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Legumes belong to one of the most important and third largest plant family Fabacae. These have unique association with root bacteria rhizobia, which are responsible for biological nitrogen fixation. Lentil (Lens culinaris) is an important pulse crop in many Asian countries including Pakistan, where the diet is mostly cereal based. Lentil has the third-highest level of protein, by weight and also contains dietary fiber, vitamin B1, and minerals (Callaway, 2004).
Main focus in agriculture is on increasing the production of food crops to meet the requirement of increasing population. Many efforts are being made in this scenario including mineral fertilization to maximize the productivity of crops. Sustainability in agriculture is also desired along with increase in productivity.
Higher plants are associated with diverse groups of bacteria (Khalid et al., 2006a). These bacteria are capable of competing soil rhizosphere for all factors required by them and during their association with plants they make effects in all possible ways i.e. positive, negative or neutral (Patel et al., 2008).
Among these who have positive effect and involved in stimulating the plant growth are termed as plant growth promoting rhizobacteria (PGPR) (Arshad and Frankenberger, 1998). Many bacteria take part in association with plants and play role in plant growth.
Along with increasing plant growth, PGPR also increase nutrient use efficiency and produce resistance against biotic and abiotic stresses (Ashraf et al., 2007). Soil productiveness and plant physical condition can be assessed by the presence and interactions of PGPR with plants in root zone (Jeffries et al., 2003). Plant growth is positively affected by hormones which change the endogenous phytohormones production. These enzymes also include 1-aminocyclopropane-1-carboxylate (ACC)-deaminase which regulates ethylene, so make effects on plant growth (Arshad and Frankenberger, 2002 and Glick, 2005). Plant growth is enhanced by lowering the ethylene concentration (Arshad and Frankenberger., 1990).
Ethylene at level i.e. 10 Âµg L-1 promote root initiation while its higher levels i.e. at 25 Âµg L-1 restrict the root growth. (Ma et al., 1998). The production /synthesis of ethylene in plants are increased in case of environmental stresses (Glick et al., 1998). The ultimate precursor of the ethylene is ACC which is produced under the stress condition and increase the level of ethylene in plants and inhibits the growth processes of plant.
So, the increased nodule development or propagation of primary or lateral roots can be achieved by using rhizobacteria, as it inhibit the ethylene production by ACC-deaminase. Therefore, co-inoculation of PGPR and rhizobium on lentil crop may improve its productivity. Co-inoculation not only reduces the production of ethylene by PGPR having ACC-deaminase but also improves the nitrogen fixation capacity, as more infection sites would be available to the rhizobium by increased root growth due to PGPR. So, co-inoculation of PGPR and rhizobia may be effectual in increasing legume crop production.
From the above discussion, the present study has been designed with the following objective:
Evaluation of the impact of co-inoculation of rhizobium with ACC-deaminase containing PGPR to enhance growth, productivity and nodulation of lentil in contrast to separate inoculation of Rhizobium and PGPR.
Lentil is from family Fabaceae and is termed as legume or pulse. In sustainable agriculture, legumes have their own importance for fixing nitrogen and are grown in crop rotation. These crops are mostly cultivated in arid and irrigated areas.
Certain biotic and abiotic factors influence plant growth including water scarcity, depletion of soil nutrients and quality of ground water (Brackish water), fungal disease, temperature variation, inadequate moisture and non-working environment of rhizobium species are the main factors which reduce the yield of legumes in Pakistan (Canci and Taker, 2009).
Ethylene is synthesized in all plants and necessary to regulate different plant growth stages (Khalid et al., 2006). Ethylene is an unsaturated hydrocarbon that mediates different plant metabolic processes (Arshad and Frankenberger, 2002). Ethylene effects on plants can be characterized by many factors e.g. ethylene concentration, growth stage of plant and the process. Plants give full positive response to low concentration of ethylene for its growth (Khalid et al., 2006b).
Ethylene is the growth limiting hormone at higher concentrations and its rapid synthesis is a result of different types of the stresses. (Arshad et al,. 2008). When plant faces any stress, the ethylene production is more, this may cease the different growth processes of plant (Norastehnia et al., 2007). The excessive production of ethylene can be used as a marker for determining stresses and also to reduce its effects on plant growth (Arshad et al., 2008). Synthesis of ethylene becomes fast, when the plant is under stress like heat, chilling, disease, physical damage, and lack of nutrients due to pathogenic attack.
A model was developed to describe different effects of stress ethylene on plants by Pierik et al. (2006). In this model, a small peak is developed after some time of stress initiation i.e. after few hours, and second peak produced after long time, i.e. after 1-3 days. The first peak is very small in comparison to second peak in magnitude and this also enables the plant to get resistance to its associated genes (Ciardi et al., 2000). It is thought that the first peak in the model shows the ACC reserve present in the plant body (Robison et al., 2001). But the second ethylene wave is so big, as it starts plant growth inhibition processes such as chlorosis, senescence and abscission. The production of the second peak of ethylene is due to enhanced transcription of ACC-synthase gene activated by different cues (Yang and Hoffman, 1984).
Nodule development and other processes of the plant are very much affected by ethylene (Shahroona et al., 2006). It also reduces growth of nodules in several legumes (Nukui et al., 2000). Cell division in plant roots also badly affected by the ethylene, and stops the division of the cortical cells (Lee and Larue., 1992 a, b) and also stops infection in legumes (Spaink, 1997). Exogenous ethylene ceases nodule formation in many plants such as Pisum sativum, Lotus japonicas, and Medicago setiva (Peters and Cristestes., 2001). Inhibition of ethylene improves nodulation (Yuhashi et al., 2000). Ethylene is produced at higher level in ripening fruits, although it is synthesized in whole plant (Arshad and Frankenberger, 2002).
The effectiveness of R. leguminosarum for improving growth and yield of lentil was studied by Huang and Erickson (2007). They reported significant increase in grain yield of lentil, seedling height and shoot and nodule biomass. Similarly, Ahmad et al. (2008) reported the effect of different methods of rhizobial inoculation on root nodulation, yield and seed protein contents of two lentil varieties (Masoor-93 and Massor-2002). The variables such as root nodulation, seed protein contents, N-contents of soil, N-contents of root and shoot of lentil, seed yield and yield components were significantly affected by inoculation with rhizobium.
Microorganisms are utilized to make plant growth better in all over the world (Khalid et al., 2009). Plant growth promoting rhizobacteria involves both direct and indirect processes to improve plant growth. Direct processes involved in nitrogen fixation, making nutrient mobile through synthesis of phosphatases, siderophores or organic acids and phytohormones (Beneduzi et al., 2008). Where as, indirect processes include the improvement in resistance mechanism of plant against pathogen and developing beneficial aspects of plant growth. Multiple traits are also found in few bacteria which enhance plant growth and one can overtake on others (Shahroona et al., 2008). Plant growth enhance by different actions and processes of PGPR on various crops (Nadeem et al., 2006; Gilick et al., 2007; Yang et al., 2009). Nodulation in legumes can be improved by using PGPR containing ACC-deaminase which may inhibit the ethylene synthesis and increase the nitrogen fixation by developing the symbiosis (kazaki et al., 2004).