PROBLEMS IN GERMINATING MACADAMIA SEED
R. A. Hamilton*
Reprint from CMS 1959
The germination behavior of Macadamia seed (M. ternifolia) of different varieties usually varies greatly and this may be confusing to those not familiar with the factors involved. Hawaii Agricultural Experiment Station personnel have done some experimental works recently on the problems of producing Macadamia seedlings. In addition, considerable information has been gained through practical experience in growing seedling nurseries at the Kona, Haleakala, and Poamoho research farms.
Behavior of seed of most varieties is rather erratic, and the germination of seed from the same variety harvested on the same day is usually spread over a period of several months. Some seed will begin to germinate after 4 or 5 weeks while the remaining visible seeds may continue to germinate as much as 4 or 5 months longer. Certain varieties have a small opening in the shell called the micropyle and it is possible for air and water to enter freely through this opening, thus permitting rapid and even germination. One such variety, HAES 503, germinates rapidly and evenly, most of the seeds germinating within 2 weeks after planting.
Effects of Husks and Shells on Germination
The green, fleshy husk that surrounds the nut must not be allowed to dry on the seed and should be removed from the seed as soon as possible after harvesting to prevent heating and consequent spoilage. Failure to remove the husk before planting may delay germination for several months. Tests conducted at the Kona Research Farm have shown that Macadamia nuts planted without removing the husk will not germinate until the husks have rotted away.
Although the husk around the seed should always be removed before planting, it is neither necessary nor advisable to crack or remove the hard shell, which surrounds the embryo. It has been noted that when the shell is cracked before planting, the majority of the embryos rot rather than germinates. It is not known whether this is due to injury to the embryo in cracking and removing the seed coat or whether some other factors are involved. Cracking the shells mechanically before planting actually lowers the percent of germinating seed and for this reason is not recommended.
Viability and Longevity
In growing Macadamia seedlings for rootstock or other purposes, it is important to know how long mature seeds remain viable after harvesting. In the past, it was usually considered, and was often stated, that freshly harvested seed must be planted as soon as possible after it matures in order to assure satisfactory germination. However, an experimental test of four varieties (Keauhou, Kakea, Kohala, and Pahau) demonstrated that husked,
Figure 1. The effect of age of Macadamia seed on germination when stored at room temperature.
Air-dry seed will germinate almost as well after 4 months in storage at ordinary room temperatures as when planted immediately. The results of this test are shown in figure 1. One variety, Keauhou, gave 39 percent germination even after 7 months in storage at room temperatures. Other varieties, such as Kakea, which gave only 16 percent germination from freshly harvested seed, germinate so poorly that they would hardly prove satisfactory for seedling production regardless of age at planting time. Keauhou, the best variety from the standpoint of germination, produced three times as many viable seedlings as Kakea, the variety with the poorest germination.
It may also be seen from figure 1 that the viability of seed stored for periods ranging from 4 months to 1 year decreased as the time in storage increased. Seed that was stored 12 months before planting failed to germinate, regardless of variety. The most satisfactory nuts for seed purposes were the No. I seeds, which are heavy enough to sink when placed in water and still have the embryo attached to the inside of the shell. Shaking the nut can test the latter condition. No. 2 seed, which floats in water and/or has the embryo loose inside the shell, germinates so poorly that it is of comparatively little value in producing seedlings, regardless of variety or age of seed. No. 1 seed has a definite advantage over No. 2 seed in germinating ability. This advantage is clearly illustrated in figure 1.
*R A. Hamilton is Associate Horticulturist HAES, College of Agriculture, University of Hawaii.
Originally published in January 1957, issue of Hawaii Farm Science. Reprinted by permission of the University of Hawaii.