Osmotic and autotoxic effects of leaf extracts on germination and seedling growth of alfalfa

Alfalfa (Medicago saliva L.) is autotoxic to seed germination and root growth of alfalfa seedlings. The objective was to determine if effects of leaf extracts were due to toxic factors, osmotic factors, or both. Rates of imbibition were slowed slightly by the leaf extract, mainly by osmotic factors. Germination at 22 degreesC occurred in 20 h when imbibed with distilled water but was progressively delayed at higher extract concentrations. Initiation of germination events occurred in less than 10 h, and seed had completed germination when water content was 65% of total weight. An exudate with toxic activity to root growth was released during imbibition. Seed imbibed in extract solution for 10 h and then transferred to water-agar medium had a slight delay in germination but no effect on root elongation rate (RER). Conversely, seed imbibed in water for 10 h before transfer to the extract showed a delay in germination and strong inhibition of RER as extract concentrations increased. Transfer to polyethylene glycol (PEG 8000) at 10 h showed some of the germination delay was due to osmotic properties. Root elongation rate was increased slightly by PEG solutions greater than -0.20 MPa, which was lower than those of the strongest extracts, -0.10 MPa in agar and -0.05 MPa on filter paper. These extracts reduced RER by up to 90% due to the toxicity factors. Inhibition was slowed mainly by osmotic factors whereas delayed seed germination and, especially, reduced root elongation were due mainly to toxic factors of the leaf extract.