Introduction: Nitrogen is the most important limiting factor, after water deficit for biomass production in natural ecosystems. Efficient use of nitrogen fertilizer is important to economical dryland wheat production and to the quality of ground and surface waters. Researchers have been looking for ways to increase the efficiency of fertilizer N use. Soil and in-season plant tissue testing for nitrogen status are a time consuming and expensive process. Chlorophyll concentration in a leaf is closely correlated with leaf N concentration so the Soil Plant Analysis Development (SPAD) chlorophyll meter is one of the most commonly used diagnostic tools to measure crop nitrogen status. The SPAD chlorophyll meter is a simple, portable, diagnostic and nondestructive light weight device used to estimate leaf chlorophyll content. Chlorophyll meter techniques provide a substantial saving in time and resources and offer a new strategy for synchronizing nitrogen application with actual crop demand. The objective of this study was to establish and analyze relationships between measurements from a SPAD instrument and the leaf pigments, as extracted at the different stages. The study also seeks to evaluate the utility of a chlorophyll meter to inform nitrogen fertilization rates of wheat (Triticum aestivum L.) genotypes in dryland condition.
Materials and Methods: This experiment was carried out in split split plot RCBD design with three replications. Treatment included: N application time (whole fertilization of N in fall, and its split fertilization as 2/3 in fall and 1/3 in spring), N rates (0, 30, 60 and 90 kg N.ha-1) as urea and 7 genotypes (Azar2, Rasad, Ohadi, HN7/OROFEN//BGN8/3/SERI/4/.., SARA-BW-F6-06-85-86-29-1, TEVEE'S'//CROW/VEE'S' and DH-2049-3. A chlorophyll meter (SPAD-Hansatech Cl-01) was used to obtain readings estimating leaf chlorophyll content (SPAD value) at tillering (GS22), shooting (29), stem elongation (GS32), flag leaf (GS39) and anthesis stages. Fresh plant leaf samples were collected from each plot for the estimation of chlorophyll a (Chla), chlorophyll b (Chlb), carotenoids (Ct), total chlorophyll (Chlt) and concentrations. The chlorophylls and carotenoids were determined by spectrophotometric analysis of chemically extracted pigments (Arnon, 1949). To examine the relationship between pigments chlorophyll and SPAD values, 30 wheat leaves were sampled. Average Chla, Chlb, Chlt, carotenoids and SPAD values were computed, and linear regression analysis was performed by least-squares method with Excel.
Results and Discussion: The results showed that, a strong positive and statistically significant (p<0.01) relationship between SPAD values and total chlorophyll (R2 = 0.93), chlorophyll a (R2 = 0.95), chlorophyll b (R2 = 0.78) and carotenoids content (R2 = 0.79) were determined. We demonstrate that the SPAD readings and plant photosynthetic pigment content per-leaf are profoundly affected by nitrogen rate and timing application. Nitrogen split application increased significantly (p<0.01) the SPAD values (8.3%), Chla (11.1%), Chlb (10.9%), Chlc (27%), Chlt (15%), (exception of carotenoids) and seed protein content (8.5%). Nitrogen application could affect significantly (p <0.01) all chlorophyll indices in plant. The ideal conditions for the chlorophyll indices in the plant were obtained using 72 kg N ha-1. Nitrogen requirement for maximum and optimum economical dryland wheat production is about 20 and 10 kg ha-1 less than nitrogen requirement for create the ideal conditions for the status of chlorophyll and its components in dryland wheat. Using chlorophyll meter, chlorophyll a (16.6 mg/g), chlorophyll b (5.9 mg/g), chlorophyll c (0.077 mg/g), total chlorophyll (23.9 mg/g) and chlorophyll a per Chlorophyll b ratio (2.78), chlorophyll index (9.4) and chlorophyll relative index (1.0) were determined in economical optimum N rate (NRD = 0). Genotype1 most suitable and Azar2 was the most inappropriate condition in terms of chlorophyll index and total chlorophyll ratio per carotenoids (optimum range 40-70) in the plant as a stress resistance index.
Conclusions: It can be concluded that, the actual chlorophyll content and its components can be estimated by using chlorophyll meter (SPAD) in dryland wheat genotypes at different growth stages. Chlorophyll components and seed protein can be improved by nitrogen fertilizer application time and rates in dryland wheat.