Legume bacteriology

• Rubber Research Institute of Malaysia, Malaysia
• Chiang Mai University, Thailand
• Malaysian Agricultural Research and Development Institute, Malaysia
• Khon Kaen University, Thailand
• CSIRO Plant Industry, Australia

Agroforestry is a major concern in the Philippines, and urgent reafforestation of some five million ha of grassland is required. However, some large-scale Bureau of Forest Development plantings of Leucaena spp. for fuelwood have failed. The National Irrigation Administration has also had difficulties establishing Leucaena in reafforestation of catchment areasanother urgent need, as erosion of hilly land is changing the characteristics of the watersheds, and resulting rapid siltation of dams in reducing their storage capacity and usefulness in flood control. Various rapidly growing legume species have potential for these acidic hilly lands, but appropriate plant nutrition is a key element in their establishment and growth, particularly the development of an effective nitrogen-fixing symbiosis with Rhizobium. This may require soil fertilisers, and the parallel development of a mycorrhizal association.
This subproject has three main aims: to determine the interrelations between Rhizobium strains and promising legume tree species and cultivars in different regions of the Philippines and select effective and competitive Rhizobium strains suitable for use as inoculants; to select strain-host cultivar combinations tolerant of soils with low pH and high aluminium levels and/or find other economic management practices to ameliorate this stress; and to determine the response to microbial inoculation of promising tree species in different field environments.
The research will concentrate on 11 tree species: Acacia mangium, A. auriculaeformis, Leucaena spp., Gliricidia sepium, Albizia falcataria, Pterocarpus indicus, Calliandra callothyrsus, Sesbania grandiflora, Enterolobium cyclocarpum, Albizia lebbek and Cassia spectabilis. Researchers at Visayas State College of Agriculture (VISCA) and University of the Philippines at Los Banos (UPLB) will conduct surveys of nodulation status in natural stands in the Philippines and collect nodules for isolation of Rhizobium strains. They will also collect soil samples for determination of pH, organic matter content and simple physical characterisation.
All of the 11 listed species will be planted at up to four locations with and without inoculation, and their patterns of nodulation and dry matter production followed through the season. Soils at these sites will be characterised, and Rhizobium isolated from the nodules at some sites. Following local authentication of the strains, the isolates will be sent to ANU for testing the interrelations between strains, host species and provenances in nodulation and nitrogen fixation. Parallel effectiveness screens will be conducted in pot trials at VISCA.
Research into methods for identifying promising Rhizobium strains will examine: use of naturally occurring antibiotic resistances; selection of mutants with high levels of antiobiotic resistance; and use of serological techniques such as fluorescent antibody labelling and ELISA (Enzyme Linked Immunoabsorbent Assay). Potentially suitable methods will be tested on isolates grown under field conditions in the Philippines.
The scientists will select Rhizobium strains and host plants tolerant of acid soils (in close collaboration with subproject A and ACIAR Project 8375), which will be tested in the Philippines in problem soils in pot and field studies, including some fertiliser treatments.
This subproject will indicate whether lack of suitable Rhizobium strains in the soil is part of the problem of establishing Leucaena on acid soils. It will also develop techniques for selecting suitable germplasm for such soils, and should indicate whether legume tree plantings in the Philippines need inoculation. The techniques developed and information obtained will have application in other countries, such as Indonesia and Australia.
Subproject F: Ecology of Rhizobium Nodulating Grain Legumes in the Philippines (8574)
The Philippines has begun a rapid increase in grain legume production particularly for soybean, with an increase of 100 000 ha by 1987. However, the recommendation to use nitrogen fertiliser for the major grain legumes grown theresoybean, groundnut, mungbean and cowpeaindicates inadequate nitrogen fixation. In recent trials, inoculation with Rhizobium produced large increases in yields of soybean and mungbean crops following sugarcane or rice, and much of the new planting will follow paddy rice.
This subproject will intensify research on the need for inoculation, particularly of soybean and mungbean, in different agroclimatic zones of the Philippines. It will examine whether Rhizobium numbers decrease during the soil flooding in paddy rice culture, how inoculum strains survive in different environmental conditions, and the effects of soil pH, farming systems (upland or lowland), fertilisers, previous cropping history and soybean germplasm type on plant nodulation in the field trials. It will also examine whether soybeans in the Philippines are predominantly nodulated by slow-growing specific Bradyrhizobium japonicum strains or by more promiscuous fast-growing strains with more widespread geographical distribution, and the effect of the host genome on this. Rhizobium isolated at UPLB will be sent to ANU for serological testing with antisera; nodulation characteristics of selected isolates will be assessed in pot trials under semi-sterile conditions with various grain legumes and, for promising strains, with several Philippine soybean and mungbean lines.
Identification of these elite strains will mostly rely on ELISA, but fluorescent antibody techniques and antibiotic markers will also be evaluated. Field trials at selected sites in the Philippines will then determine the competitiveness of identified strains on advanced and local germplasm, and inoculum survival over subsequent seasons. The team will also assess the interaction of pesticide dressings on seeds with response to inoculation, and will examine methods of inoculation to see whether any harmful effects can be overcome.
A 'soil dilution/plant infection/most probable number' method will be used to assess Rhizobium populations in the soils where the field trials are conducted, and to determine how these populations are affected by farming practices (particularly flooding), liming of acid soils and phosphorus fertiliser. Research in Australia will develop methods of counting soil populations that will be suitable for use at UPLB.
Later research will assess the response to inoculation of a local cultivar and a range of advanced selections available from plant breeders when grown in sites with contrasting Rhizobium populations, and also determine the competitiveness of the inoculum strain. If soil acidity is found to be limiting nodulation, then the potential for selecting tolerant Rhizobium and host germplasm will be explored in collaboration with ACIAR Project 8375.
This research will indicate the need to inoculate soybean and mungbean in the Philippines and whether breeders need to take more account of nodulation characteristics in their programs. Should inoculation prove to be of benefit, a follow-up phase will deal with the development of suitable inoculum production and quality-control procedures for use in the Philippines.