Drought (and extreme temperature) is an environmental occurrence that can be defined and measured by indices derived from large historical databases on precipitation and other weather variables. Several drought indices are use, as published by the Drought Mitigation Center at the University of Nebraska. Among these, the ‘Palmer Index’ seems to be widely adopted among climatologists. Information on identification and assessment of drought prone environments can be obtained from various world/country climatic maps available on the net (see 'Selected Climate and Drought Links' on this page). See an example of how some of the atmospheric and soil indices are used to describe drought conditions in Georgia (USA).

Selected Climate and Drought Links

Geographical Information Systems (GIS) is developing very fast and exciting applications useful for agronomists and environmental scientists are being developed. Even specific applications to assist breeders working in dry environments are being developed. A lightweight GIS data viewer developed by ESRI (“ArcExplorer”) is now available online. This freely downloadable software offers an easy way to perform basic GIS functions. ArcExplorer is used for a variety of display, query, and data retrieval applications and supports a wide variety of standard data sources. It can be used on its own with local data sets or as a client to Internet data and map servers. Explore the application of GIS to issues of agriculture development in dry regions as being developed at CIMMYT (search their site for ‘GIS’). GIS interfaced with GPS offer great potential in precision agriculture and similar map-based applications. Finally, a free online climate estimator for any point on the globe is available and can be a very useful tool for the agronomist and breeder. See LocClim.

Crop simulation models can reasonably estimate and quantify the impact of specific drought stress conditions on crop productivity. Such models require the input of various water regime and climate variables in order to simulate crop yield with better or lesser level of accuracy.

Crop models and user's guides are available today for:

§ Cereals - maize, wheat, rice, sorghum, barley, and millet.

§ Grain legumes - soybean, peanut, dry bean, and chickpea.

§ Root crops - cassava and potato;

§ Sugarcane, tomato, sunflower and pasture,

§ And more.

Additional information on crop simulation models and ecological models can be found at Ecobas and at the International Consortium for Agricultural Systems Applications.

The development of plant water deficit

Plant water deficit develops as the demand exceeds the supply of water. The supply is determined by the amount of water held in the soil to the depth of the crop root system. The demand for water is set by plant transpiration rate or crop evapotranspiration, which includes both plant transpiration and soil evaporation. Evapotranspiration is driven by the crop environment as well as major crop attributes such as plant architecture, leaf area and plant development. The plant functions within a physical system consisting of the soil-plant-atmosphere continuum. During the day the plant is under heavy energy load consisting mainly of the received solar radiation, ambient air temperature and humidity. While some of this radiation energy is important for photosynthesis, most of it is not utilized and it must be dissipated. It is partly dissipated by radiation emitted from the plant in the form of heat, but most of it must be dissipated by transpiration (‘latent heat’). Henceforth the term "transpirational cooling" was coined. Transpiration cause leaves to cool relative to ambient temperature when the environmental energy load on the plant is high.

waterpotentials

Fig.1. A schematic representation of the components of leaf water status during a soil drying cycle. SWP– soil water potential; LWP- leaf water potential (ψ_w); OP-s and OP-r represent two different cases of change in osmotic potential (ψ_s) with the reduction in LWP.

Water is driven through the plant from the soil to the atmosphere by the difference in water potential between the atmosphere (very low potential) and the soil (relatively high potential when wet), analogous to the flow of electrical current under the differences in electric potentials (‘Ohm’s law’). Flow is also influenced by hydraulic resistances in the plant, such as the resistance regulated by stomata in the leaves or by the conductive system (root and stem xylem elements) of the plant, or by the resistance of cells and cell walls between soil and the root xylem vessel.

As water transpires from the leaf, leaf water potential (LWP) is reduced (becoming more negative) (Fig.1). If water is available in the soil (at high soil water potential) then water will flow into the leaf to replenish the loss with only a small reduction in LWP. As soil water potential (SWP) reduces LWP must be further reduced in order to create the necessary gradient differential, which would drive the water up from the drying soil to the leaf.

