Plant "Fluorescence": Understanding Plant Health Beyond the Naked Eye

在日常農業生產與生態環境監測中，農作物或植被的健康狀況常常出現一些表面上難以察覺的變化。想象這樣一個場景：肉眼觀察并發現沒有明顯的病蟲害或營養缺陷，但田間產量卻出現了下滑。傳統的植被指數（例如NDVI）在此類情況下有時難以及時捕捉到植物內部的光合作用變化，從而導致對產量乃至生態系統健康狀態的錯誤判斷。

正是在這種背景下，日光誘導葉綠素熒光（SIF）技術應運而生，并迅速成為科學界和產業界關注的焦點。這項技術直接反映了植物光合作用的真實動態，為檢測植物壓力、預測作物生產力以及評估生態系統功能提供了一種全新的、精確的監測手段。SIF技術作為一種“植物的心電圖”，能夠捕捉到植物在光合作用過程中釋放出的微弱熒光信號。這種信號與傳統通過反射獲得的植被指數存在本質不同，其優勢在于能直接反映植物內在生理活動，而非僅僅依賴葉面積、綠度等表面特征。

In daily agricultural production and ecological environment monitoring, crops and vegetation often exhibit subtle changes in their health status that are difficult to detect with the naked eye. Imagine a scenario: you observe a field with no obvious signs of pests, diseases, or nutrient deficiencies, yet the yield is declining. Traditional vegetation indices, such as NDVI, sometimes fail to capture the underlying photosynthetic changes in plants in such cases, leading to incorrect assessments of yield and even ecosystem health.

It is against this backdrop that Solar-Induced Chlorophyll Fluorescence (SIF) technology has emerged and rapidly become a focal point in the scientific and industrial communities. This technology directly reflects the true dynamics of plant photosynthesis, providing a novel and precise monitoring tool for detecting plant stress, predicting crop productivity, and evaluating ecosystem function. SIF technology, acting as a "plant electrocardiogram", captures the faint fluorescence signals emitted by plants during photosynthesis. This signal is fundamentally different from traditional vegetation indices derived from reflection, as its advantage lies in directly reflecting the plant's intrinsic physiological activity, rather than merely relying on surface characteristics like leaf area or greenness.

土壤-植被-大氣連續統中植被尺度的生態過程和水勢梯度

Ecological Processes and Water Potential Gradients at the Vegetation Scale within the Soil-Vegetation-Atmosphere Continuum

什么是日光誘導葉綠素熒光（SIF）？

What is Solar-Induced Chlorophyll Fluorescence (SIF)?

日光誘導葉綠素熒光（Solar-induced chlorophyll fluorescence, SIF），是一種直接反映植物光合作用效率的信號，其產生過程與光合作用密切相關。簡而言之，植物在吸收太陽光能后，一部分能量用于光合作用，而另一部分能量則通過熱散發或以葉綠素熒光形式輻射出來。SIF技術通過傳感器對這些熒光信號進行捕捉和解析，從而直接衡量光合活動的強弱及其隨時間和環境條件的動態變化。

Solar-Induced Chlorophyll Fluorescence (SIF) is a signal that directly reflects the efficiency of plant photosynthesis. Its generation process is closely linked to photosynthesis. Simply put, after plants absorb solar energy, a portion of this energy is used for photosynthesis, while another portion is dissipated as heat or re-emitted as chlorophyll fluorescence. SIF technology captures and analyzes these fluorescence signals using sensors, thereby directly measuring the intensity of photosynthetic activity and its dynamic changes over time and under varying environmental conditions.

(A) 農田面積上基于 SIF 的年度作物 GPP 估計的空間詳細信息、(B) 每個網格框的農田面積比例

(A) Spatially explicit estimates of annual crop GPP based on SIF over agricultural areas, and (B) the proportion of agricultural area in each grid box

1.1 SIF信號的形成與檢測機制

Formation and Detection Mechanism of SIF Signals

在光合作用過程中，植被吸收了大量太陽能后，為維持光合平衡并防止能量過剩引起的光損傷，葉綠素會釋放出非常微弱的熒光信號。這一熒光信號主要集中在紅光（約680nm）和遠紅光（約740nm）波段，正是這一特性使得SIF成為捕捉植物內部光合作用活動的重要手段。而且，不同類型的作物和自然植被，其SIF信號強度和動態響應存在區域性差異，如同植物體內生理狀態的實時反饋。

During photosynthesis, after absorbing a large amount of solar energy, vegetation releases very faint fluorescence signals to maintain photosynthetic balance and prevent photoinhibition caused by excess energy. This fluorescence signal is primarily concentrated in the red light (around 680nm) and far-red light (around 740nm) spectral bands. This specific characteristic makes SIF a crucial means of capturing the internal photosynthetic activity of plants. Furthermore, different types of crops and natural vegetation exhibit regional variations in SIF signal intensity and dynamic response, serving as a real-time feedback of the plant's internal physiological state.

