THE ANATOMY OF LEAF ABSCISSION AND EXPERIMENTAL DEFOLIATION IN GUAYULE

Abstract

THE LEAVES of guayule (Parthenium argentatum A. Gray) contain relatively low amounts of rubber. They also contain appreciable amounts of materials that lower the quality of milled rubber when the leaves are left on the shrub during milling. Hence defoliation is a desirable step in the improvement of the milling process. Several methods of defoliation have been under consideration: one involves flash-drving followed by tumbling; another consists of immersing the plants in boiling water for a few minutes, after which the leaves fall off with a slight agitation; a third possibility was developed from experiments with the retting of intact shrubs. The anatomical aspects of these processes were studied. Since very little was known of the anatomv of leaf abscission in guayule, it was considered advisable first to investigate this process in field plants. The results of both phases of the investigation are reported in this paper. There is a considerable body of knowledge concerning the physiology and anatomy of leaf abscission. Pfeiffer (1928) in his review of the subject gives a list of citations covering 18 pages. However, there appears to have been very little work done on plants such as guayule whose abscission layer does not function actively in leaf fall. Certain of the anatomical changes that take place at the petiole base of Rhus typhina (Lee, 1911) and Mercurialis annua (Yampolsky, 1934) as well as in the perigone and style of Narcissus pseudonarcissus (Namikawa, 1926) are similar to those at the petiole base in guayule. Artschwager (1943), in his anatomical study of guayule, did not consider the abscission of leaves; however, Lloyd (1911) refers briefly to the environmental conditions affecting leaf-fall as well as to the absciss layer of leaves. The absence of details on the anatomy of leaf-fall in guayule, the general lack of information on similar plants, and the need for knowledge fundamental to the development of methods of defoliation justifies a careful examination of the problem *in this plant. MfATERIALS AND METHODS.-Leaf bases of field plants were obtained from the Hansen and Lee Field of the Salinas, California, plantings of the Emergency Rubber Project. The plants were two years old; the field was irrigated. Samples from flash-drying and retting experiments were obtained through the courtesy of the Guayule Rubber Extraction Research Unit. Samples for the study of the action of hot water in defoliation were obtained from small-scale experiments carried on in the laboratory with material from field plants. Tissue