The potential for breeding white clover (Trifolium repens L.) with improved nodulation and nitrogen fixation when grown with combined nitrogen

Abstract

The variability in nodulation and nitrogen fixation traits ofTrifolium repens (cv. S. 100) supplied with abundant nitrate nitrogen was examined using an aseptic tube culture technique. Nodule numbers varied from 0 to 20 per plant and acetylene reduction activity from 0 to 49 nmol C2H4/plant per h. Plants were selected from this variable population for high, low and zero nodule numbers. Plants within groups were intercrossed and progeny used to establish high, low and zero nodule families. These were subsequently tested for nodulation and acetylene reduction activity in the presence and absence of combined nitrogen. When grown with nitrate the control populations (unselected S. 100 and Pronitro) averaged 3.05 nodules per plant, the high family averaged 4 per plant while the low line averaged 1 nodule per plant (P<0.05). Nodule numbers were strongly correlated with acetylene reduction activity (r= +0.92) but C2H4 production was generally low, averaging respectively 4.9 and 3.1 nmol/plant per h in the high and low lines.