Na+ Transport and Toxicity in Plants

Soil salinity is an urgent and global agricultural problem, affecting more than 80 million hectares of arable land. While sodium can be benign and even beneficial to plants at low concentrations, severe toxicity symptoms are found in many species at high, saline, concentrations. Toxicity is initiated by the entry of Na+ into root systems, a process that, despite much research effort, remains poorly characterized. Currently, a scenario of rapid, energy-intensive Na+ cycling across the plasma membrane of root cells stands as the leading model describing Na+ transport under salinity. In this model, Na+ enters the cytosol passively, at very high rates, then is actively pumped back to the outer medium at nearly equal rates. However, the physiological, energetic, and toxicological foundations of rapid Na+ cycling have been called into question. The experiments in this thesis test the plausibility of this model, using the radioisotope 24Na+ in conjunction with respiratory, electrophysiological, photometric (tissue-content), and fluorescent-dye measurements. Sodium-flux pathways in root and shoot systems were measured in barley (Hordeum vulgare L.), rice (Oryza sativa L.), wheat (Triticum aestivum L.), maize (Zea mays L.), and the genetic model plant Arabidopsis thaliana L., in which transport mutants were examined. Startlingly, Na+ fluxes were resistant to alteration by powerful inhibitors, and found to be malleable only under special conditions, such as nutrient deprivation during growth, or long tracer-desorption times during measurement. In this respect, the fluxes did not resemble those of nutrient ions such as NH4+, NO3-, and K+, whose uptake behaviours are highly malleable and well-characterized. It is concluded that the rapid-cycle model of Na+ transport is not valid, and the non-malleable fluxes comprising the cycle are a phenomenon distinct from membrane transport, instead proceeding extracellularly. Consequentially, many fluxes reported in the literature may have to be reinterpreted in terms of their energetic and physiological plausibility. These findings address some fundamental questions about Na+ fluxes and provide a physiological framework for future studies of Na+ transport.