Research Journal of Recent Sciences _________________________________________________ ISSN 2277-2502 Vol. 1(6), 1-6, June (2012) Res.J.Recent Sci. International Science Congress Association 1 Improving Barley Yield Grown Under Water Stress Conditions Mostafa M. Rady 1 and Maybelle S. Gaballah 2 1 Agricultural Botany Department, Faculty of Agriculture, Fayoum University, 63514-Fayoum, EGYPT 2 Water Relations and Field Irrigation Dept., National Research Centre, Cairo, 12622, EGYPT Available online at: www.isca.in (Received 30 th April 2012, revised 2 nd May 2012, accepted 12 th May 2012) Abstract A field experiment was conducted to investigate the influences of paclobutrazol (PBZ) on leaf water potential ( w ), proline content, activities of antioxidant enzymes like superoxide dismutase (SOD) and catalase (CAT), grain yield and water use efficiency (WUE) of barley ‘cv. Giza 124’ plants subjected to water stress. Plants were treated with two regimes of irrigation water, i.e., 100% of evapotranspiration (ETc) (control) and 60% of ETc and three levels of PBZ solution (0.0 (control), 20 and 40 mg l -1 ). Leaf water potential, proline content, activities of SOD and CAT, grain yield and WUE were significantly altered by both water stress and PBZ treatments. Results indicated that PBZ (40 mg l -1 ) mitigated the water stress and significantly reduced the reduction in leaf  w as compared to non-PBZ-treated water-stressed plants. Water-stressed plants treated with PBZ (40 mg l -1 ) had significant higher proline content than water-stressed plants without PBZ treatment. Higher antioxidant enzyme activity was also observed in water-stressed plants treated by PBZ than water-stressed plants without PBZ treatments. In comparison to water-stressed plants without PBZ treatment, water-stressed plants treated with PBZ (40 mg l -1 ) had significant higher SOD and CAT activities. Furthermore, water-stressed plants treated with 40 mg l -1 of PBZ had also significant higher grain yield and WUE as compared to water-stressed plants without PBZ treatment. The results suggest that PBZ application under water stress conditions alters the equilibrium between free radical production and enzymatic defense reactions in barley by enhancing the proline content and free radical scavenging capacity. Keywords: Barley, water stress, paclobutrazol, leaf water potential, proline content, superoxide dismutase, catalase, water use efficiency. Introduction Under optimal growth conditions, reactive oxygen species (ROS) including H 2 O 2 , O 2 - and OH - are continuously produced at low levels mainly in chloroplast, mitochondria and peroxisomes of plant cells. The balance between production and removal of ROS are controlled by cellular osmo-protectants 1 and antioxidant enzyme systems 2 . Whereas, under severe abiotic stress conditions, the reductive enzymatic pathways in plants may be overwhelmed and result in damage of cell components and finally death of plant 3,4 . Attempts have been made in the past to overcome the adverse effects of drought by using plant growth regulators, which have potential to mitigate the water stress. Paclobutrazol, a derivative of triazole group is now commercially used in many tropical and sub-tropical field and vegetable crops for regulation of growth, flowering and yield 5-7 . PBZ interferes with gibberellins biosynthesis by inhibiting the oxidation of ent-kaurene to ent-kauronoic acid through inactivating cytochrome P 450-dependent oxygenase. With respect to its other functions, PBZ has been used to provide plant protection against water stress 8-10 . PBZ has biochemical effects on plants, such as detoxification of active oxygen 11,12 and reduced the decrease in leaf  w 13 and increased levels of proline 14 , antioxidants 15 and chlorophyll contents 16 . Based on these studies, the present investigation was conducted to discover the influence of PBZ on drought tolerance in barley, and to determine the interactive impacts of water stress and PBZ on grain yield and WUE in addition to leaf  w , proline content, antioxidant enzyme activities and their possible role in reducing water stress in barley. Material and Methods Plant material and treatments: Barley seeds ‘cv. G 124’ were planted in 10.5 m 2 plots on the 3 rd of December 2009. The seeds were sown at a row spacing of 15 cm at 75 kg feddan -1 . All the agronomic practices, except the irrigation were applied as commonly used in growing barley crop. All plants were fertilized with the recommended N fertilizer (75 kg N feddan -1 ) as urea (46 %N) and 50 kg feddan -1 P 2 O 5 as ammonium nonphosphate. Loamy soil having pH (1:2, w/v, soil and water solution) 7.40, EC (1:2, w/v, soil and water solution) 1.23 dSm - 1 , CaCO 3 6.12% and organic matter 1.26% was used in this experiment. All other recommended cultural practices were followed. The experiment was laid out in completely randomised split design, having two irrigation water regimes (3000 m 3 feddan -1 growing season -1 ; 100% of ETc (control) and 1800 m 3 feddan -1 growing season -1 ; 60% of ETc) and three levels of PBZ 0.0 (control), 20 and 40 mg l -1 . Irrigation treatments were isolated with 2 m fallow land to avoid the lateral movement of water