Species-specific phylloplane responses to changes in external pH

The leaf surface, known as the phylloplane, represents the initial point of contact for plants in their interactions with the above-ground environment. Although previous research has assessed how leaves respond to variations in external pH, particularly in the context of acid rain, there remains a limited understanding of the molecular mechanisms through which plants detect, respond to, and mitigate cellular damage. To examine plant responses to changes in external pH, we selected five species known to have a range of phylloplane pH values from alkaline to acidic under normal conditions, and investigated the response of the phylloplane pH to treatments at pH 6.5, 4, and 2. We found that plants were able to modify their phylloplane pH, and that this buffering ability was species-specific; however, only Gossypium species displayed strong buffering. When leaves were exposed to either pH 6.5 or pH 4, the Gossypium species alkalinized the phylloplane pH to slightly higher values than the dry control pH but, remarkably, when leaves were exposed to pH 2, they buffered the pH to 6 within 5 min. Transcriptional analysis further indicated that the responses to external pH changes varied among the five species, with differentially expressed genes associated with Ca2+-signaling pathways as well as Ca2+- and H+-ATPases pumps being highlighted. These findings also suggested that pH stress affected photosynthesis, and that both wetness and moderate pH shifts might trigger additional abiotic and biotic stress-signaling pathways.