Long-term simulated nitrogen deposition alters the plant cover dynamics of a Mediterranean rosemary shrubland in Central Spain through defoliation

Linking jasmonates with vitamin E accumulation in plants: a case study in the Mediterranean shrub Cistus albidus L

Jasmonic acid positively modulates vitamin E accumulation, but the latter can also partly influence the capacity to accumulate the jasmonic acid precursor, 12-oxo-phytodienoic acid, in white-leaved rockrose (Cistus albidus L.) plants growing in their natural habitat. This study suggests a bidirectional link between chloroplastic antioxidants and lipid peroxidation-derived hormones in plants. While vitamin E is well known for its antioxidant properties being involved in plant responses to abiotic stress, jasmonates are generally related to biotic stress responses in plants. Studying them in non-model plants under natural conditions is crucial for the knowledge on their relationship, which will help us to better understand mechanisms and limits of stress tolerance to implement better conservation strategies in vulnerable ecosystems. We studied a typical Mediterranean shrub, white-leaved rockrose (Cistus albidus) under natural conditions during three winters and we analyzed both α and γ-tocopherol, and the three main jasmonates forms 12-oxo-phytodienoic acid (OPDA), jasmonic acid (JA), and jasmonoyl-isoleucine (JA-Ile). We found that JA contents positively correlated with vitamin E accumulation, most particularly with γ-tocopherol, the precursor of α-tocopherol (the most active vitamin E form). This finding was confirmed by exogenous application of methyl jasmonate (MeJA) in leaf discs under controlled conditions, which increased γ-tocopherol when applied at 0.1 mM MeJA and α-tocopherol at 1 mM MeJA. Furthermore, a complementary meta-analysis study with previously published reports revealed a positive correlation between JA and vitamin E, although this relationship turned to be strongly species specific. A strong negative correlation was observed, however, between total tocopherols and OPDA (a JA precursor located in chloroplasts). This antagonistic effect was observed between α-tocopherol and OPDA, but not between γ-tocopherol and OPDA. It is concluded that (i) variations in jasmonates and vitamin E due to yearly, inter-individual and sun orientation-driven variability are compatible with a partial regulation of vitamin E accumulation by jasmonates, (ii) vitamin E may also exert a role in the modulation of the biosynthesis of OPDA, with a much smaller effect, if any, on other jasmonates, and (iii) a trade-off in the accumulation of vitamin E and jasmonates might occur in the regulation of biotic and abiotic stress responses in plants.

Nitrogen deposition reduces the cover of biocrust-forming lichens and soil pigment content in a semiarid Mediterranean shrubland

Biocrusts are key drivers of the structure and functioning of drylands and are very sensitive to disturbance, including atmospheric nitrogen (N) deposition. We studied the impacts of simulated N deposition on biocrust community composition and soil photosynthetic and photoprotective pigment content after five years of N application in a European semiarid Mediterranean shrubland. The experiment consisted in six experimental blocks with four plots, each receiving 0, 10, 20, or 50 kg NH₄NO₃-N ha^-1 year^-1 + 6-7 kg N ha^-1 year^-1 background. After 5 years of N application, total lichen cover decreased up to 50% compared to control conditions and these changes were only clearly evident when evaluated from a temporal perspective (i.e. as the percentage of change from the first survey in 2008 to the last survey in 2012). In contrast, moss cover did not change in response to N, suggesting that biocrust community alterations operate via species- and functional group-specific effects. Interestingly, between-year variations in biocrust cover tracked variations in autumnal precipitation, showing that these communities are more dynamic than previously thought. Biocrust species alterations in response to N were, however, often secondary when compared to the role of ecologically relevant drivers such as soil pH and shrub cover, which greatly determined the composition and inter-annual dynamics of the biocrust community. Similarly, cyanobacterial abundance and soil pigment concentration were greatly determined by biotic and abiotic interactions, soil pH for pigments, and organic matter content and shrub cover for cyanobacteria. Biocrusts, and particularly the lichen component, are highly sensitive to N deposition and their responses to pollutant N can be best understood when evaluated from a temporal and multivariate perspective, including impacts mediated by interactions with biotic and abiotic drivers.