EFFECTS OF GROWTH REGULATING SUBSTANCES AND WATER POTENTIAL ON THE DEVELOPMENT OF WOUND CALLUS IN FRAXINUS

Abstract

The rate of growth of wound callus from the cut ends of Fraxinus excelsior L. stems was reduced by lowering the water potential of the culture medium using sucrose and/or polyethylene glycol; the first decrement in potential, from 0 to – 1 bar, had a particularly depressing effect. Gibberellic acid up to 100 mg 1‐1 had no significant influence, whilst indol‐3‐yl‐acetic (IAA) appeared to exert an effect only through its interaction with the water potential. The importance of water potential in tissue development appears to have been frequently overlooked in the use of relatively high sucrose concentrations in tissue culture media, and may account in part for the frequent observation of an optimum sugar concentration for growth. Calluses produced at zero water potential were characterized by active surface growth, relatively little internal differentiation, and a semicircular section. At reduced potentials (–1 bar and lower), the calluses had suberized surfaces and were circular in section; increase in size occurred mainly through the activity of the vascular cambium and a subsurface meristem. More lignified xylem and sclercids were produced in calluses grown at –1 and –2 bars than at zero water potential; at each water potential there appeared to be an optimum IAA concentration for xylem differentiation and this optimum tended to increase as the water potential was increased. Xylem was always associated with a cambium which extended from the subtending twig. Differentiation of vascular tissues, in particular lignified xylem, appears to require physical pressure in addition to a certain concentration of auxin; at zero water potential, sufficient pressure appears to be developed when the callus reaches a certain size, whilst at lower potentials it is induced by suberization of the superficial cells of the callus restraining the expansion of the underlying tissues. Copyright © 1970, Wiley Blackwell. All rights reserved