Evaluation of phosphorus deficiency stress tolerance in oilseed sunflower pure lines (Helianthus annuus L.)

Introduction
Phosphorus is one of the essential elements in plants. In calcareous soils, the amount of available phosphorus is very low due to high pH, excessive levels of calcium carbonate, lack of organic matters, and also the insolubility of phosphate minerals under such conditions. Since a large amount of phosphorus fertilizer is constantly applied to the calcareous soils of Iran, identification of genotypes with efficient phosphorus uptake may help farmers to manage the application of chemical fertilizers. Phosphorus uptake efficient genotypes can be identified using different indices including tolerance index (TOL), mean productivity (MP), geometric mean productivity (GMP), stress tolerance index (STI; Fernandez, 1992), abiotic tolerance index (ATI; Moosavi et al., 2008), stress susceptibility index (SSI), drought index (DI), harmonic mean (HARM) and modified stress tolerance indices (kiSTI_K1S and K2STI; Naderi et al., 2008). This study was conducted to screen tolerant oilseed sunflower pure lines for phosphorus deficiency stress with higher and efficient phosphorus uptake to develop an improved genetic population for farmers’ usage.

Materials & Methods
Ninety-five oilseed sunflower pure lines were evaluated using a randomized complete block design with three replications under both optimal and deficiency conditions of absorbable phosphorus at the Urmia University research field during the 2015-2016 cropping season. This experiment was conducted in 15 kg plastic pots. Soil phosphorus concentration in phosphorus deficiency treatment was 7.240 mg.kg-1 while under optimal condition, 0.4 g.kg-1 of triple superphosphate were added. Tolerance indices were calculated for the sunflower lines based on the grain yield under optimal and deficiency conditions of absorbable phosphorus. In order to find suitable indices for screening the tolerant lines, a simple correlation coefficient was performed between the calculated tolerance indices and grain yield under optimal and deficiency conditions of phosphorus. Principal components analysis (PCA) and cluster analysis were performed to evaluate the relationship between the tolerance indices and the studied genotypes and to select the most desirable and tolerant lines.

Results & Discussion
Results illustrated that phosphorus deficiency reduced the grain yield of all genotypes and mean grain yield in phosphorus deficiency conditions was 44.88% lower than that in optimal conditions which indicates the importance of phosphorus on sunflower grain yield. The result of correlation analysis revealed a highly significant correlation between HARM, MP, GMP, and STI indices with grain yield under optimal and deficiency conditions of phosphorus. Therefore, these indices could be considered as the most suitable indices for screening tolerant lines to phosphorus deficiency conditions which also showed higher grain yield under optimal and deficiency conditions of phosphorus. Principal component analysis (PCA) showed that the first three components accounted for 94.5% of the total variation, with principal component (PC) 1 accounting for 63.1%, PC2 for 21.6% and PC3 for 9.8% of the total variation. The heatmap hierarchical clustering indicated that all the lines were clustered into three major groups, with cluster 1 (6 lines) being, in general, drought-tolerant followed by cluster 2 (29 lines) whereas, cluster 3 (60 lines) being drought-sensitive. The results of principal component and cluster analysis based on stress tolerance indices indicated that the lines 38, 94, and 95 not only showed higher tolerance in terms of tolerance indices but also they produced relatively high grain yield and had higher total phosphorus absorption values under both optimal and deficiency conditions of phosphorus, which indicated their uptake efficiency and efficient use of soil phosphorus. The results obtained from all the applied methods revealed that the lines 38 (Yp=72.23; Ys=54.37; TAPp=400.53; TAPs=106.40), 94 (Yp=70.60; Ys=34.73; TAPp=121.93; TAPs=44.00), and 95 (Yp=70.67; Ys=25.77; TAPp=36.53; TAPs=21.53) are the most desirable and tolerant ones. Therefore, these lines can be used in breeding programs to introduce new phosphorus efficient varieties with high grain yield.