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What is Drought Resistance?
It is the ability of a plant to maintain favorable water balance and turgidity even exposed to drought conditions there by avoiding stress and its consequences. Stress avoidance due to morphological anatomical characteristics which themselves are the consequences of the physiological processes induced by drought these zerophytic characteristics are quantitative and vary according to environmental conditions.
A favorable water balance under drought conditions can be achieved by transpiration before as soon as stress is experienced. These are called "water savers" or.
Accelerating water uptake sufficiently so as to replenish the lost water called as "water spenders"
A) The mechanism for conserving water:-
1. Stomatal mechanism: -Stomata of different species vary widely in their normal behaviour and range. In some species stomata remain open continuously or remain closed continuously. Many cereals open their stomata only during a short time in the early morning and remain closed during rest of the day. There is a difference in this respect between varieties of the same crop as shown by the example in two varieties of oat one is more resistant to drought open its stomata more rapidly in the early morning when moisture stress is at its minimum and photosynthesis can precede with the least loss of water (stocker 1960).
However mechanism of conserving water based on the closure of stomata will inevitable load to reduce photosynthesis and may lead to drought induced starvation injury (Leavitt, 1972).
2. Increased / Photosynthetic efficiency :- On possibility for overcoming limitations on photosynthesis, imposed bicoastal closure as means for increasing resistance to loss of water by transpiration there by transpiration there by accumulations of CO2 would be at higher rate for a given stomatal opening (Hatch & stack, 1970). A number of imperfect crop plants (maize, sugarcane sorghum prose, fox tail & finger millets) (Hatch et. al. 1987) as well as certain forage species Bermuda grass (Cynodon dactyl on) Sudan grass Bahia grass (Paspalum notatum) Rhodes grass (chloris Guyana) (Murata lyama 1963) and certain A triplex sp. fixed most of CO2 into the C4 of molic and aspartic acids so called C4 dicarboxylic acid (C4) pathway.
3. Low rate of cuticular transpiration: - The typical example is the cacturs. Thick cuticle results in low rate of transpiration.
4. Decreasing transpiration by a deposit of lipids layers on the surface of the leaves on exposure to moderate drought e.g. soybean (Levitt 1972).
5. Reduce leaf area: - The principal means of reducing water loss of xenomorphic plants is their ability to reduce their transpiring surface. Apart from the common means of keeping the aerial parts small perhaps the simplest form of this reduction of the transpiring surface is the sealing or of leaves at the time of water stress a characteristic phenomenon exhibited by many grasses. The rolling of leaves has been shown to reduce transpiration by almost 55 percent in semi conditions and by 75 percent in desert xerophytes (Stalfect - 1956).
6. Leaf surface: - Various morphological characteristics of leaves he reduce the transpiration rate and may affect survival of plants drought conditions. Leaves with thick cuticle waxy surface and the presence of spines etc. are common and effective.
7. Stomatal frequency and location: - A smaller number of stomata retard the development of water deficits. In certain species, the stom are located in depression or cavity in the leaves which is feature can further reduce transpiration by limiting the impingement of currents.
8. Effect of awns: - Awned varieties of wheat predominate in the drier at warmer regions and have been found to yield better than awnless one especially under drought conditions though there are exceptions (Gurandhacher 1963). Awns have chloroplasts stomata and so as photosynthesized. It has been found that the contribution of the away to the total dry weight matter of the kernels was 12% of that the entire plant.
B) To Improving water uptake (MC - Donough & Gauch 1959) :-
1. Efficient root system:-
The root systems of drought resistant plants are characterized by wide variety of apparent adaptations. These responded to such predominant soil conditions as the duration of soil dryness and the depth that is normally wet. Plants become adapted to dry conditions mainly by developing an extensive root system rather that structural modification of the roots (shields - 1958). The conceres "extensive root system" includes additional growth of secondary hair roots.
2. High root to top ratio (R/T):-
A high root to top ratio is very effective mean to adoption of plants to dry conditions of the growth rate of the roots considerably exceeds that of the shoots. The transpiring surface is there by reduced while root system of the individual plant obtains it's water from a large volume of soil (Simonis 1992) has shown that an increased root top ratio may actually result in greater amount of total dry matter of plants grown under dry conditions as compared a similar ones grown with full moisture.
3. Difference in osmotic potential of plants :-
Levitt (1958) has calculated a difference of 0.5% in soil moisture content that includes per manual wilting could supply a plant with enough water to keep it alive for 6 days. This could mean in certain cases the difference between survival and death.
4. Conservation of water spenders to water stress:-
Because of increased water absorption water spenders are characterized by very high rate transpiration. However as soon as the absorption rate becomes insufficient to keep up with water loss the water spenders generally develop some of the characteristics of the water savers (Cevitts - 1972).
C) Mitigating stress:-
1. Mitigating stress:-
Adoptions a drought basis mitigating effects of stress permit the plant to maintain a high internal water potential inspite of drought conditions. They therefore able to maintain cell tartar and growth avoid direct or indirect metabolic injury due to dehydration (Levit 1972).
D) Drought tolerance:-
When plant is actually submitted to low water potential it can show drought tolerance by either mitigating the actual stress induced by the moisture deficiencies or by showing high degree of tolerance to stresses.
1. High degree tolerance; Resistance to dehydration:-
The simplest method of avoiding drought induced damage is by resisting dehydration, preferably tot he extent .of maintaining turgur and at least by avoiding cell collapse after loss of turgur (Levit 1972) retain their turgur and therefore can continue to grow when exposed to drought stress. When plants are grown in their natural environment their osmotic potentials tend to be characteristic for each ecological group.
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