Measurement of the Water Potential of Intact Plant Tissues2PISUM SATIVUML.)

Abstract

Through the use of a microthermocouple psychrometer it has become possible to measure the water potential over part of the surface of a pea seed, starting about 19 hours after sowing in distilled water, or later in the case of seeds germinating at lower osmotic potentials, in both instances until well after radicles have emerged. It has been shown that the potential of air-dry seeds is well below –6,000 joules/kg but increases rapidly during imbibition, depending upon the water potential of the germination medium. Pea seeds subjected to lower external water potentials germinate at lower internal water potentials than they exhibit in distilled water. The water potentials of the seeds decrease just after radicle emergence till the radicle establishes contact with its germination medium, possibly as a result of demand for moisture during that period due to incipient cell elongation. No detectable amounts of osmotically active substances are exuded from the seed during germination; the pea seed coat restricts the entry of polyethylene glycol molecules into the seed until emergence of the radicle.