Relationships between benzyladenine uptake, endogenous free IAA levels and peroxidase activities during upright shoot induction of Cymbidium ensifoilum cv. Yuh Hwa rhizomes in vitro

Abstract

Studies were undertaken in order to improve the micropropagation ofCymbidiun ensifolium 'Yuh Hwa’. It is easy toinduce rhizomes in vitro in this cultivar but not upright shoots, which areneeded for eventual production of a flowering plant. Exposure ofCymbidium ensifolium 'Yuh Hwa’ rhizomeexplants in vitro to greater than 2.5 μM BA for 12weeks or longer inhibited both elongation of upright, aerial shoots and rootgrowth. Through transfer experiments, it was determined that the commitment toinduction of upright shoots occurred after 10–14 days of exposure to 2.5μM BA. BA supplied at 20 μM during the first 14days of culture was sufficient to induce upright shoot formation, but shootelongation did not continue under these conditions. Using radiolabelled BA,adenine was found to be a major metabolite found in the rhizome tissue at day14. BA comprised 37% of the total DPM recovered in rhizomes grown in 20μM BA compared to that of 61% of the total DPM recoveredfrom rhizomes grown in 2.5 μM BA. The total amount of BA takenup per gram fresh weight was similar for both green and etiolated rhizomesgrownin 2.5 μM BA. Rhizomes grown in 20 μM BA tookupapproximately five times as much BA as those grown in 2.5 μMBA.Free IAA levels were highest in the rhizome tip (12.7 ng/g FW) anddecreased to one fourth the value at 1 cm behind the tip. Free IAAlevels increased initially in rhizornes transferred to medium with or without20μM BA (although at different amounts) and then decreased. Whenactivated charcoal was added to the medium as well as BA there was no initialincrease in IAA. When 20 μM BA was added to the medium, solubleperoxidase activity increased while IAA concentrations remained at about thesame level over a 7-day period. Peroxidase activity in rhizomes eitherdecreasedor remained constant when activated charcoal was added to the medium containingBA.