You can use a spectral bandwidth which is very sensible to chlorophyll (around 630 nm) and another which is not chlorophyll dependent (around 730 nm, NIR). You can build a Normalized Difference Vegetation Index (NDVI) (wikipedia).

But it depends on the crops you are working on, and partitularly their optical properties.

Another way to find significant wavelength is to use principal component analysis in order to select more relevant wavelentgh

Thanking you all