Effects of Forced and Natural Leaflet Orientation On Transpiration Rates and Other Physiological Traits in Soybeans.

The objective of this study was to evaluate the effects of natural and artificially imposed leaflet orientation on transpiration rates and other physiological traits in soybeans.  The soybean variety USG 5601T was chosen for this study due to its ability to strongly orient its leaves during the day in response to sunlight.  Twenty-four plants were subjected to two treatments during the 2007 and 2008 growing seasons in Knoxville, TN.  One treatment set was restrained with netting in order to gently force the orientation of the outer canopy leaves to assume the phenotype of a plant which does not orient its leaves.  The other treatment was unrestrained and allowed to orient its leaves as normal.  Whole plant transpiration rates of 24 plants for each treatment were measured for a 24 hour period with the Dynamax Flow 32 Sap Flow instrument when the plants were in the R5 growth stage of active pod filling.  Photosynthetically active radiation, leaflet temperatures, leaflet transpiration, stomatal conductance, and photosynthesis and were measured with the Dynamax LCi Photosynthesis meter.  The unrestrained plants, which oriented their leaves, used an average of 24 grams more water per day than the restrained plants which were not allowed to orient their leaves.  Although this effect was not significant (p=0.53), it is consistent with previous findings that soybean plants with high leaflet orientation transpire more water than plants with lower leaflet orientation (Johnson et al., 2004, 2006, 2009).  High leaflet orientation allows upper canopy leaflets to maintain an average 4.5°C lower temperature due to their reduced sunlight exposure.  This may result in a lower transpiration rate for upper canopy leaflets.  However, soybean plants with high leaflet orientation allow more sunlight into the lower canopy which results in higher rates of transpiration, stomatal conductance, and photosynthesis for those leaves relative to plants with lower leaflet orientation.  The overall effect appears to be that soybean plants with higher leaflet orientation tend to have higher overall rates of transpiration, photosynthesis, and yield.