Agricultural Sciences in China 2010, 9(5): 671-680 May 2010 © 2010, CAAS. All rights reserved. Published by Elsevier Ltd. doi:10.1016/S1671-2927(09)60142-5 Nitrogen in Relation to Photosynthetic Capacity and Accumulation of Osmoprotectant and Nutrients in Brassica Genotypes Grown Under Salt Stress Manzer H Siddiqui 1, 2 , Firoz Mohammad 2 , M Nasir Khan 2 , Mohamed H Al-Whaibi 1 and Ali H A Bahkali 1 1 Department of Botany & Microbiology, College of Science, King Saud University, Riyadh-11451, Saudi Arabia 2 Plant Physiology Section, Department of Botany, Aligarh Muslim University, Aligarh-202002, India Abstract Different strategies of the application of nutrients are required to overcome the adverse effects of mustard (Brassica juncea L.) in response to NaCl stress. The objective of the present study was to determine if different added levels of nitrogen (N) in growth medium could alleviate the adverse effects of salt stress on photosynthetic capacity and accumulation of osmoprotectants and nutrients. 14 days mustard seedlings of salt-sensitive (cv. Chuutki) and salt-tolerant (cv. Radha) genotypes were fed with: (i) 0 mmol L -1 NaCl + 0 mg N kg -1 sand (control), (ii) 90 mmol L -1 NaCl + 30 mg N kg -1 sand, (iii) 90 mmol L -1 NaCl + 60 mg N kg -1 sand, (iv) 90 mmol L -1 NaCl + 90 mg N kg -1 sand and (v) 90 mmol L -1 NaCl + 120 mg N kg -1 sand. Under the condition of salinity stress, N application caused a significant ameliorative effect on both genotypes with respect to growth attributes [fresh weight (FW) and dry weight (DW)] and physio-biochemical parameters [percent water content (WC), net photosynthetic rate (P N ), stomatal conductance (g s ), total chlorophyll (Chl), carbonic anhydrase (CA) activity and malondialdehyde (MDA), nitrogen (N), potassium (K) and sodium (Na) contents, and K/Na ratio] and yield attributes (number of pods/plant, seeds/pod and seed yield/plant). The salt-tolerant genotype exhibited maximum value for growth, physio-biochemical and yield attributes at 60 mg N kg -1 sand than that of salt-sensitive genotype. These results suggest that application of N may ameliorate most of the attributes and prove to be a physiological remedy to increase the tolerance against the ill effects of salt stress in Brassicas. Key words: Brassica juncea L., carbonic anhydrase activity, malondialdehyde, net photosynthetic rate, osmoprotectants, salt stress INTRODUCTION Salinity stress is a major environmental factor that drastically affects the crop productivity throughout the world. It is a menace to both agriculture and the soil body. Progressive salinization of arid and semi- arid regions of the world is expected to have devas- tating global effects, resulting in 30% land loss within the next 25 years, and up to 50% by the year 2050 (Wang et al . 2003). It is evident that 20% of all cultivated land and nearly half of irrigated land is salt- affected, greatly reducing yield well below the ge- netic potential (Ghassemi et al . 1995; Munns and James 2003; Flowers 2004). Soil salinization is be- coming a more acute problem for sustainable devel- opment of agricultural production, primarily because of declining irrigation systems and type of irrigated water (Ghassemi et al . 1995). Saline environment is generally deficient in nitro- gen (Amonkar and Karmarkar 1995) and reduction in NO 3 - uptake could be mostly due to high Cl - content Received 4 October, 2009 Accepted 10 February, 2010 Correspondence Manzer H Siddiqui, E-mail: manzerhs@yahoo.co.in