A spaceflight experiment for the study of gravimorphogenesis and hydrotropism in cucumber seedlings

Seedlings of Cucurbitaceae plants form a protuberance, termed peg, on the t ransition zone between hypocotyl and root. Our spaceflight experiment verif ied that the lateral positioning of a peg in cucumber seedlings is modified by gravity. a has been suggested that auxin plays an important role in the gravity-controlled positioning of a peg on the ground. Furthermore, cucumb er seedlings grown in microgravity developed a number of the lateral roots that grew towards the water-containing substrate in the culture vessel, whe reas on the ground they oriented perpendicular to the primary root growing down. The response of the lateral roots in microgravity was successfully mi micked by clinorotation of cucumber seedlings on the three dimensional clin ostat However, this bending response of the lateral roots was observed only in an aeroponic culture of the seedlings but not in solid medium. We consi dered the response of the lateral roots in microgravity and on clinostat as positive hydrotropism that could easily be interfered by gravitropism on t he ground. This system with cucumber seedlings is thus a useful model of sp aceflight experiment for the study of the gravimorphogenesis, root hydrotro pism and their interaction.