SYMPLAST AS A FUNCTIONAL UNIT IN PLANT-GROWTH

This chapter presents biophysical studies on the attraction center of the elongating stem based on the integrated activities of constituent cells. It discusses the physiological anatomy of the cowpea hypocotyl, based mostly on the results obtained through electrophysiological methods. The regulatory mechanism of the elongation growth of the hypocotyl is also examined. The hypocotyl of cowpea grows in the early stage of the germination period followed by epicotyl growth in the later stage. The meristematic region, the elongating region, and the matured region are arranged, in order of cell age, along the axis from the cotyledonary node toward the base. During the germination period of cowpea, the germ as an embryonic organ develops under the supply of nutrient materials from the cotyledon, which is already in a physiologically aged state, and ceases growth within the germination period. The cells in the elongating region actively attract nutrient materials from the cotyledon and may simultaneously receive regulatory signals. Such a region is called an “attraction center.” The stem may be regarded as multiple layers of disk-like symplasts penetrated by vascular bundles. The symplasts are arranged chronologically along the germ axis. The symplast situated in a semi-macroscopic stratum plays an indispensable role in the integration of molecular, subcellular, and cellular activities into stem elongation. © 1992, Academic Press Inc.