Hyperspectral Imaging of Photosynthesis from the Single Leaf to the Complex Canopy - Understanding the Spatio- Temporal Variations of Photosynthesis within a Drought-stressed Tropical Canopy

نویسندگان

  • Uwe Rascher
  • Caroline J. Nichol
چکیده

Light use efficiency of photosynthesis adapts dynamically to environmental factors and is affected by internal and external stress factors, all of which lead to complex spatio-temporal variations of photosynthesis on various scales from the leaf to the canopy level. We tested a new, field portable hyperspectral imaging system (SOC-700), which produces 12-bit reflectance images between 440 and 880 nm with a 4nm spectral resolution. Data were filtered by smoothing the low order principal components reducing the noise of the instrument substantially. Leaf level scans of reflectance were used to detect differences in the non-photochemical energy dissipation of genetically modified Arabidopsis thaliana (L.) Heynh. mutants, which were deficient or over-expressed psbS, an intrinsic pigment-binding photosystem II subunit (also known as CP22). In a second experiment, the Photochemical Reflectance Index (PRI) was used to track drought stress induced inactivation of photosynthesis in leaves of four tropical tree species. Based on these leaf-level measurements a canopy element of the tropical rainforest in Biosphere 2 Centre was monitored using the same approach in the early morning after sunrise and two hours later when exposed to full sun. PRI developed a functional dependency only after activation of the biochemical non-photochemical energy dissipation processes were initiated after high light exposure. This study indicates the potential of hyperspectral reflectance measurements to quantify physiological adaptation of the photosynthetic apparatus and highlights the prerequisite to take structural and physiological heterogeneity of natural canopies into account. INTRODUCTION All life on Earth depends on the photosynthetic light capture and the use of this energy to convert CO2 to carbohydrates. Stress factors such as nutrient limitations, availability of water, and extreme temperatures affect the efficiency of photosynthesis by influencing the function, biosynthesis, molecular assembly, and coordination of the components of the photosynthetic apparatus (i). These molecular mechanisms occur in the context of plantand ecosystem-level responses to stress (ii). However, our knowledge of photosynthetic variability and exchange processes on the canopy or ecosystem level is still limited. Modelling studies that link global circulation models to atmospheric transport and physiological models have shown that ignoring stress responses may lead to erroneous conclusions (iii), emphasizing the need for more research concerning the effects of abiotic influences on plant ecosystem physiology. The vegetation canopy is “the functional interface between 90% of Earth’s terrestrial biomass and the atmosphere” (iv), and terrestrial photosynthetic productivity is probably the most fundamental measure of global change of highest practical relevance to humankind. Net primary productivity © EARSeL and Warsaw University, Warsaw 2005. Proceedings of 4th EARSeL Workshop on Imaging Spectroscopy. New quality in environmental studies. Zagajewski B., Sobczak M., Wrzesień M., (eds) (NPP) of terrestrial ecosystems has been a subject of increasing interest because of the importance of terrestrial carbon cycle in global carbon budget. The spatial variation of net primary productivity (NPP) across the world in different ecosystems is enormous, ranging from 30 to 1000 g C m in different ecosystems (v). On the leaf level, the biochemical model proposed by Farquhar et al. (vi) and subsequently modified (vii, viii) is generally accepted and has been widely used to interpret and model leaf photosynthesis. The bottom-up scaling approach involved empiric speciesdependent parameters and has proven effective in reproducing assimilation fluxes. The accuracy of the approach depends on the validity and robustness of the assumed scaling principles, which are strongly non-linear (ix). Additionally, natural ecosystems show distinct spatial heterogeneity in their response to stress factors, as recently shown at different tropical tree species during drought (x). Photosynthesis was differently affected by drought, with some trees having their photosynthetic efficiency reduced by 50%, while photosynthesis of other species under the same environmental conditions remained unaffected. Even though global carbon models account for special heterogeneity in NPP and GPP (gross primary productivity), they rely on the accurate measurement of their input parameters, mainly light intensity (photosynthetic active radiation at 400-700nm; PAR), fraction of absorbed light (fAPAR) and a conversion factor, which describes the efficiency of conversion of absorbed light to carbon uptake (light use efficiency, LUE). While PAR and fAPAR can reliably be estimated from modelling and remote sensing data (xi), LUE cannot be measured on the ecosystem level and causes substantial uncertainties in global carbon models (xii). Additionally spatial and temporal heterogeneity of photosynthesis in canopies is a challenging feature of biological feedback between canopy processes, solar radiation and the atmosphere (xiii). Remote sensing techniques and especially hyperspectral reflectance measurements have the potential to quantify photosynthetic efficiency of plants. Excess electrons in photosystem II are reduced via de-epoxidation of three pigments, violaxanthin, antheraxanthin, and zeaxanthin, while the accumulation of zeaxanthin serves as a quantitative indicator for non-photochemical energy dissipation (xiv, xv, xvi).The photosynthetic reflectance index (PRI) was developed to detect these bio-chemical changes and to serve as an estimate of photosynthetic light use efficiency (xvii, xviii, xix, xx). This normalized difference reflectance index, which uses two wavebands (531 and 570 nm) has been successfully used to detect changes in photosynthesis on the leaf level (xxi, xxii, xxiii, xxiv), small canopy level (xxv, xxvi, xxvii, xxviii) and recently at the ecosystem level (xxix, xxx, xxxi). Nevertheless, canopy and ecosystem measurements of PRI often have high uncertainty or failed to predict photosynthetic efficiency (xxxii). Here we present a proof-of concept approach, that PRI and light intensity can be quantified with high spatial resolution from hyperspectral reflectance images in the field, detecting changes in light use efficiency occurring within hours on the leaf to canopy level.

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تاریخ انتشار 2006