ORIGINAL RESEARCH Modulation of endogenous levels of some key organic metabolites by exogenous application of glycine betaine in drought stressed plants of sunflower (Helianthus annuus L.) Naeem Iqbal • Yasin Ashraf • Muhammad Ashraf Received: 4 January 2010 / Accepted: 22 July 2010 / Published online: 29 August 2010 Ó Springer Science+Business Media B.V. 2010 Abstract The present study was conducted to examine the changes in some key metabolites in drought-stressed sunflower plants supplied with glycine betaine externally. Imposition of drought stress at the vegetative or repro- ductive growth stages decreased the plant dry matter pro- duction and increased the accumulation of organic solutes (glycine betaine, proline, soluble proteins, free amino acids and soluble sugars) in two sunflower lines, i.e., Glushan-98 and Suncross. In general, decrease in dry matter production and increase in the endogenous levels of organic solutes, were more pronounced when drought stress applied at the vegetative stage than at the reproductive stage. Glycine betaine applied as a pre-sowing seed treatment was not found to be effective in reducing the negative effects of drought stress in sunflower plants. Foliar application of GB further enhanced the leaf endogenous levels of GB, soluble proteins and total soluble sugars in drought stressed plants without exerting any negative effects on other osmotica. However, this GB-induced increase in endogenous levels of organic solutes was found to be not associated with plant dry matter production under stress conditions. Keywords Stress tolerance Á Glycine betaine Á Sunflower Introduction Plants undergo a myriad of morphological and physiolog- ical alterations to acclimatize themselves to various envi- ronmental adversaries. One such mechanism, more promising and prevalent in plants, is the accumulation of certain organic metabolites of low molecular weight, col- lectively known as compatible solutes. These include sugars, polyols, amino acids, proline, and quaternary ammonium compounds (Ashraf and Foolad 2007). Glycine betaine (GB), an important quaternary ammo- nium compound, is considered to be one of the most pre- dominant and effective osmoprotectants. It is well established that its exogenous application might have some advantages as it improves drought tolerance in plants (Agboma et al. 1997; Gorham et al. 2000; Iqbal et al. 2008; Mahmood et al. 2009). It has been also reported earlier that rate and timing of GB application significantly affects drought tolerance ability of sunflower (Iqbal et al. 2008, 2009). This shot-gun approach of exogenous application of GB has been used to increase the drought tolerance in some other crop species (Agboma et al. 1997; Makela et al. 1998; Gorham et al. 2000). Sunflower (Helianthus annuus L.) is an important oil- seed crop grown in different parts of the world. It is mostly cultivated in arid and semi-arid regions of the world. Like other crops, water stress has also a marked adverse effect on physiological and yield attributes of sunflower (Iqbal et al. N. Iqbal Department of Botany, GC University, Faisalabad, Pakistan e-mail: naeemgc@yahoo.com Y. Ashraf Nuclear Institute for Agriculture & Biology (NIAB), Faisalabad, Pakistan e-mail: niabmyashraf@gmail.com M. Ashraf (&) Department of Botany, University of Agriculture, Faisalabad, Pakistan e-mail: ashrafbot@yahoo.com M. Ashraf King Saud University, Riyadh, Saudi Arabia 123 Plant Growth Regul (2011) 63:7–12 DOI 10.1007/s10725-010-9506-5