PHOTOSYNTHETICA 43 (2): 313-316, 2005 313 BRIEF COMMUNICATION Effect of reddening of cotton (Gossypium hirsutum L.) leaves on the ultrastructure of mesophyll cells D. STOYANOVA-KOLEVA * , A. EDREVA ** , V. VELIKOVA *** , and A. G REL + Department of Botany, Faculty of Biology, Sofia University “St. Kl. Ohridski”, Bd. D. Tzankov 8, 1164 Sofia, Bulgaria * D. Kostoff Institute of Genetics, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria ** Acad. M. Popov Institute of Plant Physiology, Bulgarian Academy of Sciences, Acad. G. Bonchev Street, Bl. 21, 1113 Sofia, Bulgaria *** Department of Bioengineering, Faculty of Engineering, Aegean University, 35100 Bornova-Izmir, Turkey + Abstract The ultrastructure of cotton leaves, exhibiting reddening as symptom of physiological disorder, was examined by means of transmission electron microscopy. Osmiophilisation of the membrane compartment was established. Massive agglom- erations on the tonoplast in the vacuole of cells under the adaxial epidermis were observed, and were referred to as elec- tron-dense osmiophilic substance, most probably of anthocyanin nature. In chloroplast stroma a zone of low electron density enclosing numerous osmiophilic aggregations of unclear chemical character was differentiated. Fragmentation and severe destruction of thylakoids in chloroplasts of reddening cotton leaves was not detected. Additional key words: chloroplast; osmophilic aggregation; peristromium; peroxisome; starch; transmission electron microscopy. —— “Red leaf disease” is a physiological disorder in cotton observed in some regions of Sudan, India (Dastur et al. 1960, Dhopte 1990), and in the last ten years in the Aegean region of Turkey, causing economic losses due to reduced yield and damaged fibre quality. The pheno- menon is provoked by abiotic stress related to disturbed soil ion equilibrium leading to K shortage and accumu- lation of Na ions in cotton leaves (Yagmur et al. 2003). As an important crop, cotton is largely studied in respect to abiotic stress constraints, such as nutrient deficiency, salinity, UV-B radiation, high temperature, etc. (Nepomuceno et al. 1998, Zhao et al. 2001, Kakani et al. 2003, Meloni et al. 2003). Scarce data, however, are avai- lable on cotton reddening and the underlying biochemi- cal, physiological, and ultrastructural mechanisms. In a previous work we reported a dramatic accumulation of anthocyanins, drop of chlorophyll content, and raise of peroxidase activity and proline content in red cotton leaves. The malonyldialdehyde content, however, was not increased, which points to a conserved membrane inte- grity. The above data suggest a stress situation in which anthocyanins may play a protective role (Edreva et al. 2002) as shown in other plants subjected to stress (Chalker-Scott 1999). A survey on functional activity of photosynthetic apparatus revealed that in red cotton leaves photosynthesis was lowered due to both decreased photochemical activity and stomatal limitations (Velikova et al. 2002). The purpose of the present paper was to study the ul- trastructure of mesophyll cells of red cotton leaves. A special emphasis was laid on the chloroplasts as structural counterparts of the functional changes occurring in the photosynthetic apparatus upon cotton reddening. Cotton (cv. Nazilli 84) plants were grown in Izmir district of Aegean region. Upper (25-d-old) leaves of plants at flowering-boll formation stage were used. Samples were taken in the middle of September when leaf reddening was fully expressed. Two stages of red- dening were assayed: light symptoms (small, slightly-red coloured spots) and severe symptoms (large, deeply red coloured spots). Green upper leaves of plants without symptoms of reddening were used as controls. ——— Received 17 June 2004, accepted 6 August 2004. * Fax: +359 2 65 66 41, e-mail: stoyanova@biofac.uni-sofia.bg Acknowledgements: We thank Tübitak and Izmir Itas-Technopark (Turkey) as well as the Bulgarian Academy of Sciences for financial support of a Joint research and development project (2000-2002).