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With the projected increase in global population, unprecedented increases in crop production will be needed and legume crops are one of the primary means of achieving these increases. The legume-Rhizobium symbiosis is the single most important source of biologically fixed nitrogen in agricultural systems but, as a biological system it is complex, and very sensitive to environmental effects, such as available soil nitrogen, soil pH (both high and low), soil salinity and extremes of soil temperature. Each of these may affect the delicate signal exchange process that occurs during symbiosis establishment. To better understand the effect of environmental factors on signal exchange and nodulation, we conducted four experiments, under controlled-environment conditions, with pea (Pisum sativum) as the model legume. The first experiment studied the effects of available nitrogen, the second the effects of low soil pH, the third the effect of soil salinity and the fourth the effects of low soil temperature. In all experiments the plants were inoculated with Rhizobium leguminosarum bv. viceae cells previously treated with 10 x 10-3 M of hesperitin or naringenin, or not treated (control). In all experiments plants were destructively sampled at 10, 20, 30 and 40 days after inoculation (in two experiments samplings were also conducted at 15 DAI), and data on plant and nodule variables were collected. To allow for a better understanding of the effects of flavonoids on nodule development an image analysis technique was developed that permitted us to measure every individual nodule at each sampling. This provided a more precise picture of nodule development over time than would have been possible with previous methods. Our results indicated that addition of flavonoids had positive effects on nodulation, both in number and size of nodules produced, and that the positive effects were greatest at the most inhibitory levels of the environmental factors tested, and at earlier sampling