Root initiation in cuttings of Agathis australis

Abstract

Rooting in Agathis australis (D.Don) Lindl (Kauri) occurred readily from hypocotyl cuttings taken from two month old seedlings and also took place in material up to ten years old although it became progressively more difficult to root with age. Material from trees older than this has not yet been tried. Root formation on older material was enhanced by uniform high humidity achieved by maintaining the cuttings in boxes covered by clear polythene. These cuttings always rooted in Spring and so the length of the lag phase (time period before there was any observable change) was determined entirely by the time of year when the cuttings were taken. Thus the lag phase varied from about a week to seven months. Constant monitoring of the soil moisture was necessary to obtain maximum rooting. The lag phase, the percentage of cuttings which rooted, and the number of roots per cutting were all modified by growing conditions, treatments applied and the age of the cutting. Cuttings rooted best under low light conditions. Although the application of αNapthalene acetic acid had beneficial effects under some circumstances, the age of the cutting material was the most critical factor. The basal 5mm of a cutting underwent a series of externally observable changes which reflected the internal events. Thus it was possible to use external appearance to identify the anatomical stage that the cutting had reached. In all cases in which root formation occurred the roots emerged laterally within 5mm of the cut base and this distance could not be altered by any of the treatments applied. There were three main types of development and in one of these very little observable change occurred. The other two were similar in that extensive swelling took place in the sub-basal region but in one the cutting rooted and in the other the swelling continued to develop but no roots formed. Over 90% rooting was routinely obtained with young material. Since the cuttings rooted within a short time of each other reproducibility, which is essential to anatomical studies, was achieved. Serial sections (12 μm) from the base to the top of the potential rooting zone were required. Thus, the problems of preparation of plant material for light microscopy included both the large number of sections and the woody nature of the material. Resin embedding proved the most satisfactory technique and a method was developed to handle the sectioning and staining of large numbers of serial sections. An analysis of these sections resulted in an integrated study of the anatomical processes. Within the first two days almost all cuttings underwent a basal wound response which consisted of limited cell division but without the formation of an external callus. At the same time parenchyma cells close to the vascular system and 4 ±1 mm from the cut base differentiated into tracheids. Subsequently vascular tissue (tracheids and phloem) developed in a downward and outward pathway into the cortex. Both a specific arrangement of induced vascular tissue and a sufficiently large number of cells were necessary for the formation of a root primordium site. A number of other developmental events occurred simultaneously in the hypocotyl, only some of which may be directly related to root formation. An increased rate of localized cell division resulted in the formation of secondary resin canals at other locations in the cortex. These, together with the primary resin canals, were obstacles to the passage of the differentiating vascular tissue through the cortex and also reduced the sites available for root primordial initiation. In some cuttings the entire basal section was filled with vascular tissue and no root primordial sites could occur and the cuttings did not root. In older plant material the number of sites was further reduced by the presence of phloem fibres, large resin canals and a band of sclerenchyma in mid cortex. These cuttings produced fewer roots, thereby supporting the argument that space for root primordial initiation is vitally important. Root excision experiments provided further evidence for the importance of the availability of root initiation sites. The removal of roots from cuttings of all ages did not result in further roots being initiated, whereas the removal of the basal 10mm of the stem resulted in the modified cutting again initiating roots. These studies have achieved two major objectives. Firstly, they provide a sound basis for optimizing the rooting of cuttings of Agathis australis. Secondly, the very detailed anatomical investigations have enabled an understanding of the complex series of interactions which take place at the base of cuttings during the rooting process.