Contrasting dynamics of water and mineral nutrients in stems shown by stable isotope tracers and cryo-SIMS

Central Division of Analytical Chemistry, Forschungszentrum Jülich, Leo Brandt Strasse, 52425 Jülich, Germany.
Plant Cell and Environment (Impact Factor: 6.96). 04/2010; 33(8):1393-407. DOI: 10.1111/j.1365-3040.2010.02157.x
Source: PubMed


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 (H218O), magnesium (26Mg), potassium (41K) and calcium (44Ca) 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.

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Available from: Peter Blümler, Sep 24, 2014
    • "Hopefully, the work of Rothwell and Dodd (2014) might stimulate opportunities for revisiting crop Ca management and perhaps shifting our focus towards understanding more about what is going on below ground and the mechanism(s) of how pea achieves its regulation of shoot Ca delivery and the p o s s i b l e i n v o l v e m e n t o f a p u t a t i v e n o v e l antitranspirant. More recent novel approaches undertaken by Metzner et al. (2010) suggest that ion exchange capacity of stem and their parenchymal tissues may not have been fully appreciated as Ca sources/ contributors to xylem sap Ca homeostasis. The structural differences between Phaseolus and Pisum stems may also have functional effective differences in their capacity to maintain Ca homeostasis in the transpiration stream. "
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    ABSTRACT: Understanding the regulation of calcium uptake, xylem transport and its impacts on growth and leaf gas exchange is a subject that has received insufficient recent attention. Calcium (Ca) is unique within the group of key elements required for plant growth in that it also has a role in cellular signalling via regulation of changes in its cytoplasmic concentration. Its mobility, within the plant, is however somewhat constricted by its chemistry and cellular signalling role, and its adsorptive capacity within the aopoplast and the xylem. Supply and demand for Ca is achieved by a homeostatic balance which if perturbed can cause a number of distinctive physiological conditions, often related to Ca deficiency. In this issue Rothwell and Dodd present experiments with bean (Phaseolus vulgaris) and pea (Pisum sativum) plants grown in a field soil exposed to the processes of soil liming (application of Ca carbonate (CaCO3). Given that there is evidence of free Ca in the xylem sap altering stomatal conductance it is reasonable to ask the question does liming elevate Ca in the transpiration stream which may explain the observed reduced growth which they hypothesise is due to Ca-induced stomatal closure. They show that liming doubled soil exchangeable Ca, reduced stomatal conductance and shoot biomass in both species compared with unlimed controls. However, xylem sap Ca concentration increased only in bean. Interestingly, the same was not true for the pea where the root xylem sap concentration remained unchanged despite an increase in soil available Ca. Given that stomatal conductance decreased in both species, but in response to a lime-induced increase in xylem sap Ca in only one; this questions the role of Ca in inducing stomatal closure. They propose that their data suggest that as yet unidentified antitranspirant causes stomatal closure in both species not the increase in xylem sap Ca per se.
    No preview · Article · Sep 2014 · Plant and Soil
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    • "The different forms of Cd in different tissues may be related to the Cd transport mechanism in the stem. In addition, the mechanisms for lateral elemental transport around vascular tissue, such as the xylem unloading mechanism and subsequent transcellular movement, are different for different elements (Metzner et al., 2010). In the present study, sulphur was actively unloaded from the xylem, whereas Cd did not leave the vascular tissue of mature leaf sheaths (Fig. 3). "
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    ABSTRACT: Participation of the intervascular transport system within the rice stem during cadmium (Cd) partitioning was investigated by characterizing 109Cd behaviour in the shoot. In addition, 45Ca, 32P, and 35S partitioning patterns were analysed for comparison with that of 109Cd. Each tracer was applied to the seedling roots for 15min, and the shoots were harvested either at 15min (i.e. immediately after tracer application) or at 48h. Distribution patterns of each element at 15min were studied to identify the primary transport pathway before remobilization was initiated. 32P was preferentially transported to completely expanded leaf blades having the highest transpiration rate. The newest leaf received minimal amounts of 32P. In contrast, the amount of 35S transported to the newest leaf was similar to that transported to the other mature leaf blades. Preferential movement towards the newest leaf was evident for 109Cd and 45Ca. These results directly indicate that elemental transport is differentially regulated in the vegetative stem as early as 15min before the elements are transported to leaves. Cd behaviour in the stem was investigated in detail by obtaining serial section images from the bottom part of shoots after 109Cd was applied to a single crown root. At 30min, the maximum amount of 109Cd was distributed in the peripheral cylinder of the longitudinal vascular bundles (PV) and, interestingly, some amount of 109Cd was transported downwards along the PV. This transport manner of 109Cd provides evidence that Cd can be loaded on the phloem at the stem immediately after Cd is transported from the root.
    Full-text · Article · Dec 2012 · Journal of Experimental Botany
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    • "Phloem sap flow would not be labeled and would thus decrease the 26 Mg root enrichment. Gottlein et al. (2005) also found in a root uptake tracing experiment lower Mg tracer recovery in roots compared to Ca. Mg was also found to diffuse less than Ca from xylem to surrounding tissues (Metzner et al. 2010). Mg may transfer more rapidly to the aboveground biomass compartment without homogenously labeling the root biomass compartment. "
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    ABSTRACT: Background and aims The sustainability of forest ecosystems may be at stake especially in forests on base-poor soils due to reduced nutrient deposition and intensified silvicultural practices. Understanding nutrient availability and cycling is therefore essential to manage forest soil fertility. This study aims to assess in a beech plot Mg and Ca vertical transfer in soil and root uptake using an isotopic tracing experiment. Methods A simulated rainfall containing a small amount (960 g Mg.ha−1; 530 g Ca.ha−1) of highly enriched 26Mg and 44Ca was sprayed on the forest floor of a 35-yr-old beech plot. The isotopic composition of fine roots and of the soil exchangeable Mg and Ca pool was monitored during 1 year. A pool and flux model (IsoMod) was developed to predict the labeling of the soil and vertical transfer of tracers. Results Tracers (44Ca and 26Mg) were immediately retained in the thin litter layer. During the following year, Mg and to a lesser extent Ca were progressively released. After 1 year, the exchangeable Mg and Ca pools of the upper mineral layer (0–5 cm) were strongly labeled (~660 ‰, representing ~55 % of the tracer input and ~370 ‰, ~41 % of the tracer input respectively). A significant proportion (~8 % 26Mg, ~2 % 44Ca) of tracer was leached through the soil, below 10 cm. This amount was much larger than what was predicted using a simple mixing model. The Ca and Mg isotopic composition of fine roots at all depths was close or lower than that of exchangeable Ca and Mg respectively. Conclusions An in situ ecosystem-scale 26Mg and 44Ca isotopic tracing experiment was successfully carried out. Tracers were at first strongly retained in the litter layer, then progressively transferred to soil horizons below. Nutrient cycling of Mg and Ca were proven to be very different. Mg had a higher mobility in the soil than Ca, and nutrient uptake sources were proven to be different.
    Full-text · Article · Aug 2012 · Plant and Soil
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