Focusing on the process:
* Yields (referring to the products of photosynthesis)
* Yellow (referring to the color of chlorophyll, which is crucial for photosynthesis)
Focusing on the importance:
* Yummy (referring to the food produced by photosynthesis)
* Youthful (referring to the energy provided by photosynthesis)
The best word will depend on the overall theme and tone of your poem.