Morphological structure of leaves and dust-retaining capability of common street trees in Guangzhou Municipality

In this study,we selected eighteen species of common street trees along main roads in Guangzhou Municipality to observe and compare their leaf surface morphology using Scanning Electron Microscopy(SEM),and to evaluate how the contact angle of water droplet on leaf surfaces measured by goniometer influence the dust-retaining capability.It was found that the maximum leaf dust-retaining capabilities per unit area of the eighteen species of trees 26 days after rainfall were 0.066—1.831g/m2,indicating significant differences(up to 27 times) among different trees.The plants with network structure on leaf surface,having high stomatal density and large opening of stomata,such as Mangifera Indica,have strong dust-retaining capability;while those whose leaf surfaces are smooth and covered by epicuticular wax with stomata arranged regularly without obvious undulation,such as Bauhinia blakeana,Swietenia mahogani,Lagerstroemia indica Linn.and Scheffera arboricola,have poor dust-retaining capability.There was a significant negative relationship between leaf contact angle and dust-retaining capability(r=-0.614).Leaves with contact angle smaller than 90° demonstrate hydrophilicity.Their maximum dust-retaining capabilities 26 days after rainfall ranged from 1.0 to 1.831 g/m2.For leaves with rugged morphological structure,hook or ridge-shaped rumple,prominence and stomata density ranging from 20 to 60,the measured contact angles are smaller,making the contact area between leaf and dust larger.Therefore,it is not easy for dust on such trees as Mangifera Indica,Bischofia Polycarpa and Ficus altissima Bl.,to detach from leaf surface and these species have a strong dust-retaining capability.On the contrary,the special surface structure and hydrophobic wax of tree leaves with larger contact angle,such as Alstonia scholaris,Chukrasia tabularis,Lagerstroemia indica Linn.,Scheffera arboricola and Bauhinia blakeana,make it difficult for dust particles to attach to leaves,resulting in small dust-retaining capabilities of less than 1.0g/m2.This study has shown that the wax content of surface foliage,stomata density and contact angle of leaves are the three main factors controlling the dust-retaining capability.It is therefore advisable to consider choosing tree species whose leaf surface morphology enhances dust-retaining capability for urban greening and thus to improve the environmental effectiveness of urban vegetation.