Citation: Kordyum, E.; Polishchuk, O.; Akimov, Y.; Brykov, V. Photosynthetic Apparatus of Hydrocharis morsus-ranae in Different Solar Lighting. Plants 2022, 11, 2658. https://doi.org/10.3390/ plants11192658 Academic Editors: Bernhard Huchzermeyer and Hans-Werner Koyro Received: 18 August 2022 Accepted: 3 October 2022 Published: 10 October 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). plants Article Photosynthetic Apparatus of Hydrocharis morsus-ranae in Different Solar Lighting Elizabeth Kordyum 1 , Oleksandr Polishchuk 1 , Yuri Akimov 1 and Vasyl Brykov 1,2, * 1 Department of Cell Biology and Anatomy, M.G. Kholodny Institute of Botany, National Academy of Sciences of Ukraine, 2 Tereschenkivska Str., 01024 Kyiv, Ukraine 2 Department of Experimental Plant Biology, Faculty of Science, Charles University, Viniˇ cná 5, 128 00 Prague, Czech Republic * Correspondence: vbrykov@gmail.com Abstract: Hydrocharis morsus-ranae is a free-floating species growing in lakes and slow-flowing rivers near the shore in Europe and Western Asia, and as an invasive plant in the USA and Canada. Light- requiring plants of this species can also grow in the shade, up to about 30% of full sunlight. In this paper we present the data about the photosynthetic apparatus of sunny and shady H. morsus-ranae plants grown in the sun and in the shade in nature. Methods of light and transmission electron microscopy, biochemistry, chlorophyll fluorescence induction as well as the principal component analysis were used. It was found that leaves of plants growing in shade differed from those in the sun with such traits as thickness of a blade, palisade and spongy parenchyma, ultrastructure of chloroplasts, and quantum efficiency of photosynthetic electron transport, the content of chlorophylls and carotenoids, anthocyanins and phenilpropanoids. By these traits, H. morsus-ranae shady plants are similar with shade-bearing plants that indicates their adaptation to light intensity lowering. The ordination plots (PCA) suggested a clear structural and functional shift of plants growing in different lighting showing relationship to light changes in the natural environment. Thus, our results displayed the high phenotypic plasticity of the H. morsus-ranae photosynthetic apparatus, which ensures its acclimation to changing light environment and wide distribution of this species. Keywords: acclimation; anthocyanin; chloroplast; chlorophyll induction; granum; pigment complex; plasticity; principal component analysis; shade; sunlight 1. Introduction Hydrocharis morsus-ranae L. are monocotyledonous, dioecious, stolon-rosette free- floating aquatic plants (hydrophytes) of lakes, ponds and slow-flowing rivers, growing near the shore [1]. Flowers are white, reaching almost 2 cm in diameter. Male (staminate) flowers are usually gathered in inflorescences of three, female (pistillate) flowers are solitary. Heart shaped leaves with the spongy lower side and long petioles are collected in the rosette and float on the water surface. The color of leaves is usually a light or pure green or becoming brownish black at senescence. It can vary depending on the level of sunlight or shade, and plant age. In autumn, thin stolons with large buds (turions) at the ends appear. Turions fall to the bottom of the water body, where they remain until spring. In spring, turions develop new plants [2–5]. Micromorphology of vegetative and generative organs were described by Cutter and Feldman [6], Seago et al. [7], Tsyrenova et al. [8], Efremov et al. [9]. H. morsus-ranae is native for Europe and Western Asia, and it widely naturalized outside its native range in the USA and Canada as an invasive species because of its significant ability to overgrow areas in a short time by vegetative and seed propagation [4,10]. In Ukraine, H. morsus-ranae is usual in the Polissya (northern forest) and Forest-Steppe physiographic regions, sporadically—in Steppe and on the Zakarpatska plain [11]. Plants 2022, 11, 2658. https://doi.org/10.3390/plants11192658 https://www.mdpi.com/journal/plants