Interactions between Hydrotropism and Gravitropism in the Primary Seminal Roots of Triticum eastivum L.

Abstract It has been proposed that hydrotropism interacts with gravitropism in seedling roots; that is, roots which are highly gravitropic show less hydrotropism (Takahashi and Suge, 1991 Physiologia Plantarum82: 24-31; Takahashi and Scott, 1993 Plant, Cell and Environment16: 99-103). Here, we examine varietal differences in the hydrotropic response and its interaction with gravitropism in wheat roots. Primary seminal roots of wheat (Triticum aestivum L.) were hydrotropically stimulated by different moisture gradients established by placing wet cheesecloth and saturated solutions of different salts in closed chambers. From equations obtained by relative humidity (RH) at different distances from the wet cheesecloth, moisture gradients at the root-tip level were estimated to be 0·03 to 1·84% RH mm-1, depending upon the salt introduced into the chamber. The roots showed positive hydrotropism in response to 0·67% RH mm-1, and the response apparently increased as the gradient was strengthened. When the primary seminal roots of 12 cultivars were exposed to a moisture gradient of 1·84% RH mm-1, hydrotropic response significantly differed depending upon the cultivar tested. Among the cultivars, the roots of Norin 11, Norin 15, Norin 117, and Norin 125 responded hydrotropically more strongly than the others. These roots, with the exception of Norin 11, showed a less vigorous response to gravity compared to the remaining cultivars. However, the roots of Norin 20, Norin 38, and Norin 107 were relatively unresponsive to both a moisture gradient and to gravity. Thus, the primary seminal roots of wheat respond hydrotropically, and the responsiveness differs among cultivars. However, the varietal difference in hydrotropic response cannot be explained solely by converse differences in responsiveness to gravity.