Mechanical Conditioning for Control of Growth and Quality of Vegetable Transplants

Physical disturbance of plants or plant parts results in a mechanical stress response from the plant. Mechanical stress applied naturally or under controlled conditions inhibits growth in mass and dimensions of major plant parts. Wind and precipitation are important natural sources of mechanical stress. Under field conditions, cultivated plants experience these natural mechanical stresses plus those caused by irrigation and contact with farm machinery or workers during cultivation. Some horticultural practices, such as staking of trees or potted plants, pinching or pruning stems, deadheading flowers, bagging potted plants, or training branches, are mechanical stresses that affect plant growth. Handling practices in plant science research, such as cross-pollination or tagging of flowers (Frizzell et al., 1960) or repeated attachment of leaf chambers for measurement of water status or photosynthetic rates (Marler and Mickelbart, 1992), also can affect plant growth and physiology. Grace (1977) extensively studied effects of wind on the growth and physiology of grasses. Jaffe (1973) termed plant response to tactile (touching, rubbing, brushing) mechanical stress “thigmomorphogenesis,” whereas plant response to a nontactile (shaking, vibration, wind) treatment was termed “seismomorphogenesis” by Mitchell et al. (1975). However, only recently has plant response to mechanical stress been studied to control plant growth in controlled environments, such as greenhouses and growth chambers (Beyl and Mitchell, 1977b; Hammer et al., 1974).