Investigation of rootstocks for seed production in Eucalyptus nitens.

Abstract

Rootstock age and size, combined with nutrients, temperature, and humidity are major factors thought to play an important role in grafting success in eucalypts. Research was conducted on the grafting success of interspecies and sub-species rootstocks, and three scions were chosen for their differential flowering ability, combined with six different rootstocks, chosen for their precocity of flowering, or as representatives of Eucalyptus nitens by itself or in a hybrid. The agro-meteorological conditions found inside the greenhouse tunnel were monitored and grafted plants were placed in three positions at different distances from the wet wall. The grafting environment was optimized by controlling the temperature and humidity of the greenhouse tunnel to achieve optimum grafting conditions. Overall, 44% of the grafted plants survived. The best rootstock host for grafting E. nitens scion was the hybrid with a strong E. nitens appearance (R5) at 67% survival rate. The two grafting periods had no significant effect on the grafting success, nor did the position of the plants in the greenhouse tunnel. For flower initiation, environmental factors such as day length (light and dark periods), temperature, and topography may affect the flowering characteristics of E. nitens, factors which are often geographically specific. Research was also conducted on the use of interspecies and sub-species rootstocks for early flowering in E. nitens and to monitor the impact of light, cold temperatures (via chilling units) and site location on floral induction. Six rootstocks were selected from Eucalyptus taxa and provenances, including three species and two hybrids. Three scions were selected from three Eucalyptus provenances: Tallaganda, Barren Mountain, and Barrington Tops. Exposed sites with good air drainage and low winter day/night temperature amplitudes were considered ‘good flowering sites’. Of the six rootstocks selected, amongst the ungrafted trees, the hybrid with a strong E. nitens appearance (R5) had the highest budding percentage while amongst the grafted trees, the hybrid rootstock with a strong E. grandis appearance (R2) induced the highest budding percentage on the E. nitens scions. This study provides further insight into the selection of rootstocks and site conditions that yielded good flowering results within the subtropical climatic conditions found in South Africa. Overall, the study showed that grafting on to dwarfing and precocious rootstocks, as well as suitable planting sites are effective in inducing early flowering in E. nitens. The optimized ii methods developed in this study will be important for E. nitens breeding and future flowering research.