Origin of adventitious roots in black walnut (Juglans nigra) softwood cuttings rooted under optimized conditions in a fog chamber

摘要

High-quality black walnut (Juglans nigra L.) logs are of great economic value and are used in the manufacture of high-end products. Indigenous to the central hardwood region, black walnut has been commercially cultivated for many years, and genetic improvement and selections have resulted in superior timber genotypes. The recalcitrance of black walnut cuttings to form adventitious roots is the greatest hurdle for mass propagation of improved material. The goal of this research was to improve the frequency of adventitious root formation in black walnut cuttings, and investigate anatomical changes during root development. Softwood cuttings (15-20 cm) were collected from juvenile and mature sources of elite genotypes, dipped for 60 s in 31.1, 62.2, or 93.2 mM indole-3-butyric acid-potassium salt (K-IBA), or 36.9, 73.8, or 110.7 mM indole-3-butyric acid (IBA), and then inserted into a moist medium consisting of 3 perlite: 1 coarse vermiculite (v/v). Cuttings were placed in bench-top fog chambers or a mist bench for 5 weeks. To visualize anatomical changes during root formation, stems were fixed in formaldehyde, embedded in paraffin, serially sectioned, and stained on sequential days throughout root development. Rooting was greatest (72%) for cuttings exposed to 93.2 mM K-IBA and placed in the fog chamber, while cuttings treated with IBA rooted at lower frequencies (16-22.2%). Cuttings in the mist bench often deteriorated and rooted at lower frequencies independent of the auxin type. Anatomical analysis revealed adventitious root initials by day 16 and root primordia formation by day 18. Rooted cuttings survived acclimatization to the greenhouse.