Ecotypic differentiation in regeneratory plasticity

Background/Question/Methods Damage to the shoot apex in plants typically releases apical inhibition and activates dormant meristems. Several studies have shown that shoot damage can result in increased branching and compensatory ability, particularly when undamaged plants have strong inherent commitment to apical dominance. We tested the hypothesis that plant's responsiveness to apical damage depends on its probability to encounter shoot and root competition. We compared the regeneratory responses to apical damage of con-specific plants from two contrasting environments: Mediterranean habitats, in which primary productivity and light-competition are relatively high and seasons are long; and arid habitats, where soil resources are more limited and unpredictable, light competition is reduced, and season length is shorter and less predictable. Two annual legumes- Medicago truncatula (recumbent with indeterminant growth and bloom) and Trifloium purpureum (erect with determinate growth and bloom) were subjected to variable water availability, simulated vegetative shade, and shoot clipping.
Results/Conclusions In M. truncatula, undamaged Mediterranean plants exhibited greater commitment to apical dominance and reduced branching compared to desert plants, even under un-shaded conditions. Following clipping they exhibited increased branching and overcompensation while desert plants exhibited only full compensation. Conversely, desert T. purpureum plants demonstrated greater apical dominance and reduced compensatory growth following clipping than Mediterranean plants. The results demonstrate intricate interactive effects of growth habit and ecotypic differentiation on the functional role of apical dominance in controlling plastic response to damage. While in the indeterminant M. truncatula, apical control enables plants to preempt highly probable light competition, in T. purpureum it restrains risky growth into the less predictable season-end typical to desert environments.