Growth rates and auxin effects in graviresponding gynophores of the peanut, Arachis hypogaea (Fabaceae)

The gynophore of the peanut plant (Arachis hypogaea) is a specialized organ that carries and buries the fertilized ovules into the soil in order for seed and fruit development to occur underground. The rates of growth of vertically and horizontally oriented gynophores were measured using a time-lapse video imaging system. We found that the region of maximum extension growth due to elongation (termed the Central Elongation Zone) is located on average at 2-5 mm from the tip. In the first 0-4 h after horizontal reorientation (gravistimulation), new zones of growth emerge on the upper surface, while the elongation zone of the lower side decreases in size and magnitude. Four to six hours after reorientation the zones of maximum growth are almost equal in size and location on the upper and lower sides. The growth rate and the gravitropic response decreased dramatically, upon the excision of the ovule region (terminal 1.5 mm), but a gravitropic growth response could be restored by applying the auxin indole-3-acetic acid exogenously to the excised tip. The addition of napthylphthalamic acid (an auxin transport inhibitor) at the ovule region allowed some growth to occur, but the gynophores do not respond normally to gravity, upon horizontal reorientation. We discuss the role of auxin in the gravitropic response of the gynophore.