Soybean Grain Weight and Oil Composition: Differential Response to Incident Radiation on Leaves and Pods.

Soybean grains are green during most of their filling period showing their capacity to utilize light. Although their photosynthetic activity does not contribute significantly to their carbon (C) economy, it can regulate grain composition by providing cofactors for the regulation of lipid biosynthesis and storage.

In sunflower, C available for oil synthesis accounted for the effect of radiation on fatty acid (FA) composition. However, such a mechanism has not been found in soybean.

Oxygen plays an essential role in FA synthesis by increasing the activity of enzymes involved in FA desaturation, e.g. light raises oleate desaturase (ODS) activity through the release of oxygen from grain photosynthesis. Therefore, FA composition of soybean oil would result from a complex combination of the effects of light on leaves, main source of C; and local effects on pods, by increasing oxygen production.

The objective was to address the effect of radiation that reaches the different structures of the plant (leaves and/or pods) on grain weight and composition of soybean oil.

Treatments were applied to modify radiation intercepted by: i) whole plant (80% shading-S_T), ii) leaves (total defoliation-D), iii) pods (paper hood-S_P) and, iv) leaves and pods (defoliation + paper hood - D*S_P), v) untreated plants (C).

Grain weight (GW) was reduced by D and S_T. These treatments increased saturated and poli-unsaturated FA, while they reduced oleic acid percentage. S_P did not produce any effect on GW but diminished linolenic acid percentage. When defoliation and pod shading were combined (D*S_P), FA composition was similar to D treatment but GW was lower.

 In conclusion, when photoassimilates from leaves are scarce, pod photosynthesis contributes to C economy of the grains. The response of FA composition of soybean oil to solar radiation is mediated not only by the availability of C but also by local effects on pods.