عنوان مقاله [English]
Excessive use of chemical fertilizers threatens the environment and leads to production of unsafe food products. Currently, the market share of organic vegetables is constantly increasing due to customer demand for safer and healthier food. Therefore, it is necessary to find alternatives instead of using chemical fertilizers in plant production.
Lettuce (Lactuca sativa L.), belongs to Asteraceae family is considered as one of the most popular salad vegetables as a cool season crop. It is also one of the most important vegetables due to its rapid growth and commercial value. Plant growth promoting bacteria promote plant growth directly by facilitating nutrient uptake through fixing nitrogen, solubiliszation of phosphorus, production of hormones and iron uptake. Arbuscular mycorrhizal fungi establish symbioses with plant roots which help to improve nutrient uptake by the host plant and alter its physiology to withstand external abiotic factors and pathogens. Arbuscular mycorrhizal fungi interactions with bacteria have been reported to enhance plant growth through phosphate solubilization, nitrogen fixation, increased AMF spore germination and colonization of plant roots. The potential of co-inoculation of these two organisms in promoting the growth of horticultural crops like tomato, strawberry, lettuce and spinach have also been reported.
Materials and Methods
In order to compare the effect of growth-promoting bacteria, mycorrhiza fungi and chemical fertilizers on morphological and physiological properties of lettuce (Lactuca sativa cv. New Red Fire), the experiment was carried out in a completely randomized design with three replications. Experiment treatments consisted of 14 treatments including three levels of potassium solubilizing bacteria (Pseudomonas vancouverensis, Pseudomonas koreensis, Pseudomonas vancouverensis + Pseudomonas koreensis) + 100% N and P, three levels of phosphorus solubilizing bacteria (Pantoea agglomerans, Pseudomonas putida and Pseudomonas putida + Pantoea agglomerans) + 100% N and K, and one level of potassium and phosphorus solubilizing combined bacteria (Pantoea agglomerans+ Pseudomonas koreensis+ Pseudomonas putida+ Pseudomonas vancouverensis) + 100% N, mycorrhiza fungi combination of three Glomus species (G. mosseae, G. etunicatum and G. intraradices) + 100% N and K and five chemical fertilizer levels (N100P100K100, N100P100K0, N100P0K100, N100P25K100 and N100P100K25) according to soil test results, and control treatment without bio-mineral fertilizers.
The “New Red Fire” lettuce seeds were surface sterilized with 0.5% (v/v) sodium hypochlorite for 10 min, and germinated at 20ºC. After germination, seedlings of similar size were transplanted singly into pots containing agricultural soil. Plants were grown under greenhouse condition of 60/70% (day/night) relative humidity, 15/18 °C (day/night) temperature. After a growth period of 75 days, plants were removed from the pots. The root system was separated from the shoot and washed to remove adhered soil. Root and shoot fresh weight, chlorophyll, vitamin C, anthocyanin, TA, TSS and nitrate contents were measured.
The results showed that application of potassium and phosphorus solubilizing bacteria and mycorrhiza fungi significantly increased plant growth compared to control plant. The highest fresh shoot weight (increase of 42.3%) and crown diameter (increase of 14.4%) was obtained with application of combined potassium and phosphorus solubilizing bacteria treatment + 100% N fertilizer compared to control plants. Application of Pseudomonas koreensis + 100% N and P resulted in the maximum content of anthocyanin (increase of 62.2%) and total soluble solids (increase of 82.5%) compared to control plants. Ascorbic acid, a well-known antioxidant and organic compound, is an essential vitamin can be obtained from fruits and vegetables. The highest vitamin C content (32.3 mg 100 mL-1) was observed at plant treated with N100P100K100 fertilizer. The maximum titratable acidity content was obtained with application of Pseudomonas vancouverensis + 100% N and P, and combined potassium and phosphorus solubilizing bacteria treatment +100% N fertilizer. The nitrate content is an important quality index of leafy vegetables; low nitrate content is associated with enhanced quality. Application of chemical fertilizers significantly increased nitrate accumulation compared to biofertilizers and control plants, so that the minimum nitrate accumulation content was observed in control plant and combined potassium and phosphorus solubilizing bacteria treatment + 100% N fertilizer (21.14 µg g-1FW), respectively.
With continuous development of economy and society, people pay an increasing attention to the quality of fruits and vegetables. Improving the crop yield means that attention must also be given to improving vegetables quality, resulting in chemical fertilizer nutrition problem. To conclude, our study suggested that after the control treatment (without using any chemical fertilizers), which had the lowest accumulation of nitrate, application of growth-promoting bacteria and mycorrhizal fungi in combination with nitrogen fertilizer caused lower nitrate accumulation than chemical fertilizers treatments. So, the use of combined and pure bacteria treatments and mycorrhizal fungus treatment can be used to improve the growth, quality and antioxidant properties and increase the concentration of nutrients and also reducing nitrate accumulation in lettuce.