Seeing the leaves: how to improve understanding of leaf physiology using remote sensing data

What is it about?

Leaves are reactive to various external changes, including light levels and temperature. When light absorption exceeds photosynthesis rate, leaves adjust their physiology to favour protection of their photosynthetic apparatus from the excess light. Leaves must react to both daily and seasonal light variation. In response to daily light variation, plants adjust their energy distribution using the xanthophyll cycle. In this cycle, chemical changes to the ‘xanthophyll pigments’, a group of yellow and orange plant pigments, allow excess energy from light absorption to be dissipated without the vital photosynthetic apparatus sustaining damage. PHOTOCHEMICAL REFLECTANCE INDEX The Photochemical Reflectance Index (PRI) is a measure of light reflectance influenced by leaf pigments, including xanthophylls and chlorophylls. Various studies have indicated that PRI could be measured by satellites in space as an indicator of light usage by plants. However, remote sensing studies rarely investigate the physiology correlating with PRI. More physiological experiments are needed to verify the reliability of remote PRI measurement. This study looked at two evergreen species over two years to understand the physiological changes underlying PRI variance. CHANGING OF THE SEASONS The study found that three processes affect PRI in evergreens: • xanthophyll cycle – the 'facultative' response to light change on a diurnal time scale • carotenoid: chlorophyll (chemicals that absorb light for photosynthesis) ratio – the 'constitutive' response to light change on a seasonal time scale • a process of rapid response to deep cold within minutes The researchers discovered that the constitutive response, to seasonal conditions, was the primary cause of PRI variance over the life of a conifer needle. Such seasonal effects have not been well studied in the past, so this paper represents an important step towards understanding both basic plant responses and their detectability with remote sensing.

Featured Image