Author : Kyi Pyar Aye

Summary : Drought is one of the fundamental issues in the current climatic environment and one of the most severe abiotic stresses in many areas of the world. Roots are critical for water and nutrient intake, yet they are rarely the main target of breeding efforts This study aimed to identify drought tolerance wheat genotypes and to examine the correlations among drought related traits. The root and shoot traits of 150 wheat genotypes were tested at the seedling stage to evaluate the genetic variability of wheat root architecture at Experiment and Lecture Building-2 Laboratory of Department of Plant Breeding, Physiology and Ecology, Yezin Agricultural University. The materials were tested in Randomized Complete Block design with three replications. The estimates of phenotypic coefficient of variation (PCV) and genotypic coefficient of variation (GCV) were observed to be high for root fresh weight. The root angle (0.99) was the highest heritability value which indicates the relative effectiveness of selection based on the phenotypic expression of a trait. According to the cluster analysis and principal component analysis, the 150 wheat genotypes were classified into three groups and nine principal components. The highest root length, seminal root number, root fresh weight, root dry weight and root shoot ratio were observed in cluster III while the lowest root angle was found in the same cluster. Moreover, the highest contributions of root angle, root length, and seminal root number were observed in PC3, which had an eigenvalue greater than one. The root angle, root length, and seminal root number contributed significantly to the overall variability, indicating that PC3 an important component for distinguishing drought tolerance wheat genotypes. Meanwhile, these genotypes could be selected for drought tolerance as a deeper root was recognized as an important factor in increased drought tolerance, together with a narrower root angle which could be considered suitable for drought tolerance in wheat breeding programs. The 21 wheat genotypes from cluster III of Experiment I, along with two check varieties (Yezin-11 and Yezin-12), were evaluated in experiment II. The experimental materials were treated with -0.407 MPa of Polyethylene Glycol 6000 (18% of PEG 6000) induced osmotic stress and a control treatment (distilled water) during germination and early seedling growth stages. The analysis of variance indicated highly significant differences among genotypes, treatments and genotypes x treatments for all seedling traits except mean germination time. The 19 out of 21 genotypes exhibited potential drought tolerance under -0.407 MPa of PEG 6000 solution because they possessed highly desirable traits such as seedling vigour index II, root length and root dry weight. Correlation coefficients among seedling traits were significant and positive for all traits except mean germination time which had no significant correlation with any of other traits. The results of principal component analysis showed that principal components with eigenvalues greater than one were deemed important and had the total variation of 83.87%. Seedling vigour index II, root length, and root dry weight exhibited a strong positive correlation, with higher PCA1 and lower PCA2 scores. Therefore, 17 genotypes possessing these seedling traits could serve as good indicators of drought tolerance in breeding programs. In Experiment III, the selected drought tolerant genotypes from Experiment II, along with three check varieties (Yezin-9, Yezin-11 and Yezin-12), were evaluated under controlled rainout shelter (pot experiment) and field conditions at the Regional Research Centre (Aungban), Department of Agricultural Research. The experiments were conducted using a separate randomized complete block design with three replications under both non-stress and stress conditions. The analysis of variance was exhibited highly significant differences among wheat genotypes for all observed traits under both pot and field conditions. Genotypes such as 20th HTWYT-8, 29th SAWYT-7, 29th SAWYT-9, 29th SAWYT-17, and 29th SAWYT-37 expressed consistent of yields under non-stress and stress conditions with higher mean productivity (MP), geometric mean productivity (GMP), and stress tolerance index (STI) values. These indices strongly correlated with yield in both conditions, supporting their use for selecting genotypes in drought fluctuating environments. Additionally, low canopy temperature, high SPAD values, sustained relative water content (RWC), and larger flag leaf area observed in these genotypes made them valuable direct selection criteria for stress­prone environments and drought resilience breeding.