Ch. Poschenrieder

Plant water relations as affected by heavy metal stress: A review

Abstract High metal availability, arising from mining and industrial activities, disposal of sewage sludge or soil acidification, is an increasing problem in agriculture and forestry. Metal toxicity causes multiple direct and indirect effects in plants which concern practically all physiological functions. In this review we consider the effects of excess heavy metals and aluminium on those functions which will alter plant water relations. After a brief comment on the metal effects in cell walls and plasmalemma, and their consequences for cell expansion growth, the influences of high metal availability on the factors which regulate water entry and water exit in plants are considered. Emphasis is placed on the importance of distinguishing between low water availability in mine and serpentine soils and toxicity effects in plants which may impair the regulation of a plants water household. Examples on water relations of both plants grown on metalliferous soil and hydroponics are discussed. The effects of met...

Localization of zinc and cadmium in Thlaspi caerulescens (Brassicaceae), a metallophyte that can hyperaccumulate both metals

Summary The compartmentation of Zn and Cd in roots of Thlaspi caerulescens J. & C. Presl, a metallophyte that hyperaccumulates both metals, was investigated by means of EDAX. Two methods for sample preparation, Na 2 S-fixation and freeze-substitution, were compared. Na 2 S-fixation was not suitable for preventing the loss of metal ions from the samples. According to our results from freeze-substituted samples, Cd accumulated mainly in the apoplast and, to a lesser extent, in vacuoles, whereas Zn was principally found in vacuoles and, to a lesser extent, in cell walls.

Water Relations in Heavy Metal Stressed Plants

Almost every plant process is affected directly or indirectly by the water supply, and water can be considered as a major factor in the regulation of plant growth (Kramer and Boyer 1995). Therefore, many investigations on plant responses to environmental stresses play considerable attention to water relations from the cell to the whole plant and community level. The influence of excess ions on plant water relations has mainly been investigated in plants exposed to high concentrations of Na+, Cl−, and other ions that cause adverse effects in plants at concentrations in the 0.1–1 M range. The interest in water relation studies under salt stress is obvious, because of the significant influence of such high ion concentrations on the osmotic potential of the substrate, and the difficulties for water acquisition by plants under those circumstances. In addition to osmotic stress, ionspecific effects of high salt concentrations on water relations and growth are well documented (Levitt 1980; Montero et al. 1997, 1998).

Cadmium-Induced Decrease of Water Stress Resistance in Bush Bean Plants (Phaseolus vulgaris L. cv. Contender) I. Effects of Cd on Water Potential, Relative Water Content, and Cell Wall Elasticity

Summary The leaf water potential (ψ w ) and the relative water content (RWC) of primary and first trifoliate leaves of bean plants ( Phaseolus vulgaris L. cv. Contender), grown with different Cd concentrations in the medium, were analysed under both normal water supply and water stress conditions. With these data, Richter plots were drawn, from which the maximum turgor pressure, the turgor loss point, the bulk elastic modulus, and the structure coefficient were derived. Cd generally decreased the water stress tolerance of plants, causing turgor loss at higher RWC and ψ w than in non-treated control plants. Cd increased the bulk elastic modulus and therefore decreased the cell wall elasticity. Low cell wall elasticity seems to be an important causes of the low water stress tolerance in Cd-toxic plants.

Silicon amelioration of aluminium toxicity in teosinte (Zea mays L. ssp. mexicana)

The influence of different Al concentrations, (0, 60 and 120 μM Al) on growth and internal concentrations of Al, Si and selected organic acids was analysed in plants of teosinte (Zea mays L. ssp. mexicana), a wild form of maize from acid soils from Mexico. The plants were grown in nutrient solutions (pH 4.0) with or without 4 μM silicon. Analysis with the GEOCHEM speciation program did not reveal differences between free activities of Al3+ in solutions with and without 4 μM Si, but solutions with Si yielded lower concentrations of monomeric Al species, [Al]mono, when analysed by a modified aluminon method. Plants grown on solutions with similar [Al]mono, but differing in silicon, showed highly significant differences in growth and tissue concentrations of Al and organic acids. Silicon prevented growth inhibition at [Al]mono concentrations as high as 35 μM, while plants grown without Si suffered severe growth reductions with 33 μM [Al]mono. In solutions with similar [Al]mono concentrations plants with Si had lower tissue Al concentrations and higher concentrations of malic acid than plants without Si. In view of both the significant influence of Si on the response of plants to Al toxicity and the fact that some soluble Si is always present in soil solutions, the addition of low Si concentrations to nutrient solutions used for Al-tolerance screening is recommended.

Chromium III‐iron interaction in Fe‐deficient and Fe‐sufficient bean plants. I. Growth and nutrient content

Abstract The influence of low Cr III concentrations (0, 0.25, 0.5, and 1.0 μM as CrCl3–6H2O) on growth, chlorophyll and carotenoid content, the concentration of selected mineral nutrients (Fe, Mn, P), and Cr content was determined in both Fe‐control (10 μM Fe as Fe‐EDTA) and Fe‐deficient (no Fe supply) bush bean (Phaseolus vulgaris L.) plants grown hydroponically. Chromium significantly enhanced growth of both Fe‐control and Fe‐deficient plants. In Fe‐deficient plants, Cr significantly reduced chlorosis in young leaves, increasing the concentration of chlorophyll and carotenoids. This beneficial effect of low Cr concentrations was neither correlated to changes of Mn, P, or Fe tissue concentrations nor to Cr‐induced alterations of the Fe/Mn and P/Fe ratios. As total Fe was analyzed, the effect of Cr on the subcellular Fe distribution or on the Fe2+/Fe3+ ratio can not be excluded.

