Response of Radiata Pine to Salt Stress. II. Localization of Chloride

Abstract

Seedlings of Pinus radiata D. Don (radiata pine) were subjected to a stepwise decrease in the osmotic potential of their rooting medium by the addition of CaCl₂, NaCl or polyethylene glycol 4000. The location of chloride within the root, stem and needle tissue was determined at the ultrastructural level using a silver precipitation technique.

Three main sites of chloride deposition were detected. In the roots, large amounts of precipitated chloride were observed in the hyphae of the mycorrhizas as well as between the cell wall and the plasmalemma of the outer cortical cells, suggesting that chloride was in part prevented from entering the root at these sites. In the stem, chloride was precipitated in the ray cells and in the tracheids where it occurred both in the cell lumen and in the inner layer of the secondary wall (the S2). Since tracheids constitute the largest proportion of a pine tree, this site could constitute a considerable sink into which excess chloride could be deposited, thus removing it from cellular metabolism. In the needles, chloride was precipitated in the epidermal and sub-epidermal cells, and in the mesophyll it was mainly located in the cell walls. In all living cells in the seedlings, chloride was precipitated mainly in the vacuoles. It was restricted to small discrete deposits in the cytoplasm and was absent from the ground plasm and organelles.

These observations support the hypothesis that, although radiata pine cannot tolerate high concentrations of chloride in vital tissues, it is adapted to avoid or delay its accumulation in such sites.