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Growth rates and auxin effects in graviresponding gynophores of the peanut, Arachis hypogaea(Fabaceae)¹

^a Universityof California at Berkeley, Department of Plant and Microbial Biology,431 Koshland Hall, Berkeley, California 94720

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

Key Words: Arachishypogaea • auxin • Fabaceae • gravitropism • growthrates • gynophore • indole-3-aceticacid • napthylphthalamicacid • peanut

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