Author : Parinya Sebunruang

Summary : 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: 1) to test the expression of supernodulation and prolonged juvenile growth characteristics that might be useful in further pushing yield ceilings in soybean; 2) to determine the combined effects of supernodulation and prolonged juvenile growth characteristics on biomass yields; and 3) to select the 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 NTS382, the supernodulating mutant, increased vegetative dry weight. The substantial amount of available photosynthate is 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 daylength, due to sufficient production of photosynthate to support both vegetative and nodule development. NPD139, the non-nodulating mutant, and its grafts with tropical varieties failed to increase vegetative dry weight accumulation becuase of its inherent abnormal growth, even in the presence of applied labelled nitrogen (15 N). Nodule number of NTS382 was highest, but it did not show any advantage in biological nitrogen fixation over Bragg, UPLSy10 and IPBSy91-10-01. NTS382 due to the lower level of nitrate condition, when grafted with scions from UPLSy10 and IPBSy91-10-01, had a reduced number of nodules but size was larger. Grafting of 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.