Physical model inversion of the green spectral region to track assimilation rate in almond trees with an airborne nano-hyperspectral imager

Abstract

Significant advances toward the remote sensing of photosynthetic activity have been achieved in the last decades, including sensor design and radiative transfer model (RTM) development. Nevertheless, finding methods to accurately quantify carbon assimilation across species and spatial scales remains a challenge. Most methods are either empirical and not transferable across scales or can only be applied if highly complex input data are available. Under stress, the photosynthetic rate is limited by the maximum carboxylation rate (Vcₘₐₓ), which is determined by the leaf biochemistry and the environmental conditions. Vcₘₐₓ has been connected to plant photoprotective mechanisms, photosynthetic activity and chlorophyll fluorescence emission. Recent RTM developments such as the Soil-Canopy Observation of Photosynthesis and Energy fluxes (SCOPE) model allow the simulation of the sun-induced chlorophyll fluorescence (SIF) and Vcₘₐₓ effects on the canopy spectrum. This development provides an approach to retrieve Vcₘₐₓ through RTM model inversion and track assimilation rate. In this study we explore SIF, narrow-band indices and RTM inversion to track changes in photosynthetic efficiency as a function of vegetation stress. We use hyperspectral imagery acquired over an almond orchard under different management strategies which affected the assimilation rates measured in the field. Vcₘₐₓ used as an indicator of assimilation was retrieved through SCOPE model inversion from pure-tree crown hyperspectral data. The relationships between field-measured assimilation rates and Vcₘₐₓ retrieved from model inversion were higher (r² = 0.7–0.8) than when SIF was used alone (r² = 0.5–0.6) or when traditional vegetation indices were used (r² = 0.3–0.5). The method was proved successful when applied to two independent datasets acquired at two different dates throughout the season, ensuring its robustness and transferability. When applied to both dates simultaneously, the results showed a unique significant trend between the assimilation measured in the field and Vcₘₐₓ derived using SCOPE (r² = 0.56, p < 0.001). This work demonstrates that tracking assimilation in almond trees is feasible using hyperspectral imagery linked to radiative transfer-photosynthesis models.

Plant phenotyping relevance

航空ハイパースペクトル画像とSCOPEモデル逆解析により、樹冠のVcmaxおよび光合成同化速度を推定する植物表現型計測法を開発・検証しており、手法が研究の中心である。

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