Effects of Leaflet Orientation and Root Morphology on Physiological Traits and Yield in Soybeans.

Drought is the most important abiotic stress adversely affecting soybean (Glycine max (L.) Merr.) yield. Leaflet orientation has been shown to reduce leaflet temperatures and transpiration while root morphology has been related to slower wilting phenotypes. The objective of this study was to investigate effects of leaflet orientation and rooting morphology on whole plant transpiration, yield, water use efficiency, and other physiological traits in soybeans using grafting techniques, population lines, near-isogenic lines, and restrained leaf canopy experiments. Experiments were conducted in Knoxville, TN with additional yield trial plots at Springfield, Spring Hill, and Milan, TN. Data were collected on whole plant transpiration, seed yield, leaflet orientation phenotype, root morphology, PAR, SAR, maturity, height, lodging, biomass accumulation, leaf area, photosynthesis, canopy light penetration, seed size, seed protein and oil. Grafting experiments revealed that plant shoots affected many of the measured traits but did not condition root phenotype. Root effects on measured traits were not significant. Effects of scion and root morphologies on measured traits could not be separated from genetic differences of the lines grafted. Population line analysis found no clear association between leaflet orientation and transpiration or yield. Leaflet orientation associated with some traits but those associations reflected the phenotype of parental lines, suggesting genetic linkage. Orienting leaves had cooler temperatures relative to leaves receiving direct sunlight. High orienting lines allowed more sunlight penetration into lower canopy which had a positive effect on mid-canopy photosynthesis. Leaflet orientation correlated with root morphology. Overall, root morphology had little effect on measured traits. Leaflet orientation and root morphology frequency distributions approximated normal distributions suggesting traits are polygenic. Experiments with near-isogenic lines detected no consistent patterns or significant effects due to differing leaflet orientation and root morphology on measured traits. This may have been due to lack of prominent differences in leaflet and root phenotypes between isogenic line pairs. Restrained canopy evaluations revealed no statistical differences in whole plant transpiration rates between plants allowed to orient leaves versus those with leaflets restrained. Further research is needed to investigate the effects of leaflet orientation and root morphology on yield and other physiological characteristics in soybean.