Association of endophytic diazotrophs in rice genotypes from varied soil environment and estimation of their biological nitrogen fixation potential using 15N dilution methods

Date

10-1995

Abstract

Biological nitrogen fixation system comprised of endophytic diazotrophs to rice is likely to have the advantage over free-living and associate systems since the N2-fixed through them may be directly assimilated by the plant without much loss. Using different rice genotypes and different soils three pot experiments and a laboratory experiment were conducted with the following objectives, a) to study the diversity of endophytic microorganisms in roots and submerged portion of selected genotypes, b) to isolate the endophytes and assess their nitrogenase activity by ARA and nif-PCR, c) to study the invasion and colonization of selected rice endophytes on rice using light microscopy, and d) to quantify the biologically fixed nitrogen in rice genotypes using 15N dilution methods.

All rice genotypes tested harboured endophytes both in the root and submerged stem. Among the rice genotypes, Oking seroni harbored the highest number of root endophytes while IR 42 and IR. 74 harbored the highest number of stem endophytes. The incidence of endophytes over total heterotrophs in the root was 0.003-0.113% and 1.5-5.0% in the stem. Submerged stems of IR 74 harbored the highest number of endophytic diazotrophs. King'B media favored the growth of the highest number of root endophytes. Root endophytes did not grow in LGIP medium whereas stem endophytes grew well in this medium. Rice root extract media seems to be effective in isolating endophytic diazotrophs. Screening of rice endophyte collection revealed that 14% of the isolates harbored nifD sequences, 10% of the isolates were ARA positive.

Endophytic isolates were effective in bringing out changes in root hair morphology and able to invade the lateral roots and epidermal cells. The colonization pattern of endophytes on IR 42 rice can be summarized as follows, 1) distribution of individual bacteria or discrete colonies on epidermal cells and root hair, 2) longitudinal distribution with 2-7 cells arranged end to end along epidermal cells, 3) distribution at emerging sites of laterals and root hairs, and 4) localization at the cortical cells of the root (intracellular).

The δ15N values perfectly correlated with atom % 15N excess values of enriched 15N for the rice genotypes grown in Casiguran and Maahas soils. Milagrosa, OR 142-99 and Pinidwa showed consistently lower δ15N as well as 15N enrichment. There was highly significant correlation between the Ndfa calculated by using reference genotype and 15N enrichment of soil available N in all the three soils under 15N enrichment method. This suggests that without a suitable non-fixing reference plant, Ndfa can be more reliably calculated by using 15N enrichment of soil available N. The percent Ndfa for the rice genotypes estimated using reference plant across three 15N enriched soils available N. The percent Ndfa for the rice genotypes estimated using reference plant across three 15N enriched soils ranged from 4.2-32.2% and 6.1-44.6% under natural 15N abundance method. The genotypes Milagrosa, Oking seroni, IR 29337-36-3, OR 142-99 and Pinidwa gave high percent Ndfa using both methods. In addition, IR 42 showed high percent Ndfa using natural 15N abundance method. OR 142-99, a potential N2 fixing genotype, showed high nitrogen use efficiency and high N accumulation rate. Correlation of endophytic bacterial counts with 15N parameters revealed a significant positive correlation of endophytic diazotrophs' bacterial counts in root and stem with total N uptake and percent Ndfa. This clearly indicates the involvement of both root and stem endophytes in dinitrogen fixation.

Document Type

Dissertation

Degree

Doctor of Philosophy in Soil Science

College

Graduate School (GS)

Adviser/Committee Chair

Erlinda S. Paterno

Co-adviser

Jagdish K. Ladha

Committee Member

Restituta P. Robles, Henry P. Samonte

Language

English

LC Subject

Rice--Soils, Nitrogen--Fixation

Location

UPLB Main Library Special Collections Section (USCS)

Call Number

LG 996 1995 S72 M35

Notes

Doctor of Philosophy (Soil Science)

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