Xerophyte
A xerophyte (from Greek ξηρός xeros 'dry' + φυτόν phuton 'plant') is a species of plant that has adaptations to survive in an environment with little liquid water.
Some species called resurrection plants can survive long periods of extreme dryness or desiccation of their tissues, during which their metabolic activity may effectively shut down.
[1] Xerophytes such as cacti are capable of withstanding extended periods of dry conditions as they have deep-spreading roots and capacity to store water.
The succulent xerophyte Zygophyllum xanthoxylum, for example, has specialised protein transporters in its cells which allows storage of excess ions in their vacuoles to maintain normal cytosolic pH and ionic composition.
These factors include infrequent raining, intense sunlight and very warm weather leading to faster water evaporation.
Water deficiency usually reaches 60–70% of their fresh weight, as a result of which the growth process of the whole plant is hindered during cell elongation.
Most of these plants are small, roundish, dense shrubs represented by species of Papilionaceae, some inconspicuous Compositae, a few Zygophyllaceae and some grasses.
For example, Caragana korshinskii, Artemisia sphaerocephala, and Hedysarum scoparium are shrubs potent in the semi-arid regions of the northwest China desert.
These psammophile shrubs are not only edible to grazing animals in the area, they also play a vital role in the stabilisation of desert sand dunes.
Compared to other dominant arid xerophytes, an adult R. soongorica, bush has a strong resistance to water scarcity, hence, it is considered a super-xerophytes.
However, it is vital that plants living in dry conditions are adapted so as to decrease the size of the open stomata, lower the rate of transpiration, and consequently reduce water loss to the environment.
[7] In brief, the rate of transpiration is governed by the number of stomata, stomatal aperture i.e. the size of the stoma opening, leaf area (allowing for more stomata), temperature differential, the relative humidity, the presence of wind or air movement, the light intensity, and the presence of a waxy cuticle.
It is important to note, that whilst it is vital to keep stomata closed, they have to be opened for gaseous exchange in respiration and photosynthesis.
These dissimilarities are due to natural selection and eco-adaptation as the seeds and plants of each species evolve to suit their surrounding.
An example is the white chalky epicuticular wax coating of Dudleya brittonii, which has the highest ultraviolet light (UV) reflectivity of any known naturally-occurring biological substance.
Some examples are Antizoma miersiana, Hermannia disermifolia and Galenia africana which are xerophytes from the same region in Namaqualand, but have different cuticle ultrastructures.
[11] Despite the many stresses, xerophytes have the ability to survive and thrive in drought conditions due to their physiological and biochemical specialties.
Examples are the heavily scented and flammable resins (volatile organic compounds) of some chaparral plants, such as Malosma laurina, or the chalky wax of Dudleya pulverulenta.
[11] Most plants have the ability to close their stomata at the start of water stress, at least partially, to restrict rates of transpiration.
Zeaxanthin dissociates light-channelling from the photosynthesis reaction - light energy in the form of photons will not be transmitted into the photosynthetic pathway anymore.
It is also dubbed the "dark" carboxylation mechanism because plants in arid regions collect carbon dioxide at night when the stomata open, and store the gases to be used for photosynthesis in the presence of light during the day.
Although most xerophytes are quite small, this mechanism allows a positive carbon balance in the plants to sustain life and growth.
Prime examples of plants employing the CAM mechanism are the pineapple, Agave Americana, and Aeonium haworthii.
[11] Exceptions exist, however, such as the ocotillo which will shed its leaves during prolonged dry seasons in the desert, then re-leaf when conditions have improved.
[citation needed] A plant's root mass itself may also hold organic material that retains water, as in the case of the arrowweed (Pluchea sericea).
In fact, in northwest China, the seeds of three shrub species namely Caragana korshinskii, Artemisia sphaerocephala, and Hedysarum scoparium are dispersed across the region.
Many xerophytic plants produce colourful vibrant flowers and are used for decoration and ornamental purposes in gardens and in homes.
A study has shown that xerophytic plants which employ the CAM mechanism can solve micro-climate problems in buildings of humid countries.
During desiccation, the levels of the sugars sucrose, raffinose, and galactinol increase; they may have a crucial role in protecting the cells against damage caused by reactive oxygen species (ROS) and oxidative stress.
A glycoside found in Haberlea rhodopensis called myconoside is extracted and used in cosmetic creams as a source of anti-oxidant as well as to increase elasticity of the human skin.
