Received 16 April 2016; accepted 17 June 2016; published 20 June 2016
Wheat is one of the three cereals which has the most cultivated land in the world and dedicated to itself about one-third of the world’s arable land (Emam, 2007)  . This plant has an important role in feeding people around the world (Ying Li et al., 2016)  . According to the statistics of Food and Agriculture Organization (FAO) in 2010 the cultivated land of wheat equals 217 million hectares, with production amounted to 653 million tons and about 44% of its production is in Asian countries and due to the large share of production, Asia is the world’s largest producer of wheat. In Iran according to available statistics 6.4 million hectares of country lands devoted to wheat that 62.5% of them were used for dryland farming. Despite vast cultivated land, less than 31% of wheat production comes from rainfed farms (Ministry of Agriculture Statistics 2013)  , and this reveals the importance of attention to rainfed wheat. In semi-arid areas production depends mainly on rain. The most important factors limiting growth and yield of plants in these areas are limitation of water resources and minimal access to nutrients (Zhao-Hui, 2008; Lee et al., 2001)   .
In dryland farming food must be provided based on soil moisture regimes. Since it is impossible to accurately predict rainfall and changes in time and amount of rainfall is high, so the exact determination of the required fertilizer for wheat in dryland conditions, especially nitrogen fertilizer is one of the most difficult and risky decisions that farmers in these areas should take (Arnon, 1972)  .
Nitrogen is one of the most important macronutrients that are used highly in today’s input agriculture (Latiri- souki et al., 1998  ; Lawlor, 1995  ). Nitrogen at the cellular level increases the number and size of cells and increases the leaf area and the efficiency of radiation absorption by plant (Jenkyn and Luch, 1998  ; Marbet, 2000  ). Rise of nitrogen also increases the durability of the leaf area and biomass (Basso & Ritchie, 2005  ; Yang et al., 2001  ). Wheat production in arid and semi-arid areas largely depends on nitrogen management. In these areas, due to the low soil organic matter, nitrogen is the most important limiting element of yield. Nitrogen mineralization in these areas is not sufficient for the needs of cereals and the use of appropriate amounts of nitrogen fertilizer to increase yield in these areas is essential (Ryan, 2002  ; Ryan et al., 2008  ).
Suitable nitrogen application in dryland conditions can increase the power of plant to deal with the drought in these conditions (Fischer, 1973  ; Langdate et al., 1973  ). Results of several studies have shown that nitrogen fertilizer increases grain yield and its protein. Asadi et al., (2013)  investigated the effects of different levels of nitrogen and competition on grain yield and reported that with increase of nitrogen grain yield increased.
Dandan Liu & Yan Shi, (2013)  investigated different amounts of nitrogen fertilizer on yield and grain quality and reported that grain quality and yield increased with the increase of nitrogen fertilizer. Emam et al., (2011)  also reported the increase of yield and yield components in rainfed wheat by increase of nitrogen fertilizer.
On the other hand excessive use of nitrogen fertilizer in rainfed wheat leads to an increase in the growth period, production of more vegetative organs and ultimately decreases of grain yield, increase of the number of infertile grains and decrease of harvest index (Basso & Ritchie, 2005  ; Fischer, 1973  ). So determining the appropriate level of nitrogen in dry lands which causes the maximum yield has particular importance in dryland areas.
2. Methods and Materials
To study the effects of different levels of nitrogen fertilizer on yield and yield components of cultivars rainfed wheat a study was done in the crop year 2012-2013 as factorial in a randomized complete block design in 2 research stations of Kohne Kand city in Bojnord (latitude 37 degrees 28 minutes north and longitude 57 degrees 20 minutes east, and altitude of 1070 meters) and Shirvan (latitude 37 degrees 19 minutes north and longitude 58 degrees 07 minutes east, and altitude of 1148 meters) research station with 3 replications. The treatments of study contain nitrogen fertilizer in 4 levels (75, 50, 25 and 0 kilograms nitrogen per hectare) and 4 cultivars of rainfed wheat (Sabalan, Cross Sabalan, Azar 2 and Rasad). To prepare the land for planting first plowing was done by chisel plow and then the disc was used to grind clods, and then by the use of leveler land was leveled. After that land was divided to plots with a length of 5 and a width of 2 meters. In per plot 8 plant lines with a spacing of 20 cm are considered and lines next to each plot were considered as marginal effect. Planting was done on October 23 with a density of 300 plants in per square meter (common planting date and density in region) with the spraying method. Consumable fertilizers were used based on fertilizer recommendations, including nitrogen in stages (sowing, tillering and stem elongation and along with rain) and phosphorus as 50 kilograms per hectare that was used at the same time with planting. Tribenuron was used to fight with weeds.
