Contrasting dynamics of water and mineral nutrients in stems shown by stable isotope tracers and cryo-SIMS.
ABSTRACT Lateral exchange of water and nutrients between xylem and surrounding tissues helps to de-couple uptake from utilization in all parts of a plant. We studied the dynamics of these exchanges, using stable isotope tracers for water (H(2)(18)O), magnesium ((26)Mg), potassium ((41)K) and calcium ((44)Ca) delivered via a cut stem for various periods to the transpiration stream of bean shoots (Phaseolus vulgaris cv. Fardenlosa Shiny). Tracers were subsequently mapped in stem cross-sections with cryo-secondary ion mass spectrometry. The water tracer equilibrated within minutes across the entire cross-section. In contrast, the nutrient tracers showed a very heterogeneous exchange between xylem vessels and the different stem tissues, even after 4 h. Dynamics of nutrients in the tissues revealed a fast and extensive exchange of nutrients in the xylem parenchyma, with, for example, calcium being completely replaced by tracer in less than 5 min. Dilution of potassium tracer during its 30 s transit in xylem sap through the stem showed that potassium concentration was up-regulated over many hours, to the extent that some of it was probably supplied by phloem recirculation from the shoot.
- [show abstract] [hide abstract]
ABSTRACT: Yoshikami and Hagins first suggested that calcium is sequestered within membranous disks in the outer segments of vertebrate rods and that the bleaching of visual pigment molecules by light causes the release of Ca from the disks. Once released, the Ca was postulated to bind to Na+ channels or carrier molecules in the plasma membrane to produce the electrical response. This theory, termed the 'calcium hypothesis', is supported by much evidence but remains controversial, largely because of the difficulty in measuring calcium in rods and of demonstrating light-induced release. Here we describe direct measurements of total rod Ca using a new microprobe method, called laser micro-mass analysis, or LAMMA . Using this technique, we show that rods contain large amounts of Ca concentrated in their outer segments. Physiological levels of illumination produce a graded efflux of rod Ca content, amounting to about 10(4) ions per rhodopsin molecule bleached in dim light. As light does not change the rate of Ca influx, the total Ca content of the rod decreases. In bright light, as much as half the total Ca leaves the rod during only 1 min of illumination.Nature 04/1984; 309(5965):268-70. · 38.60 Impact Factor
- Journal of Experimental Botany - J EXP BOT. 01/1998; 49(322):775-788.
- [show abstract] [hide abstract]
ABSTRACT: The structures of heavy and light water at ambient conditions are investigated with the combined techniques of x-ray diffraction, neutron diffraction, and computer simulation. It is found that heavy water is a more structured liquid than light water. We find the OH bond length in H2O is approximately 3% longer than the OD bond length in D2O. This is a much larger change than current predictions. Corresponding to this, the hydrogen bond in light water is approximately 4% shorter than in heavy water, while the intermolecular HH distance is approximately 2% longer.Physical Review Letters 09/2008; 101(6):065502. · 7.94 Impact Factor