The leaf cells contain various organic and inorganic solutes, which determine the leaf osmotic potential (OP). OP is lower (more negative) than LWP and the difference between the two is turgor potential. Turgor is lost (null value) when LWP=OP (in Fig.1, at about –3.0 and -4.0 MPa on days 8 and 16, respectively).

Leaf turgor is associated with cellular growth and function. When turgor becomes null cells collapse and the leaf wilts, though it is not dead. Stomata are responsive (among other factors) to turgor, closing to reduce transpiration. The reduction in stomatal conductance causes also a reduction CO₂ fixation and photosynthetic assimilation and an increase in leaf temperature. The increased leaf temperature may reach a level causing heat damage to the leaf especially under hot conditions.

Turgor maintenance and transpiration are therefore crucial to plants under drought stress. Turgor can be sustained by keeping a high LWP through water uptake from the drying soil or by reducing OP through solute accumulation (osmotic adjustment). In Fig.1 OP-s represents a variety with little osmotic adjustment (OA), where turgor is lost at a LWP of –3.0 MP on day 8, as compare with variety OP-r where due to greater OA turgor is lost only on day 16 at LWP of -4.0 Mpa. Hence, OA delays turgor loss and wilting. Further details on OA are given in the Drought Mitigation/Drought Resistance page on this section.

Turgor can also be maintained by cell wall hardening during the development of water deficit. While cell wall hardening helps to sustain turgor it impedes cell growth. This is only one of the many examples where the maintenance of plant water status under drought stress might partly be achieved at the cost of reduced growth.

Besides the factors controlling transpiration at the single leaf level, a most dominant factor in controlling whole plant and crop transpiration is total leaf area. When grown in a pot a large plant will require irrigation more frequently than a smaller one for the same pot volume. A major avenue by which plant evolution served to adapt plants to dry environments is by reduced plant size and growth rate, typical of many xerophytic plants. It is also a common observation that when sever water deficit develops lower (older) leaves are desiccated and die first so as to reduce leaf area and water requirement, while upper (younger) leaves retain open stomata and carbon assimilation. This behaviour is typical of sorghum, a relatively drought resistant crop plant.

At the crop level plant size and the demand for water is mainly expressed by leaf area index (LAI), which is the total area of live leaves per unit ground surface (m^-2m^-2). Crop evapotranspiration (ET) increases with LAI until LAI reaches a maximum threshold beyond which ET does not increase. As the crop matures and leaves senesce, LAI is reduced and so does ET.

Cellular turgor is not the only important transducer of whole plant water stress. The growth regulating hormone abscisic acid (ABA) is produced in the shoot in response to desiccation, causing many of the known expressions and consequences of plant water deficit such as arrested growth, stomatal closure, and reproductive failure. ABA is also produced in the root in direct response to the drying soil and its hardness as it dries (e.g. #8058). ABA flows with the transpiration stream to the shoot. Since the soil may dry around only some of the roots (typically in the top-soil) while most roots are in wet soil, root ABA exported to the shoot may cause stomatal closure or arrested growth before any water deficit develops in the shoot. This "hormonal or chemical root signal" may therefore serve as an "early warning system" to the plant. The function and value of this “root signal” in a crop plant subjected to drought stress has not yet been fully resolved. Without entering a long review and discussion it can be postulated that low ABA production or low shoot sensitivity to ABA might be beneficial in most cases of a crop in an agricultural ecosystem.

The overall agronomic role of ABA is still under debate and unresolved. In the evolutionary sense it may well serve as a major signal to place the plant in a dormant and a life conserving state before it enters sever desiccation.

Sensing and estimating plant stress at the whole crop level is difficult mainly because of the need to integrate an estimate based on the whole canopy. Remote sensing technology enabled to develop the Crop Water Stress Index (CWSI). This index is developed mainly from measuring the canopy temperature with an infrared thermometer. Infrared remote measurement of leaf temperature is based on a relationship between leaf temperature and transpiration. Generally, as transpiration rate is reduced due to plant water deficit so does leaf temperature rises relative to air temperature. CWSI is mainly applied to irrigation scheduling, but can be applied to non-irrigated conditions. Infrared thermometry is also being used in selection work towards breeding for drought resistance. See under Mitigation/ Methods of Breeding for Resistance.

