Though this paper is directly related to the use of digital camera in plant ecological studies, it is more focused in the correlation between the data from the digital camera and the measurement of photosynthesis.  Since the objective was this correlation it did not need to focus on shape nor texture of the plant.

It does not touch on any classification issues, but it has a very interesting approach on how to compare measurements taken in the lab to measurements taken in the field.  Could be something we could on our digital camera NDVI experiments.