Plant-Water Stress Indicators: An Assessment

Plant-Water Stress Indicators: An Assessment P. Krishnan Division of Agricultural Physics, Indian Agricultural Research Institute, New Delhi 110012 Stress is defined as a measurable alteration of a physiological or biochemical steady state that is induced by an environmental change, and that renders the individual, population or community more vulnerable to further environmental change (Bayne, 1975). Deficiency or surplus of water is one of the important stresses to crop plants. Globally, there is a growing demand for water that has ushered the need for its efûcient and judicious utilization in all sectors including agriculture, which is the single largest consumer of water. To meet the demand of water by agriculture, there are numerous infrastructure developments including irrigation projects. In India, the majority of irrigation projects perform at a very low efûciency. More efûcient water management technologies are required, not only to meet the current demand but also the future demands. Among different water management technologies, regularization of water supply is very important at the agricultural field level. The successful regularization of water largely depends on supplying water adequately to the crop need. Otherwise, its deficiency can cause higher yield losses. Therefore, the early detection of plant stress becomes very important to avoid water deficits in order to ensure better growth and yields of all crops. It allows the initiation of actions to counteract conditions causing the plant stress, especially to avoid a moderate stress from becoming severe. The early detection of plant stress can be achieved by observing certain symptoms before stress actually ensues. These symptoms are essentially indicators that are sensitive and can give early warning when a mild stress is developing. The water-stress indicators in crop plants can be categorized as (i) soil-based measures and (ii) plant-based measures. The soil-based measures require sophisticated instruments and some of them are time consuming. The estimation of soil moisture or crop evapo-transpiration from climatic parameters provides objective criteria for irrigation management. But the methods of estimating evapotranspiration require huge climatic data. These climatic data are seldom available and not applied by the common crop growers. Similarly, the estimation of crop water requirement through soil moisture requires soil moisture measurement at several locations. It is a time consuming exercise. These measures are of questionable value in fields with great spatial variation as they may not give proper assessment of crop water need. For some of these reasons, indicators that measure the plants directly tend to be preferred. Nevertheless, the plant growth parameters that take several days to express themselves cannot be related to a unique soil water measurement. Hence, the choice of indicators for plant-water stress assumes greater significance. As early as 1965, Scholander and his associates applied the technique of using a pressure chamber to allow easy measurement of plant moisture stress (PMS). The interesting feature of PMS is that it includes air temperature, wind speed, humidity, and soil moisture, which are all integrated by the plant into one single value. Principally, the measure of PMS gives an evaluation of the moisture status of a plant from the plants point of view. No additional interpretation of other data is necessary. The pressure chamber technique has gained wide use and acceptance among researchers and instructors. Other 122

instruments used to estimate the plants water balance such as the neutron probe, tensiometer, infrared thermometer and pan evaporimeter do not measure directly how the plant reacts to all of the components of its environment. The premise of plant basedwater stress indicator approach is that it considers the plant water status. Because the plant water is a good integrator of the soil, water and climatic parameters, this might be considered as the ideal criterion.leaf Water Potential (LWP) and stomatal conductance are two important and useful indicators among different measures based on plant water status.the LWP is the amount of stress a particular leaf is under at that moment. Measuring the LWP gives an indication of a plants ability to grow and function under water stress condition. The measurements of LWP can be dependable for some species, especially when there is not very much transpiration and the plant has a small canopy. Other indicators like canopy temperature, leaf reflectance, chlorophyll fluorescence, etc., have shown promises but are still impractical. To some extent, the vegetative growth indicators offer several advantages. When water stress occurs, plant tissue expansion is the first to be affected. Hence, these vegetative growth indicators are sensitive, easy to observe or measure, and tend to correlate well with crop yield and quality. Plants can also adapt to water deficits. The osmotic adjustment is recognized as one of the most important adaptive mechanisms of many crop plants to water deficits. In fact, it is a major component of drought tolerance mechanism. Turgor is maintained at lower water status, which, in turn, enables plants to maintain processes such as cell enlargement and stomatal opening. The osmotic adjustment is a highly complex and integrated system of adaptation of plants to water deficits; it strongly depends on the rate of plant water stress. This osmotic adjustment requires time, and any fast reduction in plant water status does not allow time for adjustment. This is very significant when genotypes are compared for their osmotic adjustment capacity. However, the importance of time and the rate of stress for the development of osmotic adjustment imply that it may not be a very effective mechanism of drought resistance. Under conditions where the development of drought is by nature very rapid, such as very light tropical or sandy soils of very low water holding capacity, the osmotic adjustment is not active (Blum, 1997). As the selection criteria for drought resistance breeding, these traits are of limited value due to the loss and extensive nature of physiological measurements (osmometry, thermocouple psychrometry or pressure-volume relations). Their practicality for screening large number of genotypes which is required in a breeding program still remains smal. The direct gravimetric measurement of seasonal crop water use efficiency (WUE) in the field or in pots is labour intensive. More importantly, data on genetic variations in WUE has been scant. Hence, the use of WUE as a potential selection criterion in breeding has been largely theoretical. Although the recent developments in theory and application of isotope discrimination (Ä) measurement as an estimate of WUE in plants allowed the acquisition of relatively large databases on the genetic diversity for WUE in relation to plant production in different environments, the measure of WUE is a questionable selection criterion for improving yields in water deficit environments. Plant production under drought stress depends not only on WUE but largely on the genotypes capacity to sustain transpiration (Blum, 1997). Furthermore, there may be a negative association between WUE and transpiration, such that relatively drought-resistant genotypes that sustain transpiration and maintain plant water status may present relatively low WUE as compared with susceptible ones. The advantages of selecting for isotope discrimination ( ) measurement are numerous, which include: (i) it is highly heritable, (ii) there is substantial genetic variation, (iii) the genotype x environment interactions are small, and (iv) its measurement is non-destructive and must be measured early in the plants life. Thus, the selected plants can be used for hybridization. But, the chief drawback in its extensive use has been the cost of the measurements. 123