Several direct measurements can be performed on single plants in order to assess their water status, stress status, the repercussions of water deficit and its physiological consequences. Here only the most essential and widely used measures of plant water status are mentioned. For additional information check the section on Methods of Breeding for Resistance. For technical data check the instruments and materials section in the Web Resources Page.

Pressure Chamber.jpg

Fig.2. The pressure chamber; (A) pressurized gas cylinder; (B) the chamber pressurizing valve; (C) pressure gage; (D) the chamber; (E) magnifying glass.

Leaf water potential (LWP) can be measured in detached leaves or tissues by quickly sampling the leaves and putting the sample into the measuring instrument. Measurement can be done by the pressure chamber (Fig.2), which is suitable also for fieldwork. When a leaf (or a stem) is cut off a plant, the sap is sucked back into the xylem, since it is under tension. That tension is broadly equal to LWP. The detached leaf is therefore sealed in a steel chamber with only the cut end (petiole) protruding out. Pressure is applied to the chamber (from a pressure source such as a compressed nitrogen gas). When the sap meniscus appears at the xylem surface the pressure is recorded and taken as the xylem (leaf water) potential. Typical LWP of live transpiring leaves can range from about -0.3 MPa to –2.5 MPa.

The thermocouple psychrometer had to be initially built by the scientists themselves until commercial variation were put to the market. It is not suitable for fieldwork. With the basic design the tissue sample is sealed in a small chamber containing a thermocouple. After an equilibration period a cooling current is applied to the thermocouple in order to condense water on the junction. The amount of condensed water is proportional to the water potential of the tissue. That water is allowed to evaporate causing a change in the thermocouple output. That output is calibrated for water potential, using salt solutions.

The pressure probe for measuring turgor pressure is limited to work with singular cells. With this method a small capillary tube filled with oil is used to puncture a cell. The oil is pushed back into the tube in proportion to the cellular turgor pressure. Applying and measuring a balancing pressure to the probe estimate pressure.

Relative water content (RWC) is a veteran method that has recently gained favour over LWP as a very relevant physiological measure of plant water deficit. Its advantage is that it accounts for the effect of OA in affecting plant water status. Two plants with the same LWP can have different RWC if they differ for OA. The detailed protocol for RWC is available on this site.

Osmotic potential (OP) is determined in freeze-thawed killed tissue, which serves to release all cellular solutes. The potential recorded in such a sample by the thermocouple psychrometer is OP. Alternatively if a drop of solute can be obtained by squeezing the killed tissue or by centrifugation then it can be measured for OP with a standard (micro) osmometer. Turgor is estimated as the difference between LWP and OP. When LWP=OP turgor is null. When this is attempted both LWP and OP should best be measured by the same technique, for example with the thermocouple psychrometer.

Osmotic adjustment (OA) is defined as the net accumulation of solutes after the plant has been exposed to a predetermined rate of water deficit. The reduction in OP during water deficit is not an estimate of OA because it is caused by both cellular water loss (a mere concentration effect) and real cellular solute accumulation. The increase in OP in the latter case over OP in fully hydrated none stressed plant is OA. OA is proportional (non-linearly) to the rate of water deficit (LWP) over time. The method therefore entails careful application of drought stress for an extended period of time to a predetermined level of water deficit, best measured by RWC. OP is then measured in the tissue and OA is calculated as the difference between measured OP and OP estimated for a non-stressed fully turgid state. Alternatively, water deficit is applied to a predetermined level of stress (say 70% RWC) after which the plant is fully rehydrated (typically overnight). OP is then measured and compared with that of a fully hydrated non-stressed plant. The difference between the two measures of OP is OA, measured by the “rehydration method”. A more accurate but a resources demanding method involves the derivation of OA from the relationship between OP and RWC during a drying cycle (see the comparison of methods). Typical OA values for crop plants can range from null to around 1.5 MPa.

