Chlorine

You may not realise it but chlorine is classed as a plant micronutrient meaning that it is essential for the proper growth of plants. It is important to note though, that it is the ion (Cl^‑) rather than the gas (Cl₂) that is used by plants. In particular, chlorine is important for plant photosynthesis as it is involved in the opening and closing of stomata (pores in leaves that enable plants to take in and release carbon dioxide, oxygen and other gases as required. It also helps ensure leaves are firm.

https://plantprobs.net/plant/nutrientImbalances/chlorine.html