A test subject

The team of scientists found that a remote laser’s green scanning light can detect a subtle but important shift in the biochemistry of a stressed leaf.

Specifically, the laser can pick up the presence of two pigments that aid photosynthesis. These yellow-orange molecules, called violaxanthin and zeaxanthin, help a plant moderate light. In high light, zeaxanthin protects a plant from excess sunlight by draining off light, acting a bit like an overflow valve; in low light, the plant converts zeaxanthin to violaxanthin, which cannot drain off light, to insure full use of light. “One moment from the next, plants are sensing their environment and converting between these pigments,” Logan said, for instance, when drifting clouds obscure the sun. “And that shift changes the optical quality of the leaf, which can be measured.”

Stress, however, can interfere with photosynthesis and this delicate interplay between pigments. A stressed plant won’t be able to consume as much light for photosynthesis, resulting in excess light absorption that can damage cells. “Thus, stress leads to a greater need for plants to siphon off light. In response, stress generally induces plants to convert more violaxanthin to zeaxanthin. And, we can measure that conversion remotely via the laser,” Logan said.