Microelement localization in leaves in Populus spp. and tolerance mechanisms to boron-salt toxicity
Phytoremediation has been proposed as a low-cost, environmentally friendly alternative to clean highly contaminated sites. Phytoremediation using trees offer the advantage of metal immobilization (phytostabilization), high hydraulic pumping pressures, produce large plant biomass, absorption of total dissolved salts within their tissue, long life spans, less frequent irrigation and, a massive root system that helps soil attachment. Hybrid poplar varieties are long-lived, perennial, fast growing, and produce a large amount of foliage to harvest. Boron is an essential micronutrient for plants. Certain locations, such as California and Turkey, have suffered from high boron efflux from irrigation waters. It has been identified some hybrid poplar clones that can withstand elevated concentrations of boron and salinity. Most studies are based on root to shoot metal translocation mechanisms but there is little evidence of data showing detoxification strategies in the ability of tissues to tolerate high B concentrations. It is essential to investigate the compartmentation, microelement localization and tolerance/detoxification mechanisms of contaminants at tree crown and leaf tissues, at a sub-cellular level to identify the strategies and cellular structures in charge of controlling the stress reactions in previously selected hybrid tolerant populus trees. The aim in this study was to describe the contaminant allocation in leaf tissues of specialized tolerant and sensitive poplar clones utilizing different histochemical and micro-analytical methods. In the present study, multiple element contamination effects on poplar leaves were ascertained using histochemical observations and X-ray micro analytical method. In addition, foliar element concentration was measured and compared with biomass, and physical observations. Results showed that biomass but not the tree height is affected regarding the metal concentration. Most effects (i.e necrosis) and visible symptoms were observed more apparent in leaf apexes and in younger leaves than in other leaf parts. Also, leaf branch position closer to the main stem and leaf tip and margins presented more contamination symptoms. Intercellular abiotic deposits are located on a water/mineral element route and are associated to local cell wall thickenings and wart-like protrusions - both are typical stress reactions. Condensed tannins were involved in metal chelating. Antioxidant functions from phenolics were involved in metal detoxification. Pectin was a major sink for boron. At the principal accumulation site of contaminants, coincidence of a naturally resistant tissue on the pathway of an overloaded nutrient flux, in the lower leaf blade tissues, there is a 2-3 cell layer-thick hypodermis. Hypodermis is a specialized tissue, with low cell physiological activity, and, being not involved in vital processes such as photosynthesis and assimilates transport, is an ideal site for B & salt detoxification. Stress and tolerance reactions indicate that cells in the hypodermis structure are well suited for accumulation. Cell wall thickening and various symplastic defense mechanisms were observed on both locations were the study was implemented and thus appear to be constitutive in foliage of these clones. Foliage concentrations detected several metal contaminations (noteworthy Na, B, Cl, Se). Micro analytical analysis confirmed the metal allocation as described in histochemical observations. It is distinguished a tendency from most elements to accumulate in the lower mesophyll. For several reasons, micro analytical analysis should be considered as tendencies. Better techniques should be implemented to characterize the allocation of metals in the leaf tissues, especially for Boron. Implementation of molecular methods for a better understanding of the efflux transporters should complement the results obtained in this study.
Plant biology|Cellular biology|Environmental science
Arriaga, Daniel, "Microelement localization in leaves in Populus spp. and tolerance mechanisms to boron-salt toxicity" (2012). ETD Collection for University of Texas, El Paso. AAI1533206.