Do Species with Strong Apical Dominance Incur a Cost in Terms of Suppressed Potential Fecundity or Biomass?

Abstract

Plants typically allocate axillary meristems to one of three principal fates: growth (G), reproduction (R), or inactivity (I). The latter is commonly enforced by ‘apical dominance’, promoting a growth form that favours vertical shoot extension — mediated by the effects of auxin produced in the shoot apical meristem. When the latter is removed however (e.g. by herbivores), meristem allocation may change, thus affecting plant architecture, biomass accumulation, and/or reproductive effort. Fecundity and/or plant mass may consequently suffer (under-compensate), remain unaffected (compensate), or increase (overcompensation). The latter signifies a ‘cost of apical dominance’, but one that may be outweighed by several potential benefits from having apical dominance intact. I removed the shoot apical meristem for replicate plants early in the growing season within natural populations of 22 herbaceous angiosperm species with a conspicuously vertical growth form commonly found in eastern Ontario to: (i) test for a cost of apical dominance; (ii) examine effects on leaf size and leafing intensity; and (iii) explore effects of between-species variation in leafing intensity on propensity for a cost of apical dominance. Clipped and unclipped plants had their near neighbours removed, and were harvested after flowering production had finished but before seed dispersal. Dry mass was measured separately for above-ground body size, leaves, seeds, and fruit; and counts were recorded for each type of meristem, and number of leaves, fruits, and seeds per plant. I predicted that: (i) species with a strong apically-dominant growth form would respond to shoot apical meristem removal with greater branching intensity, and thus over-compensation in terms of fecundity and/or biomass; and (ii) overcompensation is enabled by production of more but smaller leaves, and hence with a larger bank of axillary meristems available for deployment in branching and/or fruit production. Widely variable compensatory capacities were recorded, suggesting effects of uncontrolled between-species variation in phenology, life history traits, and natural susceptibility to herbivory. I also found no significant between-species relationships for compensatory response versus mean leaf size or leafing intensity. Overall, the results point to species-specific treatment effects on meristem allocation patterns, and no generalized ‘cost of apical dominance’ in herbaceous plants.