اثر سطوح مختلف مولیبدن بر برخی صفات بیوشیمیایی و تجمع نیترات در گیاه اسفناج (spinacea oleracea l)

نگرانی قابل توجهی در مورد بروز بیماری‌های ناشی از مصرف سبزی‌های حاوی نیترات و نیتریت زیاد مانند اسفناج وجود دارد و این هنگامی اتفاق می‌افتد که میزان جذب نیترات از میزان مصرف آن توسط گیاه فراتر رود. در این پژوهش اثر سطوح مختلف مولیبدن بر میزان انباشت نیترات در اسفناج در شرایط کشت بدون خاک بررسی شد. این مطالعه به‌صورت یک آزمایش به‌صورت فاکتوریل در قالب طرح کاملاً تصادفی در سه تکرار انجام گردید. فاکتور اول شامل نیترات به‌صورت نمک نیترات کلسیم در سه سطح شامل صفر (نیترات هوگلند)، 10 و 20 درصد نیترات هوگلند و فاکتور دوم نیز شامل غلظت مولیبدن به‌صورت نمک مولیبدات سدیم در چهار سطح شامل صفر، 0/5، 1/5 و 3 میکرومولار بودند. صفات بیوشیمیایی مورد مطالعه شامل (ph، اسیدیته قابل تیتراسیون و مواد جامد محلول) و عناصر غذایی (نیتروژن، فسفر، آهن، پتاسیم و مولیبدن) میزان نیترات و فعالیت آنزیم نیترات ردوکتاز در ریشه و برگ بودند. براساس نتایج اسیدیته قابل تیتراسیون، مواد جامد محلول و ph با افزایش مولیبدن تا سطح 3 میکرومولار افزایش یافت، اما کاربرد هم‌زمان مولیبدن و نیترات اثر معنی‌داری بر صفات مذکور نداشت. بیش‌ترین میزان نیترات برگ (2474/3 ‌گرم در کیلوگرم وزن تر) و ریشه (9191/2 گرم در کیلوگرم وزن تر) در تیمار ترکیبی نیترات 20 درصد و مولیبدن شاهد مشاهده گردید. از طرفی بیشترین میزان فعالیت آنزیم نیترات ردوکتاز برگ در مولیبدن 3 و 1/5 میکرومولار و هر سه سطح نیترات بود. بیشترین میزان عناصر نیتروژن (3/14 درصد)، فسفر (0/064 درصد)، آهن (0/004 درصد) و پتاسیم (0/78 درصد) در تیمار ترکیبی مولیبدن 3 و نیترات 20 درصد اضافی مشاهده گردید و میزان عنصر مولیبدن در تیمار مولیبدن 3 میکرومولار و نیترات شاهد در بیش‌ترین حالت خود (0/549 میلی‌گرم در کیلوگرم) بود. به‌طور کلی کاربرد مولیبدن در محلول غذایی اثر کاهنده روی تجمع نیترات و اثر افزاینده روی فعالیت آنزیم نیترات ردوکتاز در داخل گیاه اسفناج داشت، بنابراین افزودن 3 میکرومولار مولیبدات سدیم به محلول غذایی هوگلند در سیستم هیدروپونیک برای تولید اسفناج جهت دست‌یابی حداقل میزان انباشت نیترات توصیه می‌گردد.

the effect of different molybdenum levels on some biochemical traits and nitrate accumulation in the spinach plant (spinacea oleracea l.)

extended abstract 1.    introduction: there is significant concern regarding the occurrence of diseases caused by the consumption of vegetables  high in nitrate and nitrite, such as spinach. nitrate accumulation is a common problem in most leafy vegetables and it happens when the amount of nitrate (no-3) absorption exceeds the amount consumed by the plant. different nitrogen (n) levels significantly affect the activities and expressions of n metabolism enzymes and concentration of mineral elements in crop plants. molybdenum (mo) is an essential micronutrient for plants, playing a critical role in various physiological processes, growth, and development. although plants require molybdenum in very small amounts compared to macronutrients, it is crucial for several key functions within plants. molybdenum is an essential component of enzymes involved in nitrogen metabolism and other biochemical processes. molybdenum deficiency affects the amount of nitrogen in plants, and its presence reduces nitrate accumulation in their tissues. this element is an essential part of the nitrate reductase enzyme necessary to reduce nitrate nitrogen to the amino form, resulting in more and effective utilization of nitrogen by plants. nitrogenase and nitrate reductase, two enzymes crucial to nitrogen-fixing and nitrogen reduction, need molybdenum to carry out their respective activities. molybdenum is required as a co-factor for nitrogenase enzymes in symbiotic n fixation, where it catalyzes the redox process that transforms elemental n into ammonium (nh4+) ions. in this study, the effect of different molybdenum concentration on nitrate accumulation in spinach plant under soilless conditions was investigated.2.    materials and methods: this study was conducted as a factorial experiment with two factors in a completely randomized design with  three replications. the first factor was the concentration of  nitrogen in the  form of calcium nitrate salt [ca (no3) 2.4h2o] at three levels including 0 (control), 10 and 20%. and the second factor was  molybdenum concentration in the  form of sodium molybdate salt [na2mo4o] at four levels including 0 (control), 0.5, 1.5 and 3 μm, all added to the hoagland’s standard solution. therefore, 12 treatments were applied, and three plants were considered for each replication, so that 108 plants were totally used. depending on the plants growth rate, the nutrient solution was applied once a day at the beginning of the experiment, but it was increased to 2 times a day at the end of the experiment, when the plants biomass had been increased. the harvesting index in spinach were the size and the number of leaves before flowering, which is recognized as the commercial harvesting stage. finally, the measured traits included certain biochemical properties, some mineral elements,  and the amount of nitrate as well as the activity of nitrate reductase enzyme in root and leaf.3.    results and discussion: based on the obtained results, titratable acidity (ta), soluble solids contents (tss) and ph increased with increasing molybdenum up to the level of 3 μm, and the simultaneous application of molybdenum and nitrate did not have a significant effect on the mentioned traits. the interaction effect between nitrate and molybdenum on leaf and root nitrate levels, and leaf nitrate reductase enzyme activity was significant at the probability level of 1%. the highest amount of nitrate in leaf (3.2474 g kg-1 fw) and root (2.9191 g k-1g fw) was observed in the combined treatment of 20% nitrate and control  molybdenum. on the other hand, the highest level of nitrate reductase enzyme activity was observed when 3 and 1.5 μm molybdenum applied for each of  three level nitrate. the highest amount of  nitrogen (3.14%), phosphorus (0.064%), iron (0.004%) and potassium (0.78%) elements was obtained from the combined treatment of 3 μm molybdenum and 20% nitrate. the amount of molybdenum element in the control nitrate and 3 μm molybdenum treatment was at its highest level (0.549 mg kg-1 fw).4.    conclusion: in general, the use of sodium molybdate can be recommended as the way to reduce the accumulation of nitrate and, besides, molybdenum enrichment in spinach increases the nutritional value of this plant. based on the results, the use of molybdenum in nutrient solution had a reducing effect on nitrate accumulation and an increasing effect on nitrate reductase enzyme activity inside the spinach plant. therefore, adding 3 μm sodium molybdate to the hoagland nutrient solution in the hydroponic system, can be recommended for growing spinach to achieve the minimum amount of nitrate accumulation.