The influence of drought, shade and availability of mineral nutrients on exudate phenolics of Dittrichia viscosa

Exudate phenolics are thought to defend leaf surfaces against microorganisms and interfere with neighboring plants if leached to the soil. We investigated the effects of resource availability on growth and accumulation of epicuticular water soluble phenolics in potted seedlings of Dittrichia viscosa grown in the field under two levels of nutrients, water and photon fluence rates. As expected, growth was enhanced under additional nutrients and suppressed under water stress. Under shade, aboveground biomass accumulation was not affected, yet the total leaf area was considerably increased at the expense of dry mass invested on area basis. Glandular hair density was not affected by any treatment. The concentration of leaf exudate phenolics was increased under water stress and decreased under additional nutrients and shade, in accordance to the prevailing hypotheses predicting environmental effects oncarbon-based defensive secondary metabolites. Yet, when we considered total epicuticular phenolic production on a plant basis (i.e. the amount leachable to the soil, which better indicates its allelopathic potential), the changes in total leaf area compensated (shade) or over-compensated(nutrients, water stress) the changes in concentration. Thus, total leachable epicuticular phenolics were higher under additional nutrients and lower under water stress. We conclude that environmental factors and the corresponding physiological constrains in the production of exudate phenolics may act in opposite directions when surface protection and interference with neighbors is considered.