Investigations on the endogenous activity of gibberellins and cytokinins of malformed mango (Mangifera indica L.) in relation to histological disorders. I. Vegetative malformation.
Raafat A., El-Deeb S. H., Neumann K. H.
Author Affiliation: Agric. Botany Dept., Faculty of Agriculture, Ain Shams University, Shoubra El-Kheima, Cairo, Egypt.
Angewandte Botanik 69 : 3-9
Abstract : Vegetative malformation of mango is a serious disorder that affects both young seedlings and adult trees. It is manifested by the excessive production of abnormal vegetative buds on terminal twigs, leading to the production of a large number of shoots and the formation of bunchy tops. Healthy and malformed 1- and 2-year-old seedlings of several mango cultivars were investigated for their gibberellin and cytokinin activities in relation to histology. The bunchy top of malformed seedlings was characterized by a large number of thick, short vegetative shoots developed at the growing tip, together with the disappearance of apical dominance. Numerous buds were also produced at the axils of leaves which failed to attain normal growth. The normal shoot tip was dome-shaped in appearance while the malformed one was rather flat, with lateral buds more close to the main growing-tip than in healthy apices. The stem vascular tissues of the malformed seedling showed a clear increase in both phloem and xylem areas with an increased number of secondary xylem vessels but with a reduced diameter. Also the functioning sieve elements of the stem decreased with malformation in both number and size, together with a higher rate of sieve element obliteration. On the other hand, a reduction in xylem vessel number and radial dimension of both xylem and phloem occurred in the leaf midrib of the malformed seedling. These histological changes were accompanied by lower gibberellin and higher cytokinin activities in the gall-like growth of bunchy-top than in the normal shoot apex. Meanwhile, higher gibberellin activities occurred, especially in leaves of 1-year-old malformed seedlings. These leaves were reduced in the width of the lamina. Gibberellins of both bunchy tops and leaves, but not the stem, of malformed seedlings showed a clear shift to the non-polar types (Rfs 0.8-1.0).