Evaluation of bread wheat (Triticum aestivum L.) genotypes for drought tolerance using morpho-physiological traits under drought-stressed and well-watered conditions

**Background:** Bread wheat (Triticum aestivum L.) is a critical global food crop, but its production in Ethiopia—the largest wheat producer in sub-Saharan Africa—is severely constrained by drought, especially since over 90% is grown under rain-fed conditions. Drought stress can reduce wheat grain yield by up to 72% when occurring at tillering. Developing drought-tolerant genotypes through morpho-physiological screening is a cost-effective strategy to boost production and meet rising domestic demand, which currently exceeds local supply by over 1 million metric tonnes annually.

**Methods:** The study was conducted at Wachemo University Crop Research Farm, Hosanna, Ethiopia (7°33’13.9" N, 37°53’2" E, 2177 masl) from July to December in 2020/21 and 2021/22. One hundred ninety-six bread wheat genotypes (188 advanced lines + 8 checks) were evaluated under four testing environments: greenhouse and field in each of two years. A 14×14 lattice design with two replicates was used. Plants were maintained at 80–100% field capacity (FC) until 75% heading, after which drought-stressed plots received 35% FC while well-watered controls continued at 80–100% FC until physiological maturity. Five morphological traits (flag leaf size, angle, rolling, waxiness, disease severity) were scored visually using standard scales. Fourteen physiological traits were measured: canopy temperature depression (CTD) at heading, anthesis, milking, dough, and ripening stages; SPAD chlorophyll content at the same five stages; relative water content (RWC); excised leaf water retention (ELWR); relative water loss (RWL); and leaf membrane stability index (LMSI). Data were analyzed using PROC GLM in SAS, with Pearson correlation coefficients and principal component analysis performed in R.

**Key Results:** Significant (p<0.01) genotypic differences were found for all traits under both water regimes. Under drought stress, flag leaf size categories were: large (56 genotypes), intermediate (91), and small (49); no genotype had very large leaves. Flag leaf angles under drought were droopy (51 genotypes, 26.3%), semi-erect (97, 49.2%), and erect (48, 24.5%). Full flag leaf rolling was observed in 42 genotypes (21.4%) under drought vs. 28 (14.3%) under well-watered conditions. Strong leaf waxiness was found in 52 genotypes (26.5%) under drought vs. 45 (23.0%) under well-watered conditions. Disease resistance under drought: 15 genotypes (7.7%) were highly resistant, 65 (33.2%) resistant, 51 (26.0%) moderately resistant.

For physiological traits, drought stress reduced CTDH by 21.2% on average across genotypes. The highest CTDH under drought was 3.05°C (ETBW8491) and lowest 1.52°C (ETBW8984). SPAD readings declined by 15.9–18.7% across growth stages under drought. Genotype Alidoro maintained the highest SPAD values at all stages under both conditions (e.g., SPADH: 58.53 well-watered, 52.47 drought). Mean RWC was 65.48% under well-watered and 63.12% under drought (15.3% reduction). Alidoro had the highest RWC under both conditions (76.75% well-watered, 67.6% drought). RWL was lowest in Alidoro (30.1%) under drought, indicating superior water retention. LMSI ranged from 37.75% (ETBW9441) to 69.13% (Alidoro) under drought. Broad-sense heritability ranged from 43.02% (RWL) to 87.29% (LMSI) across water regimes.

Correlation analysis showed CTD traits were positively correlated with each other and with SPAD, RWC, ELWR, and LMSI under both regimes, but negatively correlated with RWL. SPAD values at all stages were negatively and significantly (p<0.01) correlated with RWL under both conditions. PCA revealed the first three components explained 92.0% (drought-stressed) and 88.3% (well-watered) of total variation. Biplot analysis identified Alidoro, ET-13A2, Kingbird, Tsehay, ETBW8816, ETBW9027, ETBW9402, ETBW8394, and ETBW8725 as drought-tolerant genotypes associated with high CTD, SPAD, RWC, ELWR, and LMSI.

**Clinical Implications:** This study provides a robust framework for screening drought-tolerant bread wheat genotypes using easily measurable morpho-physiological traits. The identified genotypes—particularly Alidoro, ET-13A2, Kingbird, and Tsehay—and traits (high CTD, SPAD, RWC, ELWR, LMSI; low RWL; narrow erect waxy leaves with full rolling) can be directly exploited in Ethiopian and international breeding programs to develop high-yielding, drought-resilient wheat cultivars. This is critical for food security in sub-Saharan Africa, where wheat demand is rising and drought frequency is increasing due to climate change.