Abstract

The legume–Rhizobium symbiosis is the most important source of biologically fixed nitrogen in agricultural systems. However, it is complex and sensitive to environmental effects, including available mineral nitrogen, soil salinity, and low root-zone temperature. How these factors inhibit the symbiosis is not well understood. If the effects are mostly on the early stages of nodulation, addition of signal molecule(s) may overcome it. Pisum sativum seeds were germinated and the seedlings were inoculated with bacterial culture and cultivated under controlled environment conditions, studying each of the above nodulation-inhibitory factors, under 3 levels: control (little or no inhibitory condition), and moderately or severely inhibitory conditions. Aspects of nodule development (size and number) were measured with a scanner-based technology. All of the environmental conditions studied had effects on both nodule establishment and development. The addition of either hesperitin or naringenin frequently modified nodule development, most markedly under saline conditions. Flavonoid additions had only small effects under high nitrogen availability conditions, and stronger effects under saline and low root zone temperature conditions.