Remote Sensing of Biotic and Abiotic Plant Stress

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  • Ann. Rev. Phylopalhol. 1986. 24:265-87

    REMOTE SENSING OF BIOTIC AND

    ABIOTIC PLANT STRESS!

    Ray D . Jackson

    US Water Conservation Laboratory, Agricultural Research Service, US Department of Agriculture, Phoenix, Arizona 85040

    INTRODUCTION

    The maximum yield of plants, determined by their genetic potential, is seldom achieved because factors such as insufficient water or nutrients, adverse climatic conditions, plant diseases, and insect damage will limit growth at some stage. Plants subjected to these biotic and abiotic constraints

    are said to be stressed. The term "stress" can be defined as any disturbance

    that adversely influences growth. It is axiomatic that high yields can only be obtained if plant stress is kept to a minimum. The problem is, then, to detect stress as early as possible so that management practices can be instigated to

    minimize its effect on the harvestable yield of the crop. Physiological and anatomical changes take place within plants as a result of

    stress. If transpiration is restricted from lack of water or a vascular disease, leaf temperatures will increase because of less cooling by transpired water as it evaporates from the leaf surfaces. Leaf color may change as a result of physiological changes caused by a water deficiency or a change in nutrient status. Plant pathogens may change leaf color by causing chemical changes within plant cells or by growing on plant surfaces. Morphological changes such as leaf curl or droop may result from the action of any of several stress factors. Insects and pathogens can change morphological characteristics by ingesting or detaching plant material.

    Both leaf expansion and leaf senescence are sensitive to water stress (92). Plants subjected to water deficits that were not too severe or prolonged can, upon relief of the stress, resume expansion rates similar to those in non-

    ITbe US Government has the right to retain a nonexclusive, royalty-free license in and to any copyright covering this paper.

    265

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  • 266 JACKSON

    stressed plants (6). Thus, a history of stress periods is necessary because the cumulative effect of stress on the final leaf area determines the yield (92).

    For years the most widespread method of stress detection has been visual survey. Experienced growers have the ability to detect subtle changes in plant color or a slight droop or curl of plant leaves, all indicators of stress. Irrigations are often scheduled when leaves feel warm to the touch. The limitations of these methods are that few people have either the experience or the insight to detect these signs and that fields are generally too large to be adequately surveyed by eye. Furthermore, by the time visual and tactile signs are evident, yield-limiting damage may already have occurred.

    Whether or not they are detectable by sight or touch, changes that take place as a result of stress affect the amount and direction of radiation reflected and emitted from plants. Remote-sensing techniques are capable of measuring that radiation and therefore offer the possibility of quantitatively assessing plant stress caused by biotic and abiotic factors.

    Remote sensing of plant canopies involves the detection of electromagnetic radiation coming from a complex matrix of plant leaves and stems above a background of soil and plant litter. The objective of many remote-sensing measurements is to extract information concerning plant condition from the composite scene. To complicate the situation, part of the plant-soil framework is exposed to the direct solar beam, while the remaining part is exposed to diffuse skylight and radiation that may have been transmitted through upperplant parts. Thus, illumination of the different plant and soil surfaces changes as the sun zenith changes, yielding a composite scene reflectance that may change throughout the day (51, 72). Add to this complexity the fact that plant morphology may change because of stress, and it becomes readily apparent that extraction of information from plant canopies concerning plant stress is not simple. However, it is the intriguing complexity of the problem that will motivate the research needed for its solution. By addressing this challenging problem, we will increase our understanding of the basic physics and biology of plant canopies. Thus, techniques will evolve that combine the complex factors into a system that quantitatively evaluates plant stress and identifies its cause, and does so in a time frame that allows management practices to be initiated, and yield maximized.

    This report reviews some fundamental aspects of the interaction of radiation with plant canopies, describes some remote-sensing instruments and techniques for measuring radiation reflected and emitted from vegetative surfaces, discusses recent reports concerning the detection and quantification of stresses by remote measurements, and attempts to evaluate the status of and potential for remote detection of biotic and abiotic stresses.

