Soybean genetic improvement in yield and the effect of late-season shading and nitrogen source and supply
Genetic improvement in soybean [Glycine max (L.) Merr.] yield has been associated with both assimilate and N accumulation [especially from dinitrogen (N2) fixation] during the seed-filling period (SFP). Therefore, the physiological factors associated with genetic improvement may be dependent on abundant assimilate and N supply. The objectives of this study were to quantify genetic improvement in yield under: (i) assimilate limiting conditions, (ii) under low N fertility, and (iii) when either inorganic N fertilizer or N2 fixation is the main source of N. A randomized complete block experiment was conducted at two locations in 1998 and one in 1999. The main plot factor included no shade or a 63% shade treatment imposed after the R4/R5 developmental stage. The split plot factors were three N treatments: (i) inoculated, (ii) uninoculated with no additional fertilizer, and (iii) uninoculated plus fertilizer N. The split-split plots were four cultivars, representing a pair of older and a pair of newer cultivars. Shading reduced dry matter (DM) accumulation, and both shading and N limitation reduced N accumulation and seed yield. However, the newer cultivars consistently maintained their yield advantage over the older cultivars under both shading and N limiting conditions as well as when the source of N was soil available N or N2 fixation. The results of the study suggest that the physiological factors that contribute to genetic improvement in yield are not dependent on the level of incident radiation during the SFP, or the source or availability of N during the SFP.