Introduction: Guar (Cyamopsis tetragonoloba L.) is an emerging, drought-tolerant legume with substantial multi-purpose potential, valued across food, pharmaceutical, and industrial sectors due to its galactomannan gum content. Introducing this crop into the cropping systems of arid and semi-arid zones offers a potential strategy for agricultural diversification under resource-constrained scenarios. However, maximizing its grain yield potential in new agro-ecosystems heavily relies on optimizing agronomic practices, specifically planting dates and plant density configurations. Planting date critically determines the environmental conditions encountered by the crop during key phenological phases, while plant density regulates intra-specific competition and shapes canopy architecture. Gaining a comprehensive understanding of how these dual factors concurrently influence agronomic plasticity and phenological dynamics is essential. Therefore, this study was initiated to evaluate the interactive effects of planting dates and plant population densities on the growth trajectories, developmental phases, and ultimate grain yield of guar in a semi-arid region.

Materials and Methods: To achieve these objectives, a field experiment was conducted during the 2023 growing season at two distinct research stations of the University of Birjand (Birjand and Sarayan), located in South Khorasan Province, Iran. The trial was established as a split-plot layout based on a randomized complete block design (RCBD) with three replications at both experimental sites. The treatments comprised three distinct planting dates designated as the main plots: early planting (May 12), mid-planting (June 12), and late planting (July 13). The subplots were assigned to five plant population densities: 10, 20, 30, 40, and 50 plants m-2. Standard agronomic practices, including uniform land preparation and precise siphon tube irrigation, were maintained across all experimental units. Phenological developmental stages were monitored using cumulative growing degree-days (GDD). At crop maturity, quantitative and morpho-architectural traits—including plant height, height to the first lateral branch, stem diameter, number of lateral branches, pods per plant, seeds per pod, and total grain yield—were evaluated. The collected data were subjected to combined analysis of variance (ANOVA), and treatment means were compared.

Results and Discussion: The combined analysis of variance revealed that the three-way interaction of location × planting date × plant density was highly significant (P < 0.01) for grain yield and major yield attributes, indicating highly divergent, location-specific ecophysiological responses. Delaying the planting date to July 13 accelerated thermal unit accumulation, compressing the developmental window from emergence to physiological maturity by 55 days (a 34% reduction), which limited vegetative growth and exposed critical reproductive stages to early-autumn low temperatures, resulting in a 23% reduction in stem diameter and suppressed pod formation. Regarding plant density, the two experimental stations exhibited completely contrasting performance patterns. In the Birjand station, the maximum grain yield was achieved under the mid-planting date (June 12) at the lowest density (10 plants m-2), and increasing plant density in May and June sowings progressively depressed grain yield due to intense intra-specific competition. Conversely, in the Sarayan station, increasing plant density consistently stimulated grain yield across all planting dates, peaking at the highest density of 50 plants m-2 under both May and June plantings, suggesting a requirement for higher canopy populations to optimize resource capture in that environment. Furthermore, instead of linear declines, distinct non-linear adjustment patterns were observed in reproductive and structural traits—such as seeds per pod, pods per plant, and height to the first lateral branch—under specific environmental and density matrices, reflecting complex canopy-microclimate interactions that warrant further investigation.

Conclusion: In conclusion, this study underlines that the agronomic success of guar in semi-arid environments depends on a precise alignment of canopy architecture with seasonal thermal resources, requiring strictly location-specific management strategies. For the climatic conditions of Birjand, a mid-planting window (June) combined with a low density (10 plants m-2) is recommended. Conversely, for the Sarayan region, early to mid-planting dates (May and June) coupled with high plant populations (50 plants m-2) are optimal to maximize grain productivity. Late planting in July is not recommended for commercial production in either region due to severe growth compression and terminal thermal constraints.