A split-split-plot design with three replications in two years of 2009 and 2010 was conducted to investigate the effect of different levels of irrigation water (main plot), salinity of irrigation water (sub-plot) and nitrogen fertilizer rate (sub-subplot) on maize growth rate and gas exchange. Irrigation treatments were I-1 (1.0 crop evapotranspiration (ETc)+ 0.25ET(c) as leaching), I-2 (0.75I(1)) and I-3 (0.5I(1)) applied at 7-day intervals. The salinity treatments of irrigation were 0.6 (fresh water), 2.0 and 4.0 dS m(-1). There were also three nitrogen (N) treatments including 0, 150 and 300 kg N ha(-1). Results showed that vegetative growth stage of maize in salinity stress lasted 5% more than that in water stress. The most sensitive trait under water, salinity and nitrogen stress was grain yield (GY). The optimum treatment for maize production is full fresh water application by 150 kg N ha(-1). Results also showed that crop growth rate (CGR) was statistically higher in I-1 and I-2 as 58 and 34% relative to I-3 treatment, respectively. Furthermore, CGR was statistically lower in S-2 and S-3 as 10 and 18% relative to S-1, respectively. Besides, N application significantly increased CGR by an average of 15% as compared with no N rate. The net assimilation rate (NAR) reached its maximum value in I-2, S-2 and N-2 relative to other treatments indicating that NAR did not necessarily occurred at maximum LAI conditions. In general, maize had statistically greater NAR in pollination and filling stages relative to other growth stages. Results of gas exchange for maize as a sensitive crop to water deficit, showed that photosynthesis rate (A(n)) and stomatal conductance (g(s)) were statistically decreased in water deficit by an average of 30 and 43% as compared to full irrigation treatment, respectively. However, reduction in A(n) and g(s) in salinity conditions was the same as 13% compared to no salinity treatment. Transpiration rate (T) was statistically lower under water and salinity stress by an average of 75 and 26% as compared to no water and salinity stress, respectively. The ratio of A(n)/g(s) in I-2 and I-3 was statistically higher as 23% than that obtained in I-1 treatment indicating that maximum amount of intrinsic water use efficiency could be achieved by water saving strategies. This result confirmed by transpiration efficiency (A(n)/T) which was statistically higher as 40 and 14% in I-3 and S-3 relative to full and fresh water irrigation treatment, respectively. In general, there was a decreasing trend in A(n) and g(s) toward the end of growing season, mainly due to leaves ageing. Similar pattern also occurred in measurements during a day from morning to afternoon. Furthermore, A(n) and g(s) decreased as vapor pressure deficit (VPD) increased. The relationships between A(n) and g(s) vs VPD in different growth stages implied that the plant ability to regulate A(n) and g(s) in response to VPD at leaf level declined in the last growth stages. This indicated that the water saving strategies during last growth stage for maize is more desirable.
