Based on my prior knowledge, the number of grains per plant (NGL) can be an important factor in determining the grain yield of a soybean cultivar. Generally, a higher number of grains per plant would indicate a higher grain yield, while a lower number would indicate a lower yield.

To determine typical NGL values for each target class, we can analyze historical data and observations:

Possible values of feature NGL for target class 'low' (indicating low grain yield):
1. 10.5
2. 11.2
3. 9.8
4. 11.7
5. 10.9

Possible values of feature NGL for target class 'high' (indicating high grain yield):
1. 15.4
2. 14.1
3. 15.8
4. 13.9
5. 14.6

Based on this analysis, the dictionary representing the relationship between the feature NGL and the task of predicting grain yield can be as follows:

```json
{
	"low": [10.5, 11.2, 9.8, 11.7, 10.9],
	"high": [15.4, 14.1, 15.8, 13.9, 14.6]
}
```