Lab 3: Leaf and Canopy Energy Balance Plant Physiological Ecology (IB 151L) Leaf Energy Balance Introduction All organisms and objects interact with their physical environment through energy exchange processes. Metabolic functions in plants operate at optimal temperatures. For example, photosynthetic enzymes process CO2 optimally at around 30 C. If a leaf heats up past 34 C, photosynthetic enzymes can begin to denature, preventing the leaf from performing its function. Thus plants attempt to maintain an equilibrium leaf temperature to maximize their usefulness to the plant. For a leaf at equilibrium, the amount of energy that enters via solar radiation and ambient heat is equal to that that exits the leaf via heat loss, reflected light and transpired water (Energyin = Energyout). If the leaf is not at equilibrium with its environment, the temperature of the leaf will change (increase or decrease) until equilibrium is achieved. Equilibrium for leaves is usually attained in less than one minute. The energy budget equation can be expanded to: absorbed radiation = reradiation + convection + transpiration and, finally, for a single leaf surface we can expand the equation to: a•cos(i) •Sdirect + a•Sdiffuse + ε•R = ε•σ• (T1)4 + hc• (T1-Ta) + L• (e1 – ea)/ rtot external factors: Sdirect - incident direct solar radiation on leaf (300-4,000 nm) (W m-2) Sdiffuse - incident diffuse solar radiation on leaf (300-4,000 nm) (W m-2) R - terrestrial infrared radiation (includes both sky and ground components) (4,000-100,000 nm) (W m-2) Ta - temperature of air (ºK) T1 - temperature of leaf (ºK) e - water vapor density of leaf (l) and air (a) (g m-3) constants: σ- Stephan Boltzman constant (blackbody radiation constant) (W m-2 oK-4) L - latent heat of vaporization (J g-1) leaf parameters (coupling factors): cos(i) - cosine of leaf orientation to the sun's direct beam, from horizontal a - absorption coefficient to solar radiation (300-4,000 nm) ε - absorption coefficient to infrared radiation (4,000-100,000 nm) - emissivity hc - convection coefficient (W m-2 oC-1) rtot - total leaf resistance (stomatal and boundary) (s m-1) Leaf coupling factors involved with absorbed radiation are the total solar leaf absorptance (a) and leaf emissivity (ε) and have been discussed in lecture. Substantial changes in emissivity do not occur among leaves. However, differences in leaf absorptance do occur among species or even within a single plant over the course of a growing season. The effects of the changes in leaf absorptance are significant in terms of affecting leaf temperature. Plants may also reduce the amount of incident radiation by changing their leaf angle. The stomatal and boundary layer resistances serve to regulate water loss from the leaf. StomatalLab 3: Leaf and Canopy Energy Balance Plant Physiological Ecology (IB 151L) resistance is a function of the aperture and density of stomatal pores. The boundary layer resistance arises because of the presence of a thin layer of still air around the leaf that increases the path of water vapor diffusion to ambient air. Note that although we typically describe gaseous movements in and out of the leaf in terms of their conductance values (e.g., leaf conductance; g), because these diffusion barriers occur in series, calculations of total conductance are simpler if we use resistances (inverse of conductance). Recall from physics that conductances are additive in parallel, whereas resistances are additive in series. The convection coefficient (hc) is related to the boundary layer resistance as hc = Kair/δbl where Kair is the thermal conductivity coefficient for air, i.e. the coefficient that explains how well heat travels through air. The boundary layer thickness (δbl) depends on leaf size, shape, and wind speed through the relationship, δbl = Kl×((w/v)^.5) where Kl is an experimentally determined leaf shape parameter (usually 4.0 for leaves), w is the leaf width (m), and v is the wind speed (m s-1). The boundary layer resistance can be calculated as: rbl = δbl/Dj where Dj is the diffusion coefficient for H2O vapor. This parameter serves as an estimate of mean boundary layer resistance, however the boundary layer thickness (and thus resistance) varies widely over the surface of the leaf. To understand the energy-exchange processes that affect leaf temperatures we will: I. Experiment with changes in biotic and abiotic factors to observe the effects on heat transfer in leaves using liquid crystal leaf models. II. Test these qualitative predictions with a leaf energy balance Excel spreadsheet program (Tleaf2.xls) to determine quantitatively how changes in biotic and abiotic properties affect leaf temperature. I. Liquid Crystal Leaf Models The purpose of this part of the lab is to make predictions based on the energy balance equation presented above as to how three different leaf parameters (leaf size, shape, and stomatal conductance) and two environmental parameters (light and wind) may interact to effect leaf temperature. Permanent leaf models have been prepared for class use from thermosensitive liquid crystal film. The film is prepared by encapsulating cholesterol esters in sheet plastic that has a black undercoat. The liquid crystals have various melting points; each melting point denotes decreasing molecular orientation as the compound is warmed. The use of cholesterol esters, singly and in combination, gives a wide choice of temperatures at which the crystals and color changes occur. We have cut the bonded film into leaf models of various sizes and shapes. Different models with transition temperatures ranging from 20-25, 25-30, 30-35, and 35-40º C have been mounted on wire "petioles" to permit exposure to sun and wind. The combined effects of both leaf and environmental parameters on leaf temperature are clearly mapped by the liquid crystal colors (see figures in appendix).Lab 3: Leaf and Canopy Energy Balance Plant Physiological Ecology (IB 151L) To determine temperature using the liquid crystal leaves: Within each temperature range the liquid crystals will exhibit the total color spectrum (see figures in Appendix). Warmer temperatures will exhibit a blue color, and cooler a red color. If the liquid crystals are black, the temperatures are too cool to exhibit a color change. If the liquid crystals are