Various physiological damages occur in plants upon exposure to heat stress (Hasanuzzaman et al. 2013). High temperatures (heat stress, HS) are known to inhibit enzyme activity, net photosynthetic rate (P N ), and other metabolic processes leading to physiological and biochemical alterations (Salvucci and Crafts-Brander 2004, Hassan, 2006, Zhao et al. 2008, ICPP 2014. HS uncouples enzymes and metabolic pathways leading to accumulation of harmful reactive oxygen species (ROS) which are responsible for oxidative stress (Sarieva et al. 2010). The reaction centers of PSI and PSII in chloroplasts are major sites of ROS generation (Chakraborty et al. 2011). Stomatal closure, reduction in root growth, and consequent inhibition of mineral uptake are common responses to HS (Huang and Xu 2000).Nitrogen as an essential element (Sánchez et al. 2004, Cechin and De Terezinha 2004) promotes metabolic activities, such as photosynthesis, antioxidant activities, leading to enhancing in growth and yield (Zhao et al. 2008).Due to their different physiological effects, HS and N fertilization might have interactive impacts on plants. HS reduces the rate of nutrient uptake; however, this effect could be mitigated by excessive supply of N (Altenbach et al. 2003, Zhao et al. 2008, Demmig-Adams et al. 2014. Moreover, Zhang et al. (2014) reported that N nutrition control is an important way to enhance tolerance to abiotic stress. Thus, it is important to understand how the supply of N may mitigate HS.The aim of the present experiment was to study the effects of N-fertilization on alleviation of the negative impact of HS on Chl fluorescence, P N , and stomatal conductance (g s ) of French bean plants.Seeds of French bean (Phaseolus vulgaris L.) were obtained from the Department of Agronomy, Alexandria University, Egypt. They were sown in plastic pots (20 cm 2 ), five seeds in each pot with multipurpose compost in a heated glasshouse [temperature of 24/20ºC day/night, air humidity 60-65%, 16-h photoperiod with additional irradiance of 450 μmol(photon) m -2 s -1 , total irradiance was 975 μmol(photon) m -2 s -1 ]. After appearance of expanded trifoliate leaf, 12 days after sowing (DAS), plants were thinned to one seedling per pot. Pots were then transferred to eight closed climate controlled growth chambers. Two chambers were used for N fertilization as foliar spray (three concentrations of urea: 5, 10, 15 mM). Two other

The present study evaluated the physiological and biochemical mechanisms through which exogenous salicylic acid (SA) mitigates cadmium (Cd) stress in maize. The different concentrations of Cd included 0, 5, 10 µM CdCl 2 .3H 2 O. Half of plants received salicylic acid as a soil drench. Results showed that Cd exposure reduced chlorophyll contents, chlorophyll fluorescence, photosynthetic rate and activity of catalase (CAT). However, Cd stress enhanced nonphotochemical quenching (NPQ), the production of malondialdehyde (MDA) and activities of key antioxidant enzymes superoxide dismutase (SOD), ascorbate peroxidase (APX), guaiacol peroxidase (POD). The increased activities of antioxidant enzymes and concentration of MDA were reduced by exogenous application of SA as a soil drench. Our research proves that SA, as a novel endogenous plant hormone exerts beneficial physiological effects on plants as it has a dual role in scavenging active oxygen species (AOS) and modulating redox balance to enhance photosynthetic performance in cadmium-stressed maize plants. It was concluded that SA improved plant tolerance under Cd contamination and may have alleviated the inhibitory effect of Cd on photosynthesis by protecting maize plants against oxidative damage as indicated by the marked decline in net photosynthetic rates, quantum yield and chlorophyll content as well as alteration of antioxidants activity and accumulation of MDA. Collectively, results of the present study provide an insight into the alleviative role of SA in Cd-stressed maize and propose SA as a potential candidate in mitigating Cd toxicity.