Soil N is known to affect N2 fixation in legumes. Increased cowpea yields might be achieved by increasing N assimilation through greater symbiotic fixation, but identification of high‐fixing genotypes requires an effective screening technique. The purpose of this investigation was to determine the optimum level of supplemental N for maximum N2 fixation in cowpeas grown under greenhouse and field conditions, and the relative contributions of the measured variables on nitrogenase activity.
Three cowpea cultivars were tested for N2 fixation, N2 (C2H2), under greenhouse conditions with 0, 5, 10, and 20 ppm N applied as Ca(NO3)2, and under field conditions with 0, 11.2, 22.4, and 44.8 kg/ha N applied as Ca(NO3)2. The soil was Vertic Albaqualf, fine, Montmorillonitic thermic. Nitrogenase activity decreased with increasing rates of N. Path analysis indicated that nitrate effects on N2 fixation activity were primarily on nodule mass, with the inhibition of nodule growth rather than nodule initiation. Nodule number was not a reliable predictor of N2 fixation potential of cowpea genotypes and was negatively correlated with nitrogenase activity. The results clearly indicated no advantage of applied nitrate in promoting maximum genotypic differences for N2 fixation.
Intact, flowering, rose plants have been regenerated in vitro from excised embryos of crosses between 'Bridal Pink' (the maternal parent) and several pollen parents. Explanted embryonic tissues developed into an organogenic Callus which formed adventitious shoots after several months only on a modified, half-strength Murashige & Skoog medium containing 1.0 ~tM BA and 0.05 #M NAA. These shoots could be separated, grown individually, rooted in a medium with no BA or NAA, with 1.0/~M IBA, and transplanted to greenhouse media. Embryos ranging in age from 21 to 35 days post-pollination formed organogenic callus that eventually regenerated adventitious shoots. Histological examination of normally-developing embryos showed that well-defined embryonic axes were beginning to develop at approximately 20-25 days postpollination. Analysis of populations of regenerated plants from different crosses showed differences in flower color, growth habit, peduncle length, and petal number. This system may be useful for irradiation-mutation breeding and/or for the development of transgenic rose plants using Agrobacterium tumefaciens.
The extent of intraspecific variability for N2 fixation among 100 southernpea (cowpea) genotypes was determined, and a screening technique was developed to measure the character. Significant differences in N2 fixation efficiency were found among host plant genotypes following application of a standard commercial mixed strain Rhizobium inoculant. These differences were obtained whether the criterion used was nodule mass, nodule number of nitrogenase activity, as measured by the acetylene reduction assay. This variability is evidence of genetic control of the trait and suggests the possibility of breeding for increased N2 fixation efficiency in cowpea.
Diurnal and seasonal patterns of nitrogen fixation N2(C2H2) in southernpea were delineated, using 7 genotypes which differed in potential to fix nitrogen. Diurnal activity peaked at 1200 hours, at both 34 and 53 days after planting (DAP). Significant differences in total activity between genotypes were observed, with maximum activity generally coincident with full flowering. High fixing genotypes were higher in total nitrogenase activity throughout the growing season, than were the low fixing genotypes. Peak activity for the latter was found at 34 DAP, while the former peaked at 46 DAP. Mean nodule mass, nodule number and plant dry weight were greatest 53 DAP.
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