Effects of Arbuscular Mycorrhizal Fungi and Ammonium: Nitrate Ratios on Growth and Pungency of Onion Seedlings

ABSTRACT

A pot experiment was conducted to study the growth and pungency of Allium cepa L. grown in Perlite as affected by colonization by the arbuscular mycorrhizal (AM) fungi Glomus versiforme and Glomus intraradices BEG141 and by ammonium:nitrate (NH₄ ⁺:NO₃ ⁻) ratios of 3:1, 1:1, and 1:3 in 4 mM solutions. Plants were harvested when bulb formation commenced. In general, mycorrhizal colonization resulted in increased shoot dry weight, shoot length, sheath diameter, root nitrogen (N) and phosphorus (P) content (except with G. intraradices and a NH₄ ⁺:NO₃ ⁻ ratio of 1:3), shoot N and P concentrations (except with G. versiforme and a NH₄ ⁺:NO₃= ratio of 3:1) and content. Plants inoculated with G. versiforme had higher growth parameters and N and P content than those with G. intraradices, whereas N and P concentrations showed the opposite trends. Growth parameters and N and P content of non-mycorrhizal plants were highest at a NH₄ ⁺:NO₃= ratio of 1:3, while those of plants inoculated with G. versiforme or G. intraradices were highest at a ratio of NH₄ ⁺:NO₃ ⁻ 3:1 or 1:1. Neither mycorrhizal colonization nor proportion of inorganic N species significantly affected bulb enzyme-produced pyruvate or total or organic sulfur (S) concentrations in plant shoots. Colonization by AM fungi made a substantial contribution to onion growth and may not have been directly related to bulb pungency at early stages of plant growth. However, the influence of AM fungi on plant N and P metabolism may have implications for onion flavor at later stages of plant growth.

Keywords:

• plant growth
• enzyme-produced pyruvate
• ammonium: nitrate ratios
• arbuscular mycorrhiza
• onion

ACKNOWLEDGMENTS

This work was funded by the National Science Foundation of China [Projects 30200168 and GZ051-11(155)].

Notes

^a By analysis of variance; ∗∗∗, P < 0.001; ∗, P < 0.05; NS, not significant.

^b ND, not determined: no colonization observed on roots of uninoculated controls.

^a By analysis of variance; ∗∗∗, P < 0.001; ∗∗, P < 0.01; NS, not significant.

^a By analysis of variance; ∗∗∗, P < 0.001; ∗∗, P < 0.01; ∗, NS, not significant.

^a By analysis of variance; NS, not significant.