Studying the effects of salinity stress on wheat yield and physiological indices under field conditions

Introduction: Wheat, as one of the most strategically important field crops, plays a vital role in ensuring food security for the world’s growing population. However, the production of this valuable crop is continually challenged by various constraints, including abiotic stresses. Among these, soil and water salinity represent a widespread threat, particularly in arid and semi-arid regions such as Iran, posing a serious risk to wheat cultivation. Salinity stress disrupts plant physiological and biochemical processes through a reduction in soil water potential (osmotic effect) and the accumulation of toxic ions such as sodium and chloride (ionic toxicity), ultimately leading to a marked decline in grain yield. This study investigates the effects of different levels of irrigation water salinity on physiological traits (chlorophyll content, proline accumulation, and electrolyte leakage), yield components (number of grains per spike and thousand-grain weight), and final grain yield of the wheat cultivar Chamran 2 under actual field conditions in the semi-arid climate of Ilam, Iran.
Materials and methods: This field experiment was conducted during the 2023–2024 growing season in Ilam (46°26′ E longitude, 33°38′ N latitude, 1174 m above sea level), which has a semi-arid climate (according to the De Martonne classification). The experiment was arranged in a randomized complete block design with four irrigation water salinity treatments (control and 2, 4, 6, and 8 dS m⁻¹) and three replications. Seeds of wheat cultivar Chamran 2 were sown at a density of 400 seeds m⁻² in 3 × 4 m plots (16 rows spaced at 25 cm apart). Salinity stress was imposed from the 3–4 leaf stage until full physiological maturity, with a 15% leaching fraction applied to minimize salt accumulation in the root zone. Flag leaves were sampled at the pollination stage, and total chlorophyll (using 80% acetone and spectrophotometry), proline content (via the ninhydrin method), and electrolyte leakage were measured. Grain yield and its components were determined by harvesting a 2 m² area in each plot. Data were analyzed using SAS software, and mean comparisons were performed using Duncan’s multiple range test at the 5% probability level.
Results: The results clearly demonstrate that irrigation water salinity exerts a profound and statistically significant effect on all morphological, yield-related, and physiological traits of wheat. The uniformity of the field bed (non-significant block effect) and the high precision of the experimental execution (CV < 10%) confirm the reliability of the data. Increasing the electrical conductivity of irrigation water from 2 to 8 dS m⁻¹ reduced grain yield by 42% (from 6250 to 3610 kg ha⁻¹), which was attributed to concurrent reductions in thousand-grain weight (43%) and grain number per spike (35%). At the physiological level, total chlorophyll content declined by 58%, reaching 1.60 mg g⁻¹ fresh weight, indicating severe chloroplast damage. Conversely, proline, as a defensive mechanism, increased nearly fourfold (from 4.2 to 16.5 µmol g⁻¹), while electrolyte leakage, reflecting membrane degradation, rose from 18.5% to 52.6%. Strong correlations between grain yield and chlorophyll content (r = 0.92) and between grain yield and electrolyte leakage (r = –0.94) confirm that membrane disruption and reduced photosynthetic capacity are the primary mechanisms underlying yield reduction under saline conditions. Overall, wheat plants exhibited severe yield losses and irreversible physiological impairments at salinity levels exceeding 6 dS m⁻¹.
Conclusion: Salinity stress, through its deleterious effects on key physiological processes—including chloroplast degradation (58% reduction in chlorophyll), increased electrolyte leakage, and proline accumulation—substantially diminishes yield components and, consequently, grain yield in wheat. The strong correlations among traits suggest that physiological indicators such as chlorophyll content and electrolyte leakage can serve as reliable criteria for screening salt-tolerant genotypes. Given the 42% yield reduction observed at 8 dS m⁻¹ salinity, effective water resource management and the adoption of salt-tolerant cultivars are imperative for sustainable wheat production in arid and semi-arid regions.