Some Basic Research Problems in Macadamia
Lois E. James*
While it is not the primary objective of basic research to provide the basis of technological or practical advancements, there are times when the practical consequences of investigations are immediately evident. Though the purpose of this paper is to present possibilities of practical consequence in some research problems in Macadamia, it should be remembered that of possibly greater value are the new questions that arise in the process of these investigations. It is the continual solving of these ever-increasing problems that adds to the store of new knowledge. Greenewalt of the DuPont Company has likened the pool of basic scientific knowledge to a savings account from which we make withdrawals as we convert that knowledge to practical ends through applied research (2). This paper is limited to a consideration of a few investigations, some of which are in progress at Whittier College, in which practical application is inherent in their solution. In part, the investigations, which have been undertaken, concern seedling characteristics, which some day may be correlated with crop production and other characteristics of the mature tree.
THE SHOOT-ROOT RATIO OF YOUNG SEEDLINGS OF MACADAMIA
Preliminary investigation of the shoot-root ratios of young seedlings of Macadamia has been made. Nuts of the 1959 fall crop were obtained from a large specimen of Macadamia tetraphylla L. Johnson planted in Whittier in 1922. The nuts of this tree are excellent appearing and uniform in size. The seeds were germinated in vermiculite with controlled temperature. Measurements taken on 104 germinating seeds and young seedlings reveal a characteristic growth pattern with certain distinctive variations. As is the case in most seeds, the root is initiated prior to the shoot. The young root undergoes a progressively increasing rate of growth resulting in rapid elongation. A relative increase in growth rate of the young root was found to be characteristic of all of the seedlings, but the actual length of the root prior to shoot emergence was found to be highly variable. Root lengths from 5.5 cm. to 14 cm. were observed to occur without any sign of the emergence of the shoot. With the emergence and elongation of the shoot, the growth rate of the root decreases. For a while, with the relative decrease in the rate of the growth of the root, there is an increase in the rate of the growth of the shoot. Eventually, the growth rates become about equal, and at this time, in the seedlings investigated, 94% were found to have longer roots than shoots and 6% were found to have longer shoots than roots. Studies of this nature will be repeated with seeds of other selections and follow-up studies will be made to determine possible correlation's of growth variations with mature vegetation. In some species of plants flowering is dependent upon a given amount of vegetative growth, but in other species, superimposed upon a minimum amount of vegetative growth, is a set of external or environmental influences. The two environmental conditions of primary importance in the initiation of flowering are temperature and the relative length of day and night. Floral initiation in macadamia presents a bewildering picture. In both commercial species of Macadamia the amount of vegetative growth prior to flowering is immensely variable. A tree less than 3 feet tall with a stem of but 1’ 2 inches may sport an inflorescence or two and a vigorous appearing 12 foot tree may not have had a single inflorescence. An interesting account of the ever-bearing tendencies of Macadamia in Hawaii has been given by Hamilton (3). The explanation of an everbearing tendency is a challenge, since on the surface at least, there is no neat categorized explanation of day length induction or vernalization as we find in cockleburs, beets, lettuce, and many others (5). An understanding of the factors controlling floral initiation has important implications for the development of any crop plant. The cause of floral initiation in Macadamia is simply not known, only the variability is recognized. Since it would appear that the photoperiod for Macadamia is not a deciding factor, other factors should be investigated and it might well be that a certain degree of control could be exercised by the grower, or at least a prediction of the bearing qualities of a given seedling in a given environmental situation. On the other hand, the metabolism of the tree may be such that selection remains the only tool of the grower. A study of some of the aspects of floral initiation is being contemplated at Whittier.
