Physiological gradients in citrus fruits

Abstract

Abstract does not appear. First page follows. Introduction The various portions of citrus fruits, particularly the regions near the calyx and stylar ends, differ not only in their morphology but also in their physical and chemical characters. The active acidity of various organs of plants forms a gradient that has been given considerable attention by Haas,(11) Gustafson,(9), (10) and Hurd-Karrer.(17),(18) The present paper presents evidence of the existence of physical and chemical differences and gradients in citrus and other fruits, and attempts to explain these differences in citrus fruits by a consideration of the structure and function of the parts concerned. The functions of the vascular system of plants are support, and the conduction and distribution of solutes. Soluble materials that remain in the vascular stream after lateral absorption by the cells adjacent to the tracheal tubes, concentrate in the tissues near the terminals of the vascular strands. Cells in this region necessarily receive the surplus of the solutes. The location of these terminals is not always easily recognized because of branching and anastomosing, but the concentrations of ions as revealed by chemical analyses may indicate regions in which numerous vascular strands terminate. Ross(26) has described the analogy of the morphology of the fruit carpels and that of the citrus leaf. The main lateral veins of the leaf join at the margin and proceed to the leaf tip, which region, as shown by Haas, (13) becomes the principal place of deposition for soluble materials. It will be shown that similar regions are found in the citrus fruit, the structure of which is composed of a number of modified leaves coalesced to form the whole. Mature leaves on twigs will be considered only as they may affect the gradients in the fruits. Susceptibility to abscission decreases as fruit matures, injuries due to malnutrition manifesting themselves by other symptoms. Localization of these injuries in certain parts of the fruit suggested a study of the physical and chemical composition of the susceptible and resistant parts.