Evaluation of Nitrogen Status in Dryland Wheat (Triticum aestivum L.) Genotypes Using 15N

IntroductionNitrogen (N) is the most important input in many Iranian cropping systems and applying the optimal amount of N in the right place at the right time is a significant challenge for wheat growers. Previous research results indicated that nitrogen fertilization can increase the use of soil moisture, which lead to increase of wheat grain yield in dryland area. Heat and drought stress are the most environmental factors that cause significant yield and quality reduction of dryland wheat by disruption of plant metabolism. Although these stresses, in most cases, are impossible to control, farmers can reduce their unfavorable effects by optimizing nitrogen fertilizer applications. Nitrogen use efficiency in different wheat genotypes depends on plant characteristics such as root system distribution, plant growth stages, plant nutrient requirements, soil moisture and temperature, soil nutrient content and nutrient interactions, some of these factors can be improved by nitrogen fertilizer management. Recently, the use of 15N in soil-plant systems is a great help to identify the actual uptake of nutrients from fertilizers. This technique determine nitrogen rate and application times, 15N derived from fertilizers , 5N use efficiency, fate of N residuals in soil profile and N accumulation and redistribution in plant organs exactly. In addition, many other new indices have been identified with high sensitivity to changes nitrogen status in plant such as remote sensing techniques. These indices are capable of higher accuracy and easy identification of nitrogen status in plant for instance insufficient, sufficient and excessive conditions. The most important of these indicators can be cited nitrogen stress index (NSI). There are high negative correlations between yield and grain components and are also high significant positive correlations among nitrogen uptake, nitrogen status in plant and plant relative chlorophyll meter (RCM). Therefore, nitrogen stress index (NSI) can identify the critical periods of nitrogen stress in dryland wheat that provide reasonable recommendations for the lifting of nitrogen nutrition restrictions.
Materials and MethodsTo determine effect of rates and times of nitrogen applications on the production and nitrogen status of dryland wheat, this study was conducted as split-split plot design based on randomized complete block design with three replications as which 15N application times (fall, 2/3 in fall and 1/3 in spring) were assigned to the main plots and N rates were arranged to the sub plot (0, 30, 60 and 90 kg ha-1), and 7 wheat genotypes to the sub-sub plots (Azar2, Ohadi, Rasad and 4 other genotypes as genotype1 to genotype4) in three replications in Dryland Agricultural Research Institute (DARI) during 2011-12 cropping seasons. In three elongation (ZGS32), flowering (ZGS64) and maturity (ZGS87) stages, we determined wheat dry matter accumulation and nitrogen concentration. Nitrogen stress index (NSI) was calculated by plant N concentration and dry matter using Data fit 9 software. Plant chlorophyll content also was measured in the three upper developed leaves randomly in each plot by chlorophyll meter (SPAD-Hansatech, Cl-01 model) in three mentioned steps. Total nitrogen and 15N/14N isotopic ratio was determined in grain and straw by mass spectrometry method.
Results and DiscussionThe results showed that nitrogen application time had no significant effect on yield and nitrogen uptake parameters. But, nitrogen application significantly increased dry matter (2052 kg ha-1), grain yield (1053 kg ha-1), biological yield (3403 kg ha-1), nitrogen uptake (21.8%), nitrogen uptake from fertilizer (10.3%), 15N% in plant (1.75 atom percent), N derived from fertilizers (21.9%), nitrogen stress index (0.3) and grain protein content (1.3) on average. Application of N60 was suitable rate to reduce nitrogen stress and optimal production of dryland wheat genotypes. Azar2 was the most desirable genotype while genotype1 and genotype2 were inappropriate genotypes in this respect.
ConclusionsThe nitrogen stress index (NSI) and 15N derived from fertilizers (%Ndff) were the best indices to determine nitrogen status and nitrogen application times and rates for dryland wheat genotypes. The nitrogen status in plant was an effective factor for increase of grain protein.