Processes Modulating Calcium Distribution in Citrus Leaves. An Investigation Using X-Ray Microanalysis with Strontium as a Tracer

Abstract

Citrus leaves accumulate large amounts of calcium that must be compartmented effectively to prevent stomatal closure by extracellular Ca 21 and interference with Ca 21 -based cell signaling pathways. Using x-ray microanalysis, the distribution of calcium between vacuoles in different cell types of leaves of rough lemon (Citrus jambhiri Lush.) was investigated. Calcium was accumulated principally in palisade, spongy mesophyll, and crystal-containing idioblast cells. It was low in epidermal and bundle sheath cells. Potassium showed the reverse distribution. Rubidium and strontium were used as tracers to examine the pathways by which potassium and calcium reached these cells. Comparisons of strontium and calcium distribution indicated that strontium is a good tracer for calcium, but rubidium did not mirror the potassium distribution pattern. The amount of strontium accumulated was highest in palisade cells, lowest in bundle sheath and epidermal cells, and intermediate in the spongy mesophyll. Accumulation of strontium in palisade and spongy mesophyll was accompanied by loss of potassium from these cells and its accumulation in the bundle sheath. Strontium moved apoplastically from the xylem to all cell types, and manipulation of water loss from the adaxial leaf surface suggested that diffusion is responsible for strontium movement to this side of the leaf. The results highlight the importance of palisade and spongy mesophyll as repositories for calcium and suggest that calcium distribution between different cell types is the result of differential rates of uptake. This tracer technique can provide important information about the ion uptake and accumulation properties of cells in intact leaves.Studies of the elemental composition of vacuoles in leaf cells have shown that nutrients are asymmetrically distributed between different cell types (Leigh and Tomos, 1993;Leigh, 2001). This has been studied in most detail in barley (Hordeum vulgare), where phosphorus is largely restricted to mesophyll cells, while calcium and chlorine are mainly in epidermal cells, with potassium and nitrate more evenly distributed (Dietz et al., 1992;Leigh and Storey, 1993;Williams et al., 1993;Fricke et al., 1994). Other species also show differences in vacuole composition between different cell types. In wheat, the patterns of distribution are similar to those in barley (Hodson and Sangster, 1988), but in other species they are not. In Lupinus luteus leaflets, phosphorus was found at higher concentrations in epidermal cells than in palisade and spongy mesophyll (Treeby et al., 1987), while in sorghum, phosphorus was highest in bundle sheath cells and lowest in the adaxial epidermis, these latter cells containing the highest calcium levels (Boursier and Läuchli, 1989). In the calcicoles, Centaurea scabiosa and Leontodon hispidus, calcium was highest in palisade, spongy mesophyll, and trichomes, and was excluded from the epidermal cells (De Silva et al., 1996).Although the physiological reasons for these distributions remain largely unexpla...

Cell-Specific Vacuolar Calcium Storage Mediated by CAX1 Regulates Apoplastic Calcium Concentration, Gas Exchange, and Plant Productivity in Arabidopsis

“…p-b, peak over background ratio as defined by Storey and Leigh (2004). n = 4 cells per leaf 8, four separate plants (mean + SE).…”

“…While at the other, low vacuolar Ca concentration ([Ca] vac ) may result from reduced exposure to apoplastic Ca 2+ . Studies of rough lemon (Citrus jambhiri) in which all leaf cells were exposed to strontium ions (as a tracer for Ca 2+ ) or high apoplastic [Ca 2+ ] ([Ca 2+ ] apo ) demonstrated that only certain cells had the ability to accumulate significant [Ca] vac (Storey and Leigh, 2004). This suggests transport capabilities of different cell types determine the ability to store Ca 2+ .…”

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