Pigment disorganization in Salvia rosmarinus under drought and nitrogen addition in a Mediterranean shrubland

Mediterranean basin shrublands are biodiversity hotspots that are currently at risk due to concurrent global change drivers. Here, we investigated the simultaneous effects of a prolonged drought period and increased atmospheric nitrogen (N) deposition on photosynthetic and photoprotective pigments in rosemary (Salvia rosmarinus Spenn.), a widespread iconic Mediterranean woody species, in central Spain. We examined the concentrations and degree of coordination (coupling) of eight photosynthetic and photoprotective pigments under four N addition levels (0, 10, 20, and 50 kg N ha-1 year-1), sampled once immediately after an extended natural summer drought event and again after a two-month recovery period during which plants received ambient precipitation. Our results showed that prolonged summer drought had a pronounced impact, leading to a reduction in chlorophyll, lutein, neoxanthin, and β-carotene concentrations, while increasing the concentrations of photoprotective pigments associated with the xanthophyll cycle, particularly antheraxanthin and zeaxanthin. Drought also caused a disorganisation (decoupling) of pigments, indicating poor metabolic homeostasis under extreme climatic conditions. On the other hand, recovery from drought resulted in increased coordination (coupling). The effects of N addition were minor and mostly associated with greater β-carotene concentrations after drought that we interpreted as indicative of greater oxidative stress. Our findings suggest that extreme drought events will likely have greater effects on pigment metabolism in Mediterranean woody plants than increased N deposition, although interactions between the two global change drivers could amplify physiological disruptions. Our results are among the first to show the metabolic decoupling of plants to concurrent global change drivers, paving the way for new studies focusing on understanding the coordination of physiological responses under stress rather than solely focusing on physiological responses per se.