Vol. 19, No. 4 YADAV et al 323 Journal of Agrometeorology 19 (4) : 323-327 (December 2017) Solar radiation is the primary energy source for plant life which determines the crop growth and development. It plays an important role in many plant physiological and bio- chemical processes like chlorophyll synthesis, enzyme activation, photosynthesis that regulating the growth and development of plants. Apart from global warming, global dimming has received prominent attention because it causes climatic and environmental implications. As a consequence of increase in aerosol content, cloud cover and air pollutants, light dimming or reduction in global radiation have become major challenges for crop production (Mu et al ., 2010). Long-term observations of the surface global solar radiation in recent decades have shown a widespread declining trend in many parts of the world (Liepert, 2002). Stanhill and Cohen (2001) reported averaged globally reduction of 0.51 W m -2 , or 2.7 per cent decade, since last 50 years. Liepert (2002) also observed 19 Wm -2 or 10 per cent decline in surface solar radiation in the United States from 1961 to 1990. India also shows a continuous and steady light dimming since 1960s (Ramanathan et al ., 2005; Mukherjee et al. , 2014 and 2015). Plant growth and development is adversely affected by reduced light intensity due to shading. Radiation interception, temperature, humidity and wind profile under reduced light condition are determining factor for wheat growth and yield and also the way through which these micrometeorological parameters alters the crop phenology, leaf area index, biomass production, RUE and grain yield (Healey et al. , 1998). Keeping in view the importance of radiation dimming, the present study was undertaken to quantify the effect of global radiation dimming, on morphology and yield of wheat crop in Delhi, NCR region of India. MATERIALS AND METHODS A field experiment was conducted at research farm of Indian Agricultural Research Institute (IARI), New Delhi (28°37' N latitude, 77°09' E longitude and altitude of 228.16 m above mean sea level) during two consecutive rabi season (2014-15 and 2015-16). The soil of experimental site was sandy loam having pH 7.3, organic carbon 0.47 per cent, available nitrogen 170.6 kg ha -1 , available phosphorus 18.6 kg ha -1 and available potassium 275.0 kg ha -1 . Three wheat varieties of wheat (HD 2967, WR 544 and PBW 502) were sown on 25 th November and 21 th November in 2014-15 and 2015-16, respectively. A dose of 120:60:60 kg ha -1 of N:P:K was applied (three split dose of N as basal, CRI and flowering). Five solar radiation reduction treatments i.e. R1 (control), R2 (20% shading), R3 (35% shading), R4 (50% shading) and R5 (75% shading) were applied from tillering stage to maturity. Shading nets were installed on 13 th January and 10 th January for the year 2014-15 and 2015-16, respectively after tillering stage. Effect of dimming of global radiation on morphology and yield of wheat crop in Delhi BRIJESH YADAV, JOYDEEP MUKHERJEE, V.K. SEHGAL, D.K. DAS and PRAMEELA KRISHNAN Division of Agricultural Physics, ICAR- Indian Agricultural Research Institute, New Delhi 110012, India email: mjoydeep2k@yahoo.com ABSTRACT Long-term observations of the surface global solar radiation in recent decades have shown a widespread declining trend in many parts of the world. Experient was carried out at the farm of ICAR- Indian Agricultural Research Institute, New Delhi during rabi season of 2014-15 and 2015-16 to study response of three cultivars (HD 2967, WR 544 and PBW 502) of wheat under five reduced solar radiation levels (0, 20, 35, 50, 75% shading). Plant height, internode distance and leaf area index (LAI) were measured at different DAS throughout the crop growth period. The result shows that plant height and internode distance increased under 75% shading treatment. The highest leaf area index (LAI) was observed in 35% shading treatment, followed by 20% shading and control. Final biomass, grain yield and harvest index decreased significantly with shading and was lowest in 75% shading treatment. Among the cultivars, HD 2967 performed better under reduced radiation condition as compared to other cultivars. Key words: Wheat, radiation, shading, yield, internode, harvest index