The gram-negative soil bacteria Rhizobium spp. infect and establish a nitrogen-fixing symbiosis with legume crops which involves the mutual exchange of diffusable signal molecules. In this study, Rhizobium leguminosarum containing a nod-lacZ gene fusion was used to screen the most effective plant-to-bacteria signal molecules for pea and lentil and the induction conditions. Out of a number of signal compounds including apigenin, daidzein, genistein, hesperetin, kaempferol, luteolin, naringenin, and rutin, hesperetin and naringenin were found to be the most effective plant-to-bacteria signal molecules. The induction of nod genes was temperature-dependent, where nod gene induction was decreased with dropping incubation temperature. The combination of hesperetin at 7 microM and naringenin at 3 microM resulted in better induction of nod gene activities compared to either hesperetin or naringenin alone. Nodulation and plant dry matter accumulation of pea and lentil plants receiving preinduced R. leguminosarum were higher than those of plants receiving uninduced R. leguminosarum cells in controlled environment growth chamber conditions. Preinduced Rhizobium with hesperetin at a concentration of 10 microM increased nodule number on average by 60.5% and dry matter accumulation by 14% in field pea at 17 degrees C, while it was 32% and 9% at 24 degrees C, respectively. Similarly, averaged over two rhizobial strains, a 59% and 6% increase in nodule number and biomass production at 17 degrees C, and a 39% and 27% at 24 degrees C, were obtained from lentil inoculated with hesperetin-induced R. leguminosarum, respectively.

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Previous studies showed that inoculation of soybean [Glycine max (L.) Merr] with Bradyrhizobium japonicum preactivated with plant‐to‐bacteria signal molecules increased nodule number, particularly at low root zone temperatures, thereby improving plant seasonal nitrogen fixation and final grain and protein yield under cool spring conditions. Two products carrying this technology, SoyaSignalTM and Affix+TM, were designed and tested at 127 locations in Canada and the United States from 1994 to 1999. A summary of the field test results shows that preincubation of B. japonicum with genistein and daidzein, as well as directly increasing the genistein and daidzein concentration in the soybean root rhizosphere, gave an average final grain yield increase of 7 %. The success of SoyaSignal technology was temperature dependent. The plants responded better to the SoyaSignal products when grown under cool soil conditions. Application of SoyaSignal to early planted soybean (before the soil temperature rose above 17.5 °C) increased yields by an average of 10 %. The responses declined with delayed planting dates. Soybean genotypes with high yield potential had greater yield increases than those with low yield potential. As the ratio of return to cost for SoyaSignal technology was 5.3 : 1 over the 127 site‐years, SoyaSignal technology can be used as a tool to improve soybean yield in production areas with cool springs.

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