Inoculation with arbuscular mycorrhizal fungi enhances growth of Litchi chinensis Sonn. trees after propagation by air-layering

Lychee (Litchi chinensis Sonn.) is typically propagated by air-layering mature tree branches which are potted in fertilized, soil-free media after cutting. The size of these branches, low phosphorus retention by pot substrates, and fertilization all might combine to preclude benefits of arbuscular mycorrhizas to lychee.

In order to examine the potential of lychee to benefit from arbuscular mycorrhizas in an agriculturally realistic context, lychee air-layers were grown for 469 days in ca. 95-1 pots of soil-free substrate inoculated with field-collected arbuscular mycorrhizal roots or not at two different levels of phosphorus fertilization.

High phosphorus fertilization (a one-time addition of ca. 1.32g 1^-1 slow-release triple-superphosphate) had no detectable effects on mycorrhiza formation, lychee survival, net CO₂ assimilation, or growth. Inoculation with indigenous South Florida arbuscular mycorrhizal fungi improved leaflet expansion as early as 120 after inoculation, and subsequently enhanced height growth and leaf production but did not affect stem diameter growth, net CO₂ assimilation, or survival.

At harvest, although mycorrhizal colonization was low (average 7.4% colonized root length), mycorrhizal plants had 39% higher above-ground dry weight than control plants. Below-ground dry weights did not differ, but innoculated plants had lower fine root to dry leaf weight ratios than control plants. Leaflets of inoculated plants had higher concentrations of P, K, Cu, and Zn, and lower concentrations of Ca, Mg, and Mn than those of control plants, but total Kjeldahl nitrogen and iron concentrations did not differ significantly 10 months after inoculation. Mycorrhiza enhancement of lychee growth occurred even though phosphorus clearly was not limiting for growth.

Our observations suggest that in this soil-free medium, arbuscular mycorrhizal fungus enhancement of copper and iron nutrition improved lychee growth.