Acid or base? How do plants regulate the ecology of their phylloplane?

Abstract

Plants interface with and modify the external environment across their surfaces, and in so doing, can control or mitigate the impacts of abiotic stresses and also mediate their interactions with other organisms. Botanically, it is known that plant roots have a multi-faceted ability to modify rhizosphere conditions like pH, a factor with a large effect on a plant’s biotic interactions with microbes. But plants can also modify pH levels on the surfaces of their leaves. Plants can neutralize acid rain inputs in a period of hours, and either acidify or alkalinize the pH of neutral water droplets in minutes. The pH of the phylloplane—that is, the outermost surface of the leaf—varies across species, from incredibly acidic (carnivorous plants: as low as pH 1) to exceptionally alkaline (species in the plant family, Malvaceae, up to pH 11). However, most species mildly acidify droplets on the phylloplane by 1.5 orders of magnitude in pH. Just as rhizosphere pH helps shape the plant microbiome and is known to influence belowground interactions, so too can phylloplane pH influence aboveground interactions in plant canopies. In this review, we discuss phylloplane pH regulation from the physiological, molecular, evolutionary, and ecological perspectives and address knowledge gaps and identify future research directions.

Abstract

Plants alter external environmental conditions in many ways. Well-known is the fact that roots alter the pH of the surrounding soil. However, less appreciated is the fact that plants also alter pH levels on their leaf surfaces. Our review explores this little-known topic; we discuss variation in leaf surface pH across a diversity of plants, the physiological ways by which plants can modify leaf surface pH and the importance of this trait to ecological interactions, including those with microbes and insects. This topic presents many unanswered questions awaiting future work, with relevance to agriculture as well as wild ecosystems.