UTILIZATION OF THE ACETYLENE-REDUCTION ASSAY TO SCREEN FOR TOLERANCE OF SYMBIOTIC N-2 FIXATION TO LIMITING P NUTRITION IN COMMON BEAN

The limitation of symbiotic nitrogen fixation due to P deficiency rest ricts the development of a sustainable agriculture, particularly in Me diterranean and tropical soils. Common bean genotypes, APN18, BAT271, PVA846, POT51, G2633 and G12168, were grown in an aerated N-free nutri ent solution at low (72 mu mol plant(-1) week(-1)) and control P suppl ies (360 mu mol plant(-1) week(-1)). Nitrogenase activity was estimate d by in situ measurements of acetylene reduction activity (ARA) in a f low-through system. During the assays, maximum values of ARA (peak ARA ) were reached between 20 and 30 min after exposure to C2H2, depending on P treatment and growth stage. Thereafter, a decline in C2H4 evolut ion was observed. This decline was most pronounced in low-P plants and there was a significant genotypic effect. ARA per plant was decreased by P deficiency, mostly because nodulation was delayed and the number and mass of nodules were reduced. The ARA decrease during pod filling was also activated by P deficiency. ARA per g dry weight nodule was i ncreased by P deficiency in G2633 and G12168, unchanged in APN18, BAT2 71 and POT51 and decreased in PVA846. Except for the climbing type IV G2633, total N at harvest for all P treatments was correlated with the cumulative value of peak ARA and with peak ARA at early pod-filling w hich was the highest peak ARA throughout the growth cycle of type III bushy genotypes. We conclude that if phenology and growth habit are ca refully considered, peak ARA is a reliable screen of genotypes for N-2 fixation tolerance to P deficiency. Selection of lines with early nod ulation under P deficiency is also advisable, and the effect of P defi ciency on the nodule functioning needs to be considered.