The Effect of Shading, Defoliation, and Seeding Rate On Source and Sink Reactions in Determinate and Indeterminate Soybean Cultivars.

Research was conducted at three locations in North Carolina in 2010 and 2011 in order to examine source and sink relationships. The objectives of this research are to determine if different cultivars are more likely to overcome source limitations by translocation of starch from the leaves to the seed, and to examine how the treatments imposed enhanced or reduced source limitations or created sink limited conditions. 

Treatments were applied to two determinate and two indeterminate cultivars and consisted of three different seeding rates (49,400, 296,400, 741,000 plants ha^-1), shade at two different periods during reproduction (R1-R5 & R5-R8), thinning the high seeding rate at R5, and defoliating the low seeding rate at R5.  Five plants from each treatment were taken at intervals from R5 to maturity. Biomass of entire plants and individual pods were measured along with pods per plant. The amount of dry matter produced per pod per growing degree day was compared to the rate of increase in individual pod weight per plant in order to determine source and sink relationships.

If the increase in weight per pod is equal to the amount of dry matter per pod per GDD, this indicates that the total amount of assimilate produced by the plant during this period is being translocated directly to the pods. If the increase in weight per pod is greater than the rate of plant growth per pod, then remobilization of assimilate must be occurring resulting in source limited situation. In contrast, if the increase in weight per pod is less than the rate of plant growth per pod then the excess assimilate is being stored in organs other than the pod. This indicates a sink limiting situation. To determine whether the plants were source or sink limiting, the rate of increase in weight per pod was divided by the increase in weight per plant per pod per GDD.  This ratio was then compared to a ratio of 1 which, as described above, indicates neither a source or sink limiting situation. Proc GLM was used to determine which cultivars and treatments had a ratio that differed significantly from 1. In addition, ratios for each cultivar and treatment were compared with each other to determine significant differences.

There were variety by treatment interactions at all locations. At Plymouth in 2010 and at Kinston in 2011, the most consistent treatments were the low seeding rate which tended to have the highest source-sink ratio when compared to the remaining treatments, and the high seeding rate thinned, which consistently had the lowest source-sink ratio, which was not significantly different from 1. At Plymouth in 2011, the high seeding rate treatment had the highest source-sink ratio, with no consistent differences among the thinned high seeding rate or the medium seeding rate shaded early or late. Among cultivars, the only consistent trend occurred at Plymouth in 2011, where the determinate cultivars had lower source-sink ratios compared to the indeterminate cultivars.