Abstract
Leaf fluctuating asymmetry (FA) is widely used as an environmental stress index, including pollution. Besides FA, leaf bilateral traits can have directional asymmetry (DA) and antisymmetry (AS), which are considered hereditary. Leaf FA transitioning to DA/AS or mixed asymmetry, under air pollution, has been insufficiently investigated. This study analysed leaf asymmetry types in Tilia cordata Mill. and Betula pendula Roth under traffic air pollution over several years. In addition, the relations of such transitions to pollution, and their effect on FA-integrated index, were studied. The asymmetry types of all studied leaf traits varied with air pollution increase, as well as in control trees in different years. T. cordata most often had FA transition to DA/mixed asymmetry, while B. pendula rarely had a mixed asymmetry and FA transitions to DA/AS were observed with the same frequency. Air pollution impacted FA transitions to other asymmetry types. In most cases their frequency changed non-monotonically that corresponded to hormesis and paradoxical effects. However, FA integrated index in studied trees did not depend on change of leaf asymmetry type. Thus, DA and AS in studied plants were not exclusively hereditary. Hence, the changes of leaf asymmetry type should be considered when using leaf FA in environment assessment.
Highlights
Developmental stability characterizes the ability of an organism to maintain the trajectory of development within certain bounds [1,2]
We showed that all the studied leaf traits of T. cordata and B. pendula changed
We showed that all the studied leaf traits of T. cordata and B. pendula the asymmetry type both in trees that grew in polluted plots and control ones, previously fluctuating asymmetry (FA)
Summary
Developmental stability characterizes the ability of an organism to maintain the trajectory of development within certain bounds [1,2]. FA has a mostly non-hereditary nature, but it is often observed in the background of hereditary types of asymmetry, such as antisymmetry (AS) and directional asymmetry (DA) [2]. FA is characterized by a normal distribution of R - L differences with a mean of zero [1]. DA reflects a consistent bias of a trait within a species towards greater development on one side of the body than on the other; the coiling and associated anatomical asymmetry of gastropods or the flatfish asymmetry are typical examples. DA has normally distributed R - L differences around a mean that is significantly different from zero [1]
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