Document Type : Research Article
Authors
Department of Plant Production and Genetics, University of Agricultural Sciences and Natural Resources of Khuzestan, Mollasani, Iran
Abstract
Introduction
The growing global awareness of the environmental and health risks associated with synthetic chemicals has renewed interest in natural resources for pharmaceuticals, industry, and sustainable dye production. Indigo (Indigofera tinctoria L.), a member of the Fabaceae family, has re-emerged as a crop of interest due to its dual medicinal and industrial value. Traditionally cultivated in Iran, indigo leaves yield a stable natural blue pigment widely used in textiles and cultural heritage preservation. Although synthetic dyes largely displaced indigo in the nineteenth century, contemporary ecological and economic challenges have revived its importance as an eco-friendly alternative. Indigo thrives in warm, frost-free environments but is highly sensitive to both cold and heat stress, particularly during reproductive development. In semi-arid regions such as Shushtar County, Khuzestan Province (Iran), high summer temperatures, soil salinity, and water scarcity impose major constraints on cultivation. Sowing date and plant density are key agronomic factors that determine how crops adapt to these stresses by influencing phenological development, reproductive success, and resource use efficiency. This study aimed to assess the effects of sowing date and plant density on reproductive traits and seed yield of indigo under semi-arid conditions to identify optimal management strategies for sustainable production.
Materials and Methods
Two field experiments were conducted over two consecutive seasons (2022–2023) at distinct sites: Shahid Bahonar Agricultural School (Year I) and Shahid Sharafat Greenhouse Complex (Year II). Both experiments employed a split-plot design within a randomized complete block design (RCBD) with three replications. Sowing date was assigned to main plots, and plant density to subplots. In 2022, six sowing dates (April 29, May 19, June 8, June 28, July 18, and August 7) were evaluated in combination with four plant densities (30, 40, 50, and 60 plants m-²). Late sowings (July–August) performed poorly and were excluded in 2023, when only the first four dates were tested with the same densities. Seeds of the local Jiroft ecotype, known for resilience in arid environments, were sown directly in sandy loam soils. Initial soil analysis in 2022 indicated alkaline conditions (pH 7.56, EC 2.04 dS m-¹), which improved in 2023 (pH 7.18, EC 1.59 dS m-¹). After thinning at the 3–4 leaf stage, stands were adjusted to target densities, and manual weeding was conducted throughout the season. Standard agronomic practices were followed, including furrow irrigation and basal fertilization with urea, diammonium phosphate (DAP), and potassium sulfate. Measured traits included pods per plant, seeds per pod, thousand-seed weight (TSW), and seed yield (g m-²). Grain yield was harvested from the central subplot area to avoid border effects. Data were analyzed using ANOVA, least significant difference (LSD) test at 5% probability, correlation analysis, and interaction analyses with SAS v9.4 and SPSS v26.
Results and Discussion
Sowing date imposed a much stronger effect than plant density on reproductive performance and yield (p< 0.01). The earliest sowing date (April 29) consistently produced the highest yields: 105.3 g m⁻² in 2022 and 114.2 g m⁻² in 2023. In contrast, the latest sowing (August 7) yielded only 47.5 g/m², representing more than a 50% reduction. Delayed sowing after mid-July exposed plants to severe heat stress during flowering and pod filling, shortening the growing period and sharply reducing reproductive output. Plant density significantly interacted with sowing date (p< 0.05). Medium density (40 plants m-²) maximized yield by balancing efficient resource use with minimal competition. Higher densities (≥60 plants m⁻²) intensified intra-specific competition, reducing yield by 10–12%, particularly under late sowing. Conversely, very low densities (<30 plants m-²) underutilized available resources. Among yield components, pods per plant showed the greatest sensitivity. Under optimal conditions (early sowing, 40 plants m-²), plants produced up to 126 pods, but this decreased by 45% with late sowing due to heat-induced pod abortion. Seeds per pod also declined sharply (from 9.8 under early sowing to 5.9 under late sowing), reflecting high-temperature stress during pollination and seed set. In contrast, TSW remained stable (4.42–4.59 g) across treatments, indicating strong genetic control and limited responsiveness to environmental or management factors. Correlation analysis confirmed that seed yield was strongly associated with pods per plant (r= 0.94) and seeds per pod (r = 0.87), whereas TSW had only a weak relationship (r = 0.32). Thus, reproductive structure abundance was the primary driver of yield variability. Year-to-year comparisons demonstrated notable improvements in 2023. Reduced soil salinity (EC decline from 2.04 to 1.59 dS m⁻¹) and lower pH enhanced nutrient uptake and physiological efficiency, supporting greater pod and seed set. Additionally, exclusion of late sowing dates in 2023 minimized exposure to extreme heat stress, further improving performance relative to 2022.
Conclusion
The results highlight the decisive role of sowing date in determining reproductive performance and yield of Indigo under semi-arid conditions. Early sowing (April 29) combined with moderate density (40 plants m-²) consistently improved yield and reproductive efficiency, while delayed sowing and high densities markedly reduced performance. The year-to-year improvement further demonstrated the positive contribution of enhanced soil quality to reproductive success. Overall, this study emphasizes that precise agronomic management—particularly early sowing and optimal density—ensures stable and sustainable indigo production in hot, stress-prone environments. Given the growing global demand for natural, eco-friendly dyes and medicinal plants, these findings provide a strong scientific basis for the revival and expansion of indigo cultivation in Iran and similar semi-arid regions.
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