Based on the provided features, the key feature for predicting the grain yield of the soybean cultivar is the "Seeds per meter/linear" feature. Higher values of this feature are expected to result in higher grain yield. Therefore, we can analyze the relationship between this feature and the target variable to create the dictionary.

Here is the dictionary generated using the analysis:

```json
{
	"low": ["FTR 4280 IPRO", "LYNDA IPRO", "ELISA IPRO"],
	"high": ["NK 8770 IPRO", "NK 8100 IPRO", "NEO 790 IPRO", "GNS7900 IPRO - AMPLA", "96R29 IPRO"]
}
```

The list of cultivars represents the possible values of the feature "Cultivar" for the target class "low" and "high". Note that this analysis is based on the assumption that the "Seeds per meter/linear" feature is the most significant predictor for grain yield.