Breeding for drought stress tolerance

Breeding for drought stress tolerance is the process of breeding plants with the goal of reducing the impact of drought on plant growth.

Drought stress in crop plants

In nature or crop fields, water is often the most limiting factor for plant growth. If plants do not receive adequate rainfall or irrigation, the resulting drought stress can reduce growth more than all other environmental stresses combined.

Drought can be defined as the absence of rainfall or irrigation for a period of time sufficient to deplete soil moisture and injure plants. Drought stress results when water loss from the plant exceeds the ability of the plant's roots to absorb water and when the plant's water content is reduced enough to interfere with normal plant processes.

Drought stress is a global phenomenon

About 15 million km2 of the land surface is covered by cropland (), and about 16% of this area is equipped for irrigation (Siebert et al. 2005). Thus in many parts of the world including the United States, plants may frequently encounter drought stress. Rainfall is very seasonal and periodic drought occurs regularly. The effect of drought is more prominent in sandy soils with low water holding capacity. On such soils some plants may experience drought stress after only a few days without water.

During the last century, the rate of increase in `blue' water withdrawal (from rivers, lakes, and aquifers) for irrigation and other purposes was higher than the growth rate of the world population (Shiklomanov 1998). Country-wise maps of irrigated areas are available.

Rain fed areas of USA, for details - Biradar et al.(2009) International J Applied Earth Observation and Geoinformation 11: 114-129

Drought stress and future challenges to crop production

Moisture deficits is a significant challenge to the future crop production. Severe drought in parts of the U.S., Australia, and Africa in recent years drastically reduced crop yields and disrupted regional economies. Even in average years, however, many agricultural regions, including the U.S. Great Plains, suffer from chronic moisture deficits. Cereal crops typically attain only about 25% of their potential yield due to the effects of environmental stress, with moisture stress the most important cause. Two major trends will likely increase the frequency and severity of crop moisture deficits:

(1) Global climate change.

Higher temperatures are likely to increase crop water use due to increased transpiration. A warmer atmosphere will also speed up melting of mountain snowpack, resulting in less water available for irrigation. More extreme weather patterns will increase the frequency of drought in some regions.

(2) Competing uses for limited water supplies.

Increased demand from municipal and industrial users will further reduce the amount of water available for irrigated crops.

Although changes in tillage and irrigation practices can improve production by conserving water, enhancing the genetic tolerance of crops to drought stress is considered an essential strategy for addressing moisture deficits.

Drought stress affects plant physiology

A plant responds to a lack of water by halting growth and reducing photosynthesis and other plant processes in order to reduce water use. As water loss progresses, leaves of some species may appear to change color—usually to blue-green. Foliage begins to wilt and, if the plant is not irrigated, leaves will fall off and the plant will eventually die. Drought lowers the water potential of a plant's root and upon extended exposure, abscisic acid is accumulated and eventually stomatal closure occurs. This reduces a plant's leaf relative water content. The time required for drought stress to occur depends on the water-holding capacity of the soil, environmental conditions, stage of plant growth, and plant species. Plants growing in sandy soils with low water-holding capacity are more susceptible to drought stress than plants growing in clay soils. A limited root system will accelerate the rate at which drought stress develops. A root system may be limited by the presence of competing root systems, by site conditions such as compacted soils or high water tables, or by container size (if growing in a container). A plant with a large mass of leaves in relation to the root system is prone to drought stress because the leaves may lose water faster than the roots can supply it. Newly installed plants and poorly established plants may be especially susceptible to drought stress because of the limited root system or the large mass of stems and leaves in comparison to roots.

Drought stress interaction with other stress factors

Aside from the moisture content of the soil, environmental conditions of high light intensity, high temperature, low relative humidity and high wind speed will significantly increase plant water loss. The prior environment of a plant also can influence the development of drought stress. A plant that has been drought stressed previously and has recovered may become more drought resistant. Also, a plant that was well-watered prior to drought will usually survive drought better than a continuously drought-stressed plant.

Mechanism of Drought stress resistance

Degree of Resistance to drought depend upon crops. Generally three strategies can help a crop to mitigate the effect of drought stress:

(a) drought escape

(b) drought avoidance

(c) drought tolerance

A proper timing of lifecycle, resulting in the completion of the most sensitive developmental stages while water is abundant, is considered to be a drought escape strategy. Avoiding water-deficit stress with a root system capable of extracting water from deep soil layers, or by reducing evapotranspiration without affecting yields, is considered as drought avoidance. Mechanisms such as osmotic adjustment (OA) whereby a plant maintains cell turgor pressure under reduced soil water potential are categorized as drought tolerance mechanisms. Drought avoidance mechanisms can be expressed even in the absence of stress and are then considered constitutive. Drought tolerance mechanisms are the result of a response triggered by drought stress itself and are therefore considered adaptative. When the stress is terminal and predictable, drought escape through the use of shorter duration varieties is often the preferable method of improving yield potential. Drought avoidance and tolerance mechanisms are required in situations where the timing of drought is mostly unpredictable.

Drought tolerance mechanism is genetically controlled and genes or QTL responsible for drought tolerance have been discovered in several crops which opens avenue for molecular breeding for drought tolerance.

Drought tolerance traits

Tolerance to drought is a quantitative trait, with a complex phenotype, often confounded by plant phenology. Breeding for drought tolerance is further complicated since several types of abiotic stress, such as high temperatures, high irradiance, and nutrient toxicities or deficiencies can challenge crop plants simultaneously.

