Genotypic Variability among Diverse Red clover Cultivars for Nitrogen Fixation and Transfer
Thilakarathna, Ranaweera Mudiyanselae Malinda Sameera
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Legumes fix atmospheric nitrogen (N) via symbiotic biological N fixation where part of the N fixed by legumes can be transferred to non-legumes. Identification of genotypic variability for N transfer among different legume cultivars enables improving N transfer to non-legumes under mixed stands. Six diverse red clover (RC) cultivars which include three diploid (AC Christie, Tapani and CRS15) and three tetraploid (Tempus, CRS18, CRS39) were selected to evaluate genotypic variability for N transfer. The above RC cultivars were characterized for root hair deformation, nodulation, growth, and N uptake under different levels of N supply during the growing period and for starter N supply under in vitro conditions. Significant genotypic differences among the RC cultivars were found for the above attributes where the cultivars responded differently to N applications during early growth. The above RC cultivars were also evaluated for root exudate N content in the form of NO3--N, NH4+-N and dissolved organic N (DON) during early growth under in vitro conditions. Significant genotypic differences were found for root exudate inorganic and organic N content. In general, root exudate DON content was greater than the inorganic N content and positively correlated with average nodule dry weight and shoot N concentration. The NH4+-N and NO3--N content in root exudates were positively correlated with active nodule number and root growth parameters respectively. Nitrogen fixation, N transfer ability and soil N profiles of the above six RC cultivars were evaluated with bluegrass under field conditions for two post establishment years. Significant genotypic differences were found for N fixation and transfer but, these attributes were not associated with the ploidy nature of the selected RC cultivars. Generally, N transfer increased as the season and production year advanced. Soil mineral N and potential N leaching were affected differently by the RC cultivars included in this study under mixed stands, thus showing genotypic differences for soil N cycling. The results of investigations in this thesis highlight the dynamics of N flow between legumes and companion grasses and may assist in developing management protocols and plant breeding strategies to identify genotypes with efficient N cycling profiles.