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Plants which live in arid or semi-arid regions are required to pass through periods of water shortage. To survive this factor, they adapt to water stress. Such plants are called xerophytes. These plants can safely endure water deficiency without injury (Henckel, 1964). To be able to effectively review plant adaptations to excessive water stress, we must first define what a drought resistant plant, or xerophyte, is. Drought-resistant plants are those that can grow, mature, and reproduce normally under the presence of drought conditions that would normally result in plant cells becoming plasmolysed (Henckel, 1964). This ability is due to a number of adaptive mechanisms evolved under the pressures of environmental conditions and natural selection (Henckel, 1964).
I. Drought associated problems in plants
A slight water deficiency in plants is considered normal and does not impair photosyntetic processes or cell functioning. Severe water stress can result in fatal injuries to the plant (Henckel, 1964). Dehydration in plants causes an increase protoplasmic viscosity and interferes with the phosphorylation of ADP, ultimately inhibiting energy production (Henckel, 1964). During drought, plants not only suffer from dehydration of their cells and tissues, but also from a considerable increase in overall body temperature. Therefore, drought resisting adaptations often incorporate mechanisms to prevent overheating (heat-resistance), as well as water loss.
Dehydration and overheating have the potential to alter a plants metabolism and sub-microscopic structure of the protoplasm (Henckel, 1964). It is hypothesised that injuries to a plant can differ depending on the rate of dehydration. Rapid dehydration causes mechanical injuries to the protoplasm after water loss. Under gradual dehydration, death results from metabolic disturbances (Henckel, 1964).
II. Types of xerophytes
Xerophytes can be divided into three key categories: evaders, avoiders, and tolerant plants. The primary trait that characterises the evader group is the possession of a dormant state at some point in its lifecycle. This most commonly involves the plant remaining as a seed during unfavourable seasons. Evaders mainly constitute ephemeral species, such as Eschscholtzia californica, the Californian poppy, and a few perennial species, like those belonging to the genus Calochortus, Mariposa lilies.
Tolerant plants are those that can undergo, what is sometimes intense, tissue dehydration without death. Examples of tolerant plants include some bryophytes, lichens, clubmosses, and epiphytic ferns. Finally, avoiders are species with drought resisting mechanisms that facilitate a reduction in water loss, an increase in water uptake, an increased water storage capacity, or advanced water translocation mechanisms. It is the avoiders that are considered the true xerophytes, or euxerophytes
III. Xerophytic adaptations
There are numerous morphological, anatomical, and physiological adaptations that have evolved to drought stress. No one adaptation is capable of completely protecting a plant from drought, therefore, xerophytes rely on combinations of adaptations to survive desert conditions.
Amongst the most recognised adaptations to promote drought resistance are those pertaining to the leaves, which focus on reducing transpiration. Thickened epidermis, sunken stomata, or waxy a protective coatings on leaves are examples of morphological leaf adaptations. The reduction of leaf surface area by the possession of very small leaves or none at all is another adaptation to reduce water loss. Alternatively, plants may shed their leaves during unfavourable periods to reduce transpiration. Reflective leaf structures such as pale or waxy surfaces, hairs, or scales can also aid in reducing this process.
Stems and tissues
The use of the plant stem for all photosynthetic processes reduces the overall surface area to volume ratio of the plant. Water storage tissue can be found underground, but it is more commonly found as succulent above ground limbs. Certain xerophytes are capable of storing water in enlarge vacuoles within their cells. These plants are known as succulents and are characterised by thick, fleshy leaves, or in the case of cacti, photosynthetic stems. The intercellular spaces of the plant can also be reduced to restrict the area of exposed internal surfaces, while an increase in mechanical and vascular tissue help to prevent wilting of the plant. In addition to these tissue adaptations, they raised osmotic potential can be expressed in order to increase the plants ability to uptake water from the soil. Tissues found in some groups of plants, such as lichens and bryophytes, can expressed a general tolerance to dehydration.
