Radiacao_lambers

4\n\nLeaf Energy Budgets: Effects of Radiation and Temperature\n\n4A. The Plant’s Energy Balance\n\n1. Introduction\n\nTemperature is a major environmental factor that determines plant distribution. Temperature affects virtually all plant processes, ranging from enzymatically catalyzed reactions and membrane transport to physical processes such as transpiration and the volatilization of specific compounds. Species differ in the activation energy of particular reactions and, consequently, in the temperature responses of most physiological process (e.g., photosynthesis, respiration, biosynthesis). Given the pivotal role of temperature in the ecophysiology of plants, it is critical to understand the factors that determine plant temperature. Air temperature in the habitat provides a gross approximation of plant temperature. Air temperature in a plant’s microclimate, however, may differ substantially from air temperature measured by standard meteorological methods. The actual temperature of a plant or organ often deviates substantially from that of the surrounding air. We can only understand the temperature regime of plants and, therefore, the physiological responses of plants to their thermal environment through study of microclimate and the plant’s energy balance.\n\n2. Energy Inputs and Outputs\n\n2.1 Short Wave Overview of a Leaf’s Energy Balance\n\nMost leaves effectively absorb the short-wave radiation (SR) emitted by the sun. A relatively small fraction of incident solar radiation is reflected, transmitted, or utilized for processes other than just heating. In bright sunlight, the net absorption of solar radiation (SRnet) is the main energy input to a leaf. If such a leaf had no means to dissipate this energy, then its temperature would reach 100 °C in less than 1 minute. Thus, processes that govern heat loss by a plant are critical for maintaining a suitable temperature for physiological functioning.\n\nHeat loss occurs by several processes (Figure 1). A leaf emits long-wave infrared radiation (LR). At the same time, however, it absorbs LR emitted by surrounding objects and the sky. The net effect of emission and absorption (LRnet) may be negative or positive, depending on whether it constitutes an export or import of energy, respectively. When there is a temperature difference between leaf and air, convective heat transfer (C) takes place in the direction of the temperature gradient. Another major component of the energy balance is cooling caused by transpiration (LE; where E is the energy required per unit evaporation and E is the rate of evaporation). This is called transpiration when it concerns the surface of a living plant). In addition, metabolic processes are involved in the energy balance. Respiration and other metabolic processes (M) generate heat, and energy is consumed when absorbed SR is used in photosynthesis (A), but this is typically small compared with other components of the energy balance, and usually ignored. When the temperature rises in response to sunlight, most components of the energy balance that contribute to cooling increase in magnitude until energy gain and 226 4. Leaf Energy Budgets\n\nFIGURE 1. Schematic representation of the components of the energy balance of a leaf consisting of short-wave radiation (SR), long-wave radiation (LR), both incident (in) and emitted (em), convective heat transfer (C), and evaporative heat loss (LE). Reflection (r), transmission (tr), and fluorescent emission (FL) are only given for SR incident on the upper side of the leaf. A and M are CO2 assimilation and heat-producing metabolic processes, respectively.\n\nloss are in balance. At this point, the leaf has reached an equilibrium temperature (steady state), and the sum of all components of the energy balance must equal zero:\n\nSRnet + LRnet + C + LE + M = 0 (1)\n\nAny change in the components of the energy balance will alter leaf temperature. For a correct description of the time course of change, a heat storage term must be included; however, storage capacity is low in most leaves, and response times of leaf temperature to changing conditions are typically minutes or less. Exceptions are more bulky plant parts such as succulent leaves, tree trunks and branches, but these are not dealt with here.