Evaluation of yield and drought tolerance indices of cow cockle (Vaccaria hispanica (Mill.) Rauschert) ecotypes

The cow cockle (Vaccaria hispanica (Mill.) Rauschert) is an annual plant of the Caryophyllaceae family with a chromosome number of (2n=2x=30). This plant grows upright to a height of 30 to 100 cm with several branches and has great potential for producing medicinal products due to its triterpenoid saponins. In arid and semi-arid regions, plants experience periods of lack of moisture during their growth period and must be able to tolerate these periods to produce a proper yield. This experiment was aimed at investigating the drought stress responses of different V. hispanica ecotypes and evaluating their drought stress tolerance.

This experiment was performed as a split-plot experiment based on a randomized complete block design with three replications in 2020-2021 at Parsabad Agricultural Station. The main plots were allocated to three treatments of non-stress, moderate and severe stress (irrigation cycles of 7, 10, and 14 days, respectively), and sub-plots were assigned to eight native cow cockle ecotypes collected from different northwestern regions of Iran. Seeds were sown directly in the soil at a rate of 5.5 to 8 kg per hectare with a depth of 1-2 cm, and a row spacing of 20 cm. Irrigation operations were performed similarly for all experimental units up to the beginning of stem elongation, after which the irrigation intervals for applying stress were increased. At maturity, seed yield, biomass, and harvest index were recorded and various tolerance and sensitivity indices were calculated using the yield of each ecotype under non-stress conditions (Yp) and stress conditions (Ys).

The results of the analysis of variance showed that the effects of irrigation regime, ecotype and their interaction were significant in terms of seed yield, biological yield and harvest index. Ecotype 6 had the lowest decrease in grain yield per unit area and harvest index under stress conditions. In terms of yield stability and drought tolerance indices, harmonic mean yield, geometric mean yield and mean productivity, the lowest values in moderate and severe stress conditions were related to ecotypes 7 and 1, respectively, and the highest values of these indices were related to ecotype 6 in both stress conditions. Estimation of SIIG index for different ecotypes showed that ecotype 6 had the highest amount of the SIIG in both moderate and severe stress conditions. Ecotypes 4 and 3 in moderate and severe stress conditions were in the next rank and were classified as ecotypes with relatively high drought tolerance. In moderate stress conditions, ecotypes 7 and 8 and in severe stress conditions, ecotypes 1 and 7 with the lowest SIIG showed high sensitivity to drought stress. Factor analysis based on principal component analysis (PCA) showed that the first two factors with specific values greater than one, in both moderate and severe stress conditions had 95.58% and 97.18% of the total variance, respectively. According to the MGIDI index, in moderate stress conditions, ecotype 6 had the lowest value and was considered the most tolerant ecotype to drought stress, followed by ecotypes 4 and 3. In severe stress conditions, after ecotype 6, ecotypes 3 and 4 were in the next rank.

Estimation of the ideal genotype selection index (SIIG) and the multi-trait genotype–ideotype distance index (MGIDI) for different ecotypes based on all tolerance indices showed that ecotype 6 had the highest SIIG and the lowest MGIDI in both stress conditions followed by ecotypes 4 and 3. Ecotypes 1, 7, and 8 with the lowest SIIG and the highest MGIDI showed high sensitivity to drought stress. Therefore, among the studied ecotypes, ecotype 6 can be considered as the most tolerant ecotype to drought stress. At all, it can be concluded that the studied ecotypes have sufficient genetic diversity to be used in breeding programs with the aim of obtaining lines with higher tolerance to drought stress. The use of SIIG and MGIDI indices in crop selection programs can also be considered more.