Introduction
Earliness and tallness are two important traits for improving grain yield of rain-fed wheat cultivars under insufficient rainfall conditions. Earliness is a drought escape mechanism that enables early-heading plants to complete their life cycle before the onset of drought stress. As a result, these plants achieve higher grain yields under such conditions. Additionally, tall wheat cultivars tend to perform better under severe drought stress compared to semi-dwarf cultivars.
Materials and methods
In this study, early heading was transferred from the Australian cultivar, Excalibur, to three Iranian wheat varieties named, Roshan, Mahdavi, and Kalahdari, to have near isogenic lines (NILs) for early heading in these three genetic backgrounds. In the parallel breeding program, tall genes were transferred from Roshan into Excalibur to have NILs for plant height in the background of Excalibur. The resulting near-isogenic lines, along with the control varieties (Karim and Koohdasht), were evaluated in a randomized complete block design with four replications under rain-fed condition of Kazeroon.
Results and Discussion
Mean comparison of the isogenic lines showed that tallness character was significantly improved grain yield (0.29 t. ha^-1) in the rain-fed conditions of Kazeroon. However, early maturity had a much better impact on grain yield under these conditions, where significantly increasing 1000-grain weight (6.51 g) and grain yield (0.71 t. ha^-1). Regression analysis results also indicated the greater importance of the early maturity compared to tallness. Moreover, days to heading and plant height explained 72% and 9% of the variation of grain yield under rain-fed conditions of Kazeroon, respectively. The early heading and tall lines resulting from the present breeding program, known as early heading Kalahdari and early heading Roshan, showed acceptable grain yield (2.19 and 2.18 t. ha^-1, respectively) production under rain-fed condition of Kazeroon, and had no significantly difference grain yield with control varieties, Karim and Koohdasht (2.25 and 1.90 t. ha^-1, respectively). These breeding lines can potentially perform better than the control varieties in low-rainfall years. Introducing these lines into this region can enhance yield stability by diversifying the available wheat varieties in rain-fed condition of Kazeroon.
Conclusion
In conclusion, Under the rain-fed conditions of Kazerun, both plant height and early maturity significantly influenced grain yield. In the 2022–2023 growing season, when rainfall was adequate and the regional average yield was approximately 2 t ha⁻¹, early maturity had a greater effect on yield than plant height. In contrast, during drier years, when semi-dwarf cultivars may become too short for efficient harvesting, plant height may be the more important trait. These findings highlight the value of incorporating both traits into breeding programs, with particular emphasis on early maturity under favorable rainfall conditions. Among the evaluated materials, the breeding lines Kalhaydari + Earliness and Roshan + Earliness showed excellent adaptation to the rain-fed conditions of Kazerun. Their release as new cultivars could enhance varietal diversity and improve the stability of wheat production in the region.