Integrated assessment of adventitious rhizogenesis in Eucalyptus: root quality index and rooting dynamics

Abstract

Clonal propagation of eucalypts has considerable importance due to the increasing demands for short rotation tree crops. Rooting quality is important as it governs the soil exploitation capacity of the plant and the anchorage of trees which are susceptible to wind damage. This study assesses the quality of adventitious rooting by coppice cuttings of commercially important Eucalyptus clones using multiple attribute ranking of the differences in parameters of root growth. The effects of different concentrations of indole-3-butyric acid (IBA) on root development were observed. Cuttings were treated with 500, 1000, 4000, 6000 mg L−1 IBA and tannic acid for 10 s, 2 h and 24 h. Total length of root systems, number of roots, shoot to root ratios, number of root segments, extent of forking, rooting percentage, average root diameters, and number of root tips were measured. Grey relational analysis was used to create a comparability sequence to rank treatments. Reducing the concentration of auxin (IBA) and increasing the length of exposure produced better quality roots in Eucalyptus camaldulensis and interspecific hybrids (reciprocal hybrids of E. tereticornis and E. camaldulensis), while the opposite was observed with E. tereticornis clones. A root quality index was proposed, based on the Dickson quality index for the assessment of root system characteristics and considered total mass, shoot: root ratios, total length of root systems, and average root diameters. It has the advantage of implementation convenience. A positive correlation was obtained between grey relational analysis grades and root quality index. Rooting dynamics were studied by evaluating the total length of the root system at seven-day intervals and plotting daily current and medium increments using regression analysis. The curves showed the variation in growth rates among the different clones, and their intersection gave the optimal time of permanence (time at which further growth is restricted) which varied considerably. The highest daily current increment was 35–40 days for all clones.