A novel combined spectral index for estimating the ratio of carotenoid to chlorophyll content to monitor crop physiological and phenological status

Authors: ZHOU, XIANFENG - Chinese Academy Of Sciences; HUANG, WENJIANG - Chinese Academy Of Sciences; ZHANG, JINGCHENG - Hangzhou Dianzi University; KONG, WEIPING - Chinese Academy Of Sciences; Huang, Yanbo

Submitted to: International Journal of Applied Earth Observation and Geoinformation
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/17/2018
Publication Date: 10/25/2018
Citation: Zhou, X., Huang, W., Zhang, J., Kong, W., Huang, Y. 2018. A novel combined spectral index for estimating the ratio of carotenoid to chlorophyll content to monitor crop physiological and phenological status. European Journal of Agronomy. 76:128-142. https://doi.org/10.1016/j.jag.2018.10.012.
DOI: https://doi.org/10.1016/j.jag.2018.10.012

Interpretive Summary: Diagnoses of plant physiological and phenological status is important in crop production management. Scientists from Chinese Academy of Sciences, Tuscia University and USDA-ARS Crop Production Systems Research Unit, Stoneville, Mississippi have conducted a collaborative study to assess a new method to accurately estimate the ratio between carotenoids and chlorophylls contents measured from the crop plants. In this study an integrated carotenoids/chlorophylls ratio index was proposed and evaluated with the highest accuracy in estimation of the ratio compared to other spectral indices for winter wheat and maize. The success of this study could provide valuable insight into diagnoses of plant physiological and phenological status.

Technical Abstract: Accurate estimation of the ratio between carotenoids and chlorophylls contents (Cx+c/Ca+b) is crucial and could provide valuable insight into diagnoses of plant physiological and phenological status. Published studies have extensively focused on semi-empirical approaches, such as spectral index method for Cx+c/Ca+b ratio assessment with spectroscopy techniques. However, spectral indices established in previous researches that generally rely on limited measured data might lack predictive accuracy for Cx+c/Ca+b ratio estimation in various species and different physiological conditions. In this study, we proposed and verified a hypothesis that Cx+c/Ca+b ratio could be assessed by an integrated carotenoids/chlorophylls ratio index (CCRI) in the form of the carotenoid index (CARI) divided by the red-edge chlorophyll index (CIred-edge) using abundant synthetic data simulated with PROSPECT-5 model, and its performance in Cx+c/Ca+b ratio assessment was further evaluated and compared with published spectral indices with extensive experimental datasets at both leaf and canopy level, including the ANGERS dataset, and the winter wheat and the maize data acquired in field experiments. Results show that CCRI was the most linearly correlated with Cx+c/Ca+b ratio using the PROSPECT-5 simulations (R2 = 0.959, RMSE = 0.027) among all the selected spectral indices. Calibration and validation results using the ANGERS and winter wheat data with leaf level spectrum showed that CCRI achieved the most accurate estimation(R2 = 0.515, RMSE = 0.035), suggesting that CCRI was robust and accurate when used for Cx+c/Ca+b ratio assessment regardless of various plant species and status. CCRI also showed excellent accuracy (R2 = 0.540, RMSE = 0.034) for Cx+c/Ca+b ratio estimation in both calibration and validation processes with the winter wheat and maize canopy spectra, which further supported the robustness and capacity of CCRI in Cx+c/Ca+b ratio retrieval. We suggest that the newly proposed CCRI was an optimal indicator for Cx+c/Ca+b ratio assessment.