Eucalyptus is a genus of woody plants which has gained great relevance in the last decades as a rapid source of cellulose pulp and solid wood (Albaugh et al., 2016). Eucalyptus species currently lead the hardwood production market globally, including the countries of the Southern region of South America. Specifically, in the case of Argentina, of the 1.1 million hectares forested, 22% corresponds to Eucalyptus (MAGyP, 2014), with an upward trend, being Eucalyptus grandis Hill ex Maiden the most cultivated Eucalyptus species in the area (Frangi et al., 2016).

The traditional method for obtaining seedlings is the germination of seeds originated in a seed orchard (Skolmen & Ledig, 1990). However, nurseries have obtained -through crossbreeding- elite specimens whose special traits should be kept (de Assiss, 2001). With this objective, the forestry industry is in a process of transition towards the cultivation of clones. Thus, rapid vegetative propagation techniques for producing E. grandis clones are required by forestry companies nationally and worldwide in a strategic path towards enhancement of overall productivity (FAO, 2008).

As conventional macropropagation schemes limit the productivity of eucalyptus plantations due to inefficient rooting rates or excessive rooting timespans (Trueman et al., 2018), in vitro propagation techniques are replacing the macropropagation protocols (Nakhooda & Jain, 2016). Shoot culture is probably the most common technique for E. grandis micropropagation (Nakhooda & Jain, 2016; Trueman et al., 2018,) and comprises the following stages, each of which takes between 2 and 6 weeks: shoot culture, shoot multiplication, shoot elongation, root formation and acclimatization (Trueman et al., 2018), with an average time required of around 6 months. A possible way to shorten the duration of these protocols is the condensation of some stages in the same time period. For example, rooting of micropropagated shoots could be combined with the acclimatization in a hydroponic system.

The culture of Eucalyptus in hydroponic systems has been consistently reported for decades with different purposes: phytoremediation (Iori et al., 2017), drought or salinity stress studies (Nawaz et al., 2016), fertilization experiments (Niu et al., 2015) or in combination with different microorganisms (Egerton-Warburton, 2015). Interestingly, hydroponic systems have never been employed for rooting of micropropagated Eucalyptus shoots and, to our knowledge, it has not been reported in other tree species either. It has been described, however, for rooting micropropagated shoots in shrub species such as Gypsophila paniculata (Wang et al., 2013), Rubus fruticosus (Clapa et al., 2013), Rosa hybrida (Clapa et al., 2013), or Rhododendron sp. (Zaytseva et al., 2018) and herbaceous species such as Spathiphyllum sp. (Dewir et al., 2005) or Solanum tuberosum (Piao et al., 2004).

The aim of the present work was to develop a new and faster micropropagation protocol for Eucalyptus grandis that simultaneously would allow the direct rooting and acclimatization of the micropropagated shoots in a hydroponic system.

To develop our rapid micropropagation protocol, we used nodal segments of seed-derived E. grandis seedlings as primary explants (Trueman et al., 2018). Micropropagation using nodal segments is based on the existence of axillary buds at the base of each leaf axil and their ability to grow. We supplemented MS culture medium with 1 mg BA to promote the growth of axillary buds, obtaining a success rate of 73 ± 9% (Fig. 1B), shoots that grew. Concentrations between 0.1 and 1.5 mg l^-1 of BAP are normally used in the nodal segment culture of Eucalyptus (Trueman et al., 2018). Our initial success rate is strong compared to previous research: Watt et al., 2003 obtained success rates between 67% and 78% at the initiation of nodal segments of three different genotypes of E. grandis; in the case of hybrids of E. grandis with other Eucalyptus species, the success rate varied between 18% (E. grandis x E. camaldulensis) and 91.5% (E. grandis x E. urophylla) (Watt et al., 2003; Watt, 2014). Moreover, as Eucalyptus has multiple buds within each leaf axil (Jasrai et al., 1999), we obtained an average of 1.73 ± 1.03 micropropagated shoots in each nodal segment, with a length of 0.76 ± 0.44 cm (Fig. 1C). Other authors have obtained between 0,5 and 3 shoots per nodal segment in hybrids of Eucalyptus grandis (Watt et al., 1995; Borges et al. 2011). The next step among Eucalyptus micropropagation protocols, involves the multiplication and/or elongation of the obtained shoots (2-6 weeks and 3-4 week of duration respectively), followed by their rooting (Table S1 [suppl.]; Trueman et al., 2018). In our protocol, however, suppressing these two steps and directly moving forward to the rooting of micro-propagated shoots (in a hydroponic system) reduced the extension of the whole protocol to 8 weeks. It should be noted, nonetheless, that a reduction in the number of total plants obtained was detected. Remarkably, there are no previous reports in the literature where Eucalyptus micropropagated shoots had been successfully rooted in a hydroponic system.

The rooting rate that we obtained was 46 ± 4 % after 4 weeks of incubation (Figs. 1D and 1E). Not surprisingly, the contact between the micropropagated shoots and the hydroponic solution was key to achieve a good shoot rooting. The short length of the micropropagated shoots (0.76 ± 0.44 cm, Fig. 1C) made this contact difficult, so the replacement of the styrofoam lid (20 mm) by a 1 mm thick high-impact polystyrene plaque was critical to the success of the rooting process.

With the aim of comparing the rooting rate obtained here with the rates published elsewhere, it is important to note that the latter ones tend to closely relate with the E. grandis or E. grandis hybrid genotypes utilized. From the gathered literature, the in vitro rooting rate varies between 100%, in genotypes of E. grandis (Nakhooda et al., 2011; 2014), to 35% in a hybrid of E. grandis x E. urophylla (de Oliveira et al., 2017). Other authors have obtained a range of varying in vitro rooting rates: 90% or 80% in genotypes of E. grandis (Hajari et al., 2006; Almeida et al., 2015), 36.7% to 90% in hybrids of E. grandis x E. nitens (Mokotedi et al., 2000; Watt, 2014), 35% to 90.0% in hybrids of E. grandis x E. urophylla (Jones & van Staden, 1994; Yang et al., 1995; Whitehouse et al., 2002; Hajari et al., 2006; Watt, 2014;) and 70% in a hybrid of E. grandis x E. camaldulensis (Whitehouse et al., 2002). Hence, the rooting rate that we have obtained (46±4%) was lower-intermediate compared to the rates already published.

The main advantage of rooting in a hydroponic system is that the rooting and the acclimatization can be achieved simultaneously, while the Eucalyptus shoots rooted in vitro (conventional protocols) must be acclimatized in a further additional step. By performing these two steps in a combined manner, we reduced the overall duration of the protocol by 4 weeks. Indeed, the acclimatization success rates are also typically related to the micropropagated genotype utilized, varying between 20% to 100% (Trueman et al., 2018). Thus, the acclimatization not only prolongs the time of the micropropagation protocol, but also reduces the total number of seedlings obtained. In the case of our method, however, all the shoots rooted in the hydroponic system survived and continued their normal growth (Figs. 1F and 1G).

In conclusion, the use of a hydroponic system for ex vitro rooting of E. grandis micropropagated shoots allowed us to obtain seedlings faster than conventional protocols and with an acceptable success rate. This rapid micropropagation protocol has the potential of being useful for propagating elite genotypes with commercial purposes or for performing Eucalyptus physiological studies.