Physiologia Plantarum 145: 474–484. 2012 Copyright © Physiologia Plantarum 2012, ISSN 0031-9317 Differential response of radish plants to supplemental ultraviolet-B radiation under varying NPK levels: chlorophyll fluorescence, gas exchange and antioxidants Suruchi Singh, Rima Kumari, Madhoolika Agrawal ∗ and Shashi Bhushan Agrawal Laboratory of Air Pollution and Global Climate Change, Department of Botany, Banaras Hindu University,Varanasi-221005, Uttar Pradesh, India Correspondence *Corresponding author, e-mail: madhoo58@yahoo.com Received 14 November 2011 doi:10.1111/j.1399-3054.2012.01589.x Current and projected increases in ultraviolet-B (UV-B; 280–315 nm) radi- ation may alter crop growth and yield by modifying the physiological and biochemical functions. This study was conducted to assess the possibility of alleviating the negative effects of supplemental UV-B (sUV-B; 7.2 kJ m −2 day −1 ; 280–315 nm) on radish (Raphanus sativus var Pusa Himani) by mod- ifying soil nitrogen (N), phosphorus (P) and potassium (K) levels. The N, P and K treatments were recommended dose of N, P and K, 1.5 times recom- mended dose of N, P and K, 1.5 times recommended dose of N and 1.5 times recommended dose of K. Plants showed variations in their response to UV- B radiation under varying soil NPK levels. The minimum damaging effects of sUV-B on photosynthesis rate and stomatal conductance coupled with minimum reduction in chlorophyll content were recorded for plants grown at recommended dose of NPK. Flavonoids increased under sUV-B except in plants grown at 1.5 times recommended dose of N. Lipid peroxidation (LPO) also increased in response to sUV-B at all NPK levels with maximum at 1.5 times recommended dose of K and minimum at recommended dose of NPK. This study revealed that sUV-B radiation negatively affected the radish plants by reducing the photosynthetic efficiency and increasing LPO. The plants grown at 1.5 times recommended dose of NPK/N/K could not enhance antioxidative potential to the extent as recorded at recommended dose of NPK and hence showed more sensitivity to sUV-B. Introduction Increase in solar ultraviolet-B (UV-B; 280–315 nm) has raised concerns due to potential damaging impact on crop plants (Kakani et al. 2003). The three-dimensional chemistry-climate models estimates indicate that ground level UV-B radiation is currently near its maximum level (Surabhi et al. 2009). The amount of UV-B radiation reaching tropical latitudes is higher than temperate lati- tudes (Surabhi et al. 2009). Abbreviations – AN, photosynthetic rate; APX, ascorbate peroxidase; DAG, days after germination; F m , maximum fluorescence; F o , initial fluorescence; F v , variable fluorescence; g s , stomatal conductance; LPO, lipid peroxidation; MDA, malondialdehyde; O • – 2 , superoxide anion; • OH, hydroxyl radical; PAL, phenylalanine ammonia lyase; POX, peroxidase; PS II, photosystem II; ROS, reactive oxygen species; SOD, superoxide dismutase; sUV-B, supplemental UV-B (280–315 nm); TCA, trichloroacetic acid; UV-B, ultraviolet-B (280–315 nm); UV-B BE , biologically effective ultraviolet-B; WUE, water use efficiency. Higher photosynthetic plants need sunlight and being sessile are inevitably exposed to UV-B radiation. Even a small increase in incident UV-B radiation can have significant biological effects because it is read- ily absorbed by a number of important macromolecules such as nucleic acids, proteins, lipids and phytohor- mones (Rozema et al. 1997). UV-B radiation can impair all major processes of photosynthesis including photo- chemical reaction in thylakoid membrane, enzymatic processes in Calvin cycle and stomatal limitations to 474 Physiol. Plant. 145, 2012