The temporary immersion system as an efficient technique for in vitro propagation of ‘Cascade’ hop

Humulus lupulus L. (hop) is an economically important species whose mass propagation has recently increased, due to the multiple interests that its cultivars present in several fields of application. The in vitro propagation techniques can represent a valid alternative to propagate hop plants on a large scale, and to supply a standard biomass for bioactive compound extraction. In fact, extracts obtained from hop vitro-derived plantlets have antioxidant, antimicrobial and antiviral properties. Temporary immersion system (TIS) allow the plant propagation by alternating immersions of microcuttings in liquid culture medium with dry periods, thus avoiding gas accumulation, through forced ventilation. In this study, the use of TIS (Plantform™ bioreactor) as alternative to solid medium, both in presence or absence of sucrose, was evaluated for in vitro propagation of hop ‘Cascade’, considering its effect on morpho-physiological parameters, on total phenolic content and antioxidant activity. At the end of the experiment, results obtained evidenced that microcuttings cultured in Plantform™ bioreactor produced a significantly higher number of shoots (+25.3%), roots (+77.4%), shoot and root elongation (respectively, +26.1 and +58.4%), and relative growth rate index (+2.6 times), when compared to those plants grown in solid medium. The presence of sucrose did not seem to have significantly influenced neither the number of shoots or roots, but it increased the length of both organs (respectively, +45.7 and +33.4%). Different results were obtained regarding the polyphenol content and antioxidant activity; in fact, it seems that plantlets grown in solid medium with sucrose contained significantly higher content of polyphenols and manifested a higher antioxidant activity. The temporary immersion technique has many positive potentials for in vitro propagation; this paper presents preliminary aspects on hops with development prospects for strengthening the production sector.