There is no definitive answer to this question, as different types of cells may have different requirements for ions or salts during photosynthesis.
However, one possible answer is:
Potassium (K+) is an ion that needs to go into the cell once photosynthesis starts upon day break. Potassium is involved in regulating the opening and closing of the stomata, which are pores on the surface of the leaf that allow gas exchange. When photosynthesis starts, the stomata need to open to let in carbon dioxide and release oxygen. Potassium helps to maintain the water balance and the turgor pressure of the guard cells that surround the stomata. Potassium also plays a role in the synthesis of ATP, which is the energy currency of the cell, and in the transport of sugars, which are the products of photosynthesis.