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Ureide content and growth of cowpea [Vigna unguiculata (L.) walp. var tuy] plants as affected Bradyrhizobium strains and inoculum position

Abstract:

The effect of three effective Bradyrhizobium sp. cowpea strains and inoculum position on biomass production, nodulation and ureide contents were determined in 45-day-old cowpea plants. Treatments were: single inoculations, where each one of the Bradyrhizobium strains (I-125,I-38 or I-700) was placed at 2 or 10cm from the surface of the sterilized sand; double inoculations, in which the same strain was located at 2 and 10cm; and dual-strain inoculations, where I-125 was placed at 2 or 10cm together with I-700 or I-38 either at 10 or 2 cm. Distance between inocula was enough to ensure a root zone free of nodules between inoculum sites. Analysis of nodulation profiles in plants exposed to different treatments indicated that the strain I-125 showed a greater motility and formed a larger nodule mass than I-38 and I-700. In single inoculations I-38 and I-700 produced an equal nodule mass regardless of inoculum position, in contrast to I-125 which showed reduced nodulation at 10 cm. In plants doubly or dual-strain inoculated with I-125 and I-700 about 73% of the total nodule mass was found in the upper root segments. However, due to a dry weight reduction in both nodule masses the total nodule mass per plant resembled that of single inoculations. An inhibition of delayed nodulation was only detected when I-38 was included in the double, or dual-strain inoculations with I-125. Thus, in cowpea the expression of the autoregulatory mechanism upon subsequent nodulation is strain dependent. In plants harbouring the strains I-38 or I-125, nodule mass position did not affect the ureide contents in nodules or in the petiole of the first trifoliolate leaf. An effect of inoculum position on nodule ureide contents was found in plants inoculated with I-700, which in turn contained up to three times more ureide than plants receiving other inoculation treatments. The increased concentrations of ureide recorded in I-700 inoculated plants were not translated into higher dry matter yields, pointing toward a threshold level effect. For all treatments combinations, the position of nodules on the roots had no effect on plant dry matter production, although plants inoculated with I-38 showed the highest biomass. Correlation analyses carried out between variables indicated sink-source relationships between nodule or petiole ureide contents and plant biomass. A significant positive correlation was detected between nodules and petiole ureide content, while non-significant correlations were found between nodule mass and plant biomass or between total nodule mass and nodule ureide contents. © 1993.