Water stressin plants generally causes stomatal closure, which interrupts the energy dissipation, resulting in the rise of leaf temperature. Because of stomatal closure, there is a decrease in leaf transpiration. The leaf or canopy temperature is used as an indicator of plant water stress (Jackson et al., 1981; Jackson, 1982). Idso et al. (1981) used non-water-stressed baselines established from relationship of canopyairtemperature difference of a well-watered crop and vapour pressure deficit (VPD) for estimating crop water stress index(cwsi). Essentially, the calculation of CWSI is based on three main environmental variables: plant canopy temperature (Tc), air temperature (Ta) and atmospheric vapor pressure deficiency. All these variables have much influence on water used by plants (Braunworth, 1989). The application of canopyair temperature difference based approach is appropriate for crop water stress determination. This is a non destructive, non-contact, reliable, provides considerably precise estimation and represents actual crop water stress. Thus, the CWSI which is derived from canopy - air temperature differences (Tc - Ta) versus the air vapor pressure deficit (AVPD), is one of promising tools for quantifying crop water stress. Idso (1982) defined the non-water-stressed baseline for 26 different species for clear sky conditions. These baselines could be different for various phonological stages in certain crops. Hence, different baselines should be developed, such as the baselines for pre and post head stages of winter wheat crop. The baselines can be strongly location dependent, and perhaps even species and variety dependent (Gardner et al. 1992). Additionally, the data from a single day measurement may not be sufficient enough to determine the non-water-stressed baselines. Because it responds to day-to-day atmospheric conditions, the canopy-air temperature difference cannot be used in irrigation scheduling. Gardner and Shock (1989) suggested that the AVPD in the range of 1±6 kpa is necessary to define a baseline that could be used in many locations. Kjelgaard et al. (1996) developed a model for determining integrated daily evapotranspiration (ET) rates with possible applications for determining irrigation requirements (how much to irrigate) as a complement to CWSI measurements (when to irrigate). Both techniques are important irrigation scheduling tools which use much of the same data. There may be different non-water-stressed baselines for different crops, which are evident from many other empirical studies (Alves and Pereiea, 2000), and these need to be determined for each agroclimatic zone. Kirda (2000) showed the water stress tolerance of crop atdifferent growth stages under deficit irrigation scheduling. From a study using three winter wheat genotyopes in Turkey, Orta et al. (2004) developedthe baseline equations, which can be used to quantify crop waterstress index (CWSI) for evaluating crop water stress and to schedule irrigation.the limitations of water stress indices can be assessed using directional thermal infrared (TIR) measurements and 3D simulations (Luquet et al. 2004). Mahan et al. (2005) even determined the temperature and time thresholds for BIOTIC (Biological Identified Optimal Temperature Interactive Console) irrigation of peanut. The infrared sensors installed in the field focus on the plants to record canopy temperatures. Periodic transmission of accumulated date to the base controller within the field will help the controller to determine if time and temperature thresholds have been reached. After reaching the threshold, an irrigate signal is transmitted to the producers computer or cell phone. Water stress in crop plants is usually detected only after it becomes visually apparent. Unfortunately, this is often too late to avoid a reduction in crop yield. The expensive spectrometers with ranges beyond 1000 nm have been used to determine water stress in plants by analyzing reflectance measurements at several key wavelengths called water bands. The most prominent water bands are at 1400 and 1900 nm. The reflectance at these wavelengths has been shown to correspond to water content in the plant tissues (Penuelas et al., 1999). However, the spectrometers with wavelength ranges high enough to measure 124

