Introductory Crop Physiology (1+1)

Hardening (Acclimation)

Plants exposed to low water potential, high light levels and such other factor as high phosphorous and low nitrogen fertilization become drought tolerant or hardy compared to plants of the same species not treated in this way. This is, they become to acclimation to drought, a process of considerable important to agriculture. This is a good example of a conditioning effect.
Actively growing plants, especially herbaceous species, are damaged or killed by temperature of only -1 to -5oC, but many of the plants can be acclimated to survive winter temperature of -25oC or lower.
Frost hardness typically develops during exposure to relatively low temperature (5oC) for several days.
Temperature down to -3oC are sometime required for maximum acclimation.
Short days also promote acclimation in several species and these are indications that a stimulus may move from leaf tissue to the stem.
The development of frost hardiness is a metabolic process required an energy source. Apparently this can be provided by light and photosynthesis.
Factors that promote more rapid growth inhibit acclimation: high nitrogen in the soil, pruning irrigations and so on.
In general, non-growing or slowly growing plants are more resistant to several environmental extremes, including air pollution.
Water stressed plants are more resistant to air pollution partially because their stomates are closed.