Effects of mechanical stress and plant density on mechanical characteristics, growth, and lifetime reproduction of tobacco plants.

Plastic increases in stem elongation in dense vegetation are generally believed to be induced by canopy shading, but because plants protect each other from wind, shielding (reduced mechanical stress) could also play a role. To address this issue, tobacco Nicotiana tabacum plants were subjected to two levels of mechanical stress, 0 (control) or 40 (flexed) daily flexures, and grown solitarily, in a dense monostand (with plants of only one mechanical treatment), or in a mixed stand (flexed and control plants grown together). Flexed plants produced shorter and thicker stems with a lower Young's modulus than control plants, while dense-stand plants had relatively taller and thinner stems than solitary ones. Flexing effects on stem characteristics were independent of stand density. Growth, reproduction, and survival of solitary plants were not affected by flexing, while in the monostand growth was slightly reduced. But in the mixed stand, flexed plants were readily shaded by controls and had considerably lower growth, survival, and reproduction rates. These results suggest that wind shielding indeed plays a role in the plastic increase in stem elongation of plants in dense vegetation and that this response can have important consequences for competitive ability and lifetime seed production.