Abstract
When a cucumber seed is positioned horizontally on the ground during germination, one peg develops on the lower side. Development of another peg on the upper side is suppressed by gravity. We aimed to reveal the ultrastructural changes during gravity perception and peg formation.
Since the fixation of the germinating seeds was hampered by the seed coat, we developed a new experimental system in which the seed coat was removed 4h after onset of imbibition. Then the cotyledons, the tip of the radicle (ca.0.5mm), and an apical portion of the shoot (ca.0.2mm) were excised. The remaining segment (ca.2.2mm in length) was placed horizontally. Surprisingly, size, timing and position of peg development in this system were apparently not different from the intact seed, although the developed root did not exhibit gravitropism. Conspicuous starch grains appeared first in epidermis and cortical cells after 8h of imbibition.
