Leaf surface traits related to differential particle adsorption - A case study of two tropical legumes

We aimed to test a set of epidermal traits in two legume species with contrasting chemical, physical, and micromorphological leaf-surface features to assess which ones would determine higher PM retention. For that, we performed a biomonitoring study in southeastern Brazil at the Steel Valley Metropolitan Region, where there is predominance of steel industry and one of the largest vehicle fleets in the country. A reference station was installed at a neighboring park. We evaluated leaf-surface roughness at two hierarchical levels, leaf wettability, epidermal-cell anticlinal-wall undulation, epidermal-cell perimeter, and the micromorphology and chemical composition of epicuticular waxes. Particle retention was significantly higher in Paubrasilia echinata than in Libidibia ferrea var. leiostachya, the former of which has lower roughness given by both the epidermal tissue (macro-roughness; 0.6 vs 2.6 μm) and epicuticular waxes (micro-roughness; 68 vs 220 nm), higher leaf wettability (82° vs 143°), lower epidermal-cell undulation index (1.2 vs 1.8), lower epidermal-cell perimeter (93 vs 146 μm), wax deposition in the form of a smooth layer (as opposed to densely aggregated rosettes of vertical platelets), and more polar wax chemical constitution (68% vs 47% of polar compounds). While all of the assessed traits contributed to particle retention, canonical loadings revealed that macro-roughness was the trait that contributed the most to the retention of PM_2.5 (ca = 1.47; r = -0.56), PM₁₀ (ca = 1.08; r = -0.61), PM₁₀₀ (ca = -4.95; r = -0.39) and TSP (ca = 0.98; r = -0.62), although this trait was shown by factor analysis to be secondary in distinguishing between species (0.92 contribution to the second axis). Our findings shed new light on the criteria that should be considered when selecting species for green infrastructure aiming to reduce urban air pollution, as well as on novel possibilities for PM biomonitoring in the tropics.