The influence of sucrose (5, 10, 20, 30 g l-1), nitrogen salts - KNO3, NH4NO3 (25%, 50%, 100% in relation to the MS medium) and temperature (15 °C, 20 °C) on the growth of the main shoot and the activation and development of axillary buds in Syringa vulgaris in vitro was investigated. Different ratios of sucrose/nitrogen salts in the MS medium had a limited effect on the length of the main shoot of lilac plantlets. Also, the concentration of sucrose and nitrogen salts in the medium did not significantly affect the formation of nodes on the main or axillary shoots. The outgrowth of axillary shoots depended on the sucrose and nitrogen salts concentrations and temperature. Among the various sucrose/nitrogen salts relations, the highest number of axillary shoots (4.2) was found in the plantlets growing at a temperature of 20 °C, on a medium with a low level of sucrose (5 g l-1) and 100% strength of KNO3 and NH4NO3. Increased levels of sucrose in the medium significantly reduced the development of axillary buds in lilac plantlets growing at either temperature. By contrast, high levels of sucrose increased the fresh weight of lilac shoots. Different levels of nitrogen salts in the medium containing the same level of sucrose had no significant effect on the fresh weight of lilac shoots. On the other hand, at all levels of sucrose, the increased strength of nitrogen salts in the culture medium significantly enhanced the emergence and growth of axillary shoots. Increased strength of nitrogen salts in the medium appeared to counteract, at least partially, the inhibitory effect of a high sucrose level on the growth of axillary buds in Syringa vulgaris. There was clearly an interaction between the levels of sucrose and nitrogen salts such that a medium with a low sucrose to nitrogen ratio promoted axillary branching, whereas a medium with a high sucrose to nitrogen ratio inhibited the growth of axillary shoots. The different ratios of sucrose/nitrogen salts in the MS medium and the temperature affected the morphology of lilac plantlets. Increased supply of sucrose strongly stimulated leafsurface area, but the levels of nitrogen salts had a limited effect on leaf size. The plant- lets cultured at a temperature of 15 °C had bigger leaves than the plantlets at 20 °C. Low-sucrose treatments, irrespective of the level of nitrogen salts, induced a compact and branched habit of shoots and inhibited root formation. Increasing sucrose content in the medium resulted in a spontaneous formation of roots on the plantlets cultured in the presence of low levels of nitrogen salts.
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