these water bands are expensive. It is also difficult to measure water stress at these wavelengths from satellites due to high levels of water absorption in the earths atmosphere. Although the 970 nm bands have historically been considered to measure water stress, Penuelas et al., (1993) found that it can be a useful water status indictor for complete canopies where the Leaf Area Index (LAI) does not vary greatly. Presently, there are now several less expensive silicone diode spectrometers available that can accurately measure wavelengths upto 1000 nm. The leaf reflectance can be correlated with the actual soil and plant water content to develop a method to quantify water stress based on these reflectance values. This would help to track plant water stress more accurately and serve as an important water stress indicator. The time these measurements are taken to determine crop water stress is always very important. During the night, water in the soil has time to become redistributed around the roots. So the plants may show no signs of water stress in the morning even if the soil moisture reserve is very low. Therefore, it is best to take measurements in the afternoon, ideally at the same time each day. Additionally, under cloudy conditions, the crop water demand is low, and again, the crop may show no signs of water stress. Establishing the importance of particular trait is very difficult and time consuming. The nature of water stress is such that its timing and intensity is unpredictable from year to year. This makes that any strategy for choosing a particular plant stress indicator for field crops is complex and unpredictable. On every occasion, the selection of a suitable indicator to characterise plant water stress has to be subject to the accuracy, reproducibility, affordability and feasibility. References Alves, I. and Pereira, L.S. (2000) Non-water-stressed baselines for irrigation scheduling with infrared thermometers: A new approach. Irrigation Science 19:101106. Bayne, B.L. (1975) Aspects of physiological condition in Mytilusedulis L., with respect to the effects of oxygen tension and salinity. In: Proc. 9th European Marine BiologySymposium (H. Barnes, ed.), Aberdeen University Press, Aberdeen, pp. 213-238. Blum, A. (1997). Crop responses to drought and the interpretation of adaptation. In: Drought Tolerance in Higher Plants (E. Belhassen, ed.). Kluwer Academic Publishers, Dordrecht, pp. 57-70. Braunworth, Jr., W.S. (1989) The possible use of the crop water stress index as an indicator of evapotranspiration deficits and yield reductions in sweet corn. Journal of the American Society for Horticultural Science 114:542-546. Gardner, B.R. and Shock, C.C. (1989) Interpreting the crop water stress index. ASAE Paper 89-2642. Gardner, R.B., Nielsen, D.C. and Shock, C.C. (1992) Infrared thermometry and the crop water stress index. II. Sampling procedures and interpretation. Journal of Production Agriculture 5:466-475. Idso, S.B. and Reginato, R.J. (1982) Soil and atmosphere-induced plant water stress in cotton as inferred from foliage temperatures. Water Resources Research 18:1143-1148. Idso, S.B., Jackson, R.D., Pinter, P.J., Reginato, R.J. and Hatfield, J.L. (1981) Normalizing the stress-degree-day parameter for environmental variability. Agricultural Meteorology 24:45-55. Jackson, R.D. (1982) Canopy temperature and crop water stress. Advances in Irrigation Research 1:43-85. Jackson, R.D., Idso, S.B., Reginato, R.J. and Pinter Jr., P.J. (1981) Canopy temperature as a drought stress indicator. Water Resources Research 17:1133-1138. Kirda, C. (2000) Deûcit irrigation scheduling based on plant growth stages showing water stress tolerance. Deûcit Irrigation Practices, FAO Water Reports 22. Kjelgaard, J.F., Stockle, C.O. and Evans, R.G. (1996) Accuracy of canopy temperature energy balance for determining daily evapotranspiration. Irrigation Science 16:149-157. 125

Luquet, D., Vidal, A., Dauzat, J., Begue, A., Olioso, A. andclouvel, P. (2004) Using directional TIR measurements and 3D simulations to assess the limitations and opportunities of water stress indices. Remote Sensing of Environment 90:5362. Mahan, J.R., Burke, J.J., Wanjura, D.F. and Upchurch, D.R. (2005) Determination of temperature and time thresholds for BIOTIC irrigation of peanut on the southern high plains of Texas. Irrigation Science 23: 145 152. Orta, A.H., Baser, I., Sehirali, S., Erdem, T. anderdem, Y. (2004) Use of infrared thermometry for developing baseline equations and scheduling irrigation in wheat. Cereal Research Communications 32:363370. Penuelas, J. and Inoue, Y. (1999) Reflectance indices indicative of changes in water and pigment contents of peanut and wheat leaves. Photosynthetica 36: 355-360. Penuelas, J., Filella, I., Biel, C., Serrano, L. and Save, R. (1993). The reflectance at the 950-970 nm region as an indicator of plant water status. International Journal of Remote Sensing 14: 1887 1905. 126

Model Training Course on Assessment of Soil-Plant-Atmosphere System for Improving Resource Use Efficiency in Agriculture TB-ICN:95/2012 September 4-11, 2012 Training Manual Sponsored by Directorate of Extension Ministry of Agriculture, Govt. of India, New Delhi Course Director Kalikinkar Bandyopadhyay Coordinator Sanatan Pradhan Organised by Division of Agricultural Physics Indian Agricultural Research Institute New Delhi-110 012