TURGOR, GROWTH AND RHEOLOGICAL GRADIENTS OF WHEAT ROOTS FOLLOWING OSMOTIC-STRESS

The growth rate of hydroponically grown wheat roots was reduced by mannitol solutions of various osmotic pressures. For example, following 24 h exposure to 0.96 MPa mannitol root elongation was reduced from 1.2 mm h-1 to 0.1 mm h-1. Mature cell length was reduced from 290-mu-m in unstressed roots to 100-mu-m in 0.96 MPa mannitol. This indicates a reduction in cell production rate from about 4 per h in the unstressed roots to 1 per h in the highest stress treatment. The growing zone extended over the apical 4.5 mm in unstressed roots but became shorter as growth ceased in the proximal regions at higher levels of osmotic stress. The turgor pressure along the apical 5.0 mm of unstressed roots was between 0.5 and 0.6 MPa but declined to 0.41 MPa over the next 5.0 mm. Following 24 h in 0.48 (200 mol m-3) or 0.72 MPa (300 mol m-3 ) mannitol, turgor along the apical 5.0 mm was indistinguishable from that of unstressed roots but turgor declined more steeply in the region 5-10 mm from the tip. At the highest level of stress (0.96 MPa or 400 mol m-3 mannitol) turgor declined steeply within the apical 2.0 mm. Following 24 h of osmotic stress a comparison of the growth and turgor profiles indicated that the cell walls in the proximal region of the growing zone had become harder, i.e. the yield threshold had increased and/or wall extensibility had decreased. In short-term experiments the growth rate of the whole root was reduced from 1.18 mm h-1 to 0.4 mm h-1 immediately following immersion in 0.48 MPa mannitol but then recovered to 0.9 mm h-1 after 3-4 h. The same treatment reduced turgor at both 2.0 mm and 7.0 mm from the tip of the root. Turgor then recovered faster (within 3 h) and more completely at 2.0 mm than at 7.0 mm.