Dedra A. Christensen and Dr. Phil S. Allen, Botany and Range Sciences
After seed imbibition, water travels preferentially to those tissues most actively involved in germination. The majority of water travels to the embryo of the seed, which includes both the radical (primary root), and shoot meristem (rapidly dividing tissue that develops into the shoot of the plant). A smaller volume of water travels to the endosperm, a starchy storage tissue which serves as an energy source during early plant growth. We have used a fluorescent tracer dye, fluorescence microscope, and photometer to characterize more specifically the areas where water concentrates during imbibition, and to trace the path of water movement from the distal end of the seed to the embryo.
A preliminary experiment was done to verify that germination was not affected by pyrene trisulfonate, (PTS), the fluorescent dye used in these experiments. This was done by imbibing an equal number of seeds in either water or PTS solution and recording the number of seeds germinated at 5 hour intervals between 16 and 36 hours.
To characterize water movement, all seeds were imbibed under standard conditions. For each replication, twelve seeds were imbibed in a petri dish containing one germination blotter and 7 nil of 0.2% PTS. Twelve small wells were made in the blotter so that each seed could be imbibed with only its distal end in contact with the dye. Seeds were imbibed for 0, 4, 8, 12, and 16 hours to allow monitoring of the change in fluorescence (change in water content) of specific tissues at different imbibition times. After imbibition, seeds were bisected longitudinally and placed face down on a microscope slide for viewing with an inverted fluorescence microscope. Photometer readings were taken at four locations within the seed: the radical, shoot meristem, embryo/endosperm interface, and the center of the endosperm. Fluorescence was measured as a percentage of standard, the standard being a fluorescent slide. To determine the path of water movement from the distal end of the seed to the embryo, cross sections of 12-hour seeds were photographed through the microscope.
Photometer readings indicated an increase in fluorescence from 0 to 16 hours of 109% in the radical, 24% in the shoot meristem, 15% at the embryo/endosperm interface, and 2% in the center of the endosperm. Cross-sections showed that water traveled under and through the lemma and palea, two thin layers of non-living cells which cover the seed.
These results show that water is directed first, and in the largest amounts, to the section of the embryo where the primary root, or radical, will be formed. It is directed secondarily, and in much smaller amounts, to the shoot meristem. The pathway for water movement to the embryo is not through a layer of living cells surrounding the endosperm known as the aleurone layer as originally hypothesized. Instead, water travels primarily by capillary action through the lemma and palea.