干旱胁迫会使得植物的光合作用受到抑制，从而威胁着植物的正常生长状态，可能导致农作物减产等粮食安全问题和经济损失。叶绿素荧光与光合作用密切相关，因此有望提高干旱胁迫监测的精度。干旱胁迫是逐渐加重的，如何利用叶绿素荧光的日变化信息实现对早期干旱更可靠的遥感观测仍是一个有待解决的关键问题。本文通过水分控制实验，研究了玉米冠层日光诱导叶绿素荧光（Sun-induced Chlorophyll Fluorescence，SIF）从不旱到干旱胁迫逐渐加重最后复水的演进过程中的响应规律，并基于日变化特征提出了一种新的干旱指数，对比了荧光和常用植被指数对干旱胁迫的敏感性。并利用荧光校正植被指数（Fluorescence Correction Vegetation Index，FVCI）和近红外反射率指数（Near-infrared Reflectance of Terrestrial Vegetation，NIR_V）对冠层SIF中冠层结构和植被生理的贡献进行了解耦，初步定量分析了两者在不同干旱胁迫程度下的变化趋势。

本文的研究内容如下：

（1）结果表明，随着干旱胁迫的增加，SIF的日变化峰值出现的时间逐渐由中午向上午前移，荧光产量（SIF_y，SIF yield）在上午以逐步增加的斜率下降，在下午13:00（本地时间）达到最小值，因此，单时相的SIF值很难准确地反映干旱胁迫程度。而常用植被指数：归一化差分植被指数（Normalized Difference Vegetation Index，NDVI）和增强植被指数（Enhanced Vegetation Index，EVI），随着干旱胁迫程度的加重，日变化最小值都是在正午附近达到最低值，且干旱胁迫最严重时，NDVI的变化范围为0.8~0.95左右，EVI的变化范围为0.5~0.85左右，而SIF_A（SIF in the O₂A）和SIF_B（SIF in the O₂B）的变化范围分别为0.4~1.1（mW m^-2 nm^-1 sr^-1）左右和0.25~0.5（mW m^-2 nm^-1 sr^-1）左右，荧光参数在干旱胁迫时的日变化差异大于植被指数。因此，在干旱胁迫演进过程中SIF的日变化特征，相对于其强度值可以提供更丰富、更可靠的信息。

（2）分析植被指数和SIF参数日变化发现，不同干旱胁迫程度时，早晨8点左右对干旱胁迫敏感性较低，在13点左右最为敏感。因此，利用日内变化特征，本文提出了一种新的干旱指数——晨午比指数（Noon Morning Ratio，NMR）。结果表明：NMR_NDVI的变化范围为0.88~0.99，NMR_EVI的变化范围为0.69~0.95，而NMR_SIF-Ay和NMR_SIF-By的变化范围为0.45~0.96和0.48~1.02，在不同干旱阶段NMR_SIF对干旱胁迫程度的敏感性均明显高于NMR_NDVI和NMR_EVI。这表明，利用多时相的测量值计算的NMR评估干旱胁迫是可靠的，并且与单时相的SIF相比，利用多时相的SIF日变化特征更优于NDVI和EVI，可以为监测干旱胁迫提供一种更可靠的方式。

（3）利用FCVI解耦了冠层结构和植被生理对冠层远红光SIF的贡献。结果表明，植物的生理状态和冠层结构在干旱胁迫下共同影响着SIF日变化。对荧光发射效率（Fluorescence Emission Efficiency，ΦF）和FCVI计算NMR发现，NMRΦF在干旱时日变化幅度小于结构参数NMR_FCVI。在不同干旱胁迫程度下，两者的贡献比例存在差异。

Drought stress is a process of gradual evolution. Drought stress will inhibit the photosynthesis of plants, which will threaten the normal growth state of plants, and may lead to crop reduction and other food security problems and economic losses. The degree of drought stress is gradually increasing. How to use sun-induced chlorophyll fluorescence (SIF), which is closely related to photosynthesis, for more reliable and timely remote sensing of drought is an unsolved problem. In this study, the response of maize canopy SIF to the evolution of drought stress was studied through water control experiments. Based on the study of diurnal variation characteristics, a new drought index was proposed to evaluate the sensitivity of vegetation index and fluorescence to drought stress. The fluorescence correction vegetation index (FCVI) and the near infrared reflectance of terrestrial vegetation (NIR_V) were used to decouple the contribution of canopy structure and vegetation physiology in the canopy SIF, and the variation trend of them under different drought stress levels was preliminarily quantitatively analyzed. The research contents of this paper are as follows:

(1) The results showed that with the increase of drought stress, the peak time of daily variation of SIF gradually moved forward from noon to the morning, the fluorescence yield (SIF_y) gradually increased in the morning, and reached the minimum at 13:00 p.m. (local time) Therefore, it is difficult to accurately reflect the degree of drought stress. However, the normalized difference vegetation index (NDVI) and enhanced vegetation index (EVI) are commonly used. With the increase of drought stress, the minimum value of daily variation reaches the minimum value near noon, and the range of NDVI is about 0.8 ~ 0.95 when drought stress is the most serious, EVI ranged from 0.5 to 0.85, while SIF_A and SIF_B ranged from 0.4 to 1.1 (mW m^-2 nm^-1 sr^-1) and 0.25 to 0.5 (mW m^-2 nm^-1 sr^-1), respectively. The diurnal variation of fluorescence parameters under drought stress was greater than that of vegetation index. Therefore, the diurnal variation of SIF during the evolution of drought stress can provide more abundant and reliable information than its intensity.

 (2) Based on the analysis of the daily variation of vegetation index and SIF parameters, it was found that the sensitivity to drought stress was low at about 8 a.m. and the most sensitive at about 13 a.m. under different degrees of drought stress. Therefore, a new drought index, the noon morning ratio (NMR), is proposed based on the characteristics of diurnal variation. The results showed that the variation range of NMR_NDVI was 0.88 ~ 0.99, the variation range of NMR_EVI was 0.69 ~ 0.95, and the variation ranges of NMR_SIF-Ay and NMR_SIF-By by were 0.45 ~ 0.96 and 0.48 ~ 1.02. The sensitivity of NMR_SIF to drought stress was significantly higher than that of NMR_NDVI and NMR_EVI at different drought stages. The results show that the NMR calculated by multi temporal measurements is reliable in evaluating drought stress, and the diurnal variation characteristics of multi temporal SIF are better than NDVI and EVI compared with single temporal SIF, which can provide a more reliable way for monitoring drought stress.

 (3) FCVI was used to decouple the structural and physiological contributions of far red SIF. The results showed that plant physiological status and canopy structure affected the daily variation of SIF under drought stress.

The NMR calculation of the fluorescence emission efficiency (ΦF) and FCVI shows that the variation range ofNMRΦF is smaller than that of the structural parameter NMR_FCVI in dry days. Under different drought stress degree, the contribution ratio of the two is different.