HYDROPONIC AND TISSUE-CULTURE EVALUATION OF ALFALFA (MEDICAGO-SATIVA L) SUBPOPULATIONS SELECTED FOR PHOSPHORUS EFFICIENCY

Alfalfa (Medicago sativa L.) populations were selected for phosphorus (P) efficiency from the cultivar Rangelander using both tissue culture and hydroponic screening techniques. This paper compares the progress realized from bidirectional selection programs for P-efficiency (PE) and P-inefficiency (PI) and for screening techniques. Populations were evaluated both as whole plants in hydroponics and as callus tissue cu ltures at high (322.9 mu M) and low (8.1 mu M) P levels, with either f ixed or inorganic nitrogen (N) sources. Hydroponically selected PE pop ulations had greater shoot dry weights than PI populations when grown hydroponically with low P and inorganic N, but not with fixed N. The t issue culture selected PE population had a greater shoot dry weight th an the PI population when grown hydroponically with low P and fixed N, but not with inorganic N. At high P, no yield differences were observ ed between populations. Root dry weights were not different between po pulations at either P level. No yield differences were detected betwee n any selected population and the unselected parent, Rangelander, beca use selection progress per cycle of selection was small. PE and PI pop ulations were not different when grown as callus tissue cultures with low P. Inbreeding depression appeared to reduce shoot yields and nodul ation by Rhizobium meliloti of selected and control populations compar ed to Rangelander, Although progress was limited, bidirectional select ion enabled us to detect significant yield differences between PE and PI populations developed using both screening techniques. If the paire d populations resulting from bidirectional selection had not been deve loped, both screening methods would have appeared ineffective.