Pea (Pisum sativum L. cv. Onmard) and cotton (Gossypium hirsutum L. cv. Campo) seedlings were treated with two concentrations (200 and 500 mg/l) of sodium tungstate (Na2WO4) and the developmental effects were investigated. Tungstate retarded seedling growth rate and stopped root elongation in both species. Seedling growth recovered when tungstate was removed, but primary roots continued to be stunted, while lateral root initiation and growth were stimulated. Tungstate induced premature vacuolation in cells of the root apical meristem, with vacuoles having an unusual semi-circular or cap-like shape around the nucleus. In control roots, the nuclei were spherical with prominent nucleoli bearing several randomly distributed fibrillar centres. In the tungstate-treated cells nuclei contained spherical nucleoli with a big nucleolar vacuole. Occasionally, cytoplasmic components, such as mitochondria, were entrapped in the nucleoplasm of interphasic cells of the treated roots. In these roots, most cell plates were fused to only one lateral parental wall suggesting a non-uniform centrifugal extension. The vesicles in these cell plates were dark and fused to each other at a much lower rate than in the dividing cells of the untreated seedlings. Phragmoplast and cortical microtubules were abundant in the untreated cells, but scarcely detected in the treated ones. All these observations are consistent with the view that tungstate causes considerable toxic effects to pea and cotton seedlings.
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