Leaf cooling curves: measuring leaf temperature in sunlight
Andrea Leigh A D , John D. Close B , Marilyn C. Ball C , Katharina Siebke C and Adrienne B. Nicotra AA School of Botany and Zoology, The Australian National University, Canberra, ACT 0200, Australia.
B Department of Physics and the Australian Centre for Quantum Atom Optics, The Australian National University, Canberra, ACT 0200, Australia.
C Ecosystem Dynamics, Research School of Biological Sciences, The Australian National University, Canberra, ACT 0200, Australia.
D Corresponding author. Email: andrea.leigh@anu.edu.au
E This paper originates from a presentation at ECOFIZZ 2005, North Stradbroke Island, Queensland, Australia, November 2005.
Functional Plant Biology 33(5) 515-519 https://doi.org/10.1071/FP05300
Submitted: 12 December 2005 Accepted: 14 March 2006 Published: 2 May 2006
Abstract
Despite the obvious benefits of using thermography under field conditions, most infrared studies at the leaf level are generally conducted in the laboratory. One reason for this bias is that accuracy can potentially be compromised in sunlight because reflected radiation from the leaf might affect the calculation of the temperature measurement. We have developed a method for measuring leaf temperature in sunlight by using thermal imagery to generate cooling curves from which the time constant for cooling, τ, can be calculated. The original temperature of the sunlit leaf may be determined by extrapolating backwards in time. In the absence of specular reflection, there is close agreement between the extrapolated sunlit temperature and the sunlit temperature recorded by the camera. However, when reflected radiation is high, the difference between the initial (incorrect) temperature determined from the sunlit image and the temperature extrapolated from the cooling curve can be > 2°C. Notably, our results demonstrate a close agreement between the extrapolated sunlit temperature and the temperature of the leaf approximately 1 s after being shaded, suggesting that this shaded image provides a good estimate of the original sunlit temperature. Thus, our technique provides two means for measuring leaf surface temperature in sunlight.
Keywords: infrared imaging, reflected radiation, specular reflection, thermography, time constant.
Acknowledgments
The authors thank the Royal Botanic Gardens and Domain Trust, Mount Annan Botanic Gardens, Mount Annan, New South Wales for allowing access to the plants used for the field component of this research. We also thank two anonymous reviewers for helpful comments on an earlier version of this manuscript. This work was supported by an Australian Geographic research grant and an Australian Postgraduate Award to A Leigh and by an Australian Research Council grant to AB Nicotra, CD Schlichting and CS Jones.
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