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  • REMOTE SENSING OF PLANT STRESS 267

    BASIC ASPECTS OF REMOTE SENSING OF VEGETATION

    Radiation that reaches the earth's surface includes solar radiation within the wavelength region from 0.25 to about 3 IJ-m (this includes the direct solar beam and the diffuse skylight), and radiation emitted from the atmosphere in the wavelength region from about 3 to greater than 20 IJ-m. The energy balance at the surface can be expressed as:

    Rn = G + H + AE, 1.

    where Rn is the net (absorbed) radiation (W m-2), G is the heat flux into the surface (W m-2), H the sensible heat flux into the air above the surface (W m-2), AE is the latent heat flux to the air (W m-2), and A is the heat of vaporization.

    Expanding the net radiation into its components, we have

    2.

    where Rs! is the incoming solar radiation, Rs t is the outgoing (reflected) solar radiation, RL! is the incoming longwave radiation from the atmosphere, and RL t is the outgoing (emitted thennal) longwave radiation.

    The incoming components of net radiation are principally dependent on the solar intensity and the atmosphere, and only slightly on surface characteristics. In contrast, the outgoing components are strongly dependent on the surface, whether it is soil, vegetation, buildings, etc. The magnitude and the wavelength dependence of the reflected and emitted radiation are detennined by both the reflective properties and the temperature of the surface features. Thus, a remote measurement of the amount of reflected and emitted radiation at particular wavelengths can be used to infer properties of the surface. This forms the basis for remote sensing of vegetation and soils, and hence, for measuring plant stress.

    Spectral Reflectance and Emittance From Vegetation

    The fraction of incident energy reflected from a typical leaf over the wavelength interval from 0.4 to 2.5 IJ-m is shown in Figure 1. Little of the incident visible (0.4 to 0.7 IJ-m) or near-infrared (0.7 to 1.3 IJ-m) energy is reflected directly from the outer surface of a leaf because the cuticular wax layer is nearly transparent to radiation at these wavelengths (53). Once through this layer much of the visible part of the spectrum is absorbed, with only a small part reflected or transmitted. This is evident in Figure 1, which

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  • 268 JACKSON

    .4

    w U Z .3 I-U .2 ..... w 0::

    .1

    .4 .6 .8 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6

    WAVELENGTH (fim) Figure 1 Reflectance spectra of green vegetation.

    shows that the leaf reflectance is low in the blue (about 0.45 to 0.52 JLm), peaks in the green (about 0.52 to 0.55 JLm), and decreases to a minimum in the red (about 0.63 to 0.70 JLm). The low reflectance in the blue and red regions is generally attributed to absorption by chlorophyll. Gates et al (22) stated that chlorophyll, carotenes, and xanthophylls absorb radiation at 0.445 /Lm, but only chlorophyll absorbs in the red (near 0.645 /Lm). Thus, healthy green leaves exhibit low reflectance values in the blue and red portions of the spectrum, and an increase in reflectance in these wavebands may signal a stress condition. The green peak accounts for the green color of plants perceived by the human eye.

    The high reflectance ofleaves in the near infrared (about 0.7 to 1.3 /Lm) is apparently caused by their internal cellular structure (22, 53). Radiation is diffused and scattered through the cuticle and epidermis to the mesophyll cells and air cavities in the interior of the leaf. Radiation is further scattered by multiple reflections and refractions at the interface of hydrated cell walls with intercellular air spaces because of refractive index differences ( 1.4 for hydrated cells and 1.0 for air). From 40 to 60% of the incident near-infrared radiation is scattered upward through the surface of incidence and is designated retlected radiation, whereas the remainder is scattered downward and is designated transmitted radiation. Little, if any, is absorbed. This phenomenon has been extensively studied (22, 23, 26, 28, 53).

    The spectral region between 1.3 and 2.5 /Lm is of interest because water within the leaves absorbs radiation at these wavelengths. Within this region, called the "mid-infrared" or the "water-absorption" region, leaf reflectance

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  • REMOTE SENSING OF PLANT STRESS 269

    decreases with increasing wavelength, with minimums near 1.40 and 1.85 f.Lm, and becomes negligible beyond 2.5 f.Lm. The upper limit of 2.5 f.Lm is a result of the decrease of solar radiation with wavelength and the absorption of radiation by atmospheric water vapor. Remote sensing of reflected solar radiation is generally restricted to the spectral region from 0.4 to 2.5 f.Lm.

