The predominant emphasis on harmful effects of environmental stresses on growth of woody plants has obscured some very beneficial effects of such stresses. Slowly increasing stresses may induce physiological adjustment that protects plants from the growth inhibition and/or injury that follow when environmental stresses are abruptly imposed. In addition, short exposures of woody plants to extreme environmental conditions at critical times in their development often improve growth. Furthermore, maintaining harvested seedlings and plant products at very low temperatures extends their longevity.
Drought tolerance: Seedlings previously exposed to water stress often undergo less inhibition of growth and other processes following transplanting than do seedlings not previously exposed to such stress. Controlled wetting and drying cycles often promote early budset, dormancy, and drought tolerance. In many species increased drought tolerance following such cycles is associated with osmotic adjustment that involves accumulation of osmotically active substances. Maintenance of leaf turgor often is linked to osmotic adjustment. A reduction in osmotic volume at full turgor also results in reduced osmotic potential, even in the absence of solute accumulation. Changes in tissue elasticity may be important for turgor maintenance and drought tolerance of plants that do not adjust osmotically.
Water deficits and nutrient deficiencies promote greater relative allocation of photosynthate to root growth, ultimately resulting in plants that have higher root:shoot ratios and greater capacity to absorb water and minerals relative to the shoots that must be supported.
At the molecular level, plants respond to water stress by synthesis of certain new proteins and increased levels of synthesis of some proteins produced under well-watered conditions. Evidence has been obtained for enhanced synthesis under water stress of water-channel proteins and other proteins that may protect membranes and other important macromolecules from damage and denaturation as cells dehydrate.
Flood tolerance: Both artificial and natural flooding sometimes benefit woody plants. Flooding of orchard soils has been an essential management practice for centuries to increase fruit yields and improve fruit quality. Also, annual advances and recessions of floods are crucial for maintaining valuable riparian forests. Intermittent flooding protects bottomland forests by increasing groundwater supplies, transporting sediments necessary for creating favorable seed-beds, and regulating decomposition of organic matter. Major adaptations for flood tolerance of some woody plants include high capacity for producing adventitious roots that compensate physiologically for decay of original roots under soil anaerobiosis, facilitation of oxygen up-take through stomata and newly formed lenticels, and metabolic adjustments. Halophytes can adapt to saline water by salt tolerance, salt avoidance, or both.
Cold hardiness: Environmental stresses that inhibit plant growth, including low temperature, drought, short days, and combinations of these, induce cold hardening and hardiness in many species. Cold hardiness develops in two stages: at temperatures between 10° and 20°C in the autumn, when carbohydrates and lipids accumulate; and at subsequent freezing temperatures. The sum of many biochemical processes determines the degree of cold tolerance. Some of these processes are hormone dependent and induced by short days; others that are linked to activity of enzyme systems are temperature dependent. Short days are important for development of cold hardiness in species that set buds or respond strongly to photoperiod. Nursery managers often expose tree seedlings to moderate water stress at or near the end of the growing season. This accelerates budset, induces early dorma
