INVESTIGATIONS ON THE ENDOGENOUS ACTIVITY OF GIBBERELLINS AND CYTOKININS OF MALFORMED MANGO (MANGIFERA-INDICA L) IN RELATION TO HISTOLOGICAL DISORDERS .1. VEGETATIVE MALFORMATION

Healthy and malformed one- or two-year old mango seedlings were invest igated for their gibberellin and cytokinin activities in relation to h istology. The bunchy top of malformed seedlings was characterized by a large number of thick and short vegetative shoots developed at the gr owing tip together with the disappearance of apical dominance. Numerou s buds were also produced at the axils of leaves which failed to attai n normal growth. The normal shoot tip was dome-shaped in appearance wh ile the malformed one was rather flat, with lateral buds more close to the main growing tip than in healthy apices. The stem vascular tissue s of the malformed seedling showed a clear increase in both phloem and xylem areas with an increased number of secondary xylem vessels but w ith a reduced diameter. Also the functioning sieve elements of the ste m decreased with malformation in both number and size together with a higher rate of sieve element obliteration. On the other hand, a reduct ion in xylem vessel number and radial dimension of both xylem and phlo em occurred in the leaf midrib of the malformed seedling. These histol ogical changes were accompanied by lower gibberellin and higher cytoki nin activities in the gall-like growth of bunchy top as compared to th e normal shoot apex. Meanwhile, higher gibberellin activities occurred especially in leaves of one-year old malformed seedlings. These leave s were reduced in the width of the lamina. Gibberellins of both bunchy tops and leaves, but not the stem, of malformed seedlings showed a cl ear shifting to the nonpolar types (R(fs) 0.8-1.0).