Evaluation of Waterlogging Tolerance in Twenty-One Cultivars and Genotypes of Bread Wheat (Triticum Aestivum L.) and its Effect on some Physiological Characteristics of Shoot and Root System at the Three-Leaf Stage

Covering a staggering 215 million hectares, wheat stands as the world's most extensively cultivated crop plant. Just like its botanical counterparts, wheat operates as an obligate aerobic organism, implying its reliance on absorbing oxygen from the surrounding environment to facilitate growth, proliferation, and the successful completion of its life cycle. Annual instances of waterlogging stress inflict harm upon wheat crops, attributed to inadequate irrigation practices, subpar drainage systems, uneven field leveling, elevated groundwater levels, the presence of unyielding impermeable layers, and bouts of intense, abrupt rainfall. This adverse impact is progressively escalating, potentially influenced by the ongoing shifts in climate patterns. Consequently, the adoption of resilient cultivars and the genetic enhancement of bread wheat assume critical importance. These strategies are aimed at augmenting the wheat's capacity to effectively cope with waterlogging stress, aligning it with the mounting demands of a burgeoning global population.To achieve these goals, it is necessary to understand the factors causing waterlogging stress damage in wheat and to know the mechanisms of tolerance in this plant. The survival of root terminal meristem cells under waterlogging stress conditions is very limited, and their ability to grow again after removing the stress is also restricted. Waterlogging stress leads to the death of primary roots and reduced growth of lateral roots in wheat. However, there is variation among wheat cultivars concerning these traits. Reduced access to oxygen hampers root growth and nutrient absorption, including nitrogen. Consequently, photosynthesis and carbohydrate availability decrease, further restricting root growth.

An outdoor pot experiment was conducted to investigate the effect of waterlogging stress on shoot and root dry matter, as well as some physiological characteristics. The experiment followed a split-plot design based on randomized complete blocks with three replications. The stress was applied at the three-leaf stage, and three control levels were used: no waterlogging stress, mild stress (48 hours of waterlogging stress), and severe stress (120 hours of waterlogging stress) as the main factors. Cultivars and genotypes were also included as secondary factors.During the stress period, the water level was maintained at approximately 5 cm above the soil level. The cultivation took place outdoors in plastic pots. Data analysis was performed using SAS software, and graphs were generated using Excel software. Comparisons between treatments were based on the standard error. After testing different models, the linear regression model was ultimately employed.

Mild and severe waterlogging stress resulted in a significant decrease in shoot dry matter of 14.06% and 38.37%, respectively, across all cultivars and genotypes. Different cultivars and genotypes exhibited varying responses to waterlogging stress. To further understand the reasons for these differences, among the 21 cultivars and genotypes, Mehrgan and Sarang cultivars, as well as ms 93-16 and ms 93-6 genotypes, were selected due to their contrasting tolerance levels and yield potential. These selected cultivars and genotypes were studied to analyze specific root traits.Amidst severe waterlogging stress, a significant 38% reduction in root dry matter and a corresponding 29% decrease in root volume were recorded when compared to stress-free conditions. This closely mirrored the decline evident in shoot dry matter. Evaluation of the susceptibility index during the three-leaf stage unveiled that sole resilience was exhibited by the Aflak cultivar. In contrast, the remaining cultivars and genotypes were stratified into semi-tolerant and semi-susceptible categories.Notably, regression analysis underscored that even brief periods of waterlogging stress ushered in a reduction in dry matter. Furthermore, the elongation of the waterlogging duration magnified this decrease in dry matter, thereby mitigating the disparities across various cultivars and genotypes.

In general, cultivars that were able to sustain higher levels of photosynthetic activity during waterlogging stress demonstrated a lower percentage decrease in dry matter. Although the Mehrgan cultivar experienced a significant reduction in dry matter yield and fell into the semi-sensitive group, it consistently exhibited significantly higher dry matter yield compared to other cultivars and genotypes across all treatments.