%0 Journal Article
%T The Effects of UV Radiation on Chloroplast Clumping and Photosynthesis in the Seagrass Halophila stipulacea Grown under High-PAR Conditions
%A Yoni Sharon
%A Gal Dishon
%A Sven Beer
%J Journal of Marine Biology
%D 2011
%I Hindawi Publishing Corporation
%R 10.1155/2011/483428
%X Since potentially harmful ultraviolet radiation (UVR, 280每400ˋnm) and high photosynthetically active radiation (PAR, 400每700ˋnm) are present in the shallow waters of the Gulf of Aqaba where part of the seagrass Halophila stipulacea's population thrives, we examined the effects of high PAR with and without UVR on its photosynthesis and midday chloroplast ※clumping phenomenon§ (Sharon and Beer 2008). It was found that midday clumping occurred only under high PAR in the presence of UVR, which resulted in a 44% reduction in the absorption cross section (or absorption factor, AF) of the leaves and, accordingly, a parallel lowering of midday electron transport rates (ETR). In addition, UVR had a direct effect on the photosynthetic apparatus by lowering quantum yields and, thus, ETRs, while pigment relations remained unaltered. We conclude that the potentially harmful effects of UVR and high PAR on the photosynthetic apparatus of Halophila stipulacea are mitigated by their activation of chloroplast clumping, which functions as a means of protecting most chloroplasts from high irradiances, including UVR. 1. Introduction While the mechanisms underlying chloroplast movements in plants have been studied on the intracellular and molecular levels [1, 2] their ecophysiological role(s) is less understood. These movements are in many plants induced by blue light [3每8], but in a few studies it was suggested that also ultraviolet (UV), or ※near to UV§, wavelengths could trigger chloroplast movements [9, 10]. Interestingly, Kondo et al. [11] reported that in many cases the chloroplast clumps are formed near or around the nuclei so as to protect them from high irradiances. The only seagrass so far for which chloroplast movements have been described is Halophila stipulacea [12, 13]. In the latter work, it was found that chloroplast clumping occurred during midday only in plants growing under high irradiances (>450ˋ米molˋphotonsˋmˋ2ˋsˋ1), while lower-irradiance grown plants maintained their chloroplasts evenly dispersed during the day. It was then suggested that the chloroplast clumping during midday may have a protective role against high irradiances. In addition to high photosynthetically active radiation (PAR, 400每700ˋnm), coastal waters, especially clear tropical waters, also feature high levels of ultraviolet radiation (UVR, 280每400ˋnm) in their upper layers ([14, 15] and this study). Various marine plants respond to such high UVR levels by producing mycosporine-like amino acids (MAA, [16, 17]) and other UV absorbing compounds (i.e., flavones and flavones glycosides in
%U http://www.hindawi.com/journals/jmb/2011/483428/