My question: what aspects of a living plant can I measure to determine it's health/growth? Ideally I could measure these with a computer sensor to make data collection easier, so measuring leaves would be more of a last resort.

Background: I'm a 3rd year computer science undergraduate trying to build a system to evaluate plant health for a research project. Given my chosen field, my knowledge of living things is rather limited, so I'm sourcing as much information as I can from professors, internet forums, etc (as many experts as I can find). If it helps narrow anything down, the end product will hopefully be used in a hydroponic setup to fine tune the nutrient and water levels.

If anyone has any knowledge on this subject, I'd greatly appreciate your help!