MIGRATION OF SALTS AND WATER INTO XYLEM OF THE ROOTS OF HIGHER PLANTS

Abstract

DESPITE THE immense amount of research on absorption and movement of mineral solutes and water in plants, the mechanisms of the processes remain obscure. For the teacher and investigator a description of a possible mechanism of migration of solutes and water in roots, based upon modern research and compatible with known root structure, should be of interest. Recent reviews (Hoagland, 1936, 1937; Steward, 1935) summarize the achievements in the field of plant nutrition. Utilizing single cells, filaments, excised root systems, and whole plants, these studies describe the phenomena of salt accumulation and of the response of plants to their culture solution environment. It seems logical that they should provide clues to the mechanics of salt migration and the part played by growth and development. A tentative mechanism related to known structure and function of plant roots and designed to explain uptake of water and salts which may occur simultaneously 2 is described. The relations of absorption to organic food supply, available oxygen, and ion accumulation are reviewed. An attempt is made to evaluate the role played by root pressure and by translocation 3 of water and salts. QUANTITATIVE ASPECTS OF XYLEM EXUDATION.-An excised squash root may exude more than its own weight of sap in 24 hours, replacing many times the contents of its xylem vessels (Crafts, 1936). Collections from excised roots of two 55-dav-old Hubbard squash plants grown in aerated full Hoagland's solution (Hoagland and Broyer, 1936b) were as follows: Plant number 1, whose absorbing roots (all tissue below the culture solution level) weig-hed 81.7 grams fresh and 3.4 grams dry, exuded 243 cc. during the first 24 hours and 100 cc. during the second like period; Plant number 2, with absorbingroots weighing 136.0 grams fresh and 5.8 grams dry, exuded 40 cc. during the first houir, 343 cc. duiring the first 24 hours, and 140 cc. during a second like period. The nitrate concentration of the xylem exudate was more than twice that of the culture solution during the early stages of the experiment and decreased to an