ULTRASTRUCTURE OF THE EPIDERMIS OF DEVELOPING COTTON (GOSSYPIUM) SEEDS: SUBERIN,PITS, PLASMODESMATA,AND THEIR IMPLICATION FOR ASSIMILATE TRANSPORT INTO COTTON FIBERS

The basal part of cotton fibers (Gossypium arboreum and G. hirsutum) was studied with light and electron microscopy in order to improve the understanding of assimilate transport into the fibers during the deposition of the cellulosic secondary wall. Although the distal parts of white cotton fibers are not suberized, a variable amount of suberin was found at the fiber base. This suberin is typically deposited in concentric layers, alternating with polysaccharides. Numerous pits occur in the base of cotton fibers and in ordinary epidermal cells in the periclinal and anticlinal walls. About 25% of the length of periclinal walls is occupied by pits, but only 2% of the anticlinal walls, being pitted mainly in their proximal part. In suberized walls the deposition of suberin is not reduced in the pit region. The pits, whether or not suberized, contain plasmodesmata (22 ± 2.3 and 27 ± 3.3 · μm^−-2 in the periclinal and anticlinal walls of the white lint cultivar of G. hirsutum). Transport of assimilates into the fibers through the symplast is therefore possible. This transport may occur directly from mesophyll cells to fibers, or indirectly via ordinary epidermal cells. The minimum amount of assimilates transported into individual fibers during the phase of secondary wall deposition could be estimated (1.3 pg · fiber^−-1 · sec^−-1), as well as the corresponding symplastic flux of assimilates through the periclinal cell wall, neglecting a possible transport through the anticlinal walls (10^−-3 pg · μm^−-2 · sec^−-1). It is postulated that in the green lint genotype of G. hirsutum and in wild cotton species (not studied in this paper), the uptake of assimilates into the fibers occurs through the symplast, the apoplastic pathway being excluded by the suberization of the fibers during secondary wall formation. Although cultivated, white-linted cotton species may use the same pathway, loading of assimilates from the apoplast is theoretically also possible, and the relative contribution of both pathways has to be determined experimentally.