Rapid clonal rooting through in vitro proliferation of nodal cuttings of ‘Cascade’ Hop

The demand for female hop plants (Humulus lupulus L.) by the brewing industry, combined with the dioecious nature of the species, presents challenges for propagation via seeds. Therefore, producers rely on vegetative propagation through rhizomes or cuttings. However, these practices may lead to certain drawbacks concerning phytosanitary quality. Consequently, micropropagation emerges as an effective alternative for producing healthy and genetically uniform hop plants. The objective of this study was to evaluate the in vitro culture response and ex vitro acclimatization of ‘Cascade’ hop as an alternative to conventional multi-stage micropropagation protocols, which typically include (i) in vitro establishment, (ii) shoot multiplication on cytokinin-rich medium with repeated subcultures, (iii) a separate rooting phase, and (iv) ex vitro acclimatization. Specifically, nodal cuttings from stabilized in vitro cultures were directly grown in five different culture media supplemented solely with auxins. The rooting media, compared with a hormone-free (HF) medium, differed in concentration and the ratio of two auxinic hormones: indole-3-acetic acid (IAA) and indole-3-butyric acid (IBA). After two months of in vitro culture, it was found that, despite the absence of cytokinins in the culture medium, the explants were able to sprout and produce shoots with lengths ranging from 1.22 to 2.25 cm. Rooting-wise, the medium supplemented with 1 mL L-1 IAA produced the highest number of roots (4.98), while the longest roots were observed in the medium supplemented with 2 mL L-1 IBA. The derived in vitro hop seedlings were transferred to agriperlite after thorough washing to remove residual culture medium. The acclimatization rate was 97.8%. Our findings suggest that for non-recalcitrant species, such as hops, the multiplication phase could potentially be eliminated, resulting in significant savings in time and cost.