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PLANT NUTRITION

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

Horticulture Unit, Commonwealth Scientific and Industrial Research Organisation Plant Industry, Merbein, Victoria 3505, Australia (R.S.); and Department of Plant Sciences, University of Cambridge, Cambridge CB2 3EA, United Kingdom (R.A.L.)

Citrus leaves accumulate large amounts of calcium that must be compartmented effectively to prevent stomatal closure by extracellular Ca²⁺ and interference with Ca²⁺-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.

^* Corresponding author; e-mail richard.storey{at}csiro.au; fax 61–3–5051–3111.

Received April 30, 2004; returned for revision June 5, 2004; accepted June 7, 2004.

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