1219 AJCS 7(9):1219-1226 (2013) ISSN:1835-2707 Exogenous abscisic acid(ABA) and silicon (Si) promote salinity tolerance by reducing sodium (Na + ) transport and bypass flow in rice (Oryza sativa indica) Ali Raza Gurmani 1* , Asghari Bano 2 , Najeeb Ullah 3 , Hakim Khan 1 , Muhammad Jahangir 1 and Timothy J. Flowers 4 1 Department of Agriculture, University of Haripur, Khyber Pakhtunkhwa, Pakistan 2 Department of Plant Sciences, Quaid-i-Azam University, Islamabad Pakistan 3 Department of Plant and Food Sciences, Faculty of Agriculture and Environment, The University of Sydney, NSW 2006, Australia 4 School of Life Sciences, University of Sussex, Falmer, Brighton BN1 9QG, United Kingdom * Corresponding author: argurmani@uoh.edu.pk Abstract Excessive sodium (Na + ) and chloride (Cl - ) concentrations in rice shoots reduce seedlings growth. In a hydroponic study, we investigated the role of abscisic acid (ABA) seed pre-treatment on growth of salt-stressed rice, in the presence and/or absence of silicate (Si). Caryopses of rice IR4630 (salt tolerant) and IR15324 (salt sensitive) were soaked in an ABA solution (10 -5 M) for 24 h, and the plants were then allowed to grow in a medium without or with silicate (Si, 0 or 3 mM). Twenty-one-day-old plants were salinised with NaCl (50 mM).We found the application ABA+Si the most effective treatment for improving plant growth under saline conditions. Si treatment alone or with ABA significantly increased the plant total dry biomass under saline or non-saline conditions, whereas, the effect of ABA seed pre-treatment was significant only on total dry biomass of salt sensitive genotype IR15324. Under saline conditions, ABA, Si and ABA+Si application ameliorated plant growth via suppression of Na + accumulation in shoots and lowering down Na + /K + ratios in both the rice genotypes. In addition, treatment with Si alone or with ABA significantly reduced Na + concentrations in the leaf blades and sheaths, increased net assimilation rate and stomatal conductance of salt affected rice seedlings. We concluded that silicon can be applied as a nutrient for rice under saline or non-saline conditions. Furthermore, seed soaking with ABA has a potential for enhancing salt tolerance, particularly in the sensitive rice genotypes. Keywords: Oryza sativa L., ABA, Silicate, salt tolerance, bypass flow. Abbreviations: ABA – Abscisic acid, Si – Silicate, PTS -trisodium salt of 8-hydroxy-1, 3, 6-pyrenetrisulphonic acid, NAR – Net Assimilation Rate, WUE – Water Use Efficiency, SDW – shoot dry weight, RDW – Root Dry Weight. Introduction Salinity is one of the major factors limiting crop growth and productivity in irrigated areas of the world (Zhang et al., 2010). Plant species exhibit great variability in salinity tolerance, with some having the capacity to grow in sea water, whereas most crop plants are intolerant of just one fifth of such a concentration. Since tolerance to salt in plants is a complex trait, conventional breeding techniques have had limited success in improving this trait in crops (Flowers, 2004). Salt tolerance in plants not only depends on controlling Na + uptake and localisation but also on the acquisition of K + , which is commonly suppressed by high external Na + concentrations (Amtman et al., 2006; Zhang et al., 2010). Because of the importance of K + and Na + homeostasis in plant nutrition, K + and Na + transporters have been extensively investigated (Rodriguez and Rubio, 2006; Maathuis, 2007). However, limited attention had been paid to the factors influencing net ionic transport into shoots and the impact of transpiration. Abscisic acid (ABA) regulates various aspects of plant growth and development, such as seed maturation, dormancy and adaptation to abiotic stresses (Beaudoin et al., 2000; Sreenivasulu et al., 2012). A positive role of ABA on plant growth and induction of salt tolerance through improving ionic relations in plant tissues (Holbrook et al., 2002) has been suggested in different crops (Din and Flowers, 2002; Travaglia et al., 2010; Gurmani et al., 2011). ABA induces salt and drought tolerance through regulating stomatal behaviour, leaf initiation and leaf expansion (Halbrook et al., 2002; Sharp and LeNoble, 2002). Bohra et al., (1995) reported a strong association between ABA application and inhibition of Na + accumulation in rice tissues. Silicon (Si) is an essential nutrient for the majority of plant species (Epstein, 1999; Ma and Yamaji 2008). Increases in its uptake and accumulation have been linked with improved resistance to biotic (Rodrigue et al., 2003) and abiotic stresses (Hodson and Sangster, 2002, Tahir et al., 2010). For example, deposition of Si in the exodermis and endodermis reduces Na + uptake through apoplastic transport, and consequently improves salt tolerance in rice (Gong et al., 2006; Faiyue et al., 2010b). In the experiments we report here, we have investigated treatments that might enhance the salt tolerance of rice, since this crop is so sensitive to salt and its tolerance has been hard to change (see Singh and Flowers 2011). From preliminary studies (Din, 1997), we noticed a remarkable enhancement of salt tolerance in rice by short term ABA treatment. Here we have investigated how seed pre-treatment of rice with ABA could improve its subsequent salt tolerance by investigating physiological processes that affect Na + uptake and accumulation. Since a significant