واکنش مورفوفیزیولوژیکی ارقام گندم نان تحت تاثیر تنش زیست محیطی خشکی

The aim of this study was to evaluate and select bread wheat cultivars by assessing the yield, yield components and some physiological traits under drought stress conditions. Six cultivars were evaluated as sub-plots in 2018 to 2020 cropping seasons using a split plot design based on RCBD with three replicates. The Main plot covered the three irrigation regimes including normal irrigation (non-drought stress), terminal droght stress (irrigation up to boot- stage as moderate drought stress) and full drought stress (dryland). The results revealed that a significant reuction of the plant height (1.26 and 1.38%), fertile tillers number (1.38 and 2.81%), spike length (2.16 and 4.02%), seeds number per spike (1.25 and 3.74%), seed weight per spike (0.51 and 2.62), 1000-seed weight (12.97 and 29.90%) and grain yield (12.5 and 31%) were recorded under the moderate drought stress and full drought stress, respectively as compared to non-stress conditions. The physiological traits like soluble sugars, proline content, peroxidase and polyphenol oxidase enzyme activity were significantly influenced by the drought stress treatments. Moderate drought stress and full drought significantly increased the soluble sugar content (4 and 10%, respectively), proline content (26 and 45%, respectively), polyphenol oxidase (13 and 21%, respectively) and peroxidase enzyme activity (10 and 22%, respectively). The present study verified that the biochemical parameters need to be considered as better traits to select wheat cultivars for drought tolerance under water stress conditions. However, the wheat cultivars ‘Taktaz’ showed the highest yield potential followed by the Tirgan in both non-stress and moderate drought stress conditions. Rank sum analysis identified the most drought tolerant cultivars as ‘Taktaz’ and ‘Aftab’. The Ardabil (Moghan) cultivar is considered as the most useful wheat cultivar for future drought tolerance breeding programs or similar agro-ecologies.
Introduction
Drought is one of the major production constraints limiting potential crop yields globally. It can severely affect and reduce the yield and productivity of food crops worldwide up to 70%. Recurrent drought is the leading abiotic stress causing reduced production and productivity of wheat especially in the Iran. The mean yield of wheat in Iran is 5.5 tons' ha−1 which is much lower than world average of 6.7 tons' ha−1. The fluctuations in domestic wheat production and productivity causes a deficit in supply to meet national demand during drought years. The demand for wheat in Iran is rapidly increasing because of its growing population, urbanisation, the emergence of agro-processors and increased household income. Improving production and productivity in the low altitude, drought-prone areas of Iran would have a positive impact on national wheat production. The latter is not a realistic option because land is a finite resource and expansion of crop lands would create conflict with other land uses such as livestock grazing or human settlement. Breeding for drought tolerance is a sustainable approach to improving yields in marginal areas. Developing wheat cultivars and technologies suitable for such areas have been prioritized as mitigation strategies.
Although breeding for drought tolerance is recognised globally as an important strategy, there are still very few cultivars with stable drought tolerance expressed under diverse environmental conditions. Drought tolerance is a complex polygenic trait confounded by environmental factors that affect its expression, and the identification of superior cultivars. Furthermore, the unpredictable nature of drought manifestation and the different mechanisms adopted by plants to cope with drought stress complicate the identification of superior cultivars under variable moisture conditions. Field-based empirical selection for drought tolerance is still commonly used. However, the occurrence of drought stress at various stages of plant growth and development have differential effects on yield and yield components, and there is no consensus on the best time to impose drought stress in controlled experiments.
Therefore, the objective of this research was to study the effect of soil water stress on selected morphological, physiological, and biochemical parameters of wheat cultivars and to determine the traits, which can be used for identification of tolerant wheat cultivars under water stress conditions.

Methodology
Two experiments were conducted as a split plot in the form of a randomized complete block design with three repetitions at the Agricultural and Natural Resources Research Center of Moghan during 2018-2019 and 2019-2020. Main plot includes irrigation regime (normal irrigation until the end of the growing season (non-stress), irrigation up to boot stage (moderate drought stress) and full drought stress) and sub plot including 6 line and wheat cultivars including (Taktaz, Tirgan, Araz, Kalateh, Ehsan, Aftab). Wheat cultivars were provided by SPII (seed and plant improvement institute). The panel included 6 cultivars that were being developed for drought stress areas that had International Maize and Wheat Improvement Centre (CIMMYT) origin and five standard checks that were released as drought tolerant cultivars. Planting is usually done early to mid-November, depending on the onset of rainfall in all the study areas. Each cultivar was hand planted in 2 m long rows, spaced at 0.2 m apart. A plot for each cultivar consisted of four rows.
Conclusion
current results showed that water scarcity significantly influenced all the measured traits, while traits linked to morphology, physiology, and biochemistry of the wheat were more influenced by the cultivars. results revealed that the moderate drought stress and full drought stress significantly increased soluble sugar content (4 and 10%, respectively) and increased proline content (26 and 45%, respectively), differences in the synthesis of proline and total soluble sugar with an increase in water scarcity among the selected wheat cultivars showed the diverse ability of the tested cultivars to cope up with drought. N-93-17 and Aftab cultivars among bread wheat were found to have better capacity to endure water stress by synthesizing more proline and total soluble sugar as mitigation strategies. Overall, except for the spike length, the other morphological, physiological, and biochemical parameters could be used as effective indicators to identify tolerant wheat cultivars in the water-limited environments. Moreover, the relative effectiveness of selection could be better when two or more traits are considered than using single trait as an independent factor. Dependability of the observed traits for the selection of tolerant wheat cultivars for specific area could be recommended to be verified under different environmental conditions to solve problems related to water scarcity. Results of this investigation could also provide usable genetic variability for wheat yield improvement and would be of great importance in the selection of appropriate wheat cultivars as well as for selection of desirable parents for the breeding program to develop the wheat cultivars resistant to drought stress conditions