\n\n2.2 Short-Wave Solar Radiation\n\nAbsorption of solar radiation normally dominates the input side of the energy balance of sunlit leaves during the light. About 98% of the radiation emitted by the sun is in the range of 300–3000 nm (short-wave radiation, SR). Ultraviolet radiation (UV; 300–400 nm) has the highest energy content per quantum (shortest wavelength); it constitutes approximately 7% of solar radiation and is potentially damaging to a plant (Sect. 2.2 of Chapter 4B on effects of radiation and temperature). Plants absorb about 97% of incoming UV radiation (Fig. 2). About half of the energy content of solar radiation is in the waveband of 400–700 nm (photosynthetically active radiation, PAR), which can be used to drive photo-chemical processes (SR) ; most green leaves absorb\n\naround 85% of the incident radiation in this region, depending on the chlorophyll concentration (Fig. 2).\n\nShort-wave (solar) infrared radiation (IR; 700–3000 nm) is absorbed to a much lesser extent. This wavelength region can be divided into two parts: 700 to about 1200 nm, which is largely reflected or transmitted by a leaf and represents the largest part of IR, in terms of energy content, and 1200–3000 nm, which is largely absorbed by water in the leaf (Fig. 2). The result is that about 50% of IR is absorbed.\n\nLeaves have mechanisms that make up the amount of absorbed solar radiation (SRabs): incident (SRin), reflected (SRref), and transmitted (SRtrans) radiation. Changes in SRin can be brought about by changes in leaf orientation with respect to the sun (heliotropism or solar tracking) (Sect. 5.4.6 of Chapter 3 on plant water relations; Jurik et al. 1998). These leaf movements can be active and may orient the leaf perpendicularly to the incident radiation (diaheliotropism), thus maximizing SRin under conditions of low temperature and adequate soil moisture (Fig. 3). A most dramatic example of a mechanism that increases temperature of a specific part of a plant is\n\n2.5 227 the act of copiapoa species (barrel cactus) that increase SRin by leaning toward the north in Chile (Ehleringer et al. 1980). The effect of this orientation is that tissue temperatures of the meristematic and floral regions on the tip of the cactus receive high solar radiation loads, which result in high temperatures (30–40 °C) relative to air temperatures (15–20 °C) during winter and spring months when adequate soil moisture for growth is available. Second, absorption of solar radiation by the sides of the cactus is minimized which reduces the potential detrimental effects of leaf and heat load on the cactus, and probably balances daily quantum absorbed for photosynthesis with nighttime CO2 uptake rates during drought stress periods.\n\nWhen exposed to high temperature and water deficit, leaves may become paraheliotropic (i.e., the leaf orientation is parallel to the incident radiation), thus minimizing SRin (Fig. 3; Kao & Forseth 1992). Many desert shrubs exhibit leaf shapes relative to the sun (Smith et al. 1998). This reduces midday SRin when temperatures are warmest, and increases SRin in the mornings and afternoons, when irradiance is less and temperatures are cooler. Angels can become progressively more horizontal in wetter communities which increases SRin at midday (Ehleringer 1988). This regulation of leaf angle keeps leaf temperatures within limits, thereby reducing transpiration and photoinhibition and maximizing the rate of CO2 assimilation (Gamon & Pearcy 1989). In three wild glycine (soybean) species, paraheliotropic leaf movements respond in concert with\n\nthat of copiapoa species (barrel cactus) that increase SRin by leaning toward the north in Chile (Ehleringer et al. 1980). The effect of this orientation is that tissue temperatures of the meristematic and floral regions on the tip of the cactus receive high solar radiation loads, which result in high temperatures (30-40 °C) relative to air temperatures (15-20 °C) during winter and spring months when adequate soil moisture for growth is available. Second, absorption of solar radiation by the sides of the cactus is minimized which reduces the potential detrimental effects of leaf and heat load on the cactus, and probably balances daily quantum absorbed for photosynthesis with nighttime CO2 uptake rates during drought stress periods. photosynthetic characteristics such that water-use efficiency is enhanced and the risk of photoinhibition under water deficit is reduced (Kao & Tsai 1998). Wilting and leaf rolling are additional