Adventitious root formation in olive (<i>Olea europaea</i>L.) microshoots: anatomical evaluation and associated biochemical changes in peroxidase and polyphenol oxidase activities

SummaryTrials were performed using in vitro-cultured microshoots of the olive (Olea europaea L.) cultivar 'Galega vulgar', as initial explants, to identify histological events and modifications in peroxidase and polyphenol oxidase activities during adventitious root formation. Explant bases were submitted to a 10 s quick-dip treatment to promote rooting, using a sterile solution of 14.7 mM indole-3-butyric acid (IBA). Samples for histology and quantification of enzyme activities were collected at pre-established periods from 0 to 720 h. The first signs of modifications in stem cell morphology were observed 96 h after explant inoculation on olive culture medium (OM), with some cortical cells showing a dense cytoplasm and a large central nucleus, with visible nucleoli. The first mitotic events were observed after 144 h and evolved via two different pathways: non-specific cell division, leading to callus formation; and organised cell division, leading to the formation of root meristemoids. After 456 h, the first organised root primordia became visible. No root formation was achieved without earlier callus development, and 89% of root primordia originated from tissues other than cambial/phloem tissue. Peroxidase and polyphenol oxidase activities were recorded throughout the whole rooting process. The first significant modification in enzyme activity, with a drop from 0.19 to 0.14 ΔA490 units min–1 50 mg–1 of explant material, was observed for peroxidase within the first 4 h after IBA treatment. Subsequent changes in both enzyme activities could be correlated with different phases of the adventitious rooting process.