RESUMEN
The objective of this study was to test the hypothesis that root of maguey (Agave salmiana Otto ex Salm-Dyck) seedlings reacts during the first 24 h to low substrate water potential (Yw), by anatomical modifications. Three-4 cm root length seedlings were planted in vermiculite for 24 h at Yw between -0.03 and -2.35 MPa. Root dimensions, proline content and anatomy were evaluated. Substrate ψw between -0.65 and -2.35 MPa did not significantly affect longitudinal root growth. However, proline content significantly increased from 1.6 to 2.1 emoles mg-1. Significant reductions of transverse root area (41 percent), thickness of mucilage covering the epidermis (47 percent), thickness of epidermis (between 15 and 46 percent), area of the parenchyma (between 35 and 41 percent) and number of vessels (up to 28 percent) were observed with Yw of -2.35 MPa. In contrast, thickness of xylem wall, diameter of xylem vessels and the number of cells of the cortex of the differentiation root region significantly increased (64, 17, and 97 percent, respectively). The anatomical changes associated with low substrate Yw indicate a net increase of root apoplatic paths; structures involved in water conduction increased their diameter under low substrate Yw conditions and anatomical changes occurred during the first 24 h of water stress.