Selection for biological nitrogen fixation and nitrogen utilization in birdsfoot trefoil

Birdsfoot trefoil (Lotus corniculatus L.) is a perennial forage legume which has a lower rate of biological nitrogen fixation (BNF) than alfalfa (Medicago saliva L.). In this study, we evaluated the gain from two cycles of recurrent selection in the glasshouse in three germplasms of birdsfoot trefoil using traits associated with improved BNF or N utilization. For improved BNF, selection was based on morphological traits correlated with the acetylene reduction assay (ARA) in a nil-N soil environment. For improved N utilization, selection was based on increased top dry weight production in a N-rich soil environment. The 12 selected populations and the three parental populations were evaluated in two growth chambers in a completely randomized design with four replicates. The chambers were watered three times per week with 100 mL of a modified Hoagland's solution containing either 0.0 (nil-N environment) or 10.7 (N-rich environment) mM N. At 8 and 14 wk after seeding, the plants were individually scored for nodule mass and fibrous roots, top dry weight, ARA rate, and N yield. Correlated trait selection for improved BNF as measured by the ARA produced populations with larger plants, more fibrous roots, and greater nodule mass resulting In a 56% increase in ARA in the nil-N environment. In the N-rich environment, the populations selected for improved BNF showed a significant increase in nodule mass and a 79% increase in ARA rate. Selection for improved N utilization resulted in larger plants with more fibrous roots, larger nodule mass, higher N yield and a 77% increase in ARA in the N-rich environment. In the nli-N environment, the populations selected for improved N utilization had larger root systems, slightly better nodulation, and produced larger plants which resulted in a 58% increase in ARA rate. Correlated trait selection for traits which support BNF or N utilization could significantly improve the performance of birdsfoot trefoil in environments with fluctuating N availability.