Reduced lodging for soybean in low plant population is related to light quality

Lodging is a common production problem for soybean [Glycine mar (L,) Merr,] in the southeastern USA, Reducing plant height by planting at a low optimal population (plant population sufficient to optimize yield) is a recognized practice for reducing lodging, Environmental factors responsible for reduced plant height in response to low optimal population have not been determined, however, Plant height for determinate soybean is determined mainly during the vegetative period (emergence to Ri). Because light quality affects stem extension, our objective was to determine the roles of red/far red Light ratios and blue light irradiance during the vegetative period on plant height. Determinate cultivar Deltapine 3606 (Maturity Group VI) was planted at an optimal planting date during 1995 and 1996 at low (80 000 plants ha(-1)), medium (145 000 plants ha(-1)), and high (390 000 plants ha(-1)) plant populations on a Commerce silt loam soil (fine silty, mixed, nonacid, thermic, Aeric, Fluvaquents) near Baton Rouge, LA (30 degreesN Lat). Experimental design was a randomized complete block in a split plot arrangement with four replications. Plant populations were main plots and split plots were staked (non lodging) and unstaked (prone to lodging) treatments. Increased lodging in staked vs. unstaked plots resulted in a 10% yield loss. Across unstaked plots, decreased lodging in low vs. high population was associated with a 25% reduction in plant height and a 29% increase in main stem thickness. These morphological changes were more closely related to differences in red/far red light ratios for low vs, higher plant populations rather than to changes in blue light irradiance.