mechanisms by which leaves reduce incident radiation under conditions of water stress. The reflection component of incident radiation (SR) is typically small (approximately 5-10%; Fig. 2) and comprises reflection from the surface which is largely independent of wavelength, and internal reflection, which is wavelength-specific because of absorption by pigments along the internal pathway (Sect. 2.1.1 of Chapter 2A on photosynthesis). In some plants, surface reflection can be high due to the presence of reflecting wax layers, short white hairs, or salt crystals. Reflection can change seasonally as in the desert shrub Encelia farinosa (brittlebrush) that produces new leaves during winter with sparse hairs resulting in 80% absorption of incident radiation raising leaf temperature several degrees above ambient temperature (Fig. 4). Leaves produced in summer, however, when water is scarce, have dense reflective hairs that reduce absorptance to 30-40% of incident radiation, and reduce leaf temperature below ambient temperature. The semi-arid perennial Encelia californica (bush sunflower) from moister areas and humid coastal habitats has glabrous leaves that increase absorptance and raises leaf temperature by 5-10°C (Fig. 2 in Chapter 2A on photosynthesis). The variation in leaf hair formation throughout the year might be controlled by photoperiod and gibberellins, as it is in Arabidopsis thaliana (thalecress) (Chien & Sussex 1996). Because Encelia farinosa (brittlebush) has an optimum temperature for photosynthesis that is below summer daytime temperature, this reduction in absorptance is critical to leaf carbon balance (Ehleringer & Bjorkman 1978). Presence of leaf hairs in summer increases carbon gain and reduces water loss by 20-25% through amelioration of leaf temperature (Fig. 4). A white layer of salt excreted by salt glands on the leaves of Atriplex hymenelytra (desert holly) (Sect. 3.4.3 of Chapter 6 on mineral nutrition), similarly, reduces the absorptance and leaf temperature; it enhances CO2 assimilation and water-use efficiency because of a more favorable leaf temperature for photosynthesis (Mooney et al. 1977). The components of solar radiation relevant for the energy balance of a leaf are summarized below. SR Short-wave (solar) radiation (300-3000 nm) SRIN incident radiation (UV + PAR + IR) UV ultraviolet (300-400 nm) PAR photosynthetically active radiation (400-700 nm) IR short-wave infrared radiation (700-3000 nm) SRIN = PAR + IR + UV (2) SRF Short-wave reflected (3) SRF emitted as fluorescence (4) SRnet = SRabs - SRA - SRFL (4) Typical values of the energy balance of leaves are given in Fig. 5 (and more in Figs. 8 and 10, in Sect. 2.4). A few percent of SRabs is emitted as fluorescence (SRFL) (Box 2A.4) and the same is true for the part of SRabs used in photosynthesis (SRA) in high-light conditions. For the sake of simplicity, SRA and SRFL are generally ignored in energy-balance calculations; however, this may represent a significant Energy Inputs and Outputs FIGURE 5. Schematic representation of the long-wave radiation (LR) and short-wave radiation (SR) inputs and outputs for a sunny (left) and cloudy (right) day. Graphs show the vertical profile of incident radiation (solid line) and net radiation (blue). The lower three are leaf temperatures and the upper is the effective sky radiation temperature. Assumed is a global incident irradiance of 833 W m-2, reflectance by the surroundings of 20%, leaf absorptance of 0.6, photosynthetic rate of 8 µmol m-2 s-1 at the top of the canopy on a sunny day, leaf temperature of 25°C (top), 20°C (middle and bottom) on a sunny day, and of 17°C on a cloudy day. Total incoming radiation (SRIN) exceeds the global irradiance due to reflectance from surrounding leaves and twigs. Abbreviations as defined in text. LR Long-wave terrestrial radiation (>3 µm) LRN incident radiation LR, absorbed emitted LRem LRabs = LRN - LRem (6) TR Total radiation TRabs = SRabs + LRabs (8) TRnet = SRnet + LRnet (9) The amount of energy gained by a leaf changes dramatically through a leaf canopy, and the extent of this change depends on cloud conditions (Fig. 5). In full sun, the short-wave radiation incident on a leaf exceeds that of incoming solar short-wave radiation because of reflection from surrounding leaves and other surfaces, and the total radiation (TR) absorbed by a leaf (1224 W m-2 in Fig. 5.1) approaches that of