Measuring transpiration and stomatal conductance

There are numerous methods for measuring transpiration from single detached leaves, whole plants and whole canopies in the field. Methods vary in applicability, accuracy, cost and speed. The review of all methods is beyond the scope of this site. Many methods are simply gravimetric, based on the amount of water lost from a plant grown in a container or from a detached leaf, with known leaf area. Transpiration is expressed as mass of water lost per unit leaf area per unit time. Field methods are often based on measuring the amount of water lost from the soil profile. Various porometers were developed to measure transpiration from intact leaves in the field or the laboratory. The most common principle of function is the exposure of a humidity sensor to the transpiring leaves under standard cuvette conditions (the diffusive resistance porometer). Porometers vary in function and specifications, where some constitute only a part of a more elaborate system of monitoring total leaf gas exchange. A pressure-drop (or viscous flow) porometer was designed and used in the 1960’ to 1980’s. It is based on the relationship between stomatal conductance and the flow of pressurized air across the leaf. It seems that this system was dropped in favour of the diffusive resistance poromoeter, which is also readily available commercially.

The sap flow method is becoming popular with large trees, in which transpiration is difficult to measure by other methods. This method estimates transpiration by the velocity of a heat pulse applied to the trunk and measured above the point of applications.

See our Materials and Instruments Resources for more information from dealers of various instruments

Measuring Soil Moisture

An overview of soil-water relations is available via PowerPoint presentation (allow time for download).

Soil moisture content (by volume) and status (by potential or tension) is a major variable affecting plant water status and crop water-use. Extractable soil moisture is the amount of water that a given crop can extract from the soil to a given soil water potential and soil depth. Generally, different crops can use 50% to 80% of the extractable soil moisture before crop transpiration is reduced and plants present symptoms of water deficit. These values change with crop, soil and atmospheric conditions. The determination of soil moisture content or status is of major consideration regarding plant water relations.

Methods of testing soil moisture vary from feeling the soil by hand to remote sensing it from aerial or even space platforms. Methods vary in accuracy, cost, convenience and purpose. The major methods used in farming or in research are detailed in the sites noted below. The basic method is the ‘gravimetric’ where soil moisture content is determined by weighing the soil before and after drying in an oven. More advanced methods are represented by the ‘Time-Domain Reflectometry’ (TDR) method, which has developed in recent years into a relatively accurate and convenient method for measuring soil moisture at different depths through soil access tubes. One supplier of the system is UMS, for example. Details on the various measurement methods are available at SOWACS (Soil Water Content Sensors).

The repercussions of plant water deficit

Fig.3. A consensus scheme of stress perception and gene response. See text

It is not quite clear which are the primary sensors of cellular dehydration and their order of importance or function, be it cellular water status, pressure, bound water, hormones (mainly ABA), cellular membrane functions or other agents. It is not perfectly clear how cells perceive cellular water deficit and how cellular water deficit is transduced and transcribed into the various consequences of this stress, be it adaptation or mortality. Furthermore, it has not been clearly established which of these responses are stress adaptive and which are expressions of system degradation. The working hypothesis underlying current research in this area (Fig. 3) recognizes multiple signal transduction pathways between stress perception and gene expression. Two major possible pathways transcribe the perception of this signal; one involving ABA production and the other is ABA independent. The ABA independent pathway is not fully resolved. In the ABA dependent pathway ABA induces novel protein synthesis, which regulates numerous "stress responsive", or "ABA responsive" genes. ABA may regulate stress responsive genes without novel protein synthesis. These gene products are either functional (e.g. water-channel proteins or key enzymes) or regulatory (e.g. protein kinases) and they are involved in mediating various cellular responses some of which are recognized as adaptive. The interface between the molecular domain involving stress responsive gene expression and whole plant response to drought stress is yet to be fully understood. This interface is critical for translating molecular genetics science into advances in crop production under stress conditions. At present there are hundreds of recognized ‘drought stress responsive genes’ which are up-regulated or down-regulated under the effect of dehydration and their functions remains to be determined.

