اثر تنش خشكي بر عملكرد،‌ اجزاي عملكرد و آنتي‌اكسيدان در دو اكوتيپ سير (Allium sativum L.) با تراكم هاي كاشت مختلف

The Effects of Drought Stress on Yield, Yield Components and Anti-oxidant of Two Garlic (Allium sativum L.) Ecotypes with Different Planting Densities

تنش خشكي يكي از معمول‌ترين و مضر‌ترين تنش‌هاي غير زيستي است كه مي‌تواند به صورت جدي موجب كاهش توليد محصول گردد. به منظور بررسي اثر تنش خشكي و تراكم كاشت بر عملكرد و اجزاي عملكرد و محتوي آنتي‌اكسيدان دو اكوتيپ سير (Allium sativum L.)، آزمايشي به صورت اسپليت- فاكتوريل در قالب طرح آزمايشي بلوك كامل تصادفي با سه تكرار در مزرعه تحقيقاتي سمنان در سال زراعي 91-1390 انجام شد. اثر سه سطح تنش خشكي (100، 80 و 60 درصد نياز آبي)، به عنوان فاكتور اصلي و تركيب سطوح فاكتوريل شامل سه تراكم كاشت (30، 40 و 50 بوته در مترمربع) و دو اكوتيپ سير (طبس و طرود)‌، به عنوان فاكتور فرعي، بر محتواي آنتي‌اكسيدان، عملكرد سوخ،‌ درصد ماده خشك و اجزاي عملكرد اعم از وزن تر و وزن خشك سوخ، قطر سوخ، طول سوخ، وزن تر و وزن خشك سيرچه، قطر سيرچه، طول سيرچه و تعداد سيرچه در سوخ مطالعه گرديد. نتايج نشان داد كه اعمال تنش خشكي موجب كاهش معني‌داري بر عملكرد سوخ، درصد ماده خشك و اجزاي عملكرد شد. همچنين با افزايش تنش خشكي آنتي‌اكسيدان افزايش يافت. وزن تر (855/1 گرم) و خشك (635/0 گرم) و طول (118/2 سانتي متر) سير‌چه در اكوتيپ طرود نسبت به وزن تر (512/1 گرم) و خشك (557/0 گرم) و طول (862/1 سانتي متر) سير‌چه در اكوتيپ طبس بيشتر بود، اما تعداد سير‌چه‌‌ها در سوخ در اكوتيپ طبس (993/9 عدد) بيشتر از اين تعداد در اكوتيپ طرود (104/9 عدد) بود. عملكرد سوخ در تراكم كاشت 50 بوته در مترمربع (44/9660 كيلوگرم در هكتار) به طور معني‌داري بيشتر از دو تراكم كاشت ديگر بود. عملكرد سوخ به طور معني‌داري داراي همبستگي مثبتي با تمام اجزاي عملكرد بود. از نتايج چنين بر ‌مي‌آيد كه اعمال تنش خشكي باعث كاهش عملكرد و اجزاي عملكرد گياه سير نسبت به شاهد گرديد كه اين كاهش با تشديد سطح تنش افزايش يافت.

Introduction Drought stress reduces plant growth by affecting various physiological and biochemical processes, such as photosynthesis, respiration, translocation, ion uptake, carbohydrates, nutrient metabolism and growth promoters. Garlic (Allium sativum L.) is an annual bulb crop that has been cultivated since ancient times and was used as a spice and condiment for many centuries. Garlic is an important plant because of its pharmaceutical properties. The optimum yield of this bulb crop depends on well-managed irrigation, fertilization and cultivation practices. In the final and middle stages of growth, garlic is sensitive to water stress and low irrigation is unsuitable in these stages. This experiment was established to study the influence of drought stress and planting density on yield and its components and the non-enzymatic anti-oxidant content of two different garlic ecotypes. Materials and methods This study was conducted in 2011-2012 in a farmland at the south east of Semnan city. The experimental layout was a split-plot factorial with a randomized complete block design with three replications. The treatments were comprised of three factors: irrigation rates (60%, 80% and 100% of estimated crop evapotranspiration (ETC)) as the main plot and the factorial combination of three levels of planting density (30, 40 and 50 plants.m-2) and two ecotypes (Tabas and Toroud) as the sub-plots. To estimate the crop water requirement, different meteorological parameters were collected from Semnan weather station and were used based on FAO-56 water irrigation calculation instructions. After harvesting, ten garlic plants were sampled randomly in each plot and bulb yield components were measured. To calculate the leaves anti-oxidant content, DPPH method was used. The statistical significances of mean values were assessed by analysis of variance and LSD tests at p?0.05. All calculations were performed using SAS and Mstat-C softwares. Results and discussion Drought stress decreased bulb yield and dry matter percentage significantly. Planting density had significant effects on bulb yield and the yield of planting density of 50 plants m-2 were significantly higher than two other densities. The interaction of drought stress and ecotype factors affected the dry matter percentage. Drought stress decreased fresh and dry weight, length of bulbs and the bulb diameter significantly. Drought stress decreased fresh and dry weight, diameter, length and number of cloves significantly as well. Drought stress is an important limiting factor at the initial phase of plant growth and establishment. It affects both elongation and expansion growth (Shao et al., 2008). Fresh and dry weight and length of cloves were significantly higher in Toroud ecotype. In contrast, the number of cloves in the bulb was significantly higher in Tabas. Studying the correlation coefficients showed that the bulb yield per unit area was significantly and positively correlated with diameter, weight and length of cloves and bulbs and also the dry matter percentage at P?0.001. The maximum correlation belonged to yield at the unit area and weight of the bulb (r=0.72). In other words, any bulb-weight-increasing factor did have the highest effect on increasing the yield per unit area as well. Drought stress, increased leaf non-enzymatic anti-oxidant significantly. Anti-oxidants plays significant roles in ROS scavenging and influences cellular ROS balance. Activation of antioxidant system helps the plants to tolerate stress form induced damage. The effect of ecotype was significant on anti-oxidant content and the value were significantly higher in Toroud ecotype. Toroud ecotype showed resisting reactions against higher levels of drought stress by increasing the non-enzymatic anti-oxidant content and created tolerating mechanisms versus stress. Conclusion Drought stress reduced yield and yield parameters and increased non-enzymatic anti-oxidant content of garlic. The increment of anti-oxidant content showed the tolerance of garlic to drought stress. The maximum bulb yield was obtained at the highest planting density.