Jessica Hess and Dr. Von D. Jolley, Agronomy and Horticulture

Mycorrhizae is a symbiotic association between certain species of soil fungi and the root systems of many land plants. Most economically important crops form this association. By colonizing plant roots, mycorrhizal fungi receive a carbon supply from photosynthesis in plant tissues. The plant, by reason of the fine, extracellular, hyphal network of the fungi, is lent a greatly improved access to soil nutrients (especially phosphorus), improves its water relations, and may receive some resistance to soil-borne diseases. Some scientists assert that optimal management of mycorrhizae is an essential part of sustainable agricultural systems (1).

Maize (Zea mays L.) is a mycorrhizal plant. Previous studies in maize and many other crops indicate that different cultivars within a species can have a variable response to mycorrhizae. Some cultivars may never be colonized or may succumb to a parasitic rather than symbiotic relationship with the fungi. Others may be incapable of completing their life cycle without mycorrhizal fungi. Because of the benefits received from mycorrhizae, particularly nutrient uptake improvements, choice of cultivar may have economic and environmental repercussions in impoverished or marginal agricultural areas.

The villages of Salitrón and Corral de Piedra, in the province of Chiquimula, Guatemala, are inhabited by small-scale farmers who cultivate marginal, steeply sloped land. The communities had no electricity at the time of the study for an Honors thesis project in 1999, lack educational opportunities beyond elementary level, and only receive basic health services. Brigham Young University’s Ezra Taft Benson Agriculture and Food Institute works with the village residents to improve agriculture and nutrition. One practice implemented to increase agronomic yield of the main crops, maize and beans, is to introduce new cultivars, including hybrid maize. The Guatemalan hybrid HB-83 is currently replacing much of the native, open-pollinated maize varieties. This study was done to assess the mycorrhizal colonization differences between a commonly cultivated native maize variety, Cushpeño, and HB-83.

The plants were grown at six varied and representative agricultural sites (each a block in the randomized block design) in the two adjacent communities. Triplicate roots samples were collected from Cushpeño and HB-83 after 30 to 45 days of growth and stained for analysis of colonization at CUNORI, a satellite campus of the University of San Carlos in Chiquimula. It was discovered that HB-83 is colonized at a significantly lower rate than Cushpeño (67 and 72%, respectively, Table 1) This difference, though significant statistically (at á=0.05 level), may have little ramification for the mycorrhizal component of the agricultural system, as both rates are on the high end of a wide range of reported colonization percentages for maize (2, 3).

Although available phosphorus was relatively abundant at each site (above the highest levels at which phosphorus fertilization is recommended in the area), it was noted that the difference between the colonization percentages (Cushpeño – HB-83) is strongly, negatively correlated (R2=0.9087) with the concentration of phosphorus in the soil, indicating that HB-83 is less responsive to available phosphorus when regulating the extent of colonization. This may be a negative characteristic because colonization in the presence of available nutrients may lead to parasitism. Yet other nutrients beside phosphorus may be taken up with greater efficiency through mycorrhizae, so this conclusion cannot be drawn without further analysis. Also, mycorrhizal colonization percentages rarely correlate to the actual efficacy of the symbiosis in most crops, though maize is sometimes an exception (4).

Because mycorrhizal colonization percentages do not always correlate with actual benefit to the plant, a plant tissue analysis, which was beyond the scope and time constraints of this study, is needed to determine the actual effect of the observed differences in mycorrhizal association. Only such an analysis can reveal effects on nutrient dynamics of introducing HB-83 to the village agricultural system.

References

• Johnson, N.C. and F.L. Pfleger. 1992. p. 71. In: Mycorrhizae in Sustainable Agriculture. ASA Special Publication, Madison, WI.
• Anderson, E.L., P.D. Milner and H.M. Kunishi. 1987. Journal of Plant Nutrition 10:1349– 1356.
• Hall, I.R. 1978. New Zealand Journal of Agricultural Research 21:517–519.
• Toth, R., T. Page and R. Castleberry. 1983. Crop Science 24:994–96.TABLE 1: Mycorrhizal colonization percentages in roots of HB-83 and Cushpeño in the six blocks in the experiment.