Application of Mycorrhiza and Soil from a Permaculture System Improved Phosphorus Acquisition in Naranjilla

Abstract

Naranjilla (Solanum quitoense) is a perennial shrub plant mainly cultivated in Ecuador, Colombia, and Central America where it represents an important cash crop. Current cultivation practices not only cause deforestation and large-scale soil degradation but also make plants highly susceptible to pests and diseases. The use of arbuscular mycorrhizal fungi (AMF) can offer a possibility to overcome these problems. AMF can act beneficially in various ways, for example by improving plant nutrition and growth, water relations, soil structure and stability and protection against biotic and abiotic stresses. In this study, the impact of AMF inoculation on growth and nutrition parameters of naranjilla has been assessed. For inoculation three European reference AMF strains (Rhizoglomus irregulare, Claroideoglomus claroideum, and Cetraspora helvetica) and soils originating from three differently managed naranjilla plantations in Ecuador (conventional, organic, and permaculture) have been used. This allowed for a comparison of the performance of exotic AMF strains (reference strains) versus native consortia contained in the three soils used as inocula. To study fungal communities present in the three soils, trap cultures have been established using naranjilla as host plant. The community structures of AMF and other fungi inhabiting the roots of trap cultured naranjilla were assessed using next generation sequencing (NGS) methods. The growth response experiment has shown that two of the three reference AMF strains, a mixture of the three and soil from a permaculture site led to significantly better acquisition of phosphorus (up to 104%) compared to uninoculated controls. These results suggest that the use of AMF strains and local soils as inoculants represent a valid approach to improve nutrient uptake efficiency of naranjilla and consequently to reduce inputs of mineral fertilizers in the cultivation process. Improved phosphorus acquisition after inoculation with permaculture soil might have been caused by a higher abundance of AMF and the presence of Piriformospora indica as revealed by NGS. A higher frequency of AMF and enhanced root colonization rates in the trap cultures supplemented with permaculture soil highlight the importance of diverse agricultural systems for soil quality and crop production.

Keywords: Piriformospora indica; arbuscular mycorrhizal fungi; farming practices; fungal communities; naranjilla; next generation sequencing; permaculture.

FIGURE 1

Phosphorus (A) and nitrogen (B) content of naranjilla shoots inoculated with different species of arbuscular mycorrhizal fungi (AMF): Rhizoglomus irregulare, Claroideoglomus claroideum, Cetraspora helvetica, a mix of the three AMF species and an AMF control consisting of a sterilized AMF inocula mix; or local soil inoculants derived from: conventional (Conv soil), organic (Org soil), and permaculture (Perm soil) naranjilla plantations in Ecuador and a sterilized mix of the three soils as soil control and grown for 14 weeks (WE 14) and 22 weeks (WE 22) in a sterilized potting substrate. Different lower and upper case letters above bars indicate significant differences at WE 14 and WE 22, respectively [ANOVA, Tukey’s honest significant difference (HSD) test, α = 0.05]. Data represent means + SE [n = 4 (for AMF control treatment due to exclusion of one contaminated sample), 5 (at WE 14), and 6 at (WE 22)].

FIGURE 2

Root length colonization of naranjilla roots after 14 (black bars) and 22 (gray bars) weeks of growth as affected by inoculation with different species of AMF: Rhizoglomus irregulare, Claroideoglomus claroideum, Cetraspora helvetica, a mix of the three AMF species and an AMF control consisting of a sterilized AMF inocula mix; or local soil inoculants derived from: conventional (Conv soil), organic (Org soil), and permaculture (Perm soil) naranjilla plantations in Ecuador and a sterilized mix of the three soils as soil control. Different lower and upper case letters above bars indicate significant differences at WE 14 and WE 22, respectively (ANOVA, Tukey’s HSD test, α = 0.05). Treatments with an asterisk were excluded from statistical analysis. Data represent means + SE [n = 4 (for AMF control treatment due to exclusion of one contaminated sample), 5 (at WE 14), and 6 at (WE 22)].

FIGURE 3

Variation between naranjilla root communities from trap cultures. Differences between communities were assessed with principal coordinate analysis (PCoA) based on Bray–Curtis dissimilarity. Points in the unconstrained ordination represent individual samples and are colored by the type of soil used for the trap cultures. Data was subsampled to the sequence number of the sample with the lowest sampling depth. Percentage of variation given on each axis refers to the explained fraction of total variation in all samples. Inset: Average dissimilarity within groups. Each sample was compared with all other samples within its group and these Bray–Curtis dissimilarity were averaged for each sample and plotted by soil groups.

FIGURE 4

Root fungi profiles from trap cultures. (A) Mean abundances of fungal phyla plotted by soil groups. Data were normalized by sampling depth and expressed as percentage relative abundance. (B) Mean fungal (light gray) and Glomeromycota (dark gray) OTU richness, reported by soil groups. The dataset was independently subsampled 100 times, OTU richness determined and averaged for each sample. (C) The heatmap displays all abundant (defined as >1% mean relative abundance) OTUs in percentage relative abundance in all samples. Dendrograms summarize similarities between samples (in columns) and between OTUs (in rows) based on Bray–Curtis. Non-parametric Kruskal–Wallis test was used to examine differences between soil groups (n.s., not significant, ^∗P < 0.05, °P < 0.05 false discovery rate corrected).