Evaluation of Relationships between Stomatal Dimensions and Density with the Root System in Bread Wheat Cultivars and Lines under Rainfed Conditions

Extended Abstract Background: As one of the most important cereals, bread wheat is an essential part of food security in the world, which supplies one-fifth of the total calories of the world population. Nowadays, the yield of wheat has been affected by climate change-driven drought as one of the most important abiotic stresses that has become an important threat to food security in the world. Root and stomatal traits are especially important in breeding plants to withstand drought stress. Stomata play a key role in controlling carbon dioxide uptake and water loss through transpiration. Therefore, stomatal characteristics are used as indicators of water status and plant growth, especially in drought stress conditions. Having a wide range of physiological and morphological characteristics, roots play an essential role in absorbing water and nutrients. They are also the first organ that sends signals to control the stomata in response to dryness. Therefore, the difference in the structure of the root system can cause the difference between the performance in different cultivars. This study was conducted to investigate stomatal characteristics and their relationship with the root system and plant performance in 24 bread wheat lines and cultivars. Methods: To investigate the relationship between stomatal dimensions and density with the root system, an experiment was conducted on 24 bread wheat genotypes in the form of a randomized complete block design with three replications in the rainfed conditions of the research farm at Zanjan University Faculty of Agriculture in the crop year 2018-2019. In this experiment, PVC pipes were used to study the root system. Twelve seeds were planted in each tube, which were thinned to seven after germination. In each experimental unit, there were two tubes for each genotype, one of which was used to evaluate traits and final yield, and the second tube was used for root studies. Stomatal traits, including the length and width of stomata and number of stomata per unit area, root traits including root length, root diameter, root volume, root surface, and root biomass, and seed yield were measured in the end. The resulting data from the measured traits were analyzed in the form of a randomized complete block design, and the averages were compared using the LSD method. The data were analyzed using multivariate statistical analyses, including regression analysis, path analysis, and factor analysis, and cluster analysis was used to group genotypes. Statistical calculations were done using SAS 9.0 and SPSS 21 software. Results: The results of analysis of variance and mean comparison showed high variability among genotypes for all measured traits. The results of the mean comparison of genotypes showed that genotypes 2, 5, 8, and 16 had the highest yield, and genotype 23 had the lowest yield among the examined genotypes. The highest number of stomata on the upper and lower leaf surfaces belonged to genotypes 5 and 2. In terms of root traits, the highest diameter, volume, length, root surface, and root dry weight at a depth of 0-25 cm were recorded for genotypes 2, 3, 18, and 5, respectively. There was a high and significant correlation between the yield and the number of stomata on the upper and lower leaf surfaces, the length and width of  the stomata on the upper leaf surface, diameter, volume, dry weight, and root surface at a depth of 0-25 cm in the soil. Based on the results of stepwise regression analysis, two variables, the number of stomata on the lower leaf surface and root dry weight at a depth of more than 25 cm explained 91.4% of the changes in grain yield. According to the results of the causality analysis of the number of stomata on the lower leaf surface, the most direct effect had a positive effect on seed yield. The results of factor analysis grouped the studied traits into three factors with 82.48% variability justification. The shares of the first, second, and third factors to explaining data changes were 48.86%, 24.62%, and 8.99%, respectively. Based on the plot obtained from factor analysis, genotypes 2, 5, 8, and 16 had high values for the first and second factors. According to the coefficients of the factors, it can be claimed that the genotypes located in this area have high performance, a high number of stomata, and strong root traits, which were found at the soil depth of 0-25 cm. For this reason, these are the genotypes that could produce high yields by absorbing water from the surface layers of the soil by having a large number of stomata and carrying out more photosynthesis. Moreover, the investigated genotypes were divided into three groups from cluster analysis by the ward method and Euclidean distance. Genotypes 2, 5, 8, and 16 were placed in the first group and had the highest mean values for grain yield traits, number and width of stomata on the upper and lower leaf surfaces, and root traits including diameter, volume, and dry weight at a soil depth of 0-25 cm, and root diameter at a depth greater than 25 cm. The lowest values for stomatal length were observed in both leaf surfaces. These were the best genotypes for cultivation in dry conditions. Conclusion: A strong superficial root system can provide the plant with water from scattered rains that occur with low frequency at the end of the growth period. On the other hand, the increase in the number of stomata along with their smaller size reduces leaf pores and enables a faster response of the stomata, and the rapid response of the stomata maximizes water use efficiency. Therefore, having a strong superficial root system along with high stomatal density can increase seed yield in dry conditions.