1st Workshop on Remote Sensing of Solar Induced Vegetation FluorescenceESTEC, 19-20 June, 2002

摘要

Starting from the early paper by Heber (1969), several studies have demonstrated arnsubtle shift in leaf spectroscopic characteristics (both absorbance and reflectance) inrnresponse to rapid changes in environmental conditions. More recent work, brieflyrnreviewed here, has also demonstrated the existence of two components in the markedrnpeak centered at 505-540 nm: an irreversible component, attributed to therninterconversion of leaf xanthophylls, and a reversible component at slightly longerrnwavelengths, resulting from conformational changes induced by the buildup of a pHrngradient across the thylakoid membrane associated with photosynthetic electronrntransport.rnBoth processes (xanthophyll de-epoxidation and conformational changes) are knownrnto contribute to the dissipation of excess energy in Photosystem II (PSII). Leafrnspectroscopy could therefore provide a powerful non-invasive tool for therndetermination of leaf photosynthetic processes.rnThis led to the development of the normalized spectral index PRI (PhotochemicalrnReflectance Index; Gamon, Penuelas & Field 1992; Gamon, Serrano & Surfus 1997),rnwhich relates the functional signal at 531 nm to a reference signal at 570 nm. Thernindex has been found to track diurnal changes in xanthophyll de-epoxidation state,rnradiation use efficiency and fluorescence in response to light, both at the leaf andrnmore recently at the canopy level. A common relationship has also been reportedrnacross species and functional types, although such a generality has not always beenrnconfirmed.rnRecent reports (Stylinski et al. 2000) have also hinted of a possible link between PRIrnand leaf photosynthetic potential, possibly through the correlation betweenrnxanthophyll content and electron transport machinery in the chloroplast. Such a link,rnif confirmed, could prove very useful for the remote sensing and modelling ofrnvegetation.rnSome of these open questions were addressed in the present study.rnThe correlation between leaf function and reflectance was studied in seedlings of 10rnbroadleaf tree species (Arbutus unedo, Castanea sativa, Fraxinus angustifolia, Fagusrnsylvatica, Juglans regia, Laurus nobilis, Ligustrum vulgare, Platanus occidentalis,rnQuercus robur, Q. Ilex, Salix capraea) under controlled conditions. To avoid thernpossibility of a spurious correlation in response to light, electron transport rate wasrnmodulated through changes in ambient CO2 concentration, whilst irradiance was keptrnconstant at saturating levels. This would mimic the effects of stomatal changes underrnmidday field conditions. Leaf photosynthetic potential (Jmax, Vcmax) and electronrntransport rates were derived from the resulting A/ci curves through the Farquharrnmodel (Farquhar & von Caemmerer 1982; Farquhar, von Caemmerer & Berry 1980).rnLeaf reflectance in the visible region was continuously monitored with a ZEISS