the solar constant (1360 W m-2) (i.e., the solar energy input above the atmosphere) (Nobel 1983). TRabs declines dramatically in the absence of direct solar irradiance, whether this is due to clouds or to canopy shading. Even leaves in full sun absorb more than half their energy as long-wave radiation, and leaves in cloudy or shaded conditions receive most energy as long-wave radiation emitted by objects in their surroundings. There is a sharp decline in total net radiation gained (and therefore energy that must be dissipated) through Energy Inputs and Outputs\n\nFigure 7. Schematic representation of the flow of non-turbulent air across a leaf. The arrows indicate the relative speed and direction of air movement. As air moves across a leaf, there is a laminar sublayer (short straight arrows), followed by a turbulent region. The effective dimension (d; leaf width measured in the direction of the wind):\n\nδ = 4√d/u\n\nEquation (8) shows that small leaves have higher γba and thus tend to have temperatures closer to air temperature, than do large leaves. Compound or highly dissected leaves are functionally similar to small leaves in this respect. Under hot, dry conditions, most plants have small leaves, because they cannot support high transpiration rates and must rely largely on convective cooling to dissipate absorbed short-wave and long-wave radiations. Another mechanism that reduces boundary layer thickness is the increase of effective wind speed across leaves to flutter at low wind speeds, as in some Populus species [Populus tremula (European aspen) and Populus tremuloides (quaking aspen)]. Some plants do not \"obey the rules\". Welwitschia mirabilis (two-leafed-cannot-die) is an African long-lived desert plant with extremely large leaves (0.5-1.0 m wide and 1-2 m long) (Schulze et al. 1980). Those parts of the leaves not in contact with the ground, however, are only 4-6 °C above air temperature. A high reflectivity of leaves (56%) minimizes SRabs and relatively cooled soils beneath leaves minimize LRin. (Fig. 8). There is negligible transpiration in summer, so it is primarily through these two mechanisms of minimizing energy gain that the plant avoids serious overheating. Parts of the leaves that touch the ground have substantially higher temperatures because heat exchange at the lower surface is hampered (Fig. 8). Energy Inputs and Outputs\n\nFigure 9. Leaf temperature of Espeletia timotensis, a giant rosette plant that occurs in the Venezuelan Andes at elevations up to 4500 m. Measurements were made on plants growing in their natural environment, at different times of the day, both on intact hairy leaves and on adjacent leaves of which the hairs were partly removed. Sunrise occurred around 6:45 am. The temperature of the intact leaf becomes higher than that of the shaved leaf when global radiation exceeds 300 W m²; the thick leaf p–ack (up to 30 mm) increases boundary layer thickness and restores to convective and latent heat transfer; effects of the pubescence on solar radiation absorption are minor. At night, the temperature of the intact leaf is somewhat lower than that of shaved leaves, due to reduced convective heat transfer for any leaf (Meinzer & Goldstein 1985). Copyright Ecological Society of America.\n\nFigure 10. Results of energy-balance model calculations for leaves in different conditions (model provided by F. Schieving, Utrecht University, the Netherlands). The parameter values used for the calculations are shown below the figure. The difference in temperature between leaf and air (ΔT; left Y-axis) and the components of the energy-balance long-wave radiation (LR), convective heat exchange (C), and evaporative heat exchange (E) (right Y-axis) are plotted as a function of the leaf dimension (a, b, c), stomatal conductance (d, e), and wind speed (f) during conditions pertaining to a clear day in moist conditions (a, d, f) and in a desert environment (c, e, b) under an overcast day (e). Parameter values used for the calculations are shown below, with letters in brackets referring to the calculated scenarios and the panels in the figure. Energy Inputs and Outputs\n\nClear day (a, d, f) Clear night (b) Desert (c) Overcast day (e)\nAir temperature, °C 20 10 30 20\nSoil temperature, °C 20 20 60 20\nSky temperature, °C -20 -20 -20 20\nShort-wave radiation (SRn), W m² 800 0 800 100\nLeaf dimension, mm 100 - 100 \nWind speed, m s⁻¹ 1 