Effect of chromium VI on mineral element composition of bush beans

Abstract Bush beans (Phaseolus vulgaris L. cv Contender) were grown on perlite with nutrient solution and 0, 1, 2.5 and 5 ppm levels of Na2CrO4 Significant decrease of top growth and chlorosis in trifoliated leaves were observed for 2.5 and 5 ppm Cr, with Cr concentrations (μg/g) in tops:≥ 12.1, in roots:≥ 509.9. Cr decreased K, Na, Mg and Fe concentrations, and increased P and Mn concentrations in roots. In tops decreased N, K, Na and Fe concentrations and increased Mn and Ca concentrations were observed, Translocation of P, Zn, Cu and Fe was inhibited; Ca and Mn translocation was generally enhanced. P/Fe ratio was increased up to 60% in chlorotic plants, indicating a shift from Fe2+ to Fe3+.

A rapid increase in cytokinin levels and enhanced ethylene evolution precede Al3+-induced inhibition of root growth in bean seedlings (Phaseolus vulgaris L.)

The role of plant hormones in aluminium (Al3+)-inducedinhibition of root growth was investigated in roots of Phaseolusvulgaris L. cv. Strike. Changes in ethylene evolution and changes inthe content and composition of cytokinins (CKs), at intervals up to 150min after treatment with Al3+, were determined by gaschromatography (GC) and combined gas chromatography-mass spectrometry (GC-MS),respectively. Seedlings were cultivated in a continuously aerated nutrientsolution at pH 4.5 with and without Al3+. The growth rate ofAl3+-treated roots was significantly decreased after 360min. Ethylene evolution from excised root tips doubled after 15min of Al3+ treatment and reached a maximum 30min after treatment. Levels of CK nucleotides declined 60 to 80%after only 5 min of Al3+ treatment whereas the zeatin(Z) content increased six-fold. The increase in Z continued over the entire 150min-sampling period and reached a level 80 times higher than thatin roots not exposed to Al3+. These results show thatAl3+-induced inhibition of root growth is preceded by significantchanges in CK content and composition and enhanced ethylene evolution. Since CKcan induce ethylene production, the rapid increase in CK, particularly Z anddihydrozeatin (dZ), may contribute to inhibition of root-growth either directlyor indirectly by affecting plant hormone homeostasis.

Short‐term effects of pH and aluminium on mineral nutrition in maize varieties differing in proton and aluminium tolerance

Abstract In short‐term (24 h) nutrient solution experiments, the influence of different proton (pH 6.0 and pH 4.3) and aluminium (Al) (0, 20, and 50 μM) concentrations on root and coleoptile elongation, dry weight, and the uptake of selected mineral nutrients was studied in maize (Zea mays L.) varieties that differ in acid soil tolerance under field conditions. The acid‐soil‐tolerant maize varieties, Adour 250 and C525M, proved to be hydrogen (H+) ion sensitive, but Al tolerant, while the acid soil tolerant variety BR201F was H+ tolerant but Al sensitive. The acid soil sensitive variety HS 7777 was affected by both H+ and Al toxicity. The proton‐induced inhibition of root elongation was closely related to the proton‐induced decrease of the specific absorption rates (SAR) of boron (B), iron (Fe), magnesium (Mg), calcium (Ca), and phosphorus (P). In contrast, only the specific absorption rate of B (SARB) was significantly correlated to the Al‐induced inhibition of root elongation. It is concluded, that alte...

The role of ethylene metabolism in the short-term responses to aluminium by roots of two maize cultivars different in Al-resistance

Abstract The possible role of ethylene in the initial signal transduction of Al-induced root growth responses was investigated in two tropical maize ( Zea mays ) varieties that differ in Al resistance: ATP SR Yellow and HS 701 B. The intensity of Al toxicity effects were evaluated after short (4 and 24 h) exposure to 50 μM Al in complete low ionic strength nutrient solution. Relative root elongation rates (RER) and callose formation in root tips were used as stress indicators. Ethylene production by the root tips and 1-aminocyclopropane-1-carboxylic acid (ACC) synthase and ACC oxydase activities were analysed. After 24 h exposure to Al, both less callose production and higher RER indicated that ATP SR Yellow was more Al resistant than HS 701 B. The Al resistance of ATP SR Yellow, however, was not expressed after 4 h exposure to Al, when increased callose and decreased RER were observed. In any of the varieties and after any of the time-treatments an Al-induced increase of ethylene production was found. Our results indicate that the Al-resistance genes were not constitutively expressed in the absence of Al in the medium, but activated upon exposure to Al. An efficient protection against Al was achieved after a lag time of more than 4 h. Enhanced ethylene formation does not seem to play a role either in the Al-induced inhibition of root elongation or in the induction of the resistance mechanism.