Anatomical and physiological factors affecting adventitious root formation in Pinus radiata (D.Don) cuttings

Pinus radiata is an important plantation forest species in Tasmania and other regions of the world. The growth of P. radiata from vegetative propagules has been demonstrated to have many advantages over the conventional seedling method of propagation. The major problems associated with the vegetative propagation of P. radiata by cuttings are the seasonal variation in the percentage of cuttings forming roots and the loss of rooting ability with increased stock plant age. This study investigated the anatomical and physiological factors affecting adventitious root formation in P. radiata.. Initial investigations using controlled environment conditions indicated that anatomical and/or physiological factors rather than the propagation environment were mainly responsible for the seasonal variation in the percentage of P. radiata cuttings forming roots. Most of the mortality observed in cuttings occurred prior to callus formation, highlighting the time of adventitious root development which needed to be investigated further. In controlled environmental conditions (20°C, 95% RH, 14 hour photoperiod), callus initiation was observed approximately 4 weeks after excision and root initiation was observed at approximately 11 weeks after excision. The examination of endogenous plant growth regulator concentrations indicated that the change in the concentration of auxin during the callus formation period may be an important indicator of rooting success in P. radiata cuttings. In those cuttings which exhibited a high rooting percentage, a transient increase in auxin concentration was observed in the basal region of the cuttings 2 and 3 weeks after excision. This was not observed in cuttings which exhibited a low rooting percentage. The concentration of carbohydrates in the basal stem region of the cuttings at the time of excision related poorly to the rooting ability. However, the maintenance of sucrose concentrations in the basal tissue above approximately 15mg/g DW was considered important for the successful rooting of cuttings. The maintenance of adequate water relations in the cuttings during ARF was demonstrated to influence the rooting percentage. In those cuttings which survived and rooted, the leaf water potential was maintained above a value of -2.50MPa. However, in those cuttings which failed to survive the leaf water potential was below -2.50MPa in the week prior to cutting necrosis. In summary, the ability of the cutting to react to excision was considered more important than the absolute levels of these physiological factors at the time of excision. The physiological basis of these findings is discussed and investigated further, from which recommendations are made for improved management practices in commercial P. radiata cutting nurseries.