If you have photosynthesis, you already have photosensitivity. The process of photosynthesis will cause chemical gradients in the reaction's inputs and outputs

If you have photosynthesis, you already have photosensitivity. The process of photosynthesis will cause chemical gradients in the reaction's inputs and outputs (and those of secondary reactions) that indicate where the plant senses light. Exactly how those are leveraged, I don't know, but based on hanging out with folks who study this for a few years, I'm going to bet it's very complicated and somehow involves auxin. An overview of some plant growth influences and auxin: https://www.biologyonline.com/tutorials/plant-auxins-phototropism-geotropism https://www.biologyonline.com/tutorials/plant-auxins-phototr...