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 cultures at high (322.9 μM) and low (8.1 μM) P levels, with either fixed or inorganic nitrogen (N) sources. Hydroponically selected PE populations had greater shoot dry weights than PI populations when grown hydroponically with low P and inorganic N, but not with fixed N. The tissue culture selected PE population had a greater shoot dry weight than the PI population had greater shoot with low P and fixed N, but not with inorganic N. At high P, no yield differences were observed between populations. Root dry weights were not different between populations at either P level. No yield differences were detected between any selected population and the unselected parent, Rangelander, because selection progress per cycle of selection was small. PE and PI populations were not different when grown as callus tissue cultures with low P. Inbreeding depression appeared to reduce shoot yields and nodulation by Rhizobium mellloti of selected and control populations compared to Rangelander. Although progress was limited, bidirectional selection enabled us to detect significant yield differences between PE and PI populations developed using both screening techniques. If the paired populations resulting from bidirectional selection had not been developed, both screening methods would have appeared ineffective.