Effects of Nutrient Level on Cost and Benefit of Apical Dominance in Epilobium ciliatum

Abstract

-The disruption of apical dominance has been considered fundamental to the overcompensatory response to herbivory in many plant species. For isolated plants, such overcompensation can be interpreted as a cost of apical dominance since the growth of apically intact plants is limited by the number of active meristems. When apical dominance is disrupted, growth becomes limited more by nutrient levels and so the greatest overcompensation is generally predicted for plants grown under high nutrient levels. This relationship, although possible for plants growing free from competition, may not hold true when plants are competing for light. In competing plants, the retention of apical dominance, expressed through a strong vertical component of growth, represents a benefit, as plants may outcompete those from which apical dominance is removed. In this case, undercompensation rather than overcompensation may be expected following shoot apex removal under high nutrient levels, since high nutrient levels generally promote competition for light. We tested these predictions concerning cost and benefit of apical dominance in Epilobium ciliatum. Fitness estimates (i.e., dried biomass, capsule weight and number) were compared between apically decapitated and intact plants grown in the presence and absence of neighbors and under three different nutrient levels. Cost of apical dominance (overcompensated capsule production following shoot apex removal in the absence of neighbors) was detected only under the intermediate nutrient level, not the highest nutrient level as generally predicted. This was interpreted by recognizing that increases in nutrient level may decrease the strength of apical dominance. Thus, under high nutrient levels, architectural modifications, and thus the potential fitness benefits following the disruption of apical dominance, may be minimal. This leads to the general prediction that overcompensation (cost of apical dominance) should be most common in habitats of intermediate fertility. Benefit of apical dominance in E. ciliatum (in terms of capsule production) was most evident for competing plants grown under high nutrient levels where competition for light is expected to be most intense.