The Effect of Natural and Enriched Periphyton by Plant Growth Promoting Microorganisms on Rice Growth Indices in a Greenhouse Experiment

Traditionally, most researches on artificial wetlands focuses only on two phases, i.e., water and soil, ignoring or underestimating the importance of the microbial aggregates between them, the third phase in artificial wetlands. Filamentous algae grow substantially by attaching themselves to the soil/sediment surface and form aggregates with bacteria, fungi, and other micro- and meso-organisms (e.g., protozoa and metazoa), known as periphytic biofilms. Actually, periphytic biofilms are ubiquitous in wetlands (including paddy fields) and play a major role in regulating nutrient cycling in these ecosystems. However, their effects on rice plant growth indices have been rarely reported. Therefore, the aim of this study was to investigate the effect of natural periphyton and enriched periphyton with plant growth-promoting microorganisms on germination and rice growth indices.

To achieve the objectives of this study, first three periphyton samples were collected from paddy fields of Guilan province and grown in BG11 medium in the laboratory. Then, the superior bacteria and fungi that stimulated plant growth were isolated from the periphyton mass and the periphyton samples were enriched with these microorganisms. The greenhouse experiment was conducted in 1399 in the research greenhouse of the Department of Soil Science and Engineering, University of Tehran. For this purpose, 2.5 kg pots with a length of 25 cm and a diameter of 15 cm were selected for greenhouse experiments. Twenty rice seeds (Oryza Sativa: cultivar Hashemi) were placed directly in pots in three replications for each treatment. About 100 mL of natural periphyton suspension grown in BG11 medium (equivalent to 2 g/kg dry weight) and periphyton enriched with superior bacteria and fungi and a mixture of bacteria + fungi was added to the pots. Experimental treatments included WP treatment:  without periphyton, PN1 epipelon 13, PN2 epiphyton 8 and PN3 epipelon 6, PN1 + Bacillus cereus, PN2 + Talaromyces stipitatus and PN3 +Acinetobacter calcoaceticus + Talaromyces minioluteus. Also, phosphorus fertilizer treatment at four levels (F0: without phosphorus fertilizer, F1: supply of 50% of phosphorus fertilizer from mono calcium phosphate source, F2: supply of 100% of mono calcium phosphate and F3: supply of 100% of rock phosphate source) was added to the pots. The treatments were grown in three replications in a greenhouse with a temperature between 25–35 °C for 30 days in 80 percent humidity.

The results showed that the presence of natural and enriched periphyton in the soil increased the germination index of rice seeds compared to the control by about 1.4 and 1.9 times, respectively. The rice seedlings decreased by about 10 percent with the presence of natural periphyton, but this negative effect was lowered by enrichment of plant growth promoting microorganisms. Measurement of rice seedling height index showed that the highest measured height was related to the treatment of periphyton enriched with Bacillus cereus (PSB) at F2 level of 39 cm. In general, in natural periphyton treatments, about 15 percent of the height of rice shoots was lower than the control treatment and periphyton treatments enriched with superior microorganisms. The maximum measured root length was related to the treatment of recombinant periphyton with Bacillus cereu (PSB) with 16.95 cm. The lowest root length was measured in natural periphyton treatment (PN3) with 11.5 cm. The highest shoot to root ratio was related to periphyton treatment enriched with Bacillus cereus at F2 and F3 fertilizer levels with 0.7 and 0.68, respectively, with a statistically significant difference at level 95 percentage (p˂ 0.05) compared to other treatments. The maximum weight of rice biomass in periphyton enriched with Bacillus cereus (PSB) treatment, which was the enriched with plant growth promoting bacterium, was measured at F2 and F3 fertilizer levels with 8.7 and 8.3 g, respectively. In general, measurement of rice seedling height, root length, biomass weight and phosphorus in shoots showed that treatment of enriched periphyton with Bacillus cereus at different levels of phosphorus fertilizer had a positive effect and led to an increase of 3.5, 9.4, 6.1, and 35.8 percent compared to the control treatment, respectively.

Finally, it can be concluded that periphyton enriched with superior plant growth microorganisms can be used as a multi-functional biofertilizer in paddy fields, it increases plant growth and efficiency of fertilizer, and it can also control weeds in paddy fields.