Two rootstocks have been used in Macadamia nut production in Hawaii. These are M. integrifolia, which is also the species used for nut production in Hawaii; and M. tetraphylla, a closely related species, occasionally used for nut production in Australia.

Almost all of the trees presently in commercial production in Hawaii are grafted on M. integrifolia seedling rootstocks, but a small proportion of trees are grafted on M. tetraphylla seedlings. These trees appear to be vigorous, long-lived and productive. The use of these two rootstocks is well known in Hawaii and is documented (1, 3, 4, 5). The interaction of varieties with location has been studied by Cameron, Mouat and Hamilton (2). There is however no published account on the effect of rootstocks on production or quality of nuts.

MATERIALS AND METHODS

An experiment was carried out in 1968 to study possible effects of these root-stocks on kernel quality, in-shell nut weight and shell thickness of nuts produced by M. integrifolia clones. Characters studied were:

Kernel Quality_ the percent of no. 1 kernels as measured by flotation in tap water. This gives a good measure of minimum oil content (6).

Nut Weight_ computed at the average number of in-shell nuts per pound.

Shell Thickness_ as measured at the side of the shell approximately midway between the micropyle and funiculus.

Experimental trees, with the exception of one pair of trees at the Haleakala Experimental Farm, on Maui, were located on the island of Hawaii and consisted of used 14 paired replications. Each pair was made up of 2 comparable trees of the same clone, one grafted on ‘‘smooth-shell’’ (M. integrifolia) and the other on "rough-shell’’ (M. tetraphylla) rootstocks. Seven cultivars and 4 different locations were represented.

Random samples of 50 nuts were taken from each of the experimental trees during October and November. This is considered to be approximately mid-season in the usual harvest period for the 7 cultivars involved. Samples taken were mature, freshly harvested nuts. Nuts were picked from the ground, under the trees, while the husks were still green. Nut samples were husked the same day as harvested and dried to approximately 1½ percent moisture in a forced air oven for 75 to 100 hours at 110 F. Percent No. 1 kernels was computed as percentage of oven-dried kernels floating in tap water. Nuts per pound were computed by dividing 453.6 by the average weight in grams of in-shell, oven-dried nuts. Shell thickness was measured at the side of each shell by means of a slide caliper.

RESULTS

Data on kernel quality nut weight, and shell thickness are summarized in Table I. There was close agreement in the average weight of nuts produced on different rootstock species. Nuts from trees grafted on rough-shell rootstocks averaged 66.5 nuts per pound, while those from trees on smooth-shell rootstocks averaged 65.2 nuts per pound The observed difference of 1.3 nuts per pound is not significant.

Similarly there was no apparent effect on kernel quality, which could be attributed to the rootstock species used. Overall kernel quality was high, averaging over 90 percent No. 1 kernels on both rough-shell and smooth-shell rootstocks at all locations. There was no observable difference in kernel quality, which could logically be attributed to the rootstock used. The observed difference in shell thickness of .002 inches was also not statistically significant.

CONCLUSIONS

Analyses of the data reveals no differences in kernel quality nut weight or shell thickness. It can therefore be concluded that under the conditions of this experiment, the rootstocks used had no effect on the three characters studied. Any judgement on superiority of rootstocks should therefore be made on the basis of other factors such as yield, disease resistance, wind resistance, or nursery performance, which were not considered in this experiment.