A Cause Of Pollination Failure In Macadamia
Lois E. James *
Low fruit set is not infrequently observed in macadamia seedlings. The work of Schroeder (1) on pollination in a number of seedling clones indicates a certain degree of sell-sterility. Urata (2) has reported partial incompatibility in a majority of the macadamias investigated in Hawaii.
An interesting observation concerning fruit set has been made on a specimen of Macadamia tetraphylla L. Johnson. This tree, located at 8420 La Bajada Ave., Whittier, was planted 12 years ago and started flowering 1 years ago. During the first five years of flowing, no fruit set whatsoever occurred. The period of flowering of this particular tree is relatively short, extending only 2 to 3 weeks. During the sixth year of flowering, at a time when 3/4 of the flowers of the racemes had dropped off, the remaining fourths of the tree and as high as could be reached from the ground with a broom, were beaten with the flat side of the broom. Every raceme within reach was struck 2 or 3 times. There resulted a heavy fruit set completely limited to the distal fourths of the racemes that had been struck. Maturation of individual flowers in this tree is in an acropetal direction from the base toward the tip. The kernels were uniform, of good quality, and ripened at about the same time. The following year the tree flowered abundantly, but the broom treatment was not repeated. The tree bore only 10 nuts.
Several clusters of flowers were preserved for the study of their floral morphology. In many respects the flowers of this specimen of macadamia are like those that have been reported by previous investigators (1,2) for Macadamia integrifolia Maiden and Betche. The floral tube becomes split near the middle at a suture of two of the sepals through which the central portion of the style is protruded. Schroeder (I), in describing the flower of M. integrifolia, reports, "as the style increases in length and extends laterally the bulbous apical portion is withdrawn downward in the floral tube, passing over the anthers and collecting pollen en route." In the case of the specimen of M. tetraphylla under consideration, the central bend of the style occurs but the bulbous portion of the style is not pulled down the floral tube. In some cases the split in the suture through which the style is protruded is rapidly extended the entire length and the calyx, with the remaining sutures fully joined, is bent hack leaving the pistil entirely free (fig. 1). The bending of the floral tube is in part the result of an abscission on one side. Other flowers were observed with the floral tube completely detached from the receptacle and sitting on top of the pistil even after the style had straightened out (fig. 2).
It would appear therefore, that poor fruit set in this specimen is caused by the caducous nature of the floral tube. The calyx is torn from the receptacle by the pressure resulting from the developing pistil. If the split in the
Figure 1. The splitting of the suture and bending of the floral tube occur prior to the extension of the style.
Figure 2. Early abcission of the floral tube causes it to be carried upward by the style.
suture is extended the entire length before abscission, the corolla tube is actually pushed back out of thc way and if the corolla tube abscises before the split is extended, it remains as a cap on the end of the style. In either case, pollination cannot take place because the apical portion of the style cannot he pulled down past the anthers. Some of the abscised flowers were observed in which the anthers had not yet opened. The broom treatment served to break the floral tubes and scatter pollen throughout the flower cluster thereby effecting pollination. The failure of insect pollination in this specimen has not been considered. Certainly there was no lack of bees in the area.
Prof. of Biology, Dept. of Biology, Whittier CollegeLITERATURE CITED
1. Schroeder, C. A. Some observations on the pollination of macadamia in California. Calif. Macadamia Soc. Yrbk. 5: 1959.
2. Urata, U. Pollination requirements of Macadamia. Hawaii Agri. Exp. Sta. Tech. Bull. 22. 1954.