Roots systems and seeding cycles
Long lasting plants may utilise a large root systems, which can be either very deep or very wide ranging; nevertheless, they extract moisture from the maximum possible volume of soil. This can be juxtaposed to ephemeral herbs, which use seeds to pass through the dry season in a dormant state. This method of survival is often associated with intense growth, or even the completion of the plants life cycle within the wet season.
IV. Crassulacean Acid Metabolism
An extremely important drought resistance mechanism that is currently found in 23 families of flowering plants is a specialised pattern of photosynthesis known as crassulacean acid metabolism (CAM). It should be noted however that only species of Cactaceae and Euphorbiaceae are exclusively CAM. All plants that photosynthesise using the CAM method are succulents; however, not all succulents are CAM.
The stomatal cycle of CAM plants is inverted compared to cycles found in C3 and C4 plants. This inverted cycle results in a nocturnal uptake of CO2, which is stored as malic acid, or malate, in the enlarged cell vacuoles of the succulents. Nocturnal stomatal opening allows for the uptake of CO2, which is needed for the light dependant PCR cycle within the chloroplast, during hours when evaporative water loss is minimised. During daylight hours, the stomata are closed and the stored malate is decarboxylated to release the CO2, which can then be fixed by the PCR cycle to produce glucose. The PCR cycle of CAM plants is the same as that found in C3 plants.
While CAM plants have the advantage of being able to uptake, retain, and reassimilate CO2 during drought stress, under moist conditions, they can be expected to have a slower growth rate than C3 and C4 plants. This is due to daily carbon assimilation by CAM plants being approximately half that of C3 plants and one third that of C4 plants. CAM plants also exhibit a higher ATP requirement for photosynthesis than C3 and C4 plants.
V. Heat Resistance
Severe overheating can be extremely dangerous to plants, as symptoms include the decomposition of proteins, and the appearance of ammonia molecules in toxic amounts. High internal temperatures cause proteins in the plant to be hydrolysed to amino acids (Henckel, 1964). Overheating in plants inhibits respiration, which ultimately inhibits the formation of organic acids that react with ammonia molecules to form salts and aimides. Therefore, heat resistant plants have adapted to combat the effects of overheating by reducing their respiratory coefficients and accumulating organic acids (Henckel, 1964). Highly adapt xerophytic plants can also respond to overheating with intense protein synthesis, which is due to raised levels of nucleic acids, and intense restorative processes, which are the result of elevated respiratory rates (Henckel, 1964).
There are two primary ways in which transpiration mechanisms can be used by xerophytic plants to control internal temperatures. Due to the absorption of solar radiation, leaf temperatures tend to be higher than ambient air temperatures. Some plants, such as select desert species can tolerate tissue temperatures well above those normally endured by other plants, which enables them to reduce transpiration and heat up without causing critical damage. The other temperature control mechanism is to increase transpiration, whereby the vaporisation of water, internal leaf temperatures are lowered. This mechanism is most common in deep rooted species, as it relies heavily on the ability to tap into an adequate water supply. The result of both mechanisms is that cell turgor is maintained, allowing basic processes, which cannot occur in flaccid cells, to continue, for example photosynthesis.
As with dehydration, the factors causing injury and death to a plant after overheating may differ depending on the abruptness and intensity of the temperature increase. Ammonia poisoning is the result of a slow temperature increase, whereas rapid overheating disrupts sub-microscopic structures and impedes the coagulation of protoplasmic proteins (Henckel, 1964).
The physiological problems that are brought about in plants due to drought stress cannot be solved by one adaptation, but by a combination of many. While some adaptations provide an advantage for one plant species over another in a select habitat, that advantage may become a disadvantage when removed from its context. This factor provides insight into the evolutionary persistence of the three modes of photosynthesis (C3, C4, and CAM) and their biogeographic distributions. The wide range and combinations of drought resistance mechanisms found within xerophytes can be analysed to give perspective and support to the evolutionary history of drought resistant plant taxa.