Cost of apical dominance in two monocarpic herbs, Erysimum strictum and Rhinanthus minor

Apical dominance is advantageous in conditions favoring rapid growth in height and unbranched architecture. The cost of apical dominance, on the other hand, should be expressed in conditions where fecundity increases along with an increasing number of branches. Apical damage can be used to measure such costs: when suppressed meristems are released from apical inhibition, the vegetative and reproductive productivity of initially unbranched plants should increase owing to the regrowth and increased branch development following damage. We studied these regrowth responses in two monocarpic herbs, Erysimum strictum P. Gaertn., B. Mey., and Scherb. and Rhinanthus minor L., after both apical damage (10% of the shoot cut) and more extensive damage (50 and 75% cutting). Both species tolerated apical damage, although severe damage had detrimental effects on the performance of both, especially R. minor Apical damage had positive effects on most of the measured performance parameters of Erysimum. However, the success of seed germination collapsed, presumably due to delayed flowering and less successful pollination. The response was parallel in Rhinanthus; apical damage affected neither the vegetative biomass nor fecundity, but heavier damage, especially 75% clipping, led to severe reductions in most performance measures. The differences in regrowth responses are presumably due to the different habitat requirements of the species. Rhinanthus prefers relatively dense vegetation and starts to branch and produce flowers after a certain threshold in height has been reached, whereas Erysimum prefers gaps in vegetation. In this species, unbranched architecture may be favored in closed vegetation and branched architecture in less competitive habitats. In both species, fruit production correlated positively with the number of branches in both control and clipped plants, which is consistent with the assumption of the cost of apical dominance.