The effects of growth promoting rhizobacteria and nitrogen fertilizer on growth indices of bean (Phaseolus vulgaris L.) under water shortage stress conditions

Common bean (Phaseolus vulgaris L.) is the world’s most important food legume crop. This staple considered as a nearly perfect food mainly because of its high protein content (about 25 percent) and abundant fiber, complex carbohydrates (about 60 percent), and other daily food needs such as vitamins (folate) and minerals (Cu, Ca, Fe, Mg, Mn, and Zn). Water use in agricultural production as one of the most important environmental factors affecting plant growth and development, especially in arid and semi-arid climatic conditions of Iran. 

In order to study the effect of plant growth promoting Rhizobacteria and nitrogen chemical fertilizer on some characteristics of root and growth indices of bean (Phaseolus vulgaris L.) under water stress conditions, a split-plot design based on RCBD with three replications was conducted during growing season of 2015-16 at Agricultural Research station, College of Agriculture, Ferdowsi University of Mashhad, Iran. Three levels of irrigation (100%, 75%, 50% water requirement) assigned to main plots and different types of biofertilizers (Nitroxin®, containing Azotobacter sp. and Azospirillum sp., Biophosphor®. containing Phosphate-solubilizing bacteria Bacillus sp. and Pseudomonas sp.), Nitrogen fertilizer) urea form, and Control (no fertilizer) were assigned to sub plots. Destructive sampling was performed to calculate the growth indices (such as TDW (total dry weight), LAI (leaf area index), CGR (crop growth rate), RGR (relative growth rate) and NAR (net assimilation rate)) randomly from competing plants regarding the marginal effects from 1 real completed leaf stage to the end of the growing season from 5 m2 surface (every 7 days; 12 steps). The growth indices of LAI, CGR, RGR and NAR were calculated using equations (1-4). At the end of the growing season SRL (specific root length) was determined by Tenant Modified Method. Leaf area calculated by Leaf Area Meter device (Delta T, UK). Data analysis of variance and figures preparation were done by Minitab Ver. 16, Slide Write Ver. 2, and Excel 2010 softwares. At the end, Means comparing did by Duncan's test at probability 5%.
 
Equation 1:LAI= (1/GA)[(LA2+LA1)/2]                                                                           
Equation 2:CGR= (1/GA)[(W2–W1)/(t2–t1)]                                                                                
Equation 3:RGR= (lnW2–lnW1)/(t2–t1)                                                                            
Equation 4:NAR= [(W2–W1)/(t2–t1)] [(lnLA2–lnLA1)/(LA2–LA1)]                                 
where GA is ground area (m2), LA is leaf area (m2), W is shoot dry weigh (g) and t is time (day).

According to the result, the effect of biological fertilizers especially Nitroxin significantly increased maximum values of bean`s growth indices included total dry matter (TDM max), leaf area index (LAI max), crop growth rate (CGR max), relative growth rate (RGR max) and net assimilation rate (NAR max) compared to control and even nitrogen fertilizer. So that the highest and the lowest total dry matter (TDM max) at 91 days after planting were observed in Nitroxin (370 g.m-2) and control (342 g.m-2) traits (p≤0.01), respectively. The highest and the lowest leaf area index (LAI max) at 91 days after planting were observed in Nitroxin (2.0) and control (1.7) traits (p≤0.01), respectively. The highest and the lowest crop growth rate (CGR max) at 77 days after planting was observed in Nitroxin 11.04 g.m-2.day-1 and control 8.81 g.m-2.day-1 traits (p≤0.05), respectively. The highest and the lowest relative growth rate (RGR max) at 21 days after planting was observed in Nitroxin 0.23 g.g-1.day-1 and control 0.20 g.g-1.day-1 traits (p≤0.01), respectively, and then decreased gradually. Similar to RGR, the highest and the lowest net assimilation rate (NAR max) at 21 day after planting was observed in Nitroxin 35.5 g.m-2.day-1 and control 28.4 g.m-2.day-1 traits (p≤0.05), respectively. All attributes, showed highest values at 100% water requirement treatment. Also, the effect of fertilizers (p≤0.05) and water requirement (p≤0.01) were significant on grain yield. The main and interaction effects of fertilizer and irrigation were significant (p≤0.05) on specific root length (SRL). So that, the highest specific root length (30.15 m.25cm-3 soil) were obtained from Nitroxin and 100% water requirement. 

In total, the results showed that it could be possible to produce the healthy production of bean, moreover, attain the optimum yield as equal as to conventional systems.