Simulation of Drought and Its Effect on Germination of Five Pasture Species

While studying the effect of drought on seed germination, water potentials are usually simulated by addition of various osmotic substrates to water. Studies showing the comparative effects of these simulated water potentials and of true drought are rather scarce. A study was undertaken whereby drought was simulated by sodium chloride (NaCl)), mannitol, and polyethylene glycol (PEG, 20,000 mol wt.), and its effect on the germination characteristics of Trifolium repens L., Lolium perenne L., Danthonia caespitosa Gaudich, Atriplex vesicaria Hew. ex Benth. and A. nummularia Lindl. was determined. These osmotic drought effects were compared with those of true drought. The rate and total germination of all the species declined with decreasing levels of water potential. The extent of such reduction varied considerably among species and with the type of osmotic medium. On iso-potential basis, the order of osmotic medium with respect to the severity of their effect on germination reduction in all the species was PEG > NaCl> mannitol. It was suspected that both NaCl and mannitol entered the seeds, and also that the toxic effects of NaCl were either equally or dominantly offset by the solute entry effects. Under artificial conditions when soil/seed contact and water flow properties of soil media were nonlimiting and there was no undesirable microbial activity, the equivalence of osmotic-PEG and matric potential held for all the species under study. Osmotic solutions created by PEG did serve as a convenient and satisfactory media for studying the effect of true drought on seed germination. However, caution must be exercised in extending these results to predict germination behavior in soil/water media where the so-called “nonlimiting” conditions may not always be met. The order of species with respect to drought tolerance at germination was: L. perenne > A. nummularia = D. caespitosa > A. vesicaria > T. repens. This tolerance may be a reliable indication for species as regards their ease of germination under comparatively adverse soil/ water conditions. However, it is suggested that it is not necessarily an acceptable index of drought tolerance in mature plants.