In plants as well as animals, one can distinguish three principal and different scenarios. (1) True internal tolerance to altered environmental conditions occurs when the internal conditions within tissues and cells of the organism are shifted as the external conditions change, and yet the organism's metabolism is able to proceed under these somewhat altered conditions. For example, some plants exhibit osmotic adjustment, the accumulation of solutes that do not interfere with metabolism, when soil water availability decreases and thus track the external conditions by decreases in their tissue water content. (2) In contrast, other organisms avoid internal change by compensatory mechanisms (e.g., maintenance of a constant high tissue water content as a drought progresses). (3) Finally, in the most extreme example of evasion, life cycle adjustments allow an organism to persist through unfavorable conditions/seasons in a state with minimal or no metabolic activity. Remarkably, plant species employing these contrasting strategies often coexist in the very same habitat! For example, in a hot and dry desert environment, water-storing cacti, maintaining a high internal tissue water content, can be found growing side by side with species that allow their tissues to lose considerable amounts of water and yet remain metabolically active by virtue of osmotic adjustment. After a spring rainfall, the same desert may explode with soft-leafed ephemerals that are neither equipped to store water nor to tolerate low internal water content (except for their seeds). These ephemerals complete their life cycle in an extraordinarily short period and disappear before the drought returns, leaving only highly drought tolerant seeds behind.
However, plants employing these different strategies do tend to differ in the range of environmental conditions they tolerate. For example, the proverbial cactus in the desert must periodically refill its water stores and is typically unable to survive in the most extreme deserts with extended periods of drought through which only species truly tolerant of low internal water content are able to persist. On the other hand, the seeds of the escape artist species may skip a year or more with no large rainfall events without germinating, only to repopulate the scene after an extended drought period through which no other plant life has survived.
Another scenario is found along mountain slopes in temperate climates with cold winters, where lower altitudes are populated by a mix of annuals with soft tissues versus hardy evergreen ground covers and coniferous trees. At higher altitudes, the highly resistant conifers predominate by virtue of their tolerance to altered internal conditions, enabling them to survive long winters with subfreezing temperatures and frozen soils that deprive plants of access to liquid water. Above treeline, only ground-hugging evergreens, winter-deciduous species, and very few annuals are able to survive.
Thus, acclimation to moderately stressful conditions frequently involves adjustments to maintain metabolic activity or to tolerate a modest departure from optimal internal conditions. In contrast, acclimation to the most extreme environments (like deserts, high altitudes), characterized by cycles of favorable and extremely harsh seasons, often involves the ability to shut down metabolism and survive harsh conditions in a metabolically inactive, yet highly resistant state.
In summary, major principal response types include the following:
• Fast-growing, short-lived species escape stressful seasons by completing their life cycle before the harshest conditions set in. While true in the extreme for the desert ephemerals, the same principle is used by many annual and biennial weeds and crops: when they encounter moderate stress, these species maintain a high level of metabolic activity, for example, keeping the capacity for photosynthesis up in the winter (in winter wheat and other crops), keeping leaf pores (stomata) open via osmotic adjustment under moderate drought (for more detail, see the final section, 'Oxidative stress and redox signaling as common denominators in stress perception and response'), etc.
• Drought-deciduous or winter-deciduous species permit the most vulnerable portions of the plant to senesce as the plant enters a period of unfavorable conditions (e.g., low water availability, exceedingly high temperatures, subfreezing temperatures). This can involve the loss of leaves or needles, twigs, whole branches, the entire aboveground portion of the plant, or major portions of the root system. However, during the senescence process, essential mobile resources are recovered and stored for use during the next period of conditions favorable for activity and growth.
• Slow-growing evergreens naturally undergo multiple cycles of growth during favorable seasons and coordinated inactivation of whole metabolic pathways (like photosynthesis) during unfavorable seasons, for example, hot and dry summers in the desert or winters with subfreezing temperatures.
• At the extreme end of the stress tolerance range, many plant seeds, as well as a few specialists like resurrection plants, can dry out completely and remain in this state for years before becoming revived during a substantial precipitation event.
Plant species vary greatly in their ability to respond to an increased availability of resources, depending on their evolutionary history and adaptation to stress (for more detail, see below). Those adapted to persist through more stressful conditions are generally less responsive to factors such as increased nutrient, water, or CO2 availability. Part of the success of invasive species derives from their ability to respond positively to increased resource availability and outcompete the native species genetically constrained to respond less strongly.
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