Internal Water Status and Water Transport in Seed Onion Plants1

Abstract

AbstractWater potential, transpiration rate, and stomatal resistance of the various parts of the onion (Allium cepa L.) plant were measured under field and controlled conditions.Water potentials of florets and pedicels were always lower than leaf water potentials. Water potential difference between the florets and the soil of as much as —9 bars were found. The greatest drop in water potential was found between the flowers and the upper part of the seed stalk. These differences in water potential were found even when plants were growing in soil well supplied with water.Transpiration rates of the leaves were the greatest of all plant parts, the seed stalk next (about 15 to 20% of the flux density from the leaves), and the umbel lowest, representing a very small fraction of the total. Stomatal resistance of the leaves was at all times lower than seed stalk stomatal resistances. An almost on‐off stomatal behavior by which the onion plant regulates its transpiration was found to operate in a narrow range of leaf water potential, —3 to —7 bars.The relation between flux density and water potential drop was found to be highly nonlinear in plants growing in different soils and under a wide range of soil water potential and environmental conditions. Impedance to water flow in the seed stalk was 5 to 8 times greater than in the leaves. For the same conditions potential drop between soil and root surface was less than 5 cm of water.