Plant Response to Mechanical Stress

Mechanical stress vectors of nature include wind, rain, hail, and animal movements. Production agriculture and landscape practices provide further physical insults to plants in the form of pruning, pinching, tying, guying, and trimming. Urban high-rise buildings create air turbulence and downdrafts that permanently entrain trees and shrubs to growth habits reminiscent of natural plants inhabiting seacoasts and mountain slopes. Appreciation for wind per se as a powerful factor limiting plant development has been slow to develop. It is easy to overlook the mechanical aspect of wind and precipitation on plant form and growth habit because they are not always present and because many environmental stress factors coexist with wind in the outdoor environment. The influences of other environmental stress factors on plants often mask or negate the influence of mechanical stress. Confounding natural factors include airborne sea salt, desiccation, and evaporative chilling, all of which accompany wind. Only if controlled mechanical stresses (e.g., shaking, handling, flexing) are applied to plants growing in the wind-protected confines of a greenhouse or growth chamber is it possible to separate effects of mechanical stress per se from those of other environmental stresses, such as heat, cold, drought, flooding, and/or mineral deficiencies. It is surprising how sensitive plant growth can be to brief episodes of mechanical stress, especially if the plants are grown otherwise undisturbed in a stress-free environment.