Carbohydrates Modulate the In Vitro Growth of Olive Microshoots. I. The Analysis of Shoot Growth and Branching Patterns

The rate of microshoot proliferation during the micropropagation of olive (Olea europaea L.) plants is limited by the low rates of both bud sprouting and growth of secondary shoots following subculturing. The aim of this study was to determine (1) the effects of sucrose and mannitol on shoot growth, (2) whether either of these sugars modifies the pattern of shoot development of the explants, and (3) the influence of apical dominance on explant development. Working with single-node microcuttings of Olea europaea L. cv. Maurino with two opposite axillary buds, we added 17, 34, or 68 g L-1 of sucrose or mannitol to the medium as the primary carbon source. Shoot development was classified as either (a) an outgrowth of the first bud on an explant (shoot-type A), (b) an outgrowth of the second bud (shoot-type B), or (c) an outgrowth of an axillary bud on either an A- or B-type shoot (shoot-type C). Explant survival, fresh-mass production, and patterns of shoot development were influenced by the type and concentration of sugar used. Mannitol promoted the sprouting and growth of A-, B-, and C-type shoots more than sucrose. The developmental responses observed indicate that the growth of axillary meristems of in vitro olive explants is not regulated by apical dominance. The results demonstrate that the sugar alcohol plays an important role in the developmental regulation of olive explants. Mannitol may also protect against detrimental effects associated with in vitro growth conditions.