INVESTIGATIONS ON THE ENDOGENOUS ACTIVITY OF GIBBERELLINS AND CYTOKININS OF MALFORMED MANGO (MANGIFERA-INDICA L) IN RELATION TO HISTOLOGICAL DISORDERS .2. FLORAL MALFORMATION

A study on floral mango malformation was made on the anatomical struct ure of the different parts of terminal twigs of 20-year old trees of Z ebda (resistant) and Hindy (susceptible) mango varieties at the flower bud differentiation stage before the morphological symptoms of the di sorder appeared. Parallel to this the assays of hormonal activities of both gibberellins and cytokinins were performed. The shoot apices of Hindy showed earlier differentiation of flower buds than Zebda as mani fested by the elongation of the apical meristem and the appearance of lateral branch primordia. The stem of Hindy variety was characterized by an increased dimension of the vascular bundle and areas of both phl oem and xylem, however, with reduced xylem vessels either in number or diameter. Also, this was accompanied by a higher rate of phloem oblit eration and the presence of dark companion cells in the functioning si eve elements similar to what was observed in the leaf petiole and leaf midrib which were characterized by poorly developed vascular tissue. All investigated sections revealed the absence of inter- or intracellu lar fungal or bacterial growth in the different tissues. Lower GA-acti vity characterized the shoot apex, foliage leaves and stem of Hindy va riety as compared to Zebda with clear shifting to nonpolar GA(s) in th e leaves of Hindy. Gibberellin conjugates were also clearly reduced in Hindy shoot apex and stem and were greatly elevated in the leaves. Si milarly, the cytokinin activity showed lower values in the different p arts analyzed, especially the shoot apex of Hindy variety. However, on examining the gibberellin activity of the terminal twigs of healthy H indy trees, natural existing lower values characterized the shoot apex , leaves and stem at the same developmental stage compared to the resi stant Zebda variety. These values of healthy Hindy trees were superior to those of malformed-expected ones of the same variety. It was concl uded that a decrease in the gibberellin activity of mango shoot apices , presumably to a crucial level easily attained by low-GA varieties, c ould be responsible for the triggering of the malformation syndrome, w hatever its original cause. Also the interrelationship between the cha nges in the hormonal status and anatomical variations characterizing f loral malformation in the susceptible variety was discussed.