Ronald A. Balsamo

Dehydration-induced expression of a 31-kDa dehydrin in Polypodium polypodioides (Polypodiaceae) may enable large, reversible deformation of cell walls.

Current and predicted climate changes caused by global warming compel greater understanding of the molecular mechanisms that plants use to survive drought. The desiccation-tolerant fern Polypodium polypodioides exhibits extensive cell wall folding when dried to less than 15% relative water content (RWC) and rapidly (within 24 h) rehydrates when exposed to water and high humidity. A 31-kDa putative dehydrin polypeptide expressed in partially and fully dry tissues detected via western blotting was present only during drying and rapidly dissipated (within 24 h) upon tissue rehydration. Immunostaining indicates the presence of dehydrin near the cell wall of partially and fully dried tissues. Atomic force microscopy of tracheal scalariform perforations indicates that dry vascular tissue does not undergo significant strain. Additionally, environmental scanning electron microscopy reveals differential hydrophilicity between the abaxial and adaxial leaf surfaces as well as large, reversible deformation. The ability to avoid cell wall damage in some desiccation-tolerant species may be partially attributed to cell wall localization of dehydrins enabling reversible, large cell-wall deformation. Thus, the de novo synthesis of dehydrin proteins and potential localization to the cell walls of these desiccation-tolerant species may play a role in avoiding mechanical failure during drought.

Utilization of Grasses for Potential Biofuel Production and Phytoremediation of Heavy Metal Contaminated Soils

This research focuses on investigating the use of common biofuel grasses to assess their potential as agents of long-term remediation of contaminated soils using lead as a model heavy metal ion. We present evidence demonstrating that switch grass and Timothy grass may be potentially useful for long-term phytoremediation of heavy metal contaminated soils and describe novel techniques to track and remove contaminants from inception to useful product. Enzymatic digestion and thermochemical approaches are being used to convert this lignocellulosic feedstock into useful product (sugars, ethanol, biocrude oil + biochar). Preliminary studies on enzymatic hydrolysis and fast pyrolysis of the Switchgrass materials that were grown in heavy metal contaminated soil and non-contaminated soils show that the presence of lead in the Switchgrass material feedstock does not adversely affect the outcomes of the conversion processes. These results indicate that the modest levels of contaminant uptake allow these grass species to serve as phytoremediation agents as well as feedstocks for biofuel production in areas degraded by industrial pollution.

ELECTROPHYSIOLOGY OF THE SALT GLANDS OF AVICENNIA GERMINANS

Microelectrode techniques were used on epidermal peels of Avicennia germinans to characterize functional aspects of secretion in salt glands. Increasing the concentrations of K+, Na+, or Mg++ in the bath medium decreased the measured transcellular voltage and resistance. The velocity and magnitude of these transcellular depolarizations varied: K+ > Na+ > Mg++ >> Cl-. Increasing the pH of the incubation medium from 5.5 to 7.0 reduced both the rate and the magnitude of voltage decrease when Na+ concentration was changed; tissue incubated at pH 8.0 did not form secretion droplets. Transcellular depolarizations resulting from concentration shifts of K+ from 10 to 100 mM were partially suppressed in the presence of La+++. The voltage decrease accompanying changes in K+ concentration was partially blocked by tetraethyl ammonium chloride (TEA) but not in Na+. Niflumic acid reduced the magnitude of the transcellular voltage with increased K+ concentration but did not affect the velocity. The small voltage decrease accompanying changes in Cl- concentration was not affected by either TEA or niflumic acid (NA). 2-4-Dinitrophenol (DNP), sodium azide, and vanadate markedly reduced the rapidity and the magnitude of voltage decrease accompanying increases in concentration of K+, Na+, or Mg++. Ion concentrations in secretion droplets indicated that a considerable cation flux occurred across epidermal peels of A germinans in regions where there were salt glands. Salt secretion is best explained by a modified chemiosmotic hypothesis where cation channels and/or permeases work in concert with the electrochemical proton gradient generated by the plasma membrane H+/ATPase.

Leaf biomechanics as a potential tool to predict feeding preferences of the geometric tortoise Psammobates geometricus

Food and non food plant characteristics were studied for the geometric tortoise, Psammobates geometricus, to test the hypothesis that feeding preferences in tortoises may be influenced by the mechanical properties of plant leaves. Twelve focal feeding observations made between 8 October and 31 October 2002 were synthesized with literature reports to develop species lists of food and non food grasses and geophytes in renosterveld habitat. Failure load and tensile strength were determined in the field for leaves of food and non food plants that were spatially available to the tortoises and present in relative abundance during the course of the study. Leaves of geophyte food plants were fleshy and succulent with a low failure load and tensile strength. Leaves of food plant grasses had significantly higher failure loads and tensile strengths compared to leaves of food plant geophytes. In non food plants there was no signifi cant difference in failure load between grasses and geophytes, but tensile strength was signifi cantly higher in the grasses. Non food plant grasses and geophytes overall had significantly higher failure load and tensile strength values than did food plants. Two non food plants that overlapped in mechanical strength to the food plants either have toxic/unpalatable chemicals in high amounts or are recent invasive species to the renosterveld. Thus, these studies support the hypothesis that leaf biomechanical properties may be an important factor in determining the diet of the geometric tortoise and perhaps tortoises in general.

Leaf biomechanical properties in Arabidopsis thaliana polysaccharide mutants affect drought survival

Individual sugars are the building blocks of cell wall polysaccharides, which in turn comprise a plant׳s overall architectural structure. But which sugars play the most prominent role in maintaining a plant׳s mechanical stability during large cellular deformations induced by drought? We investigated the individual contributions of several genes that are involved in the synthesis of monosaccharides which are important for cell wall structure. We then measured drought tolerance and mechanical integrity during simulated drought in Arabidopsis thaliana. To assess mechanical properties, we designed a small-scale tensile tester for measuring failure strain, ultimate tensile stress, work to failure, toughness, and elastic modulus of 6-week-old leaves in both hydrated and drought-simulated states. Col-0 mutants used in this study include those deficient in lignin, cellulose, components of hemicellulose such as xylose and fucose, the pectic components arabinose and rhamnose, as well as mutants with enhanced arabinose and total pectin content. We found that drought tolerance is correlated to the mechanical and architectural stability of leaves as they experience dehydration. Of the mutants, S096418 with mutations for reduced xylose and galactose was the least drought tolerant, while the arabinose-altered CS8578 mutants were the least affected by water loss. There were also notable correlations between drought tolerance and mechanical properties in the diminished rhamnose mutant, CS8575 and the dehydrogenase-disrupted S120106. Our findings suggest that components of hemicellulose and pectins affect leaf biomechanical properties and may play an important role in the ability of this model system to survive drought.