Host related determinants of vesicular-arbuscular mycorrhizal dependency in selected species of Leucaena and Sesbania

Abstract

Characterizing mycorrhizal dependency (MD) of plant species is essential not only for successful utilization of vesicular-arbuscular mycorrhizal (VAM) fungi in agriculture but also for determining the relationship between MD and plant characteristics. Pot experiments were conducted: i) to evaluate MD of selected species of Leucaena and Sesbania, based on P levels established in the soil solution and ii) to determine the relationship between MD and plant specific characters such as root morphology, rhizosphere pH reduction, external and internal P requirements and rate variables associated with P uptake, P utilization and growth. Since the knowledge of P levels established in soil solution is essential for studies involving VAM symbiosis, a greenhouse study was conducted to develop a P sorption curve. A phosphorus sorption curve developed by subjecting soil:sand mixture to three cycles of wetting and drying after liming and to three more cycles after the addition of P and other nutrients was observed to predict P concentrations established in soil solution more accurately than a P sorption isotherm developed in the laboratory. Mycorrhizal dependency of selected species of leucaena and Sesbania was determined at soil P levels of 0.002, 0.02 and 0.2 mg/L, in a fumigated oxisol, inoculated or uninoculated with the VAM fungus Glomus aggregatum. Based on MD values and response of plant species in terms of total dry matter yield and tissue P concentration at these soil P levels, plant species were categorized into 5 distinct MD groups, namely highly dependent, dependent, moderately dependent, marginally dependent and independent. At soil P level of 0.02 mg/L, a stepwise regression model containing root dry weight, root hair length, root density, root diameter and root hair incidence as components explained 85% of the variability in MD in these plant species. Root characteristics, such as length, surface area, density and root hair incidence were negatively correlated with MD. In contrast, root diameter was positively correlated. Since all plant species reduced pH of agar media, there appears to be no relationship between MD and the ability of these plant species to reduce rhizosphere pH. Root colonization levels and the ability to absorb P per unit root surface area were not significantly correlated with MD. Under nonmycorrhizial conditions, k. leucocephala, a plant species highly dependent on VAM fungi, was observed to have higher external and lower internal P requirements than those of Sesbania pachycarpa, a plant species marginally dependent on VAM fungi. This experiment involving six soil solution P levels indicated that MD could be predicted accurately based on soil solution P concentrations, since soil solution P concentration explained 97% of the variability in MD of plant species used in this study. A sequential harvest experiment was conducted to study the relationship between MD and rate variables associated with P uptake, P utilization and growth. The results of this study indicate that highly dependent plant species, such as L. leucocephala, have low growth rate, low P uptake rate and high P utilization efficiency under nonmycorrhizial conditions. These results illustrate that soil solution P levels could serve as a basis for categorizing plant species into distinct MD groups. Among plant characteristics studied, root morphological characters were observed to be the most important determinants of MD.