Abstract
Symbiotic fixation of atmospheric nitrogen (N2) is a complex physiological process influenced by the interaction of genetic elements in the higher plant species and rhizobia. No standardized, efficient method is available to critically examine the effect of altering the genetic elements in either component by selection and/or genetic engineering. At planting, seeds of a tropical (‘TGX-4E’) and non-tropical (‘Soma’) soybean cultivar were inoculated individually in sand-filled Conetainers® in a greenhouse with each of two strains within two rhizobial types (Bradyrhizobium japonicum and cowpea). Six weeks after inoculation, each plant was classified into one of two categories; vigorous plant with dark green leaves indicating effective nodulation and N2-fixation (+), and stunted plant with chlorotic yellow leaves indicating ineffective nodulation and no N2-fixation (-). The results indicated that this non-destructive method could be used to identify major genetic differences in the soybean and inoculant. Therefore, this method could be used to rapidly identify genetic segregants resulting from selection in plant breeding programs and/or genetic engineering. Key words: effective nodulation, rhizobia, tropical soybean type, symbiosis.
Highlights
The formation of effective nodules in soybean [Glycine max (L.) Merr.] when inoculated with compatible rhizobia leads to fixation of atmospheric nitrogen (N2) making nitrogenous fertilization of the soybean unnecessary
This study reports a rapid, inexpensive and nondestructive method to screen soybean-rhizobium combinations which have improved N2-fixation ability and have been generated by either plant breeding selection programs and/or genetic engineering
Each strain was maintained in the laboratory by sub-culturing periodically on yeast-mannitol agar (YMA) growth medium described by Vincent (1970)
Summary
The formation of effective (functional) nodules in soybean [Glycine max (L.) Merr.] when inoculated with compatible rhizobia leads to fixation of atmospheric nitrogen (N2) making nitrogenous fertilization of the soybean unnecessary. This study reports a rapid, inexpensive and nondestructive method to screen soybean-rhizobium combinations which have improved N2-fixation ability and have been generated by either plant breeding selection programs and/or genetic engineering. Two rhizobial strains of Bradyrhizobium japonicum (designated I and II) and cowpea type (designated III and IV), were used in the study.
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