Light limitation reduces tolerance to leaf damage in Datura stramonium

Question: Does light limitation reduce tolerance to leaf damage in the annual herb Datura stramonium? Hypothesis: Tolerance to leaf damage should be lower under light limitation (shade) than under full sunlight, because a reduction in leaf area and less exposure to light will limit carbon photo-assimilation and plants' ability to maintain fitness. Organism: Jimsonweed, Datura stramonium L. (Solanaceae). Methods: Damaged and undamaged plants of ten full-sib families (N = 471 plants) were grown under two different light treatments, full sunlight (control) and 35% light reduction (shade). Total seed number produced per plant was used as an estimate of maternal plant fitness. To compare the effect of light reduction on tolerance, we used the index of tolerance (the difference in seed production between the damaged and undamaged treatment of each genetic family). We also evaluated the effect of defoliation and light limitation on photosynthetic activity (estimated as the chlorophyll content index) and leaf growth compensation (i.e. total leaf area, mean area per leaf, and leaf number), compensatory characters related to tolerance. Conclusions: Light limitation exacerbates the negative effect of leaf damage on plant fitness, producing a two-fold average reduction in tolerance. Moreover, compensatory growth was lower in the light-limited environment. Under high damage and light limitation, the evolution of higher tolerance to damage can be highly constrained.