    The thermal-infrared portion of the electromagnetic spectrum extends from about 3 to about 20 f.Lm. Atmospheric water vapor strongly absorbs radiation in much of this region, affecting the range from 8 to 14 f.Lm less than it does others. Within the thermal infrared, radiation naturally emitted from all objects is readily detectable and is related to their surface temperatures. Temperature refers to the concentration of internal heat energy and is a measure of the average kinetic energy of atomic and molecular units in motion within bodies above absolute zero. The thermal energy of a substance is indicated by its absolute temperature (as measured by conventional thermometers) or its radiant or apparent temperature (as measured with radiometers). The relation between emitted radiance (R) and absolute temperature (1) can be derived from the Stefan-Boltzmann Law,

    R = EaT'\ 3.

    where E is the emISSIVIty of the surface and (J is the Stefan-Boltzmann constant (5.674 X 10-8 W m-2 K-4), with R in units of W m-2.

    Equation 3 was derived by integrating Planck's equation over the wavelength range from 0 to 00. When integrated over a small wavelength interval, the exponent of T is not 4. However, for the 8-14 f.Lm region the exponent is sufficiently near 4 that the error caused by its use is negligible (80). The combination of having an atmospheric window between 8 and 14 f.Lm and a valid relation between absolute temperature and emitted radiance allows the estimation of surface temperatures by remote means. For discussions of basic principles of thermal-infrared radiometry, see Fuchs & Tanner ( 18), Gates (2 1), and Lorenz (57).

    Another portion of the electromagnetic spectrum, where emitted radiation can be measured and used to infer properties of the surface, occurs at the millimeter and centimeter wavelengths (commonly called the microwave region). Passive microwave radiometry measures energy emitted naturally from surfaces. Such measurements have been used to estimate near-surface soil moisture (77). Active microwave radiometry, or radar, has considerable potential for remote sensing of crop condition (7, 93). Microwaves are not obstructed by clouds as are the visible and near-infrared wavelengths. This is a tremendous advantage for crop monitoring from satellites, since the measurements are not at the mercy of the weather.

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  • 270 JACKSON

    Leaf and Canopy Reflectance Characteristics

    Early studies concerning leaf reflectance properties were conducted using laboratory spectrophotometers. Excised leaves were placed such that the leaf surface would be perpendicular to the incident beam from the spectrophotom,eter. The reflectance and transmittance were measured, and the absorptance was obtained by the equation l-(reflectance + transmittance). Experiments were conducted in which plants were subjected to various stresses, and the effect of these stresses on the reflectance and transmittance properties was ascertained. A large number of experiments of this type were conducted in Weslaco, Texas (see for example, 2, 25, 27-30, 86, 87). Such studies provided a sound basis for interpretation of field measurements of reflectance characteristics of plant canopies and showed how different wavelengths of light interact with plant material. Reviews by Bauer (4), Gates et al (22), and Knipling (53) are recommended for additional information.

    Reflectance of light from a plant canopy depends not only on the reflectance properties of individual leaves and stems but also on the ways in which they are oriented and distributed. Under stress it is likely that both of these factors will change. Laboratory measurements of light spectra from leaves have shown that reflectance values at all wavelengths within the 0.4-2.5 IA-m region increased as the leaves became progressively more dehydrated (23). These results can be attributed to anatomical and physiological changes within the plant cells. Stress also causes the geometry of the plant to change (e.g. leaf droop and curl), thus exposing different fractions of vegetation and soil (both sunlit and shaded) to the radiometer. As lower leaves are exposed, canopy reflectance may be affected because reflectance properties of leaves grown in shade differ from those of leaves exposed predominately to sunlight (24).

    The relative importance of stress-induced changes in canopy architecture was studied on a cotton crop by Jackson & Ezra (47). They me...

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