Abstract
Symmetry pervades nature, but asymmetry is also rather common. Deviations from genetically programmed symmetry are usually associated with internal or external developmental disturbances and may therefore be related to imperfections in physiological processes. In this study, we test the hypotheses that the photosynthetic efficiency of individual leaves of a plant is negatively related to their asymmetry. We measured chlorophyll fluorescence in leaves of three woody species (Betula pubescens, Populus tremula and Salix caprea) in early and late summer in two localities situated ca. 1000 km apart, and we quantified the asymmetry of these leaves by a multivariate measure based on the relative positions of several landmarks. Contrary to our expectation, we found that the photochemical efficiency of photosystem II was positively correlated with leaf fluctuating asymmetry; this effect was weak but consistent across the studied plant species, localities and seasons. Our finding adds to limited evidence that within-plant variation in leaf asymmetry is associated with variation in leaf physiology. Irrespective of the underlying mechanisms, which remain unknown, the results suggest that trees may benefit even more from their asymmetric leaves, at least in terms of photosynthesis, than they do from their more symmetric leaves.
Highlights
Trees, in contrast to highly integrated organisms like animals, can be viewed as conglomerates (‘populations’) of partially autonomous and repetitive multicellular subunits called modules [1].This modular structure buffers a tree against aging and disturbances, while enhancing the resource foraging efficiency through active environmental screening and growth in favourable directions [2,3,4].The end result is a significant increase in the performance of the entire plant in terms of fitness-related traits [5]
We found weakthat evidence forclassified directional asymmetry(1.53 in the leaves of B.collection pubescens, butinvery strong that had been classified as symmetric (0.92 ± 0.05; F1,179 = 131.7, P < 0.001)
The obtained fluctuating asymmetry (FA) scores were much higher in leaf size in B. pubescens (r = −0.20, n = 80 leaves, P = 0.083) and S. caprea (r = 0.22, n = 80, P = 0.052), but leaves that had been classified as asymmetric (1.53 ± 0.05) during collection than in leaves that had not in P. tremula (r = −0.08, n = 80, P = 0.49)
Summary
In contrast to highly integrated organisms like animals, can be viewed as conglomerates (‘populations’) of partially autonomous and repetitive multicellular subunits called modules [1].This modular structure buffers a tree against aging and disturbances, while enhancing the resource foraging efficiency through active environmental screening and growth in favourable directions [2,3,4].The end result is a significant increase in the performance of the entire plant in terms of fitness-related traits [5]. In contrast to highly integrated organisms like animals, can be viewed as conglomerates (‘populations’) of partially autonomous and repetitive multicellular subunits called modules [1]. This modular structure buffers a tree against aging and disturbances, while enhancing the resource foraging efficiency through active environmental screening and growth in favourable directions [2,3,4]. The end result is a significant increase in the performance of the entire plant in terms of fitness-related traits [5]. One plastic trait that varies strongly within plant individuals is leaf shape, and especially leaf bilateral asymmetry. Increases in fluctuating asymmetry (FA; the subtle non-directional deviations from symmetry in organs that are otherwise symmetric) are thought to arise from internal or external disturbances occurring during the development of an organism (developmental instability: [11,12])
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