Combined effect of rooting medium and biostimulant application on morphological traits of micropropagated hop plantlets

Abstract

Establishing vigorous and healthy plants is the first requisite for the future success of a new hopyard. Although hop micropropagation has been extensively studied, little information is available on subsequent ex vitro rooting and acclimatization phase. For example, materials and practices used to obtain an excellent hop plantlet ready for the field are often not referenced. The major challenge is to produce well-rooted plants without excessively boosting the vegetative growth. New substrates (alternative to peat) and treatments (alternative to synthetic plant hormones) are likely required to address this issue and to increase the sustainability of the process. With this aim, we used micropropagated plantlets of cultivar Cascade and Nugget to assess the combined effect of rooting medium and biostimulant application on root and shoot development during two hardening periods: i) acclimatization in glasshouse, and ii) acclimatization in open air. Two growing media were tested: i) a standard peat-based substrate (control treatment), and ii) an experimental substrate, consisting of a mixture of the standard substrate and woodchip compost. Extracts of seaweed Codium fragile (Suringar) Hariot and plant Opuntia ficus-barbarica A. Berger were used as biostimulant agent. The experiments were set up in a completely randomized design with three replicates. Survival rate, shoot and root traits as well as rhizome development were determined. At the end of the study, the foliar biostimulant significantly increased root growth (21% in length and 46% in weight) and rhizome length (+46%), while it had a moderate effect on shoot development. Hop cultivars tested in our study highlighted a very different way to allocate plant biomass during the hardening period, with Nugget having a root-to-shoot ratio twofold higher than Cascade. Finally, even though the experimental substrate did not affect plant traits during the first acclimatization period, it had a strong negative effect during the acclimatization in open air. Our findings provide practical guidance for nursery managers seeking to produce high-quality hop plantlets using sustainable materials and treatments.