بررسی تاثیر کاربرد باکتری های ازتوباکتر کروکوکوم و سودوموناس پوتیدا بر رنگیزه های فتوسنتزی و عملکرد گندم در شرایط تنش کادمیوم

آلودگی ناشی از فلزات سنگین یکی از مشکلات اساسی جوامع بشری در تولید محصولات کشاورزی است و به‌عنوان عامل اساسی تهدید سلامت بشر به شمار می‌رود. پژوهش حاضر به‌منظور بررسی امکان کاهش اثرات سمیت ناشی از آلودگی خاک با کادمیوم به‌وسیله کاربرد گونه‌های باکتریایی جنس ازتوباکتر و سودوموناس انجام گردید. برای این منظور، غلظت‌های صفر، 75، 150 و 300 میکرومولار کلرید کادمیوم به همراه باکتری‌های محرک رشد (ازتوباکتر کروکوکوم و سودوموناس پوتیدا) به دو رقم گندم (گنبد و کریم) اعمال شدند. صفات موردبررسی شامل محتوای رنگیزه‌های فتوسنتزی (کلروفیل a، b و کل) رنگیزه‌های کمکی (کارتنوئیدها، فلاونوئیدها)، عملکرد کوانتومی، spad، وزن خشک ساقه، عملکرد دانه، وزن دانه و تعداد دانه است. آزمایش‌ در قالب طرح کامل تصادفی (crd) با سه تکرار و در شرایط گلخانه ای انجام شدند. نتایج تجزیه واریانس صفات موردبررسی نشان داد که تنش کادمیوم موجب کاهش رنگیزه‌های فتوسنتزی، فلاونوئیدها، عملکرد کوانتومی و شاخص سبزینگی گردید اما میزان کارتنوئیدها افزایش یافت. حضور باکتری‌های محرک رشد موجب تخفیف اثرات تنش کادمیوم بر گیاهان گندم شد به‌طوری‌که شاخص‌های مذکور در گیاهان تیمار تلقیح شده با باکتری‌ها به‌صورت معنی‌داری افزایش نشان دادند

Evaluation of the effect of Azotobacter crococum and Pseudomonas putida on photosynthetic pigments and wheat yield under cadmium stress

IntroductionHeavy metal pollution is one of the major problems of human societies in the production of agricultural products and is considered as a major threat to human health. Wheat is a strategic crop in human nutrition and the growing global population requires more agricultural production. To achieve high yields of crops, especially in developing countries, chemical fertilizers and pesticides are widely used, which causes excessive accumulation of heavy metals in agricultural soils and has detrimental effects on human health and other living organisms. Increasing the concentration of heavy metals in the soil on the other hand affects the toxicity and growth and yield of crops. Considering the mentioned cases and the importance of wheat as a major human food, the present study aimed to investigate the effect of plant growthpromoting rhizobacteria (GPR) in reducing the effects of cadmium in two wheat cultivars and their effect on plant pigments. It was done by examining the change in the content of plant pigments. Materials and methodsThe experiment was carried out as a factorial experiment in a completely randomized design with three replications in Research Greenhouse of Agricultural Faculty of Mohaghegh Ardabili University in 2018. Experimental treatments included cadmium chloride stress (CdCl2H2O 0, 75, 150 and 300 μM) and the effect of growth promoting bacteria (Azotobacter and Pseudomonas). The studied traits included content of photosynthetic pigments (chlorophyll a, chlorophyll b, total chlorophyll and chlorophyll a/b ratio), adjuvant pigments (carotenoids, flavonoids), quantum yield, SPAD, stem dry weight, yield, seed weight and number of seeds. Results and discussionsThe results of analysis of variance of the main photosynthetic pigments (chlorophyll a, chlorophyll b and total chlorophyll) at 24, 48 and 72 hours after cadmium treatment and shoot showed that the interactions of the cultivar in bacteria at 72 hours on chlorophyll a (5% level) and was significant on chlorophyll b and total chlorophyll (1% level) . Comparison of means showed that the highest amount of chlorophyll b (without change compared to control) and total chlorophyll (4.63% compared to control) of Karim and chlorophyll a (95.3% compared to control) in Gonbad cultivar in the range 72 hours were observed. The highest amount of chlorophyll a was 6.31 mg / g and total chlorophyll (7 mg / g) was observed from Gonbad cultivar and inoculation with Azotobacter, which was not significantly different from the control. In the case of chlorophyll b, the lowest amount (0.57 mg / g) was obtained from the use of Pseudomonas in Gonbad cultivar and bacterial inoculation in Karim cultivar had no significant effect. The superiority of Karim cultivar over Gonbad cultivar is probably due to the genetically precocious of Karim cultivar and faster maturation process. In addition, different wheat cultivars have significant differences in cadmium accumulation in their organs because plants absorb more cadmium in stress conditions by secreting siderophore to compensate for nutrient deficiencies. Gonbad cultivar is probably more sensitive due to the absorption of heavy metals, which in turn produces more reactive oxygen, and consequently oxidative stress and degradation of photosynthetic proteins and a decrease in chlorophyll content. Among bacteria, genus pseudomonas increases plant phosphorus uptake due to its high ability to dissolve insoluble mineral phosphate. Phosphorus, as an energy carrier, increases the uptake and transport of nitrogen to the leaves, as a result in the production of higher amounts of chlorophyll. Pseudomonas may have produced more chlorophyll by increasing the uptake of trace elements by stimulating the activity of the ATPase protein pump and converting insoluble phosphate into a plantusable form. The interaction of cultivar in bacteria was significant for carotenoids at 24 hours, flavonoids at 24, 72 and shoot stage at 1% level and for carotenoids at 72 hours and quantum yield at 48 hours at 5% level showed significance. Comparison of the mean interaction of cultivar in bacteria also showed that the highest amount of carotenoids (0.67 mg / g) was obtained in 24 hours from Gonbad cultivar and inoculation with Azotobacter, while in 72 hours of control treatment in Karim cultivar the highest carotenoids (0.61 mg / g) and did not differ significantly from Pseudomonas treatment. The decrease in carotenoids is probably due to their role in the nonphotochemical suppression of excited chlorophylls, which disrupts the structure of these pigments and ultimately reduces the amount of these pigments. Cadmium appears to act as a degradation agent for pigments and other macromolecules by increasing ROS accumulation and degrading photosystem II. Nonphotochemical suppression of excited chlorophylls can be another cause of degradation and reduction of auxiliary pigments. Among the main stressful effects on stem dry weight, yield, seed weight and number of seeds, the effect of cadmium on these traits (except number of seeds) was significant at the level of one percent. The interaction effect of cultivar on bacteria on stem dry weight (at 1% level) number of seeds (at 5% level) was significant and in other cases no significant. Regarding stem dry weight, Azotobacter was more effective than Pseudomonas on Karim cultivar and the number of seeds of Karim cultivar increased more in the presence of bacteria than the control. This is probably due to the higher resistance of Karim cultivar to cadmium toxicity and also due to the higher efficiency of carbon cycle enzymes in this cultivar. ConclusionsThe results showed that cadmium decreased photosynthetic pigments, increased auxiliary pigments (such as carotenoids) and decreased dry weight, yield, grain weight and number of grains in wheat plants studied. The presence of plant growth stimulating bacteria improved the photosynthetic system, dry weight, seed weight, number of seeds and yield. Application of Azotobacter growth promoting bacterium had the best results, so that in most of the studied traits, it improved the stress effects of cadmium; Therefore, Azotobacter can be used as a bacterium that reduces the stress effects of cadmium in Karim wheat.