JOURNAL OF PURE AND APPLIED MICROBIOLOGY, Nov. 2013. Vol. 7(Spl. Edn.), p. 139-148 * To whom all correspondence should be addressed. Tel.: +966563772132; Fax: +966114675833; E-mail: hayssam77@hotmail.com Nitric Oxide and Salicylic Acid Mitigate Cadmium Stress in Wheat Seedlings Mohammed O. Basalah, Hayssam M. Ali*, Mohamed H. Al-Whaibi, Manzer H. Siddiqui, Ahmed M. Sakran and Abdulaziz A. Al Sahli Botany and Microbiology Department, College of Science, King Saud University, P.O. Box 2455, Riyadh 11451, Saudi Arabia. (Received: 20 July 2013; accepted: 15 September 2013) Nitric oxide (NO) and salicylic acid (SA) are well known as signaling and ubiquitous bioactive molecules that play a key role in tolerance of plant to abiotic stress by regulating the various plant cellular mechanisms. Therefore, the present experiment was conducted to study the interactive effect of NO and/or SA in tolerance of wheat (Triticum aestivum L. cv. Samma) under cadmium (Cd) stress. The results indicated that plants grown in Cd-containing medium exhibited reduced growth characteristics ( plant height, fresh weight, dry weight and leaf area), carbonic anhydrase activity and content of essential nutrients [nitrogen (N), phosphorus (P) and potassium (K)] nutrients and chlorophyll (Chl) a and b by enhancing MDA and Cd accumulation. However, combined application of NO and SA was more effective in suppression of deleterious effect of Cd stress by inducing the activities of enzymes [carbonic anhydrase (CA), superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT)] and accumulation of osmoprotectant (Pro) and by maintaining nutrients homeostasis, leading to the reduction of MDA accumulation and improvement of photosynthetic pigments (Chl a and Chl b) resulted in a better plant growth performance under Cd stress. The present study revealed that application of both NO and SA together improved the plant growth and development by reducing formation of reactive oxygen species by improving antioxidant enzymes and CA activity and balance supply of nutrients. Key words: Cadmium stress, Nitric Oxide, Salicylic acid, Photosynthetic pigments, Essential nutrients, Carbonic anhydrase, Antioxidant, Triticum aestivum. A few studies deal with role of nitric oxide (NO) and salicylic acid (SA) in plants under heavy metal stress. Cadmium (Cd) is one of the most toxic heavy metals for animal, human and plants by entering the environment may be though phosphate fertilizers and waste disposal (Yilmaz et al., 2006; Mahmood et al., 2009). It is clear that cadmium (Cd) is a strong environmental pollutant with high toxicity to animal and plant (di Toppi and Gabbrielli, 1999). Although, the replace with the mechanisms of cytoplasmic toxicity are identical in all organisms, different plant species and verities show a wide range of plasticity in cadmium tolerance (Methwally et al., 2003). Salin, (1988) stated that Ca cannot generate reactive oxygen species (ROS) via Haber-Weiss or Fenton type reactions. Cd toxicity results in the alteration of oxidant level in plant, including the generation of toxic reactive oxygen species (ROS) like hydrogen peroxide (H 2 O 2 ), hydroxyl radical (OH - ), super oxide radical (O 2 - ) etc. (Choudhury and Sanjib, 2004). Plants have various defense mechanisms by which they can scavenge these ROS. Cd enhances the activity of some antioxidant enzymes like catalase, peroxidase, super oxide dismutase and glutathione