بررسي نقش گلايسين‌بتائين در افزايش مقاومت به خشكي گياه سورگم (Sorghum bicolor L.) در شرايط مزرعه

The role of glycine betain in increasing drought resistance of Sorghum halopens under field condition

گونه هاي گياهي در پاسخ به تنش هاي محيطي اسموليت هاي سازگاري مانند گلايسين‌بتائين، پرولين و ديگر اسيدهاي آمينه را در سيتوپلاسم سلولي تجمع مي دهند كه به‌واسطه آن فشار اسمزي را تنظيم مي كنند. در اين تحقيق نقش گلايسين‌بتائين در افزايش مقاومت به خشكي گياه سورگم تحت تيمارهاي مختلف تنش خشكي موردبررسي قرار گرفت. مطالعه حاضر در سال 1391 در مزرعه تحقيقاتي دانشگاه يزد به‌صورت طرح بلوك كامل تصادفي در سه تكرار انجام شد. تيمار هاي آزمايشي شامل غلظت‌هاي مختلف گلايسين‌بتائين (0، 100 و 200 ميلي مولار) در تركيب با دور آبياري (3، 6، 9 روزه) در نظر گرفته شد. تيمارهاي آبياري بعد از جوانه زني و استقرار بوته هاي سورگم اعمال گرديد. محلول پاشي گلايسين‌بتائين طي دو مرحله بر روي اندام هاي هوايي گياه صورت گرفت. نتايج اين تحقيق بيان گر آن است كه با افزايش غلظت گلايسين‌بتائين در تيمار آبياري 9 روزه، ميزان قند هاي محلول در گياه سورگم به‌طور معني داري در مقايسه با شاهد افزايش يافت. در دور آبياري 6 روز، با مصرف 200 ميلي مولار گلايسين بتائين، ميزان پرولين نسبت به شاهد افزايش معني داري را نشان داد. همچنين با افزايش شدت خشكي ميزان پرولين در برگ هاي سورگم كاهش يافت. كاربرد گلايسين‌بتائين در تيمار 3 روزه آبياري تأثيري در افزايش وزن تر و خشك گياه سورگم نداشت؛ اما استفاده از گلايسين‌بتائين در تيمارهاي 6 و 9 روزه آبياري باعث افزايش وزن سورگم نسبت به تيمار عدم مصرف اين ماده شد. به‌طوركلي نتايج اين تحقيق نشان دهنده نقش مثبت گلايسين‌بتائين در كاهش اثرات كم‌آبياري در گياه سورگم مي باشد.

Introduction Drought is considered as one of the most important factors limiting crop yields around the world especially in the dryland area. The response of plants water stress has been assessed using different physiological techniques, such as chlorophyll, proiline and water soluble hydrocarbons measurement. In response to environmental stress, plant species accumulate different kind of osmolytes like glycine betaine and proline in their cytoplasm and subsequently adjust their cells osmotic potential. Of different osmolytes occurring in plants in response to environmental stresses, glycinebetaine (GB) has been studied extensively. Significant advances have been made in mitigating the inhibitory effects of environmental stresses by exogenously applied glycinebetaine in different plant species. Exogenous application of glycinebetaine as foliar spray has been employed to enhance drought tolerance in a number of crops. This study was conducted to evaluate the role of application of glycine betaine (GB) in improving drought tolerance of sorghum halopens under different stress treatments.   Material and methods The study was performed in an experimental greenhouse at Yazd University in 2012 based on randomized complete block design with 3 replications. Different amount of GB i.e. 0, 100 and 200 mM in combination with various irrigation periods i.e. 3, 6 and 9 days were considered as experimental treatments. Irrigation treatments were applied after germination and establishment of sorghum seedlings. Foliar sprays of GB were applied two times during growth period of sorghum. Different plant parameters including sorghum fresh and dry weight, proline and water soluble hydrocarbons were measured. Proline content of leaves was determined according to Bates et al. (1973). Soluble carbohydrate content was determined in aqueous solution phenol sulfuric acid reagent according to Kochert (1978). After testing the normality distribution of data, two way analysis of variance was used for data analyzing and Duncan multiple range test was used to compare mean of treatment. SPSS (version 16) software and EXCEL were used for data analysis and drawing graphs.   Results and discussion Results indicate that increasing GB concentration cause a significant increase in water soluble carbohydrate at 9 days irrigation period. None significant difference in sorghum carbohydrate content was observed at 3 and 6 days irrigation period when compared to control. Gorham et al., (2000) found that exogenous GB application was very effective in increasing carbohydrate content of cotton. In another study Makhdum and Shababuddin., (2006) reported that exogenous application of GB significantly increase the carbohydrate concentration of bean. Result also indicated that at 6 days irrigation period, with increasing GB concentration the proline content of sorghum leaves significantly increased. In this irrigation treatment, application of 200 mM GB cause a 62 percent increase in proline when compare to control. Exogenous application of GB to lowaccumulating or nonaccumulating plants may help reduce adverse effects of environmental stress. Externallyapplied GB can rapidly penetrate through leaves and be transported to other organs, where it would contribute to improved stress tolerance (Ashraf and Foolad, 2007). According to our result increasing irrigation period from 3 to 6 and 9 days cause a significant decrease in fresh and dry weight of sorghum. The main consequences of drought in crop plants are reduced rate of cell division and expansion that can decrease dry matter accumulation of plant. On the other hand with the development of drought stress, as the plant photosynthesis decreased, plants need more carbohydrate compounds to adjust osmotic potential and subsequently available carbohydrate for grown and accumulation of dry material diminished. At 3 days irrigation period, application of GB had no significant effect on fresh and dry weight of sorghum whereas both GB concentrations significantly increase sorghum fresh and dry weight in water deficiency condition. Application of GB ameliorate the adverse effect of drought stress on fresh and dry weight of sorghum. This can attributed to this fact that GBtreated plants exhibited a slower decrease in leaf water potential during drought stress and developed wilting symptoms much later than untreated plants. Generally, they protect plants from stress through different courses, including contribution to cellular osmotic adjustment, detoxification of reactive oxygen species, protection of membrane integrity, and stabilization of enzymes/proteins (Bohnert and Jensen, 1996). Different studies demonstrated the positive effect of exogenous GB application in decreasing adverse effects of drought stress on dry material accumulation of some plant species such as corn (Wyn Jones, 1984), tomato (Makela et al., 1998(, bean (Webing and Rajashekar, 1999) and cotton (Gorham et al., 2000). Conclusions Overall results indicate the negative effect of drought stress on growth and development of sorghum. Application of GB ameliorate the adverse effect of drought stress and accumulation of proline and soluble carbohydrate in abovegrounds tissue of sorghum improved plant water relation under stress. However, more study on the mechanism of action of this substance under different experimental conditions is still needed. Also it is suggested that the time of application and frequency use of GB investigated in future research.