Drought is prolonged shortage of water supply due to below average rainfall or precipitation in a given area. Plants productivity and growth in mainly affected by various abiotic and biotic stresses.
Drought is one of the main abiotic stress that mainly affect plant growth and ultimately plant yield. All these changes are mainly arises due to altered physiological and metabolic functions that ultimately reduced the production of photosynthetic pigments. This is also result in the decline of harvesting of light and production of reducing power, this power is the source of energy in photosynthesis in dark reactions.
When the water available in the soil is decline and atmospheric conditions that result in the loss of water due to transpiration and evaporation. Drought stress is mainly result in the protein damage. It also cause cellular dehydration. Drought stress affecting the physiological stages that may induced negative impact on plant yield or biomass.
Drought stress is basically a moderate loss of water which result in stomatal closure and gas exchange limitations. Drought causes the turgor loss, reduced the leaf water potential and reduction of water contents. Water stress reduced the cell division and cell enlargement. Severe drought stress is characterized by the arrest of photosynthesis and disturb the plant metabolism that eventually leads towards the plant death.
Tolerance to drought stress is observed in almost all plants and it varies even within the same specie or from specie to specie. The response of the plant to drought stress include changes in growth, osmolyte accumulation, stomatal conductance and expression of various genes.
Abscisic acid (ABA) is a main stress hormone in all these physiological processes, in severe conditions its rapid accumulation and participation in biochemical and physiological processes that allows the plant to carry out its functions in stress conditions. ABA enables the plant to maintain the shoot and root growth under stress conditions.
Drought tolerance increased by maintaining root growth due to enhanced plant capacity of water uptake. Under the drought stress high yield can be achieved by maintaining the crop plant function at low water potential.
The production of reactive oxygen species is connected with different metabolic processes such as photosynthesis and aerobic metabolism. Under stress conditions its production is increased due to various mechanism as it inhibits the photorespiration and inhibit the regeneration of NADP+.
Under such conditions accumulation of ABA triggers the production of hydrogen per oxides and super oxides anion by inhibit the uptake of carbon dioxide and alters the transport of electrons in chloroplast.
Conventional plant breeding seems to be effective by using the criteria of physiological selection but this method depends upon genetic variability and it is also time consuming. The main target of crop breeding is to attain the high yield potential under drought stress.
In stress responses various molecular networks are involved such as signal transaction. Transcription factor activity also play an important role under drought stress because it involves in the regulation of stomatal conductance and stomatal response.
All these factors enables the crop plants to adapt themselves and survive under stress conditions. The response of the plant to drought stress differ at various levels of organization it depends upon the duration and intensity of stress, specie and growth stages of plant.
Causes of plant growth reduction under drought stress
- Effect on stomatal signaling due to drought stress
Activity of stomata is affected by different environmental stresses, influence the absorption of carbon dioxide and thus impact on plant growth and photosynthesis.
In response to drought stress, ion uptake and water transport system outside the membrane, control the turgor pressure that cause the changes in guard cell which ultimately results in stomatal closure.
Rapid production of ABA start during drought stress, stops the physiological responses of plants, including stomatal closure this whole process is regulated by the signal of transduction network.
Drought tolerance and water use efficiency is enhanced in several plant species by the over expression of NCED3 enzyme. During drought conditions the ABA that is accumulated in vascular tissues is transported towards guard cells due to change in Ph and by active transporter through passive diffusion.
This study indicates that the transport system of ABA plays an important role in the drought stress tolerance and in adjustment of plant growth.
During the drought stress ABA that promote the signaling pathway that stops the production of reactive oxygen species as a result cytosolic Ca2+ concentration increases and reduction in inward K+ channel and H+-ATPase takes place, which leads towards stomatal opening. K+ efflux and anions from guard cells reduced the guard cell turgor pressure as a result stomata are closed.
- Effect on photosynthesis:
During drought stress, plants basically respond through stomatal regulation anti-oxidative defense system and osmotic adjustment in order to minimize the damage that is caused by drought stress. The plant growth is retarded due to the greater intensity of drought stress that prevails for longer period of time these changes disturb the morphological structure and affect the biomass distribution pattern or even death of plant takes place.
Drought is an alarming factor that inhibit the plant growth and cause serious decline in photosynthesis. Drought stress cause decline in light saturation rate it also decrease the carbon dioxide assimilation rate which ultimately reduce the maximum velocity of carboxylation and at the end it reduce the capacity to regenerate the ribulose 1,5-bisphosphate.
