PLANT GROWTH PROMOTING ABILITIES OF TWO BACTERIAL ISOLATES ON RICE GROWN UNDER SELECTED AGROECOLOGICAL REGIONS OF LOW COUNTRY WET ZONE OF SRI LANKA

Sri Lankan rice cultivation depends on the large scale use of inorganic fertilizers with the  introduction of new improved rice varieties in 1960s. Excessive use of chemical fertilizers,  despite of the recommendation of the Department of Agriculture, Sri Lanka, is a common practice among most farmers, which is accused of environmental, health and social issues. The necessity of alternative fertilizers has emerged as an important topic in Sri Lankan agriculture. Plant growth-promoting bacteria are recognized as an eco-friendly approach to  replace inorganic fertilizer. The lack of knowledge towards development of efficient  biofertilizer for different agro-ecological regions is a major issue in developing biofertilizer for Sri Lankan rice cultivation. Therefore, the objective of this study is the characterization of two bacterial isolates (I-I and I-II) for their plant growth promoting abilities and evaluation of their potential on improved rice under greenhouse condition and field condition in Low Country Wet Zone of Sri Lanka. Both bacterial isolates exhibited potential for plant growth promoting abilities, including phosphate solubilizaion,  exopolysaccharide production, protease activity and catalase activity. Bacteria isolate one (I-1) was a Gram-positive bacterium of short rods and produced endospores while isolate two (I-2) was a Gram-negative bacterium, which did not produce endospores. The endophytic nature of inoculated bacteria was confirmed through Scanning Electron Microscopic pictures. Sri Lankan improved rice variety Bg 300 positively responded for both bacteria inoculation under in vitro condition. Above experiment was extended under greenhouse condition with three improved rice varieties of Bg 300, Bg 379/2 and At 308.  The number of grains per first panicle and days to flowering under greenhouse condition were positively affected by inoculation. The bacteria inoculated plants of Bg379/2 reported significant lower number of days to flowering (DF) of 112.5±0.86 and 114±0.28 in contrast to control plants of 118±1.22-DF.The Bg 300 plants inoculated with bacteria also flower earlier compared to control plants.The plants of Bg 300 and Bg 379/2 with I-1, produced the highest number of grains per first panicle (GP) as 134±7.35 and 157±11.2 respectively over those of the control plants of Bg 300 and Bg 379/2 (105.75±6.98 and 132±2.94 respectively). Above experiment was further extended to the field level with improved rice variety Bg 300 in two locations. The experiment was carried out with  treatments of I-1 inoculation, I-2 inoculation, recommended dose of inorganic fertilizer (F)  and recommended dose of inorganic fertilizer with each isolate (I-1+F and I-2+F) and non inoculated non-fertilizer respectively. Bacteria inoculation could replace inorganic  fertilizer for the yield of rice variety Bg 300 under field condition in the Low Country Wet Zone (WL1 and WL2). The Plots inoculated with isolate 1 produced the highest yield in Horana and Mapalana as 12.75kg ± 0.25/30 m2 and 7kg ± 0.2/30 m2  respectively. Bacterial isolate 2 produced yields of 12.5kg ± 0.3/30 m2 in Horana field and 6 kg ± 0.2/30 m2 in Mapalana, which were significantly higher than negative  control.There is a potential for tested bacteria to be further developed as a biofertilizer in the future. 

Keywords - Low Country Wet Zone of Sri Lanka, Inorganic fertilizer, Plant growth  promoting bacteria, Rice, Yield