WHEAT CULTIVARS RESPOND DIFFERENTLY TO A DRYING TOP-SOIL AND A POSSIBLE NONHYDRAULIC ROOT SIGNAL

Research has shown that when plant roots are exposed to a drying soil a non-hydraulic (chemical) signal is produced in the root and transported to the shoot, causing stomatal closure and growth retardation. This study was designed to reveal genetic diversity in wheat response to soil conditions which elicit a root signal, as the first step in the investigation of the genetic control of the production of and the response to the root signal. Five spring wheat (Triticum aestivum L.) cultivars were established in the growth chamber in soil-filled polyvinyl chloride tubes, 120 cm long and of an internal diameter of 10-2 cm. Soil was well fertilized and wet to field capacity at emergence when two treatments were imposed: (1) tubes were watered from the top as needed to eliminate stress (control); and (2) tubes had a constant water table at a soil depth of 100 to 120 cm, with no applied water. Measurements were performed on five dates on leaf water status and stomatal diffusive resistance. Above-ground biomass and grain yield per plant were determined at maturity. The water table treatment resulted in dry and hard top soil conditions which were previously indicated to elicit a possible root signal. Under these experimental conditions, cultivars differed in their leaf water status, stomatal diffusive resistance (R(S)) and plant production. In the control treatment, R(s) of cultivars increased with reductions in their relative water content (RWC) and leaf water potential (LWP), indicating the expected control of R(s) by leaf water status. Under conditions of a drying top soil, relative water content (RWC) and leaf water potential (LWP) increased in cultivars that had a higher R(s), indicating that stomatal activity was controlling leaf water status. It was therefore suggested that the drying top soil elicited a root signal which caused stomatal closure and reduced plant production. Under such conditions, two cultivars (Bethlehem and V748) consistently maintained relatively low R(s) and high plant production, despite their relatively lower RWC and LWP, as compared with cvs C97, V747 and V652. Limited observations suggest that in these two cultivars relatively fewer roots may have been exposed to the drying top soil, as compared with the other three cultivars.