Abstract
Fluxes of mineral nutrients in the xylem are strongly influenced by interactions with the surrounding stem tissues and are probably regulated by them. Toward a mechanistic understanding of these interactions, we applied stable isotope tracers of magnesium, potassium, and calcium continuously to the transpiration stream of cut bean (Phaseolus vulgaris) shoots to study their radial exchange at the cell and tissue level with stem tissues between pith and phloem. For isotope localization, we combined sample preparation with secondary ion mass spectrometry in a completely cryogenic workflow. After 20 min of application, tracers were readily detectable to various degrees in all tissues. The xylem parenchyma near the vessels exchanged freely with the vessels, its nutrient elements reaching a steady state of strong exchange with elements in the vessels within 20 min, mainly via apoplastic pathways. A slow exchange between vessels and cambium and phloem suggested that they are separated from the xylem, parenchyma, and pith, possibly by an apoplastic barrier to diffusion for nutrients (as for carbohydrates). There was little difference in these distributions when tracers were applied directly to intact xylem via a microcapillary, suggesting that xylem tension had little effect on radial exchange of these nutrients and that their movement was mainly diffusive.
Highlights
Fluxes of mineral nutrients in the xylem are strongly influenced by interactions with the surrounding stem tissues and are probably regulated by them
We used this cryogenic protocol to examine the exchange between xylem vessels and stem tissue of French bean (Phaseolus vulgaris ‘Fardenlosa Shiny’), with stable isotope tracers for potassium, calcium, and magnesium applied to the transpiration stream of a cut
With stable isotopes of naturally minor abundance of magnesium, potassium, and calcium (26Mg, 41K, and 44Ca, respectively) highly enriched to greater than 94% total element, contained 2.5 mM of each of the three elements
Summary
Fluxes of mineral nutrients in the xylem are strongly influenced by interactions with the surrounding stem tissues and are probably regulated by them. Long-distance transport of nutrients in stems is strongly influenced by the interaction of the moving xylem sap with the surrounding tissues (e.g. phloem; Stout and Hoagland, 1939; Biddulph and Markle, 1944) The importance of this radial exchange was highlighted in studies on budgets of carbon/nitrogen and mineral nutrients (Pate et al, 1979; Jeschke et al, 1985, 1991; Wolf et al, 1991). Strict cryogenic sample preparation followed by analysis with SIMS below 2130°C (cryo-SIMS) has been shown to satisfy this criterion (Metzner et al, 2008), with scanning electron microscopy of the frozen samples (cryo-SEM) for quality control and detailed anatomical information of the individual tissues We used this cryogenic protocol to examine the exchange between xylem vessels and stem tissue of French bean (Phaseolus vulgaris ‘Fardenlosa Shiny’), with stable isotope tracers for potassium, calcium, and magnesium applied to the transpiration stream of a cut
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