For those interested in this topic, you also might be interested in sensors that can measure evapotranspiration (ET). ET is directly related to photosynthesis and crop yield.

Here is a quick refresher from the textbook on plant water relations:

> Stomata are microscopic valves present in all leaves, formed by the pairing of specialized cells called guard cells on the leaf surface. Stomata are the main ports through which water vapor is lost. Of equal importance is the fact that stomata also are the main ports through which carbon dioxide gas moves from the air into the leaf and is photosynthesized into plant material and accounts for most of the plant dry matter produced. - Hsiao

Our company, Tule Technologies (YC S14) is the commercial vendor of sensors that can measure evapotranspiration. We solve many of the problems mentioned in this article.