Early growth and graft success in macadamia seedling and cutting rootstocks

Rootstocks play a vital role in growth and productivity of tree crops through water and nutrient translocation and signal transduction to the scion. Early vigour and graft compatibility of rootstocks are important for further growth and development of the scion, and hence for productivity. This study aimed to identify the variability of early growth and graft success of cuttings and seedlings of different species, cultivars and breeding lines of macadamia that will be subsequently screened in a macadamia rootstock field trial. The experiments were conducted in a mist house at Maroochy Research Facility, Nambour. The experiments comprised macadamia seedlings and cuttings of eight commercial cultivars, fifteen breeding lines, and different sources of wild germplasm of Macadamia tetraphylla, M. jansenii and M. ternifolia. All the existing cultivars and breeding lines in the study are derived from M. tetraphylla and/or M. integrifolia species. There was significant variation among the genotypes in cutting survival and growth rates for both seedlings and cuttings. The study identified a breeding line (BHI2) which produces strong cutting root systems with better striking rates (100%) than existing rootstock cultivars. Cutting survival rate was greatest in standard macadamia rootstocks (79%) and least in M. ternifolia (46%). Growth rate varied depending on the genotype and the method of propagation. 'H2' seedlings were greatest for height increase rate, while cuttings of M.jan3, M. ter1, M.ter3, BHI1, BHI2, BHY1, 'A268', 'A4', 'D4' and 'Daddow' showed greater growth in height than 'H2'. Both seedlings and cuttings of BHI2, cuttings of BDW4, M.ter1, M.jan3, BHY1, and seedlings of M.tet2 had greater stem diameter increase rate than 'H2'. Among groups, standard rootstocks had the greatest rate of increase in height and breeding progeny of high index value had the greatest rate of stem diameter increase. Graft success was significantly higher in seedlings than cuttings. Cuttings of BHI2, 'D4' and 'Daddow', and seedlings of BDW5, M.tet2, BHI3 and BHY1 outperformed 'H2' for graft success. Results from this study suggest that initial rooting in cuttings can be an indicator of graft success. After planting the grafted tree, the relationship of the pre-grafting characteristics of the rootstock with the performance of the scion can be used as tools of identifying high performing rootstocks in the macadamia breeding program.