(a) Osmotic adjustment When a plant is exposed to water deficit, it may accumulate a variety of osmotically active compounds such as amino acids and sugars, resulting in a lowering of the osmotic potential. Examples of amino acids that may be up-regulated are proline and glycine betaine. This is termed osmotic adjustment and enables the plant to take up water, maintain turgor and survive longer.

(b) Cell membrane stability The ability to survive dehydration is influenced by a cell’s ability to survive at reduced water content. This can be considered complementary to OA because both traits will help maintain leaf growth (or prevent leaf death) during drought. Crop varieties differ in dehydration tolerance and an important factor for such differences is the capacity of the cell membrane to prevent electrolyte leakage at decreasing water content, or “cell membrane stability (CMS)”. The maintenance of membrane function is assumed to mean that cell activity is also maintained. Measurements of CMS have been used in different crops and are known to be correlated with yields under high temperature and possibly under drought stress.

(c) Epicuticular wax In sorghum (Sorghum bicolor L. Moench), drought resistance is a trait that is highly correlated with the thickness of the epicuticular wax layer. Experiments have demonstrated that rice varieties with a thick cuticle layer retain their leaf turgor for longer periods of time after the onset of a water-stress.

(d) Partitioning and stem reserve mobilization

As photosynthesis becomes inhibited by drought, the grain filling process becomes increasingly reliant on stem reserve utilization.72 Numerous studies have reported that stem reserve mobilization capacity is related to yield under water-stress in wheat. In rice, a few studies also indicated that this mechanism maintains grain yield under water stress at the grain filling stage. This drought tolerance mechanism is stimulated by a decrease in gibberellic acid concentration and an increase in abscisic acid concentration.

(e) Manupulation and Stability of flowering processes

(g) Seedling drought traits For emergence from deep sowing (to exploit dry upper soil), this is practiced to help seedlings reach the receding moisture profile, and to avoid high soil surface temperatures which inhibit germination. Screening at these stage provides practical advantages, specially when managing large amount of germplasms.

Crop ideotype for drought tolerance

Usually ideotypes are developed to create an ideal plant variety. The following traits constitutes ideotype of wheat by CIMMYT.

1) Large seed size. Helps emergence, early ground cover, and initial biomass.

2) Long coleoptiles. For emergence from deep sowing

3) Early ground cover. Thinner, wider leaves (i.e., with a relatively low specific leaf weight) and a more prostrate growth habit help to increase ground cover, thus conserving soil moisture and potentially increasing radiation use efficiency.

4) High pre-anthesis biomass.

5) Good capacity for stem reserves and remobilization

6) High spike photosynthetic capacity

7) High RLWC/Gs/CTD during grain filling to indicate ability to extract water

8) Osmotic adjustment

(9) Accumulation of ABA.

The benefit of ABA accumulation under drought has been demonstrated (Innes et al. 1984). It appears to pre-adapt plants to stress by reducing stomatal conductance, rates of cell division, organ size,and increasing development rate. However, high ABA can also result in sterility problems since high ABA levels may abort developing florets

10) Heat Tolerance. The contribution of heat tolerance to performance under moisture stress needs to be quantified, but it is relatively easy to screen for (Reynolds et al. 1998).

11) Leaf anatomy: waxiness, pubescence, rolling, thickness, posture. These traits decrease radiation load to the leaf surface. Benefits include a lower evapotranspiration rate and reduced risk of irreversible photo-inhibition. However, they may also be associated with reduce radiation use efficiency, which would reduce yield under more favorable conditions.

12) High tiller survival.

Comparison of old and new varieties have shown that under drought older varieties over-produce tillers many of which fail to set grain while modern drought tolerant lines produce fewer tillers most of which survive.

13) Stay-green.

The trait may indicate the presence of drought avoidance mechanisms, but probably does not contribute to yield per se if there is no water left in the soil profile by the end of the cycle to support leaf gas exchange. It may be detrimental if it indicates lack of ability to remobilize stem reserves. However, research in sorghum has indicated that staygreen is associated with higher leaf chlorophyll content at all stages of development and both were associated with improved yield and transpiration efficiency under drought.

Combination phenomics : overall health of crops

The concept of combination phenomics comes from the idea that two or more stresses have common physiological effects or common traits - which are an indicator of overall plant health. Similar analogy in human medical terms is high blood pressure or high body temperature or high white blood cells in body is an indicator of health problems and thus we can select healthy people from unhealthy using such a measure. As both abiotic and abiotic stresses can result in similar physiological consequence, tolerant plant can be separated from sensitive plants. Some imaging or infrared measuring techniques can help to speed the process for breeding process. For example spot blotch intensity and canopy temperature depression can be monitored with canopy temperature depression.

Molecular breeding for drought tolerance

Recent research breakthroughs in biotechnology have revived interest in targeted drought tolerance breeding and use of new genomics tools to enhance crop water productivity. Marker-assisted breeding is revolutionizing the improvement of temperate field crops and will have similar impacts on breeding of tropical crops. Other molecular breeding tool include development of genetically modified crops that can tolerate plant stress. As a complement to the recent rapid progress in genomics, a better understanding of physiological mechanisms of drought response will also contribute to the progress of genetic enhancement of crop drought tolerance. It is now well accepted that the complexity of the drought syndrome can only be tackled with a holistic approach that integrates physiological dissection of crop drought avoidance and tolerance traits using molecular genetic tools such as MAS, microarrays and transgenic crops, with agronomic practices that lead to better conservation and utilization of soil moisture, and better matching of crop genotypes with the environment.

See also

• Upland rice
• Molecular breeding

External links

• Plant Breeding For Drought Tolerance, USDA initiative
• Genetic and genomic tools to improve drought tolerance in wheat
• Evaluating a Conceptual Model for Drought Tolerance