Ontogenetic shifts in resource allocation and potential defense syndromes of a tropical medicinal treelet

Abstract

Variation of plant traits across ontogeny can play an important role in shaping the vital functions of growth, reproduction and defense, depending on the priorities of resource investment. Plant functional traits in seven ontogenetic stages, following complete trajectories, of a tropical light-demanding evergreen medicinal treelet (Rauvolfia vomitoria Afzel.) were examined in a common garden experiment in the field. The highly plastic morph-physiological and potential defense traits showed complex and nonlinear variations with increasing tree height (age), revealing a mostly clear-defined transition at reproductive onset. Leaf photosynthetic rate varied in a unimodal pattern throughout ontogeny, with the highest value occurring at reproductive onset. Except for the oldest plants, priority of resource use in leaves across ontogeny shifted to enhance photosynthetic rate, photosynthetic N and P use efficiencies, and prevention of physical damage (higher leaf thickness, leaf mass per area and lignin concentration), but to decrease carbon-based chemical defense (i.e., total phenolics). Fine root phenolics were relatively stable, but lignin concentration increased throughout ontogeny. With higher levels than fine roots and aboveground parts, the concentrations of an indole alkaloid (i.e. reserpine) increased steadily in coarse roots, especially during the reproductive stages. Whereas, tree growth progressively shifted from the demand of N relative to P, based on the indirect evidence from N:P stoichiometry. Our results demonstrated that, under the conditions of no environmental interference, the pronounced trajectories in functional traits were observed throughout the life cycle of R. vomitoria plants due to strong ontogeny-induced shifts in resource allocation. The synergies or tradeoffs of the defense syndromes differed between and within the classes of defense (i.e., physical versus chemical), and also, between and within above- and below-ground tissues across the ontogenetic developmental stages, which could probably be explained by multiple defense hypotheses. The revealed variations of the growth traits and secondary metabolites of R. vomitoria plants at different developmental stages will be helpful for maximizing the reserpine yield through the suitable cultivation practices.