Seasonal chlorophyll fluorescence before and after rapid light curves in the endangered species Lophophora diffusa (Cactaceae) across two microenvironments

In desert environments, intense light stress can significantly impede the growth and survival of plants, making microenvironments formed under nurse plants crucial. Chlorophyll a fluorescence serves as a widely employed tool for detecting plant stress conditions. Here, we investigated the seasonal ecophysiological performance of Lophophora diffusa (Cactaceae) under nurse plants and in direct sunlight. Our assessment involved monitoring chlorophyll fluorescence parameters as indicators of photosynthetic efficiency over the course of one year, including the effective quantum yield of photosystem II (Φ_PSII), maximum quantum efficiency of photosystem II (F_v/F_m), electron transport rate (ETR), and non-photochemical quenching (NPQ). Higher daily PPFD levels were observed in open spaces throughout all seasons except for winter. Additionally, temperatures were lower beneath nurse plants compared to exposed sites during spring and autumn. Lophophora diffusa exhibited dynamic photoinhibition, as evidenced by lower F_v/F_m values in plants exposed to sunlight compared to those sheltered under nurse plants in spring. During rapid light curves, L. diffusa exhibited decreased maximum Φ_PSII and maximum ETR values in winter across both microenvironments, while displaying higher maximum NPQ in cacti exposed to sunlight compared to those sheltered under nurse plants in spring. High saturating light levels for ETR were observed in both environments throughout most seasons, except for winter, indicating a high tolerance to high-light conditions. Light curves induced greater stress compared to natural conditions, as evidenced by the acclimation with increased NPQ observed only after rapid light curves. These findings suggest that adult individuals of L. diffusa may not strictly rely on nurse plants for survival and have developed various mechanisms to thrive in diverse microenvironments.