EFFECT OF INCREASING DOSES OF GLYPHOSATE ON WATER USE EFFICIENCY AND PHOTOSYNTHESIS IN GLYPHOSATE-RESISTANT SOYBEANS Luiz Henrique Saes Zobiole 1, 6 ; Rubem Silvério de Oliveira Junior 2 ; Carlos Moacir Bonato 3 ; Antonio Saraiva Muniz 4 ; César de Castro 5 ; Fábio Alvares de Oliveira 5 , Jamil Constantin 2 ; Adilson de Oliveira Junior 5 1 PhD student of Postgraduate Program in Agronomy at Center for Advanced Studies in Weed Research (NAPD), State University of Maringá (UEM), Colombo Av., 5790, 87020-900, Maringá, Paraná, Brazil 2 Associated Professor, Center for Advanced Studies in Weed Research (NAPD), Agronomy Department, State University of Maringá (UEM), Colombo Av., 5790, 87020-900, Maringá, Paraná, Brazil 3 Associated Professor, Biology Department, State University of Maringá (UEM), Colombo Av., 5790, 87020-900, Maringá, Paraná, Brazil 4 Associated Professor, Agronomy Department, State University of Maringá (UEM), Colombo Av., 5790, 87020-900, Maringá, Paraná, Brazil 5 Embrapa Soybean Researcher, Postcode 231, 86001-970, Londrina, Paraná, Brazil 6 National Council for Scientific and Technology Development (CNPq) scholarship lhzobiole@uol.com.br Abstract The increase cultivated area of GR soybeans across different regions of the world have raised questions not yet answered as related secondary effects of glyphosate on physiology of GR soybeans. In this work, we attempted to measure the effect of increasing glyphosate doses on water absorption and photosynthetic parameters of soybean, cv. BRS 242 GR. Plants were grown in completed nutrient solution (Hoagland & Arnon, 1950) in a greenhouse equipped with an evaporative cooling system (25–35 : 20–22 C day/night) under natural daylight. The commercially formulated isopropylamine salts of glyphosate was sprayed on foliar doses of 450, 675, 900, 1350 and 1800 g. e.a ha -1 either in single application at four leaf stage (24 DAE) and in sequential application (24 and 36 DAE) (50%-50%) at four and five leaf stages, respectively. Using an infrared gas analyzer (IRGA), fluorometer (pulse-modulated) and chlorophylometer, before and after herbicide application net photosynthesis (A), transpiration rate (E), stomatal conductance (gs), sub-stomatal CO 2 (Ci), carboxilation efficiency (A/Ci), fluorescence (Fs), maximal fluorescence (Fms); yield of quantum efficiency (Y=Fm-Fs/Fms) and chlorophyll contents were monitored. In addition, water absorption was measured daily and biomass yield and water use efficiency (WUE) were estimated by harvesting plants at R1 stage. All measures of photosynthetic parameters (A, E, gs, Ci) and Fs, Fms and Y were affected by increasing glyphosate doses. Chlorophyll contents were reduced right after glyphosate use, however with decreased in chlorophyll content and carboxilation efficiency, (A/Ci) the values suggest that glyphosate might have interfering, in some way, in the synthesis of chlorophyll and/or in carboxilative metabolism of photosynthesis (Calvin cycle), which contributed to dry biomass reductions. Total amount of water absorbed and biomass production by plants were also decreased as glyphosate doses increased, with a more intense effect of single application, as compared to sequential. WUE was also significantly reduced with increasing glyphosate doses. Under single and sequential application, soybean plants need 13 to 20% and 8 to 14% more water to produce the same amount of biomass, respectively. Keywords GR soybean; glyphosate; water; photosynthesis Introduction Increasing cropped area in the world, especially transgenic crops, and with the effects of climate change, increasing atmospheric CO 2 and surface air temperature (Houghton et al., 2001), probably an important concern in the near future will be the water requirement of crops. Knowledge of water requirements by crops and their water use efficiency (WUE) is important for assessing effects of climate changes on crop water balance and water resources. It is expected that the predicted main changes in the global climate, like the increase in CO 2 and temperature, may lead to increases in transpiration rate by plants, which could impact dramatically the input of water required for crop production (Allen et al., 2003). Many farmers have noticed that some GR soybeans are more sensitive to water stress; others have reported visual injuries in some GR soybean varieties after use of glyphosate (Zablotowicz & Reddy, 2007). Glyphosate is a wide spectrum, foliar-applied herbicide that is translocated throughout the plant to actively growing tissues where it inhibits 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) in the shikimate pathway that is, responsible for the biosynthesis of aromatic amino acids, plant defense mechanisms, and phenolic compounds (Singh et al. 1991; Hernandez et al. 1999).