BIOLOGIA PLANTARUM 51 (1): 104-109, 2007 104 Expression of dehydrins under heat stress and their relationship with water relations of sugarcane leaves A. WAHID* and T.J. CLOSE** Department of Botany, University of Agriculture, Faisalabad-38040, Pakistan* Department of Botany and Plant Sciences, University of California, Riverside, CA-92521-0124, USA** Abstract The heat stress-induced dehydrin proteins (DHNs) expression and their relationship with the water relations of sugarcane (Saccharum officinarum L.) leaves were studied. Sugarcane seedlings were subjected to heat stress (day/night temperature of 40/35 °C) under relative humidity 60/65 % to avoid aerial desiccation and determinations made at 4, 12, 24, 36, 48, 60 and 72 h. The leaves showed a sharp decline in the water and osmotic potentials, and relative water content during first 12 h of heat stress, but a regain in their values in 24 h. The pressure potential (ψ p ) decreased initially but increased later and approached control leaves. The increase in ψ p was tightly correlated to the accumulation of free proline, glycinebetaine and soluble sugars, indicating their possible involvement in the osmotic adjustment under heat stress. Immunological detection revealed the expression of three DHNs with an apparent molecular mass of 21, 23 and 27 kDa under heat stress (48 to 72 h) and their expression was independent of the changes in the water relations of leaves. Additional key words: compatible osmotica, heat shock proteins, osmotic adjustment, relative humidity, water potential. Introduction Heat stress is often defined as a rise in temperature beyond threshold level for a time sufficient to cause irreversible damage to plant development. It is simply a transient elevation in temperature usually about 10 to 15 °C above ambient (Buchanan et al. 2000). With a rise in temperature, there is an increase in metabolic activities leading to cellular protection which increases demand for resources (Rawson 1988, Shah and Paulsen 2003). Since leaf photosynthesis is directly impinged upon by heat stress (Camejo et al. 2005), a decline in this process limits the supply of photosynthates to keep pace with normal growth (Ebrahim et al. 1998, Karim et al. 2000). Resultantly, leaf number and its expansion rate, tillering, spike development and grain filling are markedly reduced (Fischer 1985, Fokar et al. 1998, Wahid and Shabbir 2005). Elevated temperature results in rapid loss of water which may cause dehydration. It interacts with soil drought to affect cell and tissue water content and all components of water potential (Machado and Paulsen 2001). Barley seedlings subjected to heat stress initially indicated a severe depression in leaf water potential, which recovered slowly with the time (Wahid and Shabbir 2005). Sudden, not the gradual, heat stress greatly hampered the leaf relative water content and pressure potential. Plants gradually exposed to heat stress indicated an accumulation of several heat stable proteins, which appeared to be associated with heat tolerance in strawberry (Gulen and Eris 2003). Likewise, preconditioned tomato plants with heat indicated a better osmotic adjustment and maintained leaf water potential than non-preconditioned ones (Morales et al. 2003). Most common molecular response of plants submitted to heat stress is the expression of heat shock proteins (HSPs), which have a fairly wide range of molecular masses (10 - 250 kDa). They associate to various cellular structures or organelles to provide protection and act as molecular chaperones (Schoffl et al. 1999, Sanmiya et al. 2004). The dehydrin proteins (DHNs) are among other proteins that are classified as a group of late embryogenesis abundant (LEA) proteins, referred to as LEA group II, and typically accumulate late in embryo- ⎯⎯⎯⎯ Received 19 May 2005, accepted 7 January 2006. Abbreviations: DHNs - dehydrin proteins; FP - free proline; GB - glycinebetaine; RWC - relative water content; SS - water soluble sugars; ψ p - pressure potential; ψ s - osmotic potential; ψ w - water potential. Acknowledgements: First author acknowledges the Ministry of Science and Technology, Government of Pakistan for postdoctoral award, Terrance Donovan for providing the sugarcane material and R.D. Fenton for excellent help in the laboratory work. * Corresponding author; fax: (+92) 41 9200764, e-mail: drawahid2001@yahoo.com