To measure yield from the two middle rows that remain intact and of each row 0.5 meter was harvested and transported to the laboratory. Yield components were measured after harvest. 10 plants were chosen and number of spikes per square meter, number of grains per spike and weight of one thousand grains was measured. Finally data were analyzed using SAS software. Comparison of the average data was conducted at the level of 5% and with Duncan test.
3. Results and Discussions
The results showed that the effects of different cultivars and different amounts of nitrogen fertilizer on yield and yield components were significant in both regions (Table 1). In both regions Azar 2 had the highest yield and lowest yield was related to the Cross Sabalan in Shirvan and Sabalan in Bojnoord. With the increase of nitrogen fertilizer, yield was significantly increased too. However, there was no significant difference between levels 50 and 75 kilograms nitrogen per hectare (Table 3).
Moraghebi et al., (2011)  also reported that along with nitrogen increase, grain yield, grain protein and grain protein yield increased linearly. Increase of grain yield by increase of nitrogen obeys the law of diminishing returns; that means yield increases with increase of nitrogen consumption but yield increase at higher amounts of nitrogen gradually becomes less. This would correspond with the findings of other researchers (Alcozen et al., 1993  ; Sowers et al., 1994  ).
3.2. Yield Components
3.2.1. Number of Spikes per Square Meter
The effects of cultivar and different amounts of nitrogen fertilizer were significant on the number of spikes per square meter (Table 1) and Cross Sabalan has the highest number of spikes per square meter (Table 2) but it has the least yield and it shows the low role of this factor in increase of yield in rainfed conditions. Many evidences indicate that yield components of grain are more or less dependent to each other. For example, more spikesper unit area are neutral with fewer grains per spike (Evanze, 1984)  . With the increase of nitrogen in used treatments, the number of fertile tillers per plant increased too (Hosseini et al., 2011)  but the reaction of dissimilar genotypes was different (Giovanni et al., 2004)  .
3.2.2. Number of Grains per Spike
The impact of cultivar on the number of grains per spike was significant (Table 1) and Azar 2 had the most number of grains per spike. High yield in Azar 2 is because of the number of grains per spike and higher weight of thousand grains in this cultivar in comparison to others (Table 2). In dryland conditions the number of grains per spike is more important than other characteristics. This may be due to the importance of this attribute and its
Table 1. Analysis of variance of experimental characteristics.
ns, **, *: non-significant and significant, respectively, at the level of 1 per cent and five percent.
sensitivity to drought conditions and its applied pattern (Collaku, 1989)  . Shahbaz Panahi et al., (2012)  reported that the number of grains per spike in drought conditions significantly affects grain yield and with the increase of nitrogen, number of spikes per square meter and number of grains per spike increased (Table 2, Table 3). This can be attributed to the role of nitrogen in increasing growth and grain yield (Shahzad et al., 2013)  . Determining the most important characteristics and wheat yield components and their correlation with grain yield causes suitable genotypes be selected and used to increase the yield per unit area (Nabipour et al., 2002)  .
Hanchinal et al., (1994  ) suggested number of grains per wheat spike as the most critical attribute for the selection of drought resistant genotypes. According to Ehdaie & Noor Mohammadi (1994)  correlation between grain yield and its components, plant height, number of spikes per plant and number of grains per spike is positive and significant in unfavorable environmental conditions.
3.2.3. The Weight of One Thousand Grains
The effect of nitrogen and cultivar was significant on the weight of one thousand grains (Table 1). High yield of Azar 2 in comparison to other cultivars is because of the number of grains per spike and the high weight of one thousand grains in this cultivar in comparison to others (Table 2). So high weight of one thousand grains as one of the most important components of grain yield in drought conditions should be considered as a criterion for the selection of drought tolerant genotypes (Sanjari & Yazdansepas, 2009  ; Rezadoost & Roshdi, 2006  ; Yazdansepas et al., 2010  ).
Alavisiny et al., (2013)  also reviews the resistance of different wheat lines to drought conditions and stated that the role of weight of one thousand grains in yield in comparison to other yield components is more. With the increase of nitrogen, weight of one thousand grains reduced and had inverse relation with other yield components (Table 3). Usually correlation between wheat grain yield with weight of one thousand grains, plant height, number of tillers per square meter, length of the last peduncle, number of grains per spike, biological yield and harvest index is positive and significant (Mohammadi, 1998)  .
