Agricultural soils in many parts of the world are slightly to moderately are contaminated by heavy metals elements. Among them, Pb has an effect on growth and metabolism of plants with a wide range of adverse effects like decrease in root elongation and biomass, accelerated leaf senescence, inhibition of chlorophyll biosynthesis and seed germination, interferes with nutrient uptake, influence the net photosynthetic rate and respiration and alternate permeability of cell membrane. So, the usage of a fast and safe method is necessary to remove pollutants with the lowest cost and impact on the environment. One of the best methods is phytoremediation that the Brassicaceae family plants such as canola (Brassica napus L.) have a special ability to refine contaminated soils due to their much biomass production. On the other hand, bioremediation is one of the methods that has been considered in recent years to remove contaminants. In this context, it is important to use a variety of microorganisms like algae, fungi and bacteria in order to improve phytoremediation efficiency. Therefore, the purpose of this research was to evaluate the effect of Piriformospora indica and Trichoderma longibrachiatum on improving the growth and some physiological traits of canola under lead stress.
This research was done as factorial layout based on a completely randomized design with three replications. Treatments were four levels of lead from lead nitrate source (0. 500, 1000 and 1500 mg kg-1 of soil) and three treatments of fungi (control, Piriformospora indica and Trichoderma longibrachiatum). After 60 days of (Hyola 401 cultivar) planting time, coinciding with the end of vegetative growth and before flowering, Vegetative and some physiological traits were measured. To quantify the effect of lead stress regression analysis and linear and segmental models were used.
The results showed that node and leaf number, stem diameter, leaf fresh weight and stem dry weight (between 20 and 58%) were reduced as linearly and segmental model where lead levels increased from zero to 1500 mg. Also, inoculation of Piriformospora and Trichoderma fungi increased stem diameter and dry weight. Using the mentioned fungi had a positive effect on vegetative traits at different levels of lead. Accordingly, plant height, leaf area, stem and shoot fresh weight and leaf and shoot dry weight reduced from 27, 74, 68, 65, 69 and 70% in control to 12, 44, 35, 36, 52 and 52% in symbiosis with Piriformospora and 31, 48, 58, 53, 52 and 53% after applying Trichoderma at in 1500 mg as compared to the zero levels, as well. Also, the electrolyte leakage rate showed an increasing trend with 0.000084 slope Up to 500 mg and then with a 0.0018 slope. But plant inoculation with Piriformospora and Trichoderma reduced the electrolyte leakage with about two percent. In addition, the interaction between lead and fungi was significant on SPAD reading, chlorophyll a+b, relative water content (RWC) of leaves and proline. SPAD reading and chlorophyll a+b declined from nearly 18 and 35% in non-inoculation treatment to about 7 and 27% in Piriformospora, and 13 and 5% in Trichoderma in 1500 mg as compared to the control, respectively. Among morphological and physiological traits, the highest correlation (r=0.80; P<0.01) was between leaf area and RWC.
In conclusion, results indicated that vegetative traits showed more sensitivity to lead toxicity. Among these traits, the highest sensitivity was recorded in stem and shoot dry weights. Inoculation of canola seed with Piriformospora and Trichoderma fungi ameliorated this sensitivity in some traits. Accordingly, it seems that these fungi can improve slightly the adverse effect of lead toxicity and increase canola tolerance to lead stress.
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