COMPARATIVE ANATOMY OF MACADAMIA SELECTIONS
This is a report only of the nature of the work that has been undertaken at Whittier College in the comparative anatomy of macadamia selections. This work is still incomplete and at this time only a few statements to illustrate the value of this type of investigation will be made. The purpose of the investigation is to determine the nature of the inheritance of vegetative characteristics as evidenced by a study of the morphological and anatomical variability within each species and the nature of the hybrids. Furthermore, it is planned to correlate these findings with such characteristics as nut quality, fruit setting, frost resistance, etc. Investigations of this sort are not only unpredictable as to their outcome but require extensive collecting of data before definite conclusions may be drawn. On the other hand, it might be mentioned that it is not out of the realm of possibility that certain anatomical characteristics or an association of characteristics may be found in the seedling, which could provide the means of determining desirable qualities in the mature tree. Examples in the literature pointing to the practicality of this type of investigation are known. One illustration is the correlation between the size of the xylem vessels (water-conducting cells) in the rootstocks of apples and pears and the future productivity of the tree. Donald C. Duncan who has reported, in an unpublished thesis, the results of anatomical studies on 36 selections of Macadamia has carried on a part of the anatomical work. These selections include both the species Macadamia integrifolia Maiden and Betche and M. tetraphylla L. Johnson, and also hybrids of these two species. Specimens were obtained from the M & N Nursery in Vista and the Westree Nursery in Carlsbad. In certain respects, the trees chosen at these two nurseries turned out to be somewhat unfortunate for two reasons. All but 7 had a quality rating of not less than good, which resulted in insufficient data to attempt to correlate nut production with vegetative characteristics. Also the hybrid selections were probably all F 1 segregates from the same parental M. tetraphylla which would mean that making any generalizations concerning hybrids would scarcely be valid. Though many interesting anatomical and morphological differences have already been found in the different selections of macadamias, the work will be continued and broadened extensively.
UNUSUAL ROOTS IN MACADAMIA INTEGRIFOLIA
Seeds of both Macadamia tetraphylla and M. integrifolia were placed in large glass covered aquarium on wet filter paper to germinate. The root of M. tetraphylla appeared typical, or not unusual, but two types of roots were produced by M. integrifolia. One type of root was normal and like that of M. tetraphylla, but the other type gave the appearance of a string of linked sausages and was about four times the diameter of the normal appearing roots. These bulbous portions extended many centimeters and were covered with root hairs. These plants were kept in the aquarium for several months and during all this time the root hairs that were formed persisted. Tests proved that the root hairs became suberized and therefore probably did not function in absorption. Dr. Flora M. Scott of UCLA reported (oral communication) having seen what might be a similar situation in the roots of coconuts in Hawaii. Microscope slides were prepared to determine cellular differences in the two types of roots. The size of the cells was observed to be the same but there is a several fold increase in the number of cells in the swollen roots. Neither the significance of these unusual roots nor the constancy of their occurrence is known.
PROBLEMS CONCERNING THE VARIABILITY OF THE OIL CONTENT IN MACADAMIA
The variability of the oil content in macadamia nuts and the varied climates and soil conditions in which macadamia trees are being planted offer excellent opportunities in the investigations of fat in relation to environment and genetics. On the practical side, results of investigations of this type might well aid in the determination of suitable selections for specific climatic situations in California. Certainly in macadamias, the oil content of the nuts is of prime importance. Storey (7) has stated that high quality nuts are determined largely by the oil content of the kernel since both flavor and texture are highly correlated with oil content. In general, oil content of seeds varies with the successive stages of maturity and there is a progressive transformation of saturated fatty acids into acids of a greater degree of unsaturation. Different species of plants vary not only in the quantity of oil produced at any given growth period but also in the specific types of oils produced. Furthermore, on the basis of the extensive work done on oil production in soybeans, cotton, peanuts, sunflowers, and various species of tobacco (1, 6, 8), it may be said that the oil content of seeds depends probably on the combined action of heredity and environment. As summarized by McNair (4), some of the environmental influences are as follows: the total percentage and iodine value of oil is affected by the length of the growing season and even more by the character of the growing season. Different varieties are not always influenced in the same manner by environment. The comparative effect of soil types on per cent of oil formation varies with seasonal conditions. In some seasons clay soil produces a higher percentage of oil than loam, and vice versa. Under practical conditions climate is a more potent factor than soil in modifying the oil content of seeds. In some species the oil content is lowered by the application of nitrogen. In some species fertilizers lower the iodine value. Warm climates favor the formation of saturated fatty acids and cold climates favor the formation of unsaturated acids. Shade, soil moisture, and density of planting may affect both oil content and iodine value. To what extent the various environmental factors influence oil production in Macadamia is largely unknown. Now that a limited number of Macadamia selections are being recommended for commercial plantings, nuts of the same clone will become available from various soil and climate situations. So far, the only work that has been started at Whittier concerning oil formation in Macadamia consists of microchemical tests on vegetative parts of young seedlings. It is hoped to correlate these findings with the oil content of the nuts.
*Professor of Biology, Dept. of Biology, Whittier College.
LITERATURE CITED.
Williams, C. B. Cotton plant. N. C. Dept. Agr. Bull. 5: 27. 1906.
Reproduced from the California Macadamia Society Yearbook 1961 pp. 57-60