Drought is a wide-spread problem seriously influencing barley (Hordeum vulgare L.) production, but development of drought resistant cultivars is hampered by lack of effective physiological characteristic selection criteria. This study was carried out to evaluate response of gas exchange in drought resistance and drought susceptible barley cultivars under different levels of water, nitrogen and zinc supply. Two barley cultivars differing in yield performance were grown in plastic pots in semi-controlled conditions in 2009-2010. Gas exchanges characteristics including Net Photosynthesis (Pn), transpiration (E), intercellular CO2 concentration (Ci), mesophyll conductance (MC) and Photosynthetically water use efficiency (WUE PN) were measured during grain filling period. Leaf SPAD chlorophyll was determined after anthesis. Results showed that compared with Pn, E was less sensitive to drought stress. After each drought stress period and re-watering, unlike Pn, E reached the same values of those before the stress and under these conditions nitrogen fertilizer alleviated the negative influence of water stress on Pn. Nitrogen fertilization increased chlorophyll SPAD of flag leaf and affected MC especially under well watered conditions. Results showed that drought resistance in barley was obtained due to decreasing of E under drought stress.
