The Impact of Light Intensity on Plant Support Mechanisms

Herbaceous plants support themselves primarily by the hydrostatic pressure exerted by the cell wall. Additional support can be gained by increasing wall thickness and lignification. This study was designed to examine the tradeoff plants must make to support their leaves in light-limited environments. We tested the hypothesis that individuals of the herbaceous plant white snakeroot (Eupatorium rugosum) that grow in shaded woodlands, should have less support tissue than those that grow in the sun. Consequently, shaded plants would be expected to rely more heavily on water pressure to support their leaves, so slight decreases in leaf water content should cause them to wilt more readily. We measured the wilting rate of E. rugosum from three different habitats. Time-lapse videos were made of the wilting leaves and wilting rates were determined using ImageJ. In addition, we measured leaf area, water content and rate of water loss. The amount of leaf support tissue (g cm-2), was greatest in plants from the sunniest habitat (p\u3c0.0001). Time-lapse video analyses showed that there was little change in wilting rate between plants from the sun and shade habitats (p = 0.409). Plants from the forest interior lost water at a slower rate than those at the forest edge (p\u3c0.0001), so when the wilting rate was expressed as a function of water loss (θ cm2 mg-1 H2O), plants from the forest interior wilted after losing less water than those at the forest edge (p\u3c0.0001), as we hypothesized