Environmental and Experimental Botany 52 (2004) 175–184 Effect of UV-B and monocrotophos, singly and in combination, on photosynthetic activity and growth of non-heterocystous cyanobacterium Plectonema boryanum Sheo Mohan Prasad ∗ , Mohd Zeeshan Ranjan Physiology and Biochemistry Laboratory, Department of Botany, University of Allahabad, Allahabad 211002, India Accepted 30 January 2004 Abstract The impact of artificial ultraviolet-B (UV-B 280–315 nm) irradiation and monocrotophos (an organophosphorus insecticide), singly and in combination, on survival, growth and photosynthetic activities and energy transfer within the pigments has been studied in a non-heterocystous cyanobacterium Plectonema boryanum. Survival and growth of the cyanobacterium decreased with selected dose of UV-B (30 min and 90 min) and monocrotophos (5 g ml -1 ). The various doses of UV-B (30, 60, 90 and 120 min exposures) damaged the photosynthetic pigments, phycocyanin, carotenoids and chlorophyll a (Chl a); phycocyanin, however, appeared to be the main target of UV-B. This was further proved by the prominent decrease in phycocyanin absorption peak in intact cells. 14 C-fixation was found to be more sensitive to UV-B and monocrotophos, singly and in combination than photosynthetic oxygen evolution. Photosynthetic electron transport activity decreased in response to UV-B and monocrotophos individually as well as in their combined dose (UV-B + monocrotophos). This inhibition was due to a greater susceptibility of PSII than PSI. UV-B and monocrotophos either alone or in combination induced inhibition of PSII activity and its partial restoration by artificial electron donors (DPC, NH 2 OH and MnCl 2 ) suggests that these stresses blocked the electron flow at water oxidation side. However, less restoration in combination of these stresses and 90 min UV-B exposure alone indicated that the PSII reaction centre was also affected. Suppression in emission peak ( 650 nm) of phycocyanin revealed that both the stresses, either singly or in combination, altered the energy transfer efficiency of phycocyanin to PSII. The present findings suggest that the doses of UV-B radiation and monocrotophos singly and in combination, have adversely affected the photosynthetic activity of P. boryanum, lead- ing to the significant decrease in survival and growth. The interactive effect of both the stresses was, however, less than additive. © 2004 Elsevier B.V. All rights reserved. Keywords: Plectonema boryanum; UV-B irradiation; Monocrotophos; Growth; Phycobilisome; Photosynthesis; 14 C-fixation 1. Introduction Recently, the increase in solar ultraviolet-B (UV-B; 280–315 nm) radiation impinging the earth has gained ∗ Corresponding author. Tel.: +91-0532-2462048. E-mail address: zee bot auind@rediffmail.com (S.M. Prasad). an importance world over. This increase is due to an accelerating depletion of the stratospheric ozone shield which is caused by anthropogenically released atmospheric pollutants such as chlorofluorocarbons, chlorocarbons and organobromides (Crutzen, 1992; Lubin and Jensen, 1995). Several studies have demon- strated the impact of enhanced UV-B radiation on 0098-8472/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.envexpbot.2004.01.013