Leaves from two Solanaceae species with different leaf morphological traits respond differently to imidacloprid foliar spray*

Abstract

Imidacloprid (IMI) is an insecticide neonicotinoid widely used for controlling crop pests and usually applied by foliar spray. The insecticide causes changes in structural and functional aspects in different plant species. However, the foliar absorption rate, which is controlled mainly by micromorphological characteristics of the leaf surface, can be related to the response intensity. This study aimed to investigate the anatomical, physiological and biochemical responses to IMI foliar application in two Solanaceae species whose leaves have distinctive micromorphological features. Capsicum annuum has glabrous leaves, whereas Solanum aethiopicum has hairy leaves. IMI was applied using leaf pulverization. Leaf absorption and wettability tests were performed. Analyses of gas exchange, chlorophyll a fluorescence, peroxidase and superoxide dismutase enzyme activity, and lipid peroxidation were performed. Morphometric analyses of leaf blades were performed under light microscopy. 3,3-diaminobenzidina (DAB) was used for in situ detection of reactive oxygen species (ROS). In both species, IMI-treated leaves showed higher mesophyll thickness and more intense ROS staining than those in untreated leaves. Imidacloprid-treated C. annuum leaves showed more severe alterations in physiological and biochemical aspects than those in S. aethiopicum leaves. This was most evident in terms of CO2 assimilation rate, stomatal conductance, transpiration rate, and instant carboxylation efficiency. In addition, IMI-treated C. annuum leaves presented anatomical alterations including pronounced sinuosity on palisade parenchyma cell shape and increased intercellular spaces in spongy parenchyma. The most pronounced functional and structural alterations in IMI-treated C. annuum leaves was associated with their leaves micromorphological features, which increased wettability and probably IMI absorption.