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Modern living and globalization have brought up many changes in the environment. Changing lifestyle and over-dependence on combustible fuels resulted in carbon emissions and global warming. In developing economies, agriculture largely stands as a livelihood for the country's population. The present article throws light on the effect of drought on agriculture because of changing climatic patterns.

planting stragtegies in dry and rough climate

Agriculture and climate


Agriculture is a long-term process. It depends on multiple interactions with the environment right from sowing until obtaining the crop produce. Yield per hectare of the farm is dependent on the prevailing climatic conditions during the crop season. 

Temperature extremes and agriculture


Raising global temperatures and carbon emissions have already resulted in altered rainfall patterns, extreme temperatures, etc. Heat waves are the primary progression of elevating temperatures. 

Prolonged exposure to high temperatures is harmful to all living beings. Extended high-temperature exposure can cause serious damage particularly for plants in the phase of flowering. An extended exposure can cause changes in carbon assimilation and eventually growth rates. Climate deviations during flowering have a profound effect on fruit production which can bring severe losses to farmers cultivating cash crops.

Even short span temperature extremes can pose a severe threat to yield quality if they coincide with the critical developmental stages of crop. High temperatures are usually accompanied by high light intensities. Extreme light intensities beyond required levels damage the photosynthetic apparatus. So, even after the advent of ambient temperature and light intensities, the plant engages in the process of repairing rather than engaging in carbon assimilation and building biomass. 

In tropical and subtropical areas prevailing high temperatures are a global characteristic. Heat waves, reducing water availability have a combined effect further amplifying the existing severity.

Prolonged dry conditions accompanied by high temperatures can result in a severe condition termed Drought. Low or negligent precipitation along with extended high temperatures result in moisture loss from soil. Low soil moisture along with prevailing temperatures imposes abiotic water stress in plants. 

Plants adapted a multitude of physiological, morphological, and biochemical changes to cope with extreme conditions. 

Desert ephemerals 


Few plant species complete their lifecycle before the extreme climate arrives. These plants do not have any morphological structures to face drought, but they adapt rapid phenological development as an adaptation. 

They produce enough reproductive output in the form of seeds while few others maintain inactive underground structures that are not harmed by the existing harsh environment. 

On the advent of favorable conditions, the reproductive structures start a new generation and hence the species continues.

Some species adapt developmental plasticity to the environment. During the dry season they limit growth as a slow process. On the advent of wet conditions, they grow rapidly and produce large reproductive output.

Desert perennials 


Some arid area plants have a separate set of morphological adaptations. These are water savers having small leaf size or replacing the leaf count with thorns reducing water loss through transpiration. 

Few other desert plants are drought avoiders by maintaining a succulent lifestyle. They have a thick wax coating on the leaf surface and closed stomata to minimize water loss. 

Drought tolerance is an adaptive strategy that plants take as a physiological measure.

Impact on Crop plant physiology at a glance 


Living beings suffer from dehydration and overheat during dry environments. Plants are immobile and cannot run away to shade regions to protect from heat. However, they developed a multitude of adaptations to withstand drought conditions.

Plants developed a well-equipped hormonal system to communicate the existing localized stress to other parts so that the entire plant body gets ready to face the fight.

Plant hormone Abscisic acid (ABA) plays a key role in stress-mediated responses.

Plant roots sense water scarcity and an immediate synthesis of ABA takes place. Stomatal closure during high temperatures (usual noon times) is an ABA mediated response and is most intensely studied physiological adaptation.

ABA accumulation in guard cells causes an increase in calcium ions which leads to an imbalance in membrane potential. As a result, potassium ions leave guard cells into the surrounding mesophyll cells. Due to this series of ion uptake and efflux guard cells lose turgor and stomatal aperture shrinks otherwise called stomatal closure.

Stomatal closure reduces transpiration and thereby lowering pressure down the length of xylem tissues. Water is uptaken by roots because of existing negative pressures that run down the length of xylem. 

Plants lack “heart” which stands as a central pumping organ for the entire body in case of animals. The water loss through transpiration creates a negative pressure and thus roots tend to absorb water from the soil. Running over the principle of Cohesion-Tension theory water is transported to each cell throughout the plant via xylem vessels and tracheids. So, when the stomata are shut off, there is no or reduced water loss to the environment. 

Stomatal closure has an impact on growth too 


Though water loss is reduced, stomatal closure limits gaseous exchange which has a direct effect on carbon assimilation.

Usually plants grown in arid areas show stunted growth having reduced leaf size. Reduced leaf size is an adaptive feature so that only a small area is exposed to sunlight thereby less water can below. Reduced plant height, stem diameter, plant biomass has been reported in many plant species studied under drought when compared to those under normal conditions.

Rising temperatures pose a serious threat to the light-harvesting complex and photosynthetic enzymes causing a lowering photosynthetic output. This has a final impact on plant growth. 

Mastermind 


Most water is lost and the existing dehydrated interior can damage membranes and proteins. To protect the cellular proteins and membranes, plant cells accumulate small, highly soluble, non-toxic neutral molecules at molar concentrations. 