Photo damage of photosystem ll also takes place due to water stress. With increasing drought stress, photosystem ll quantum efficiency is decreased and it disturb the photochemistry of plant. Drought stress badly decrease the leaf area of all plant species as a result photosynthetic productivity also decreased.
Under drought stress conditions reduced photosynthesis is indicated by both non stomatal and stomatal limitations. The rate of photosynthesis is reduced due to decrease in photosynthetic pigments.
On photosynthesis drought effect may be direct as it alters the photosynthetic metabolism by limited diffusion through stomata or the effect may be secondary as the multiple stresses are imposed by superimposition that arise the oxidative stress. These stresses quickly change the expression of genes with physiological alteration.
- Effect on the composition of pigments:
Photosynthetic pigments plays an important role in plant growth because they harvest light for photosynthesis and involve in the reducing power production.
Chlorophyll a and chlorophyll b pigments are of great importance they are prone to soil drying. Carotenoids also plays an additional role they help the plants to tolerate the adverse conditions of drought stress.
- Chlorophyll:
Drought stress has the capacity to reduce the chlorophyll and carotenoid’s tissue concentration. Drought stress disturb the ratio of carotenoids, chlorophyll a and chlorophyll b.
Drought stress cause significant reduction in chlorophyll pigment concentration. The contents of chlorophyll decrease to higher level during severe drought stress.
Higher plant’s foliar photosynthetic rate also decreased because leaf water potential and relative water contents decreases. During drought stress chlorophyll a is not much more affected that results in significant decline in chlorophyll a and chlorophyll b ratio.
- Carotenoids:
Carotenoids are larger group of isoprenoids molecule that are produced by non-photosynthetic and photosynthetic organisms. They are classified into hydrocarbon carotenes like beta carotene and lycopene and effects of reactive oxygen specie as it is essential for the proper functioning of chloroplast.
Beta carotene e has an additional function of accessory pigment it behaves as an effective anti-oxidant molecule as it performs a unique role by protecting different biochemical processes and also sustain these processes.
In photosynthetic tissue the major role of the beta carotene is that it directly quenched the triplet chlorophyll that protect the plant from oxidative damage by preventing the production of single oxygen. Drought stress increase the production of reactive oxygen specie in thylakoid membrane.
- Effect on respiration:
Drought stress also impose negative impact on respiration rate. There are larger variation from stimulation to inhibition under different drought stress conditions. Drought stress quickly reduce the leaf growth and leaf size and ultimately reduced the components of growth respiration. It also disturb the activity of electron transport.
During severe drought stress electrons are shift towards alternative pathway from the cytochrome. The partitioning of electrons from alternative pathway is increased by 40% during drought stress condition. As a result the rate of the synthesis of the mitochondrial ATP is reduced up to 35% under severe drought stress.
However, drought stress has no major impact on different levels of mitochondrial oxidase protein. All these changes effect the plant growth and development and ultimately decrease the plant yield. Another point of great concern is that the effect of water stress on the partitioning of electrons between the cyanide and cytochrome that adversely effect the synthesis of ATP.
Drought stress effect the ratio of ATP and ADP, it takes place due to the change in ATPase and kinetics and it also disturb the balance between ATP synthesis and ATP demand. Drought is particularly connected with light that increase the oxidative stress by increasing the production of reactive oxygen species in different cell compartments.
- Transcription factors:
During drought stress various gene expression and their functions are particularly induced. In various species transcriptomic analysis have been discovered the presence of physiological processes that are connected with various drought responsive gene expression.
In different signaling pathway receptor and sensor genes that found in membrane play very important role by conveying the information to the target protein of cytoplasm through catalytic processes called phosphorylation. All these factors altogether changes the physiology of plants.
Conclusion
The severe drought stress negatively effect the plant growth. The increased hydrogen per oxide production damage the lipid and protein. However, plant has the ability to increase soluble sugar and ABA concentration that helps the plants to avoid the plants drought stress.
During growth and development plants have developed the mechanism to adapt themselves and respond to severe environmental stresses due to their sessile life cycle. Plant growth is reduced due to drought stress this is because of the stomatal closure which reduce the carbon dioxide uptake and consequently reduced the activity of photosynthesis.
In order to overcome these problems there is need to understand the different regulatory mechanism that control and increase the adaptive response of plants to drought stress. These mechanisms are ion transport, stomatal response, activation of stress signaling molecules and protect the photosynthesis from different injuries.
We should have to understand these key factors to increase the plant productivity during drought stress. There are various transgenic technologies that regulate the key genes and enables the plants drought to adapt themselves and increase their tolerance against stress conditions and also helps the plants to maintain their productivity.
Authors: Aqsa Ahmad, Iqra Basharat