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Interrelationships Among Supernodulation, Prolonged Juvenile Growth and Biomass Yields in Soybean [Glycine max (L.) Merr.]
Dissertation Abstract:
Grafting experiments in soybean were carried out to put together the supernodulating characteristic from a Bragg mutant and the juvenile genes from tropical cultivars. Experiments were conducted under natural long and short day regimes. The objectives of this study were to (1) test the expression of supernodulation and prolonged juvenile growth characteristics that might be useful in further pushing yield ceilings in soybean, (2) determine the combined effects of supernodulation and prolonged juvenile growth characteristics on biomass yields, and (3) select compatible combinations and determine cultivar sources that can be used in soybean breeding programs.
Experiment 1 under long day conditions showed that grafting tropical varieties having a prolonged vegetative period such as UPLSy10 and especially IPBSy91-10-01 on NTS3982, the supernodulating mutant, increased vegetative dry weight. The substantial amount of available photosynthate was presumed to have provided a relief in competition between vegetative growth and nodule development. Bragg, the wild type, apparently had a high adaptation potential under long day length, due to sufficient production of photosynthate to support both vegetative and nodule development. NOD139, the non-nodulating mutant, and its grafts with tropical varieties failed to increase vegetative dry weight accumulation because of its inherent abnormal growth, even in the presence of applied labelled nitrogen (15N).
Nodule number of NTS382 was highest, but it did not show any advantage in biological nitrogen fixation over Bragg, UPLSys10, and IPBSy91-10-01. NTS382 due to the lower level of nitrate condition, when grafted with scions from UPLSys10 and IPBSy91-10-01, had a reduced number of nodules but size was larger. Grafting IPBSy91-10-01 on NTS382 was able to increase seed yield, due to its extended length of vegetative development, resulting in more leaf area and higher photosynthate production to support the high sink demand.
Experiment 2 showed the pronounced effects of short day length on plants in terms of shorter length of vegetative and reproductive period and early maturity of all genotypes. It resulted in lower dry matter accumulation, a reduction in extent of nodulation and biological nitrogen fixation and less yield compared to Experiment 1. NTS382 and Bragg isolines had a substantially decreased shoot and root biomass production under short day length. The nodulation of NTS382 was greatly reduced, indicative of low supply of photosynthate, though it was still superior compared to the tropical varieties and Bragg. However, seed yield of NTS382 was increased when grafted to UPLSys10 and IPBSy91-10-01 due to prolonged vegetative period conferred by the scions, thus, overcoming the limitations imposed by short daylength, and a renewed high rate of biological nitrogen fixation.
This study successfully demonstrated the convergence of the supernodulating character and juvenile vegetative growth through grafting. It created a new plant with two demarcated genotypes but acting jointly to produce the desired effects of high nodulation, attendant nitrogen fixation and high yield. Therefore, the results of this research point to the feasibility of combining supernodulation and long juvenile traits in tropical soybean varieties. Crosses involving the supernodulating mutant, NTS382, and the source of juvenile gene from IPBSy91-10-01 and UPLSys10, could result in genotypes which combine high nodulation, good biological nitrogen fixation, ideal phonological development, and high seed yield under a tropical environment.