Some factors which may influence root formation in conifer cuttings : a thesis presented in partial fulfilment of the requirements for the degree of Master of Horticultural Science at Massey University
Seasonal fluctuations in adventitious root formation capacity of cuttings of Juniperus virginiana L. 'Skyrocket' ("easy-to-root") and J. scopulorum Sarg. 'Pathfinder' ("difficult-to-root"), as indicated by three parameters, was determined over a nine month period from late summer (February) until spring (October). Rootability rose from a low or moderate level in late summer to an optimum in mid winter after an intervening period of low potential and diminished again in spring. A less comprehensive study of Cupressus sempervirens L. 'Swane's Golden' ("easy-to-root") revealed that rootability was high in February and June but low in March. The most marked difference in the pattern of seasonal changes between the two juniper cultivars occurred in the percentage of cuttings rooted, which increased dramatically in J. virginiana 'Skyrocket' in April and remained at high levels before declining sharply in October but remained low in cuttings of J. scopulorum 'Pathfinder' until the optimum was attained suddenly in June and diminished thereafter. Generally more gradual changes occurred in the other parameters. Air temperature treatments of the stock plants and cuttings induced significant differences in level of rooting achieved in cuttings taken in mid winter. Material of J. virginiana 'Skyrocket' and J. scopulorum 'Pathfinder' from stock plants which had received normal winter chilling had a greater root regeneration potential than that from plants maintained in a heated glasshouse since autumn. The converse was true for cuttings of J. scopulorum 'Blue Haven' ("very difficult-to-root") and C. sempervirens 'Swane's Golden'. The effect of cutting environment was rather more variable but material which had been exposed to the most favourable parent environment tended to root in greatest numbers in an unheated compared with a heated air environment. Responses of J. scopulorum. 'Pathfinder' and C. sernervirens 'S.vane's Golden' under controlled environment growth cabinet conditions confirmed these results. The attainment of a high rooting percentage in cuttings of J. scopulorum 'Pathfinder' in mid winter appeared to be dependent to a large extent on exposure of the stock plants to low temperatures. Results from the seasonal study generally coincided with the commonly held opinion that phase of growth may be an important determinant of root formation potential in narrow-leaved evergreens. It was suggested that the promotion of rooting in chilled material of J. virginiana 'Skyrocket' and J. scopulorum 'Pathfinder' may have been associated with the stimulation of shoot activity brought about by that treatment but there was no conclusive evidence to support this. Analysis of endogenous growth regulator content was conducted in material from different cultivars, temperature treatments and harvest dates. Level of an IAA-like growth promoter seemed to be the least related to differences in rootability. Estimated ABA and total cytokinin content appeared to be inversely related to rootability in several instances.