1 1 1\nRelative humidity, % 65 100 30 80\nStomatal conductance, mmol m² s⁻¹ 400 0 30 -\na. Clear day b. Clear night c. Desert\n\nLeaf-air temperature difference, ΔT (°C)\n0 100 200 300 400\nLeaf dimension (mm)\n\nd. Clear day e. Overcast day f. Clear day\n\nStomatal conductance (mmol m² s⁻¹)\n0 100 200 300 400\nWind speed (m s⁻¹)\nEnergy exchange (W m²) There is no evidence for such a regulatory mechan-ism. Leaf cooling through transpiration is only beneficial at high temperatures, but then high transpiration rates may create problems for the plant if water loss is not matched by water uptake; limited water supply often coincides with high temperatures (Chapter 3 on plant water relations). Leaf cooling by transpiration also occurs at suboptimal temperatures, because stomata are open during photosynthesis which leads to an even less favorable temperature. Hence, leaf cooling must be considered a consequence of transpiration water loss that is inexorably associated with the stomatal opening required to sustain photosynthesis, rather than a mechanism to control leaf temperature, as is the case for some mechanisms.\n\nIn many situations, there is an inverse relationship between convective and evaporative heat exchange at any given irradiance. When stomatal conductance (gs) and thus evaporative heat loss decline, leaf temperature increases, which causes an increase in convective heat exchange (by increasing the temperature gradient between leaf and air). Transpiration increases also as a consequence of the higher leaf temperature and thus leaf-to-air vapor pressure difference, but that only partly compensates for the effects of lower gs (Fig. 10).\n\nA negative radiation balance of a leaf under a clear sky at night causes its temperature to drop below air temperature (negative ΔT), causing condensation of water (dew) on the leaf (positive λE); however, not all water on a leaf on a chilly morning is dew; it may also originate from guttation (Sect. 5.2 of Chapter 3 on plant water relations. Guttation water appears as drops on leaf margins (dicots and broad-leaved monocots) or leaf tips (grasses) as opposed to dew that covers the leaf surface.\n\n2.6 Metabolic Heat Generation\n\nThe metabolic component (M) refers to heat production in biochemical reactions. Its contribution in leaves is very small under most circumstances and is generally ignored in calculation of the energy balance. In some plant groups, however, the contribution of metabolism to the energy balance may be substantial. In floriferous species such as the spadix of Arecaceae and flowers of, e.g., Cycadaceae and Nympheaceae, temperatures may rise several degrees above air temperature, due to their extremely high respiration rates, which largely precedes through the alternative pathway (Sect. 2.6.1 of Chapter 2B on plant respiration). 3. Modeling the Effect of Components of the Energy Balance on Leaf Temperature\n\nThe analysis of the exact contribution of the different components of the energy balance to leaf temperature is difficult when based on measurements only. The physical relationships as described earlier can be used to calculate leaf temperature from input parameters relevant for the energy balance of a leaf. By varying parameter values, the influence of a single parameter or combination of parameters on the final leaf temperature and components of the energy balance can be analyzed in a model. Although such a model uses simplifying assumptions, the outcome of the calculations appears to describe the real situation in a satisfactory manner (Campbell & Norman 1981, Campbell & Norrman 1998). In Sects. 2.2 and 2.4, two examples were shown where energy-balance calculations were used (Figs. 4, 5, and 8). Here we develop a sensitivity analysis, investigating the effect of changes in one variable on the energy balance while keeping other variables constant.\n\nFigure 10 illustrates the result of model calculations on the basis of input parameter values provided. We use realistic values for a clear day, mid-day conditions, an overcast day, a clear night, and a clear day in a desert. The daytime scenario will have a positive short-wave radiation input (SIRn). The other components contribute to regulated loss of energy at a stable leaf temperature and are negative. SIRn is zero for the night scenario. Components of the energy balance and leaf-to-air temperature difference (ΔT) are calculated in relation to leaf dimension (d), stomatal conductance (gs), and wind speed (u) for a total of six scenarios (Fig. 10).