At the whole plant and the crop level, the important repercussions of water deficit are mediated by effects on plant phenology, phasic development, growth, carbon assimilation, assimilate partitioning and plant reproduction processes. These major effects account for most of the variation in crop yield caused by drought stress. Growth depends on cell expansion and cell division. Cell expansion is probably the most sensitive to water deficit. Cell division might be less sensitive. Cell expansion is dependent on the maintenance of turgor, cell wall extensibility and other factors possibly pertaining to ABA signalling. Reduced cell expansion as a primary response to water deficit serves to reduce plant water use but also lead to reduceed plant productivity. If the reduction in total plant water use is not sufficient to sustain turgor, then transpiration is further reduced by stomatal closure. Initially, stomatal closure reduces transpiration more than it reduces CO₂ assimilation, but at an advance stress both are reduced drastically. Wilting is an expression of turgor loss, which takes up different forms according to plant species, such as leaf rolling in the cereals (Fig. 4).

Fig.4. Drought stressed sorghum plant with rolled leaves

Reduced cell expansion also carries a primary effect on meristematic development of yield components, such as the inflorescence or the

tiller initials in the cereals - leading to potentially small reproductive organs and reduced yield. This is an irreversible structural effect that is difficult to amend by re-watering. It can however be amended to some extent by inter-organ compensation following watering, such as renewed tillering in the cereals. The meristematic tissues are generally positioned within the plant in a relatively protected environment as compared with that of a fully expanded leaf and therefore it may take a sever stress for meristem to loose its turgor. However, ABA transported from stress-affected organs can also arrest meristem development even at relatively high meristem water-status.

Water deficit causes advanced or delayed flowering, depending on the species (for example, wheat and rice, respectively). ABA may have a role in this respect as it has been shown to delay flowering in tomato and maize. A delay of up to 50 days has been seen in rice subjected to pre-flowering drought stress. The effect of drought stress on phasic development has been shown to be crucial in affecting maize yield under stress. In maize stress cause a delay in female organ development while the male inflorescence is less affected. Hence, stress causes an increase in the time interval between silking and anthesis. A short anthesis-to-silking interval (ASI) (#3347) has been shown to be a main feature of drought resistance in maize.

It is widely agreed that the reproductive growth stage is the most drought-sensitive stage. Water deficit can cause reproductive failure. Pollen or pollen mother cells are generally more sensitive to desiccation than the ovary so that male sterility is a common result of drought stress during flowering. Reduced grain set of wheat under drought stress has been ascribed to ABA accumulation in the shoot. An ABA responsive gene has been found in the floral parts of tomato. There are some interesting reports (e.g. #3074) showing that grain set of maize subjected to drought stress could be partially improved by the experimental infusion of sugar solution into the stem, leading to conclude that sugar starvation could be an additional factor in affecting grain set and ovary abortion under stress. Short supply of sugar could very well be a generally important factor in the abortion of fruit under drought stress.

Shortage of assimilates and sometimes nitrogen availability is a major cause of arrested grain and fruit growth during drought stress. Drought stress during cereal grain development reduces the duration of grain filling. If the rate of grain filling is not adjusted upward, final grain weight is reduced. Increased grain growth rate under drought stress depends on the supply of assimilates. This supply is becoming short due to the inhibition of current photosynthesis during stress. An alternative source of assimilates are pre-anthesis stem reserves in the form of sugars, starch or fructans, depending on the species. These reserves are readily utilized for grain filling and their availability may become a critical factor in sustaining grain filling and grain yield under drought stress.

Root/shoot dry weight ratio increase as plant water stress develops. The increase is mostly due to a relative reduction in shoot dry weight. However there were rare cases where an absolute amount of root dry weight increase was observed under drought stress. ABA may have a role in promoting root growth under drought stress. Osmotic adjustment has also been found to improve deeper root growth under stress. Most certainly root distribution within the soil profile changes as stress develops, in a way that helps the plant to explore deep soil moisture. In the cereals, dry topsoil inhibits the formation and establishment of new roots in the topsoil while assimilates partitioned to the root are used in furthering the growth of existing roots into deeper soil. In the small grains and rice, tillering is associated with the development of new roots from tillers. Therefore, extensive tillering is generally associated with dense and shallow roots while limited tillering tends to associate with sparser and deeper roots. This is one of the reasons why most cereal crop cultivars developed in dry regions tend to have a limited tillering habit.

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