Ultrastructural Features of Molybdenum Deficiency and Whiptail of Cauliflower Leaves: Effects of Nitrogen Source and Tungsten Substitution for Molybdenum

Abstract

Cauliflower plants were grown in sand culture with molybdenum-free nutrients: containing sodium nitrate, ammonium sulphate or ammonium nitrate as nitrogen sources and without, or with 0.1 or 0.25 ppm tungsten. Control plants were given 0.05 ppm molybdenum. After 6 weeks' growth primary visual lesions of the whiptail disorder appeared in molybdenum-deficient plants grown with ammonium sulphate. Samples of lamina from around these lesion areas, and from corresponding areas in chlorotic plants grown with nitrate or from control plants, were prepared for electron microscopy. In molybdenum-deficient plants grown with nitrate, chloroplasts became bulbous and enlarged but had reduced grana stacking and distorted expansion of intrathylakoid spaces. Decrease of electron density and appearance of cavities in the stroma were followed by spherical protrusions from the surface of the chloroplast bounded initially by both chloroplast and tonoplast membranes. These protrusions became filled with chloroplast debris or ruptured. In plants grown with ammonium, chloroplast structure appeared normal except in, or close to, lesion areas where their appearance resembled those grown with nitrate. In cells near the centre of lesions chloroplasts disintegrated into the cytoplasm. Mitochondria appeared to remain relatively undamaged. Addition of tungsten to the nutrient resulted in suppression of molybdenum-deficiency symptoms both visually and microscopically. Chloroplast integrity was preserved; cavities in the grana stacking were much less evident but areas of electron transparency persisted. The resemblance of the symptoms described to superoxide damage of lipid membranes is discussed with respect to the known biochemical changes which take place in ascorbic acid, S-methylcysteine sulphoxide and cytochrome c reductase levels as a result of molybdenum deficiency in cauliflowers.