花青素对植物反射特性的影响及遥感估算:叶片尺度

The Influence of Anthocyanin on Plant Optical Properties and Remote Sensing Estimation at the Scale of Leaf

花青素是植物三大色素之一,与植物的生长发育、生长状态以及生长环境状况密切相关,花青素动态变化对生态环境的监测和预警具有重要意义。对叶片光学特性与色素关系的深入理解是估算色素浓度的前提,然而花青素对叶片光学属性的影响并未能得到系统地分析,极大地限制了植物花青素的反演和监测。该研究从机理出发,采用包含花青素参数的新一代植物叶片辐射传输模型PROSPECT-D,结合改进的Sobol’算法,获取输入参数协同变化条件下叶片的光学表现,计算模型参数的全局敏感性指数,全面分析花青素对叶片光学属性的影响,探索花青素的光学敏感波谱区间,在此基础上,以地面实测的叶片波谱和色素数据为样本,发展并探讨叶片尺度花青素的遥感反演方法。结果表明,Sobol’全局敏感性分析结果受样本量变化的影响,总敏感性指数在样本数为3 000时达到稳定;花青素主要影响400~689 nm波谱范围的叶片反射特性,叶片对光子的反射能力随花青素含量的增加而降低;叶绿素、类胡萝卜素与花青素对叶片的反射存在协同作用,在467~589 nm波谱区间,花青素对叶片反射的贡献高于其他参数,花青素总敏感性指数在509 nm处最高(86.64%);基于叶绿素、类胡萝卜素对叶片反射能力的贡献程度,花青素的波谱敏感区间可分为三个部分:467~505 nm(花青素、叶绿素、类胡萝卜素共同影响区)、 506~541 nm(花青素、类胡萝卜素影响区)、 542~589 nm(花青素、叶绿素影响区);叶片花青素含量与敏感区间的高光谱窄波段、波谱指数之间存在显著的线性关系。在所有的窄波段中,样本花青素含量与560 nm的叶片反射率的相关性最好,相关系数为0.948,但由于花青素与叶绿素、类胡萝卜素光谱吸收波段重叠,考虑其他色素干扰的波谱指数ARI、 mARI(Sentinel-2波段参考)与花青素表现出了好的相关性(相关系数分别为0.953 2, 0.953 6),但由于波段设置的差异,不同卫星的波谱指数表现并不一致。该研究可为未来更大范围的花青素遥感估算奠定重要的理论基础。

Anthocyanin(Anth)is the third major group of leaf pigments.It can provide valuable information about plant physiology,and the information on the dynamics of their concentrations is a key to understand plants'the physiological reaction and resistance to different environmental stress factors brought about by episodic events or seasonal fluctuations.Traditionally,pigments are extracted from vegetation with spectrophotometry or high-pressure liquid chromatography,which are destructive and do not permit repeated measurements on the same samples.Optical methods monitoring plant physiological status through measuring leaf optical properties(absorbance or reflectance)possess several advantages over traditional destructive methods.However,there is a lack of research on the inversion of Anth through optical methods.Overlapping features in the specific absorption coefficient of pigments makes retrieving Anth content through remote sensing challenging.Understanding the relationship between Anth and leaf optical properties is necessary and helpful to estimate Anth content from leaves.This research used PROSPECT-D,a radiative transfer model including Anth parameter,to construct leaf spectral data in different parameter conditions.The global sensitivity analysis was conducted to quantify the influence of Anth on leaf optical properties through the modified Sobol method.we aimed to find sensitive Anth bands and calculate spectral indices relating to Anth content.Furthermore,inversing strategies based on hyperspectral narrow wavebands and spectral indices,including the Anthocyanin Reflectance Index(ARI)and modified Anthocyanin Reflectance Index(mARI),were discussed.The results showed that:(1)Anth can influence leaf optical properties in the 400~689 nm range,and leaf reflectance decreased in the visible band when Anth concentration increased.(2)The leaf reflectance in 467~589 nm was sensitive to the dynamics of Anth concentrations.There was the highest total sensitivity index of Anth at 509 nm.Chlorophyll and carotenoid had an impact on leaf reflectance in 467~589 nm.According to total sensitivity indices of chlorophyll and carotenoid,three spectral regions can be formed:467~505 nm(influenced by carotenoids,chorophylls and Anth),506~541 nm(influenced by Anth and carotenoids),542~589 nm(influenced by chorophylls and Anth).(3)Anth concentration correlated best with leaf reflectance at 560 nm.Because of the pigments'overlapping absorption features,including chorophylls and carotenoids,Anth had better relationship with spectral indices than reflectance in individual narrow wavebands.Spectral indices partly removed other plant pigments'influence on plant leaf reflectance.Consequently,they can describe the dynamics of Anth concentrations accurately.This research will provide a theory method for remote sensing estimation of Anth content at the leaf scale.