The impact of drought stress on the growth, yield, and water use efficiency of white sweet potato in autumn cultivation

Introduction
With the reduction of water resources in Iran, increasing the efficiency of water use in the agricultural sector has attracted the attention of researchers. Regarding water requirements, irrigation requirements, and crop water productivity, it is important for managers and decision-makers to adjust the cultivation information patterns of each region. In Iran, limited research has been conducted on sweet potatoes, and the extent of its production within the country remains unclear. Sweet potatoes are mostly grown in the cities of Minab, Jask, and parts of Sistan and Baluchestan province. In the study by Shamili et al. (2016), the effect of soil texture and irrigation method on improving yield and yield components of two sweet potato cultivars was investigated. Sweet potato is also used as a potential crop for animal feed and raw material in industry. Therefore, the research of Naseri et al. (2014) was conducted with the aim of cultivating sweet potato in the Minab region to produce fresh fodder, which shows the suitable ability of this plant for producing animal fodder. Given that Minab and Jask counties are significant producers of sweet potatoes in the nation, this study aimed to examine the impact of water stress on the growth and yield of sweet potato in the tropical region of Minab county.
Materials and Methods
This study was conducted in the research lysimeters of Minab Higher Education Center in 2024-2025 in a completely randomized design. The study area is located at an altitude of 29 meters above sea level at the latitude and longitude coordinates of 27° 5' 28" and 57° 5' 42", respectively. The study area is a tropical and humid region with a temperate climate in winter, and therefore most of the cultivation is done in the fall and winter. White sweet potato plant cuttings were planted at 40 cm intervals on the ridge on 2024/4/11. The lysimeters were irrigated to meet soil moisture deficiency. Irrigation treatments included (I₁) 120, (I₂) 100, (I₃) 80, and (I₄) 60 percent of the moisture requirement, and were performed in three replications. For proper establishment of the plants, the same irrigation rate was applied for all four treatments up to 40 days after planting, and then irrigation was performed based on moisture stresses. The irrigation rate was determined before irrigation and by using soil sampling from different depths. At the end of the growing season on 27/2/1404, the entire root crop as well as the plant were collected and weighed. Considering that not all produced tubers were suitable for the market and economically viable, the weight, diameter, and length of each sweet potato tuber were evaluated to determine their marketability. Those that met USDA standards were classified as marketable products. To calculate water use efficiency, the definition of irrigation water use efficiency (IWUE) was used.
Results and Discussion
Some parameters such as root weight, root dry weight, and total dry matter were significantly affected by drought stress, and other parameters such as irrigation water use efficiency based on tuber and total dry matter were not significant. The weight of the harvested tubers for treatments I₁ to I₄ was found to be 5846, 5224, 2852, and 2069 kg ha^-1, respectively. Additionally, the IWUE for all harvested tubers for treatments I₁ to I₄ were determined to be 1.20, 1.26, 0.97, and 0.83 kg m^-3, respectively. Drought stress up to 80% moisture supply (I₃) had no significant effect on yield reduction. A similar trend was observed for foliage fresh weight, with the difference that drought stress only caused a significant difference between the maximum stress (I₄) and minimum stress (I₁) treatments. Excessive water consumption in treatment I₁ only increased the weight of foliage and had no effect on the total weight of harvested tubers. Despite the fact that water stress had no effect on the IWUE of all tubers, the IWUE value for marketable tubers was significant. According to the results, the IWUE value of treatments I₁ and I₂ was at the same level, but applying stress in treatments I₃ and I₄ significantly reduced the IWUE of marketable tubers. The amount of applied water in treatment I₁ was 479 mm, treatment I₂ was 408 mm, treatment I₃ was 304 mm, and treatment I₄ was 241 mm, which was applied based on the supply of soil moisture deficiency. The actual evapotranspiration amount during the growing season was found to be 565, 494, 390, and 334 millimeters for treatments I₁ to I₄, respectively.
Conclusion
The effect of drought stress on the fresh weight of harvested tubers was significant. However, this effect was not significant on the IWUE. From this, it can be concluded that the sweet potato plant has the ability to adapt to drought conditions, such that with increasing drought stress, the IWUE remained constant and did not decrease. The product obtained in the present study was also evaluated in terms of marketability. The results showed that by separating the marketable product, the IWUE decreases. Also, by applying water stress, the IWUE decreases significantly. In other terms, water stress leads to a reduction in the quantity of marketable products, which has resulted in a notable decline in the Irrigation Water Use Efficiency (IWUE). Given the importance of economic issues, it is recommended that an economic analysis of this research be conducted to determine the impact of drought stress on crop profitability. Also, the response of other sweet potato cultivars to drought stress also requires a more comprehensive study.