They are called osmolytes or osmoprotectants since they protect cellular protein structures by hydrogen bonding. Polyol hydroxyl groups of some osmolytes form hydrogen bonds with proteins of membrane bilayer helping in maintaining cell interior as well as cell turgor.

This strategy is not only to survive the existing stress but also helps plant cells to retain back their machinery once the drought is circumvented by normal conditions. 

What a mastermind!!! 

Drought stress induces accumulation of oxygen free radicals into the cellular interior which is entirely a different subject of discussion. Plants developed a well-defined antioxidative defense system to fight the drought-induced free radicals and maintain redox balance. 

The above-discussed strategies are adapted by plants that face an unexpected drought. However, there are other strategies adapted by plants to face the expected drought. Expected drought occurs in arid areas where plants living there.

Some species of plants developed special mechanisms to survive and continue performing carbon assimilation . This is known as the C4 pathway. It was first studied in the Crassulacean family. 

They perform gaseous exchange at night. When the surrounding temperatures reduce, carbon dioxide is fixed as Malic acid (a 4-carbon molecule) by enzyme PEP carboxylase and stored in vacuoles. During day time, malic acid is broken down to release CO2 which is accessible to enzyme RUBISCO to carry out further carbon assimilation.

C4 metabolism to increase food production


Global food needs are growing day by day and there is a dreadful need for studies that can increase food grain production. With increasing pollution and changing climatic conditions more research is being conducted in the aspects that can increase food production with existing conditions.

Few approaches have been discussed in our previous article. 

MODERN FARMING METHODS  

Apart from them, studies are also focussed on molecular aspects for installing the C4 pathway in C3 plants like rice in a manner to improve photosynthetic capacity.

The idea is that food crops having a C4 pathway can withstand arid conditions and continue their photosynthetic output. This can help plants to produce food grains even despite dry conditions, high temperatures, and even when stomata are closed there is no halt for carbon assimilation.

Genetic engineering (the science of manipulating organisms genes using biotechnology) has been employed and a set of genes regulating certain biochemical processes, leaf anatomy has been inserted and expressed in rice. Gene insertion and expression are not simply expecting expertise and knowledge from different fields like physiology, systems biology, molecular biology, metabolomics, plant breeding apart from genetic engineering, and biochemistry.

Photosynthesis in C3 plants is carried out in mesophyll cells while C4 plants have compartmentalization to perform photosynthesis. Carbon dioxide is fixed as 4-carbon compound oxaloacetate then converted to malate in mesophyll cells. Malate is transported to bundle sheath cells where it is converted to carbohydrates by the Calvin cycle (light-independent steps of carbon fixation). 

Chloroplasts are restricted only mesophyll cells (photosynthetic ground tissue of plant leaves) in C3 plants while they are distributed in both bundle sheath cells (tightly packed sheath of photosynthetic cells around vascular bundles) as well as mesophyll cells in C4 plants which make the two plan types very much different. Researchers increased the number and size of chloroplasts in bundle sheath cells of rice by overexpressing the chloroplast development genes in a cell-specific manner. 

Vein spacing is different in leaves of C3 and C4 plants. Veins are spaced closely in C4 plants than those in C3 plants. C3 plants have higher mesophyll cells that push the veins apart thus increasing vein space and reducing the vein density. Higher vein density allows only limited mesophyll cells thus the equal ratio of mesophyll cells and bundle sheath cells. 

Calvin cycle should be enhanced in bundle sheath cells while it should be reduced in mesophyll cells to install the C4 mechanism. For this RUBISCO activity is reduced in mesophyll cells and increased in bundle sheath cells to confine the Calvin cycle to bundle sheath cells. Simultaneously genes coding enzymes like β-carbonic anhydrase and Phosphoenolpyruvate carboxylase (PEPC) are overexpressed in mesophyll cells. 

However, this is only a summary of creating the rice cultivar with the C4 pathway but concerning the experimental part for bringing the plant from lab to field, it requires large data relating different genes, their functions, gene silencing, overexpression, gene source and finally field experiments. Before all, understanding plant responses to different stress stimuli and the genes responsible for particular response gains first place. Without complete knowledge of phenology, gene loci in different drought-tolerant species; advancement towards developing new cultivar would not have been possible. 

The socioeconomic impact brought by drought


Drought is an outcome of long period dry conditions. Arid areas have more effect on the social life of the population living there and semiarid regions grow in becoming arid. Long-period drought conditions result in famines and migrations. 

Low precipitation over extended periods reduces groundwater and diminishes surface water resources making the drought-stricken lands unfavorable for human living. 

Farmers tend to spend more money to dig wells, drill bore wells, and buy tankers of water because the precipitation levels are already low. The obvious and direct effect would be on the farm income and has an indirect strike over local economy returns as the entire social living is at stake. 

Forest fires are unstoppable due to prevailing dry conditions in forest areas. This poses a severe threat to the environment and entire wildlife living there are disturbed. The indirect effect would be on the people depending on forest products with an extent to their social life too.

Earth bears several species that have their uniqueness in interacting with the environment. Few are tolerant of harsh conditions while others are intelligent by escaping the harsh conditions. Every response is operated by a unique set of genes.

Advancements in bioinformatics and biotechnology have boosted up the studies in understanding as well as developing the “hardier” traits. 


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