In order to study the effects level of nitrogen fertilizer on yield and physiological traits of rice cultivars (Oryza sativa L.), an experiment as factorial in RCBD with three replications was conducted during 2009 year in the Rice Research Institute, Iran, Rasht, central of Guilan and Rudsar, East of Guilan. Factors were cultivar (Khazar, Ali Kazemi and Hashemi), and nitrogen fertilizer levels (0, 30, 60, and 90 Kg N/ha). Characters measured were: leaf area index (LAI), Total of dry weight (TDW), leaf dry weight (LDW), Crop growth rate (CGR), relative growth rate (RGR), net assimilation rate (NAR), leaf area duration (LAD), leaf area ratio (LAR), leaf weight rate (LWR), specific leaf area (SLA), grain yield. Growth parameters were calculated during six growth stages by harvesting samples from leaf area index, leaf dry and total weight dry weight in all treatments. The comparison of calculated and measured amounts of growth indexes with use determination of regression coefficients (R 2 ).Results of growth analysis indicated that, nitrogen increasing rates of fertilizer caused the increment of growth indexes. Khazar and Ali Kazemi Hashemi showed higher growth indices rather than Hashemi. Results indicated that cultivar and nitrogen fertilizer significantly effected on grain yield. Khazar and Ali Kazemi have the highest grain yield among cultivars. Also, Results indicated with increasing nitrogen fertilizer application, grain yield increased significantly (17،13 and 57 %).

One way to evaluate the sustainable development of agriculture is the use of energy flow method. This method, in an agricultural production system, refers to the energy consumed during production operations and energy saved when crops have been produced. In this article, an evaluation of energy balance and energy indices in rain-fed cultivation of wheat in northern Iran (Guilan province) was carried out. The data were collected in 2011 from 72 farms in Guilan province by using a face to face questionnaire method. By using consumed data as input and total production as output, their respective equivalent energy, energy balance and energy indices were calculated. Energy efficiency (energy output to input energy ratio) for seed and straw in this study were found to be 1.609 and 1.614 respectively, showing the effective usage of energy in agro ecosystems wheat production. Energy balance efficiency (production energy to consumption energy ratio) for seed and straw in this study were found to be 0.92 and 0.79 respectively, showing the effective usage of energy in agro ecosystems wheat production.

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