photosynthetically active radiation penetrating to the mesophyll. The trichomes may thus contribute to the prevention of photoinhibition at high incident quantum flux.

Notes: The aim of the present study was to test the accuracy of the pressure-chamber technique as a method for estimating leaf-cell turgor pressures. To this end, pressure-probe measurements of cell turgor pressure (Pcell) were made on mesophyll cells of intact, attached leaves of Kalanchoë daigremontiana. Immediately following these measurements, leaves were excised and placed in a pressure chamber for the determination of balance pressure (Pbal). Cell-sap osmotic pressure (?cell) and xylem-sap osmotic pressure (?xyl) were also measured, and an average cell turgor pressure calculated as Pcell=?cell–?xyl–Pbal. The apparent value of Pbal was positively correlated with the rate of increase of chamber pressure, and there was also a time-dependent increase associated with water loss. On expressing sap from the xylem, ?xyl fell to a plateau value that was positively correlated with ?cell. Correcting for these effects yielded estimates of Pbal and ?xyl at the time of leaf excision. On average, the values of Pcell obtained with the two techniques agreed to within ±002 MPa (errors are approximate 95% confidence limits). If ?xyl were ignored, however, the calculated turgor pressures would exceed the measured values by an average of 0.074 ± 0.012MPa, or 48% at the mean measured pressure of 0.155 MPa. We conclude that the pressure-chamber technique allows a good estimate to be made of turgor pressure in mesophyll cells of K. daigremontiana, provided that ?xyl is included in the determination. The 1:1 relationship between the measured and calculated turgor pressures also implies that the weighted-average reflection coefficient for the mesophyll cell membranes is close to unity.

Notes: Abstract This article deals with the physiological ecology of the Bromeliaceae, a large neotropical family containing both terrestrial and epiphytic forms, as well as many species with crassulacean acid metabolism (CAM).The article is in two parts. In the first, we review what is known of the occurrence of CAM and C3 species in the Bromeliaceae. The photosynthetic pathways are discussed in the context of the major taxonomic divisions within the family and the great diversity of bromeliad life-forms. Of the three subfamilies, the Pitcairnioideae contain both C3 and CAM species and are essentially all terrestrial. In contrast, the Tillandsioideae are entirely epiphytic or saxicolous, with CAM species being restricted to the genus Tillandsia, And in the Bromelioideae all species show CAM, but terrestrial and epiphytic forms are found in about equal numbers. The evidence suggests that both CAM and the epiphytic habit arose more than once in the family's evolutionary history.In the second part we consider the photosynthetic ecology of the various bromeliad life-forms in more detail using the specific example of Trinidad (West Indies). CAM bromeliads tend to be centred on the drier regions of the island and C3 forms on the wetter areas. However, at any one site there is a marked vertical stratification of species within the forest profile. Based on the known habitat preferences of the bromeliads, six contrasting sites were selected for field studies in Trinidad. These ranged from arid coastal scrub to montane rain forest, the vegetational and climatic characteristics of which are described here. The constancy of δ13C values (carbon-isotope ratios) for individual CAM species in these markedly different habitats emphasized the need for ecophysiological studies to characterize environmental effects on CO2 assimilation and transpiration. The following papers in this series present the results of a comparative investigation of gas exchange and leaf water relations of CAM and C3 bromeliads in situ at the various sites.

Notes: Abstract The results described represent the first detailed measurements of gas exchange of epiphytic plants with crassulacean acid metabolism (CAM) in the humid tropics. A portable steady-state CO2 and H2O porometer was used to measure net exchange rates of CO2 and H2O vapour (JCO2, JH2O), leaf temperature (T1), air temperature (TA), air relative humidity (RH) and photosynthetically active radiation (PAR) for bromeliads in the field during the dry season in February and March 1983 on the tropical island of Trinidad. Different lengths of tubing (up to 25 m) were used so that the gas exchange could be measured of bromeliads in situ in their epiphytic habitats. Derived parameters such as leaf-air water-vapour-concentration difference (Δw), water-vapour conductance of leaves (g) and internal CO2 partial pressure (piCO2) could be calculated.The particular problems of making such measurements in the humid tropics due to high relative humidities and high dew-point temperatures are discussed. The long and often broad, strap-like leaves of bromeliads are well suited for measurements with the steady-state porometer. It is shown that CAM activity varies along the length of individual leaves, and variability between different leaves is also demonstrated.The major phases of CAM, i.e. nocturnal stomalal opening, CO2 uptake and dark fixation as malic acid (Phase I), daytime stomatal closure and light-dependent assimilation of CO2 derived from decarboxylation of the malic acid (Phase III), and late-afternoon stomatal opening with direct light-dependent assimilation of atmospheric CO2 (Phase IV) were all clearly shown by CAM bromeliads in situ. Their expression and magnitude depended on the environmental conditions. An early-morning peak of CO2 uptake as is characteristic of Phase II of CAM was not detected during the night-day transition. A bromeliad intermediate between C3 and CAM, Guzmania monostachia, showed substantial net CO2 uptake in the early morning but no net uptake integrated over the whole of the night.

Notes: Abstract Field measurements of the gas exchange of epiphytic bromeliads were made during the dry season in Trinidad in order to compare carbon assimilation with water use in CAM and C3 photosynthesis.The expression of CAM was found to be directly influenced by habitat and microclimate. The timing of nocturnal CO2 uptake was restricted to the end of the dark period in plants found at drier habitats, and stomatal conductance in two CAM species was found to respond directly to humidity or temperature. Total night-time CO2 uptake, when compared with malic-acid formation (measured as the dawn-dusk difference in acidity, ΔH+), could only account for 10–40% of the total ΔH+ accumulated. The remaining malic acid must have been derived from the refixation of respired CO2 (recycling). Within the genus Aechmea (12 samples from four species), recycling was significantly correlated with night temperature at the six sample sites. Recycling was lowest in A. fendleri (54% of ΔH+ derived from respired CO2), a CAM bromeliad with little water-storage parenchyma that is restricted to wetter, cooler regions of Trinidad.Gas-exchange rates of C3 bromeliads were found to be similar to those of the CAM bromeliads, with CO2 uptake from 1 to 3 μmol m−2 s−1 and stomatal conductances generally up to 100 mmol m−2 s−1. The midday depression of photosynthesis occurred in exposed habitats, although photosynthetically active radiation (PAR) limited photosynthesis in shaded habitats. CO2 uptake of the C3 bromeliad Guzmania lingulata was saturated at around 500 μmol m−2 s−1 PAR, suggesting that epiphytic plants found in the shaded forest understorey are shade-tolerant rather than shade-demanding.Transpiration ratios (TR) during CO2 fixation in CAM (Phase I and IV) and C3 bromeliads were compared at different sites in order to assess the efficiency of water utilization. For the epiphytes displaying marked uptake of CO2, TR were found to be lower than many previously published values. In addition, the average TR values were very similar for dark CO2 uptake in CAM (42 ± 41, n= 12), Phase IV of CAM (69 ± 36, n= 3) and for C3 photosynthesis (99 ± 73, n= 4) in these plants. It appears that recycling of respired CO2 by CAM bromeliads and efficient use of water in all phases of CO2 uptake are physiological adaptations of bromeliads to arid microclimates in the humid tropics.