Determination of spatial and temporal water relations in single leaves and canopies based on thermographic measurements

Abstract

Land plants are exposed to an exceptional challenge, because they need to take up CO₂ from a dry atmosphere, which induces water loss while taking up CO₂. As the leaf water content (LWC) is a result of an equilibrium between water uptake and water loss through transpiration, it is an important parameter in the overall plant-water relation. <br /> Water loss, and thus LWC, is mainly affected by transpiration. Water vapor released through the stomata is restricted by the leaf boundary layer, a thin air layer surrounding the leaf and acting as a heat barrier. Convection that is the heat transfer from the leaf surface into the ambient air via air movement further influences the boundary layer. The relationships between LWC, transpiration, convection, and the leaf boundary layer, are summarized in the leaf energy balance model that relates all these parameters to the leaf temperature (T_L). Therefore, thermography, non-invasive and spatial T_L measurements is a convenient method to derive information about the plant-water relations. A commonly used parameter is the crop water stress index (CWSI), which empirically relates passively measured T_L to rates of transpiration. <br /> This work introduces the active thermography where T_L is manipulated and transiently increased by exposure to short heat pulses. Resulting cooling curves are measured and quantified with the time constant (t). In theory, t is the product of the leaf heat capacity per unit area, which is proportional to LWC, and the inverse of the leaf heat transfer coefficient (h_leaf). h_leaf describes the boundary layer conductance to heat and is the sum of heat transfer coefficients for transpiration, convection, and thermal radiation. <br /> Using spring barley (<em>Hordeum vulgare</em>) and common bean (Physeolus vulgaris), it was extensively tested whether and under which conditions t can be used to derive LWC. Finally, the active thermography was transferred from the laboratory into the green-house were the applicability to detect mild drought stress on the canopy scale was tested. Under well-defined ambient conditions, t of dark-adapted leaves was proportional to LWC for the both plants species and under varying wind-speed. These relationships were used to map t of whole leaves, providing distribution maps of LWC. A comparison of active thermography measurements with active temperature measurements within a leaf gas exchange cuvette revealed an environment-depending relationship between t and h_leaf. While t in the well-ventilated gas exchange cuvette is mainly driven by changes in LWC, t of leaves exposed to wind-free conditions was related to both, LWC and h_leaf. <br /> Forward modelling of h_leaf, showed a strong impact of h_leaf on t, which significanT_Ly differed with wind and illumination. Thus, the relative contribution of the single heat transfer coefficients for the single components (transpiration, convection, and thermal radiation) greatly depended on the environmental conditions. While the heat transfer coefficient for thermal radiation was nearly negligible, the heat transfer for convection and transpiration showed a much stronger impact on h_leaf, respectively. Irrespectively of wind-speed and illumination, the heat transfer coefficient for convection contributed most to the overall h_leaf. In the dark, the heat transfer coefficient for transpiration had no impact on h_leaf. Upon illumination the impact was significant, but became less important with increasing wind-speed. <br /> Finally, the active thermography approach was transferred from the laboratory into the green-house, where the applicability for mild drought stress detection on the canopy scale was tested. Four different barley varieties were exposed to a deficit irrigation (DI) treatment. Diurnal and seasonal dynamics of t were observed and compared to the commonly used CWSI. While the CWSI mainly responded to changes in h_leaf, t responded to changes in LWC and h_leaf. Consideration of both parameters in parallel revealed an overall insight in the plant-water relations, which could be related to canopy water loss rates and biomass production. Therefore, combining t and the CWSI in one single index would be advantageous. A first attempt to develop such an index is made, and the empirical dehydration index is suggested as basis for future development. <br /> This work demonstrates, that the active thermography is a valuable tool for a wide range of applications in plant science. The measurement of t can be used to detect dynamics in the plant-water relations, when regarding the environmental conditions properly. Additionally, t may be used in modelling studies, e.g. to model leaf and canopy heat transfer processes or to model water-relations of plants with respect to hydrological cycles of whole ecosystems.