Species-specific phyllosphere responses to external pH change
The leaf surface, known as the phylloplane, represents the initial point of contact for plants in their interaction with the aboveground environment. Although prior research has assessed how leaves respond to external pH variations, 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 look at plant responses to external pH changes,  we measured the phylloplane pH for five species with variable phylloplane pH that ranged in the dry control. Moreover, we investigated the phylloplane pH in response to three pH treatments (pH 6.5, 4, and 2) and found that plants can modify their phylloplane pH, and this buffering ability is species-specific.   Among the species analyzed, only Gossypium displayed a strong buffering ability. For treatments where leaves were exposed to either pH 6.5 or pH 4, Gossypium alkalinized the phylloplane pH slightly higher than the dry control pH. Remarkably, when leaves were exposed to pH 2, Gossypium was able to buffer the pH to 6 within five minutes. Furthermore, our transcriptional analysis indicated that the responses to external pH changes varied among species, highlighting differentially expressed genes associated with calcium (Ca2+) signaling pathways, as well as Ca2+ and H+-ATPases pumps. These findings also suggest that pH stress negatively impacts photosynthesis, and that both wetness and moderate pH shifts may trigger additional abiotic and biotic stress signaling pathways. Overall design: We selected five different species with predicted pH levels ranging from alkaline to acidic to assess a broad initial phylloplane pH: Gossypium arboreum, Gossypium hirsutum, Beta vulgaris, Nepenthes bicalcarata, and Nepenthes rafflesiana.  For each species, we exposed one leaf of three biological replicates per species to one of three pH treatments (6.5, 4, 2) for 5 minutes and  we collected unsprayed leaves for each species as dry controls.