Toward a rooting protocol for eucalyptus dunnii

Ramlal, Nirvana
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Eucalyptus dunnii exhibits fast growth, low lignin and high cellulose contents, traits that are highly desirable for pulp production and are therefore favoured by the paper industry. In this study, E. dunnii roots were produced by the various methods practiced by commercial forestry viz. macrocuttings, microcuttings and seedlings. Roots produced were compared and characterized in order to understand the distinctions between Eucalyptus roots produced by these methods. Seedlings and clonal mini-cuttings were sourced from the Trahar Technology Centre, Mondi (South Africa). Plant shoots were established in culture and multiplied before the induction of roots in vitro. An optimal growth medium was determined for E. dunnii multiplication, which was 4.42g.l-1 MS (with vitamins), 25g.l-1 sucrose, 0.2g.l-1 benzyl aminopurine, 0.01g.l-1 naphthaleneacetic acid, 0.1g.l-1 biotin, 0.1g.l-1 calcium pantothenate, and 3g.l-1 gelrite (pH of 5.6-5.8). Due to the high (68%) levels of vitrification, tests were conducted to reduce or reverse this phenomenon with no success. Only healthy material was selected for rooting experiments. In vitro rooting was tested on medium containing 1.105 g.l-1 MS (with vitamins), 15 g.l-1 sucrose, 0.1 g.l-1 biotin, 0.1 g.l-1 calcium pantothenate, 4 g.l-1 gelrite and pH of 4.5-5.8 and 0.1-1mg.l-1 IBA without success. In vitro rooting (4%) was achieved on peat:vermiculite (1:2) probably due to less humidity. Comparison of micropropagated plantlets and seedlings of the same height range showed that root architecture of main roots, main side roots, number of side root and shoot masses were all statistically similar. Mini-cuttings underwent treatments before placement into seedling tray inserts containing coir:perlite (1:2). With the first treatment, the lower excised end of the shoot was dipped in 20mg.l-1 IBA, for 48 hours while being incubated in a dark and humid environment; the cutting was then placed into the insert. With the second treatment, mini-cuttings were placed directly into the prepared inserts for four weeks. Thereafter, the shoots were carefully removed and underwent the same process as with treatment one. The third treatment involved dipping mini-cuttings in Seradix 1 0.1% IBA) and placement into the prepared inserts. Treatment 4 involved three pulse treatments which were tested by placing mini-cuttings into various concentrations of IBA (20mg.l-1, 200mg.l-1, 2000mg.l-1) in solution for 150 seconds before placement into the insert. Mini-cuttings devoid of any treatment had approximately the same percentage success (26%) as those treated with Seradix 1 (25%). Mini-cuttings that underwent the 200mg.l-1pulse treatments showed the greatest percentage success (30%). Mini-cuttings treated with Seradix 1 and seedlings of various sizes (7.5-70cm) were obtained from the Trahar Technology Centre and the root architecture was analyzed and quantified. Mini-cutting derived plantlets and seedlings of the same size had approximately the same shoot:root ratio. Mini-cutting plantlets had a shoot:root ratio of 2.2:1 and seedlings, 2.8:1 in the 5-7cm height range. In the 7.1-9cm height range, cutting plantlets had a shoot:root ratio of 2.5:1; and seedlings, 2.4:1. The number of lateral roots of both seedlings and mini-cutting derived plantlets in each height range was found to be statistically similar. In the comparison of seedlings of the same height range as the mini-cutting plantlets sampled, it was observed that the root architecture differed in root length and structure. With the comparison of shoot and root growth of different heights (20 to 70cm, increasing in increments of 10cm) of E. dunnii seedlings, the length of the main roots of seedlings within all height ranges were found to be similar. The same trend was noted in lateral-root lengths and numbers. While some shoot dry masses were not significantly different from others. Shoot dry mass of seedlings gradually increased in accordance with an increase in the height of shoots. With every 10cm increase in shoot height, shoot mass would increase by approximately 50%. Although in vitro plantlets, mini-cutting plantlets and seedlings of the same height range seemed similar in shoot:root ratios and root lengths, no direct comparisons could be drawn from the study due to the varying growth conditions of the samples before analysis, as well as restrictions on root growth by containers. To fully understand root architecture and growth patterns, it is suggested that more field work is required to obtain an accurate representation at different stages of growth.