High yield in Shirvan in comparison to Bojnoord can be attributed to the difference in rainfall and soil properties in these areas that is discussed below.
Table 2. Mean comparison of yield and yield components for different cultivars.
Means with similar letters in each column are not significantly different at the 5% level of probability (Donckan).
Table 3. Mean comparison of yield and yield components for different levels of nitrogen.
Means with similar letters in each column are not significantly different at the 5% level of probability (Donckan).
3.2.4. Effects of Rainfall on Yield
Despite the high resistance of wheat to drought, rainfall and adequate water supply has a major impact on grain yield so that high yield in Shirvan in comparison to Bojnoord can be related to higher rainfall in Shirvan (Table 4). Nasabian et al., (2004)  in examining the effects of temperature and rainfall on strategic products stated that per millimeter increase in rainfall and rising temperatures, greatly increase the yield.
Azizi & Yar Ahmadi (2003)  by the use of regression model studied the relationship between climatic factors and yield of rainfed wheat in plain Syalkhor in Lorestan. Using parameters of autumn and spring rainfall rate, number of spring frost days, first autumn rainfall and number of rainy days, they showed that there is a direct relationship between the yield of rainfed wheat and variables of autumn rainfall and also number of rainy days during the humid period of the year; and there is an inverse relationship between the number of frost days in spring and the delay in the first rainfall of autumn. Norwood (2000)  examined the effect of climatic factors on the rainfed wheat growing areas in the Great Plains of Kansas in America. He identified suitable areas for growing rainfed wheat by the analysis of climatic data such as rainfall, temperature, evaporation and soil and concluded that evaporation and rainfall compared to other climatic elements have the greatest impact during the growth stages of rainfed wheat.
Verdin & Klaver (2002)  and Moeletsi & Walker (2012)  showed that rainfall and water use and evapotranspiration have significant effects on rainfed crops yield.
In addition to the amount of rainfall, time of rainfall is very important too. Higher rainfall in November in Bojnoord than in Shirvan (Table 4) caused that growing happens sooner in this region (Table 5). At stem elongation stage Bojnoord rainfall was higher than Shirvan butat the time of grain filling that coincides with May (Table 5) rainfall in Shirvan was doubled (Table 4). This indicates that in dryland conditions, rainfall and wheat access to water at the time of grain filling has the greatest impact on yield and is very important. Studies that have been carried out in Tunisia show that the relationship between rainfall, time of grain filling and the atmosphere performance is positive and significant so that for each millimeter rise in precipitation over the annual rainfall average (450 mm) will be added to the product 6 kilograms per hectare (Watts & El-Mourid, 1988)  .
Table 4. Mean, max and min temperature, amount of rainfall from planting to harvesting.
Table 5. Phenology stages.
Table 6. Soil analysis of experimental farm at 0 - 30 cm.
3.2.5. The Effects of Soil Organic Matter on Yield
Another factor influencing the increase of yield in Shirvan in comparison to Bojnoord is more soil organic matter (Table 6). Soil organic matter plays an important role in soil quality. Humus is a part of that and is essential for controlling pH, water and minerals holding capacity, increase of soil aggregate stability and reduction of water and wind erosion (Tisdal & Oades, 1982  ; Lal, 2003  ; Albercht & Rasmusen, 1998  ).
Kazemaini et al., (2008)  studied effects of nitrogen and soil organic matter on rainfed wheat yield in Shiraz and stated that with the increase of nitrogen and soil organic matter, wheat yield increased significantly.
3.2.6. Soil Salinity
Moreover, soil salinity in Bojnoord was higher than in Shirvan (Table 6). The sensitivity of the final yield to salinity, as well as other environmental stresses, is a function of the sensitivity of each of the different components of the yield to stress (Pesarrakli, 1999)  . As grain yield, total dry weight reduces in saline environment (Mostajeran et al., 2005  ). Maas & Grieve (1990)  stated that salinity changed spike final capacity which caused a significant decrease in spike length, number of spikes per unit area and also number of grains per spike.
Francois et al., (1994)  studied the effect of salinity on wheat growth and yield components in three different phonological periods (use of salinity in the growing season, before the differentiation of terminal spikelet and after the differentiation of terminal spikelet) and declared that use of salinity before the differentiation of terminal spikelet reduced the number of spikelets per spike and the number of tillers whereas the use of salinity after the differentiation of terminal spikelet only significantly reduced the number of grains and grain weight. Francois et al., (1986)  announced that salinity decreased grain yield by reducing the weight of the grain. Maas & Poss (1989)  expressed that soil salinity affected wheat yield before heading stage more than after that.
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