Final answer:
Stomatal closure in response to bacterial infection serves as a defense mechanism, minimizing the entry of additional pathogens and water loss. The hormone responsible for this response is abscisic acid.
Step-by-step explanation:
Stomata, the tiny openings on the surfaces of leaves, play a crucial role in gas exchange and water regulation in plants. When stomata close in response to bacterial infection, it is a defense mechanism because it reduces the potential for additional pathogens to enter through these openings, thereby limiting the spread of the infection. The hormone most commonly associated with the closing of stomata under these stress conditions is abscisic acid (ABA), which signals the guard cells to reduce their internal water content and become flaccid, leading to stomatal closure. This process effectively helps to minimize water loss, which is vital for the plant's survival during times of stress.