INFLUENCE OF GROWTH-REGULATORS AND LIGHT ON IN-VITRO SHOOT REGENERATION IN M.26 APPLE ROOTSTOCK

The effects of different BA and IBA concentrations and dark/light comb inations, applied during both the last proliferating subculture (LPS) and the regeneration phase (RP), on shoot regeneration from leaves wer e evaluated in M.26 apple rootstock. A positive influence on caulogene sis was found with a low cytokinin/auxin ratio in the medium during th e LPS. The increase in IBA concentration from 0.49 mu M to 4.92 mu M i n the LPS, along with the absence or the use of a low cytokinin concen tration (0.89 mu M) in the medium, enhanced the subsequent shoot regen eration from leaves. During the RP, a concentration of 22.2 mu M BA ga ve the best caulogenesis results. During both the LPS and the RP, high IBA concentrations were able to replace the combined effect of a low IBA concentration and dark treatment; this could indicate that dark tr eatments interact with the auxin metabolism in the leaf caulogenesis r esponse. Auxin application could not reproduce all the effects of dark treatments, suggesting that dark also affects other biochemical and/o r physiological aspects, such as gibberellin metabolism. Moreover, cyt okinins applied during both the LPS and the RP influenced the size of regenerated shoots.