\n\nThe calculations show that the difference between leaf–air temperature (ΔT) increases with increasing leaf width, because the boundary layer conductance decreases which results in a decrease in convective heat exchange (C) in scenarios a, b and c. On a clear day in cool humid conditions (a), net emission of long-wave radiation (LR) increases with leaf width as a result of the increasing leaf temperature (TL). Evaporative cooling (IE) is rather constant, because the high boundary layer conductance (gah) increases in small leaves, whereas the higher TL and thus larger (ul – wa), compensates for the lower gah in larger leaves. At night, (b) leaf temperature (TL) drops below air temperature (TA) because of the negative radiational balance (TRem) causing condensation at the prevailing high humidity. This 4. A Summary of Hot and Cool Topics\n\nWe have a sound understanding of the leaf energy budget as affected by leaf traits and environment. What remains to be tested experimentally, however, is whether, indeed, larger-leaved species actually operate at warmer leaf temperatures, under field conditions. If they do, then the costs must be associated with wider leaves, e.g., costs associated with tissue tolerance of higher temperatures, or higher rates of leaf respiration. If they do not operate at higher leaf temperatures, then costs of wider leaves might be incurred through avoiding radiating, e.g., by growing in shaded habitats, or having leaves with steeper angles of reflective leaf surfaces. Alternatively, there may be costs associated with greater allocation to roots, to ensure sufficient acquisition of water to sustain higher transpiration rates. A close interaction between modeling and experimental approaches should provide answers for many of the remaining ecophysiological questions.\n\nReferences\n\nCampbell, G.S. 1981. Fundamentals of radiation and temperature relations. In: Encyclopedia of plant physiology, Vol 12A, O.L. Lange, P.S. Nobel, & H. Ziegler (eds). Springer-Verlag Berlin, pp. 11–40.\nCampbell, G.S. & Norrman, J.M. 1998. An introduction to environmental biophysics. 2nd ed. Springer-Verlag, New York.\nChien, J.C. & Sussex, I.M. 1996. Differential regulation of trichome formation on the adaxial and abaxial leaf surfaces by gibberellins and photoperiod in Arabidopsis thaliana (L.) Heynh. Plant Physiol. 111: 1321–1328.\nEhleringer, J.R. 1988. Changes in leaf characteristics of species along elevational gradients on the wasatch front, Utah. Am. J. Bot. 75: 680–698.\nEhleringer, J.R. & Björkman, O. 1978. Pubescence and leaf spectral characteristics in a desert shrub, Encelia farinosa. Oecologia 36: 151–162.\nEhleringer, J.R. & Forseth, I. 1980. Solar tracking by plants. Science 210: 1009–1014.\nEhleringer, J. & Mooney, H.A. 1978. Leaf harits: Effects on physiological activity and adaptive value to a desert shrub. Oecologia 37: 183–200.\nEhleringer, J. Mooney, H.A. Gulmon, S.L., & Rundel, P. 1980. Orientation and its consequences for Agavoideae (Cactaceae) in the Atacama desert. Oecologia 26: 63–67.\nGamon, J.A. & Peery, R.W. 1989. Leaf movement, stress avoidance and photosynthesis in Vitis californica. Oecologia 79: 475–481.\nJurik, T.W., Zhang, H., & Pleasants, J.M. 1990. Ecophysiological consequences of non-random leaf orientation in the prairie cornus plant, Silphium laciniatum. Oecologia 82: 180–186.\nKao, W.-Y. & Forseth, I.N. 1992. Diurnal leaf movement, chlorophyll fluorescence and carbon assimilation in soybean grown under different nitrogen and water availabilities. Plant Cell Environ. 15: 703–710.\nKao, W.-Y. & Tsai, T.-T. 1998. Tropic leaf movements, photosynthetic gas exchange, δ13C and chlorophyll a fluorescence of three soybean species in response to water availability. Plant Cell Environ. 21: 1055–1062.\nKjellberg, B., Karlsson, S., & Kerssensons, I. 1982. Effects of heliotropic movements of flowers of Dyras ocotella L. on gynoecium temperature and seed development. Oecologia 54: 10–13.\nKörner, C. 1983. Influence of plant physiology on leaf temperature on clear midsummer days in the Snowy Mountains, south-eastern Australia. Acta Oecol. 4: 117–124.\nMeinzner, F. & Goldstein, G. 1985. Some consequences of leaf pubescence in the Andean giant rosette plant Espeletia timothiensis. Ecology 66: 512–520.