GROWTH OF SEEDLING ROOTS IN RESPONSE TO EXTERNAL OSMOTIC-STRESS BY POLYETHYLENE-GLYCOL 20,000

Seedling roots of ten plant species were grown in siliceous sand wetted with solutions of polyethylene glycol (PEG) of MW = 20,000 with osmotic potentials of 0.0, - 0.25, - 0.5 and - 1.0 MPa. After 48 h growth under controlled lighting, root elongation and root diameter were measured. Root elongation of all species was reduced by increasing levels of external osmotic stress. Dicotyledonous species were affected more than monocotyledons at potentials of -0.25 and -0.5 MPa but less at -1.0 MPa. Root diameters of all the species were thicker than those of the unstressed at potentials of -0.25 and -0.5 MPa. At a potential of -1.0 MPa the dicotyledons were still thicker, though not by as much as they were at -0.25 and -0.5 MPa. The monocotyledons, in contrast, were thinner at -1.0 MPa. There was a significant positive correlation (r = 0.81, p < 0.01) between root diameter and root elongation at -1.0 MPa potential. Species were ranked according to the relative root elongation (RRE) and relative root thickness (RRT) at the highest level of stress (-1.0 MPa). In both rankings dicotyledonous species were in the top ranks and monocotyledous species were in lower positions. The results are compared with those for the elongation and thickening of roots growing against external mechanical stress obtained in a previous study. There were good correlations between the responses observed for the two types of external stress. The implications of these findings are discussed.