Introduction:

Delayed planting of wheat is common in Orzooeieh, a warm region in southern Kerman Province, where farmers often sow wheat after grain-corn harvest in late January. Although this practice fits the local cropping system, it exposes wheat to severe terminal heat stress during spring, leading to substantial yield reductions compared with timely sowing in December. Identifying cultivars capable of maintaining acceptable yields under such conditions is therefore essential for stabilizing wheat production. Two adaptive traits, earliness and plant height, are particularly important for performance under heat and drought stress. However, their independent contributions to yield in late-planted wheat have not been well quantified. This study aimed (1) to evaluate sixteen bread-wheat cultivars and breeding lines under delayed planting in Orzooeieh and identify superior genotypes, and (2) to determine the specific effects of earliness and plant height on yield using near-isogenic lines (NILs).

Materials and Methods:

The experiment was conducted in Orzooeieh using a randomized complete block design with four replications. Three locally grown cultivars, Sirvan, Khalil, and the newly adopted Setareh, served as checks. Thirteen additional genotypes were included: six registered cultivars (Mahdavi, Roshan, Excalibur, Baharan, Talaei, and Rakhshan) and seven breeding lines developed at Shahid Bahonar University of Kerman (Semi-tall Line, Awned-Roshan, Early-heading-Mahdavi, Three-parent Line, Early-heading-Kalheidari, Semi-dwarf-Roshan, and Semi-tall-Excalibur). To isolate the effects of height and earliness, NILs were generated by repeated backcrossing and selfing so that comparisons reflected differences in a single trait. Wheat was sown in late January using 25 rows per plot (10 cm row spacing; seeding rate 200 kg ha⁻¹). Standard farmer practices for irrigation, fertilization, and pest management were applied. After removing border rows and row ends, a 30 m² area per plot was harvested to determine grain yield. Kernel number per spike and thousand-kernel weight were measured from 200 randomly sampled spikes per plot. Analysis of variance and mean comparisons were performed in R (v4.5.0). Relationships among traits were evaluated through correlation analysis.

Results and Discussion:

Analysis of variance indicated significant genotypic differences for all traits except grain-filling duration. This variation provided a strong basis for identifying heat-tolerant genotypes suitable for late planting. The highest grain yields were obtained from the Semi-tall Line, Awned-Roshan, Early-heading-Mahdavi, and Mahdavi. These genotypes significantly outperformed both check cultivars Sirvan and Khalil. Although the Three-parent Line also produced relatively high yields, its performance did not differ significantly from the checks. In contrast, Semi-tall-Excalibur exhibited very poor yield due to severe grain shattering (>60%), highlighting the importance of assessing shattering resistance under the dry, windy conditions of Orzooeieh. A key finding of this study was the strong positive correlation between plant height and grain yield (r = 0.70**), with plant height explaining 49% of the variation in yield. This pattern was confirmed by NIL comparisons: the tall Roshan NIL yielded 3.81 t ha⁻¹, significantly higher than the semi-dwarf Roshan NIL with 2.56 t ha⁻¹. Under the intense heat conditions of late sowing, dwarf and semi-dwarf cultivars ⁠— although often superior under optimal, high-input environments — suffered from reduced biomass, fewer kernels per spike, and diminished canopy development because their height fell well below the optimal range of 70–100 cm. Conversely, semi-tall and tall genotypes approached this optimal height even under stress, enabling better spike fertility and maintenance of photosynthetic area. Yield-component analysis supported these observations. The highest kernel numbers per spike (30–36) were recorded in high-yielding genotypes such as Semi-tall Line, Awned-Roshan, Mahdavi, Early-heading-Mahdavi, Excalibur, Baharan, Khalil, Rakhshan, Setareh, and Semi-tall-Excalibur. Thousand-kernel weight was also favorable in many high-yielding lines, including Early-heading-Mahdavi (48 g), Awned-Roshan (47 g), Three-parent Line (47 g), Roshan (50 g), and Semi-tall-Excalibur (52 g). NIL comparisons revealed that reduced height directly decreased kernel number per spike, confirming that plant height is a key determinant of spike fertility under high temperature. Unlike height, earliness did not show a significant correlation with grain yield. The NIL pair Mahdavi and Early-heading-Mahdavi produced statistically similar grain yields (4.31 vs. 4.36 t ha⁻¹). Nevertheless, Early-heading-Mahdavi reached the reproductive phase 6–9 days earlier than other high-yielding lines and required one fewer irrigation. Given the water scarcity and high pumping costs in Orzooeieh, this reduction in irrigation input is agronomically and economically valuable. Earlier studies likewise indicate that earliness provides substantial benefits under drought, even when not directly associated with yield increases under well-watered conditions. Integrating yield performance, yield components, phenology, and water-use considerations, Early-heading-Mahdavi was identified as the most promising genotype for late-sown environments in Orzooeieh. However, like its recurrent parent Mahdavi, it remains susceptible to yellow rust. A marker-assisted backcrossing program is currently underway to introgress resistance genes Yr5 and Yr15 into this background.

Conclusions:

This study demonstrates that under the severe terminal heat stress associated with late planting in Orzooeieh, semi-tall cultivars consistently outperform dwarf and semi-dwarf cultivars. Current farmer reliance on semi-dwarf varieties for late sowing should therefore be reconsidered. Achieving an optimal plant height of approximately 70–100 cm is more important than dwarfing alleles under heat-stress conditions, as height directly enhances kernel number and biomass production. Although earliness did not significantly increase yield, its contribution to reduced irrigation frequency and lower production costs makes it a valuable trait for farmers in water-limited environments. Considering all evaluated traits, Early-heading-Mahdavi was the most suitable genotype for delayed planting. release of this line as a new cultivar for late-sown systems in heat-prone regions.