Background

Edaphic factors constrain bean production in many areas of the world. These include large areas in both Latin America and Africa where low soil pH; limited P supply or availability, or micronutrient deficiency limits plant growth. In Africa, crop production may actually mine the soil of its nutrients, farmers commonly applying less N and P as fertilizer than is removed in the grain. Temperature and water availability also limit bean nodulation and growth. The host, the rhizobia and the symbiosis can all be affected, with tolerance to specific edaphic stresses often found in both host and micro-organism. Our goal is to identify bean cultivars and rhizobia tolerant to these different stresses, and by study of their interaction under stress, to achieve better growth and N₂ fixation.

Selection of bean varieties for N₂ fixation at low P

Differences in low P tolerance among cultivars grown in peat moss with rock phosphate as P source (I. Christiansen, 2001).

ANT22 and E295 have been identified as tolerant to growth at 5µM P, supplied via rock phosphate in sphagnum potting mix. These lines showed:

• higher N and P-use efficiency
• higher nodule P concentration
• slight rhizosphere acidification
• active phosphatase production
• rhizosphere population changes

Some microorganisms with the ability to solubilize apatite in culture medium or soil can also be used to improve phosphate availability for crop plants (I. Christiansen, 2001).

P concentration can also affect nodule occupancy, perhaps affecting plant performance (I. Christiansen, 2001), with % recovery of specific clusters of Rhizobium from bean, markedly affected by P supply (see below):

Strain and host cultivar selection for acid-pH tolerance

R.tropici UMR1899, first isolated in Colombia (Graham et al., 1982 1994), is tolerant of a range of stresses, including pH. It has been widely used as an inoculant strain for beans where soil pH is of concern, for example in Brazil. When exposed to acid pH this strain:

• Undergoes change in outer-membrane structure
• Is better able to maintain cytoplasmic pH
• Accumulates glutamate and K⁺ intracellularly
• Exploits a proton-translocating ATPase to remove H⁺ from the cell
• Produces acid-shock proteins (Aarons et al., 1991; Graham et al., 1994, Ballen et al., 1998)
  

Difference in the tolerance of UMR1899 and CIAT 1632 to culture medium of different pH.

Our program is also interested in cultivar differences in nodulation at low pH, and has identified three cultivars (Preto 143, Bico de Ouro and Capixaba precoce) that are capable of effective nodulation at low pH (Vargas and Graham, 1988). We are currently collaborating with the Universidad Nacional de la Plata in Argentina, and Murdoch University in Australia in furthering these studies.

Studies on Mn and Fe deficiency of beans in Ecuador

During our Bean/Cowpea CRSP project in Ecuador (Graham et al., 2003) it was shown that most soils in this area were deficient in zinc. Zinc chelates were recommended as foliar or soil applications and resulted in major increases in yield. Adoption of chelate application has been widespread.

Demonstration plots in Ecuador showing response to soil zinc chelate application. In this trial, yield increased 600 kg/ha with Zn application.