Effect of the reduction of phosphorus fertilizer for Citrus iyo orchards on the development of vesicular-arbuscular mycorrhizae and the quality of fruit.

ISHII, TAKAAKI¹, SUNAO KIRINO², MING ZENG³, JIRO AIKAWA³, ISAO MATSUMOTO³ & KAZUOMI KADOYA³

¹Fac. Educ., Ehime Univ., JAPAN, ²Onsen Seika Agr. Coop, JAPAN, ³Fac. Agri., Ehime Univ., JAPAN.

We reported that satsuma mandarin trees which were inoculated with vesicular-arbuscular mycorrhizal (VAM) fungi grew larger and had better fruit quality as compared with non VAM control trees under low concentrations of applied phosphorus (P) condition (Shrestha et al., 1996). The formation of VA mycorrhizae, however, is inhibited by a great amount of fertilizers, especially P. This experiment was conducted to evaluate the effects of the reduced P fertilization for Citrus iyo orchards on the development of vesicular-arbuscular mycorrhizae and the quality of fruit.

The number of VAM spores in the soil and the percentage of VAM infection in the roots increased with the reduction of P. Sugar content in the juice, the ratio of sugars to acids, and the a/b value of peel color in the plots of -P and -P+CH increased as compared with the control. The reduction of P also affected the content of carotenoids in the peel. These results suggest that we should revise large-scale consumption of chemical fertilizers and consider the effective application of fertilizers. In particular, the reduction of P fertilizer affected the growth of VAM fungi and increased the percentage of VAM infection in citrus roots. Insoluble P fertilizer such as bone dust would be useful for the propagation and maintenance of VAM fungi.