Fungal hyphae of arbuscular mycorrhiza (AM) are branching cellular filaments that form associations with nearly all land plants by intertwining themselves within root systems and facilitating nutrient exchange between the fungus and plant. AM thus increase the soil volume that roots can exploit by channeling otherwise unreachable nutrients to the plant’s root systems. Previous studies have shown that root exudates can signal AM and other microbes to move closer to the root systems and form associations. Flavonoids, a class of phenolic chemicals that are involved in UV protection in leaves, are hypothesized to participate in this signaling, and it’s conceivable that, within okra plants (Abelmoschus esculentus), there is a relationship between flavonoid levels within the roots with that in the leaves, which is also modulated by solar UV light levels. A common garden experiment with pot-grown okra and a generalized AM inoculant was conducted under ambient UV light levels over three harvest periods and under both UV blocking and transparent film to determine if 1) mycorrhizal colonization influences shoot morphology and increases growth and production, 2) mycorrhizal colonization increases leaf epidermal flavonoid levels, and 3) UV exposure of shoots affects colonization of roots by AM.
Results/Conclusions
When averaged over three harvest periods, mycorrhizal infection had no significant (P>0.05) effect on stem or leaf dry mass, leaf area, leaf mass:area ratio, and height of stems and internodes. Early in the study (harvests 1 and 2), there was no significant mycorrhizal effect on epidermal UV-A transmittance (a measure of epidermal flavonoid concentration; measured non-invasively with a UV-A PAM fluorometer); however, by the third and final harvest mycorrhizae inoculated plants exhibited significantly higher leaf flavonoid concentrations (One-way Student’s t-test, P=0.03) than non-inoculated plants. All three harvests showed a significant difference in percent mycorrhizal colonization between inoculated and non-inoculated treatments (One-way Student’s t-test, P<0.05), but there was no detectable UV treatment effect on average percent mycorrhizal colonization in the roots (P>0.05). These results indicate that mycorrhizal colonization of roots can improve UV-sunscreen protection of leaves but the mechanism and adaptive significance of this response is unclear. Further studies are needed to determine if the leaf flavonoid response results solely from improved nutrient status of the plant or if there are specific signaling pathways operating between roots and shoots that influence flavonoid synthesis and accumulation.