Can evolutionary history affect plastic responses of plants to light competition?

Plants can respond plastically to changes in the occurrence of competing neighbors, with shifts in varying traits, including shade avoidance or tolerance. However, little is known about the effect of evolutionary background on the extent of these plastic responses. In this study we compared light-competition responses of the annual plant Hymenocarpos circinnatus from six populations along a productivity gradient, from a semi-arid region, where light competition is relatively weak and homogenous, to a mesic-Mediterranean region, where light competition is stronger and heterogenous. Light competition was simulated using filters to create varying heights and densities of the surrounding vegetation. H. circinnatus plants from all populations increased their shade avoidance and shade-tolerance responses under competition with dense compared to sparse neighbors, regardless of neighbor height. However, in contrast to our predictions, H. circinnatus did not exhibit divergence across populations in the extent of these plastic responses in most of the studied traits. These results suggest that although competitive interactions in the semi-arid region are relatively weak, they might still exert strong selection pressures on a subordinate competitor such as H. circinnatus.