224 P. Mitchell et al.
response over days to weeks. Over longer time-scales, adaptation involves evolu-
tionary responses arising from genetic changes in the population that can alleviate
the impact of the stress (Lambers et al. 2008 ). Selye ( 1936 ) summarized the response
of the individual to continuous or permanent stress into a three-phase stress model
(Fig. 11.1b). The alarm phase is characterized by distress or a decline in physiologi-
cal function. The resistance phase involves recovery to the normal range in function-
ing and may include a period of acclimation that increases resistance to subsequent
stress. Finally, the exhaustion phase occurs if the stress continues or intensifies so
Fig. 11.1 Representation of the different scales at which stress defines the response of plant func-
tioning in individuals and populations. a Describes changes in plant function or system state at
both the individual and population level across a range of temporal scales. The initiation of stress
in the individual results in a stress response, followed by a period of acclimation. Over generations,
adaptation can allow further recovery from stress and some return to normal conditions or the
steady state ( solid line) or a new steady state ( dashed line; modified from Lambers et al. 2008 ). b
An individual exposed to a permanent stress exhibits three phases of stress as proposed by Selye
( 1936 ). The initial decline in plant function or distress can induce acute damage and may result
in loss of biomass or plant injury (disturbance). The period of recovery, termed eustress, results
from acclimation processes and may enable recovery back to the normal range ( dashed horizontal
line) or enhanced resistance to subsequent stress ( solid line). If stress persists, exhaustion occurs
whereby the plant shows chronic damage or death. (Modified from Steinberg et al. 2008 )