1.2 夫瑯禾費暗線的“井”填充效應

The "Filling" Effect of Fraunhofer Lines

在自然光照條件下，植被反射的光譜實際上包含了兩個部分：一部分是葉片對入射太陽光的反射，另一部分則是植被自身發射的日光誘導葉綠素熒光（SIF）。雖然SIF的信號非常微弱，通常不到太陽入射能量的1%，遠弱于植被的反射光，但在特定的波段，我們可以巧妙地利用一個自然現象來提取SIF信號。這個自然現象就是“夫瑯禾費暗線”。由于太陽和地球大氣層的吸收作用，太陽光譜中存在一些非常狹窄且強度較低的“暗線”（寬度為0.1~10nm），這些就是夫瑯禾費暗線。在這些暗線位置，太陽光的能量顯著低于周圍波段。

葉綠素熒光的光譜輻射恰好會填充這些暗線區域，使得在這些暗線位置，SIF的相對比例顯著增大。因此，將太陽輻射和植被反射光譜中某個波段的夫瑯禾費暗線與相鄰的波譜之間的相對強度進行比較，二者之間的差異就可以用來反演出SIF的強度。這就像在太陽光譜的“凹陷處”找到了SIF留下的“填補”痕跡，通過測量這種填補的程度，我們就能估算出SIF的強度。

Under natural illumination, the spectrum reflected by vegetation actually comprises two components: one is the reflection of incident solar light by the leaves, and the other is the Solar-Induced Chlorophyll Fluorescence (SIF) emitted by the vegetation itself. Although the SIF signal is very weak, typically less than 1% of the incoming solar energy and much weaker than reflected light, we can cleverly utilize a natural phenomenon to extract the SIF signal at specific wavelengths. This phenomenon is known as "Fraunhofer Lines". Due to the absorption by the sun and the Earth's atmosphere, the solar spectrum contains very narrow and less intense "dark lines" (with widths ranging from 0.1 to 10nm). These are the Fraunhofer lines. At these dark line positions, the solar energy is significantly lower than in the surrounding spectral regions.

The spectral radiation of chlorophyll fluorescence happens to "fill in" these dark line regions, causing the relative proportion of SIF to significantly increase at these Fraunhofer line positions. Therefore, by comparing the relative intensity between a Fraunhofer line and its adjacent spectral region in both the incoming solar radiation and the vegetation's reflected radiance spectra, the difference can be used to retrieve the intensity of SIF. This is akin to finding the "filling" trace left by SIF in the "dips" of the solar spectrum. By measuring the extent of this filling, we can estimate the intensity of SIF.

葉綠素熒光（SIF）對夫瑯和費暗線的“井”填充效應

The "In-filling" Effect of Chlorophyll Fluorescence (SIF) on Fraunhofer Lines

SIF：植物的心電圖

SIF: The Plant's Electrocardiogram

從前面的介紹可知，SIF能夠更直接、更敏感地反映植物光合作用的效率和健康狀況。SIF可以看作是植物的“心電圖”，用來監測植物生理活動的“脈搏”。憑借這個特性，SIF在眾多領域展現出強大的應用潛力，以下是它的主要應用方向：

As discussed, SIF provides a more direct and sensitive reflection of plant photosynthetic efficiency and health status. SIF can be considered the plant's "electrocardiogram," used to monitor the "pulse" of its physiological activity. With this characteristic, SIF demonstrates powerful application potential in numerous fields. Here are its main application areas:

農業領域

• 監測作物生長：SIF能夠實時反映作物的生長活力，幫助了解作物的生長狀態，及時調整管理措施。

• 診斷病蟲害和脅迫：在作物表現出肉眼可見的病蟲害或水分脅迫癥狀之前，SIF信號可能已經發生變化，為早期預警和精準施策提供依據。

• 優化施肥灌溉：根據SIF數據評估作物對養分和水分的需求，實現精準施肥和灌溉，提高資源利用效率，降低生產成本。

• 產量預估： SIF與作物最終產量之間存在良好的相關性，利用SIF數據可以更準確地預估作物產量，為農業生產決策提供支持。

Agricultural

• Monitoring Crop Growth: SIF can reflect crop growth vitality in real-time, helping to understand the growth status of crops and adjust management measures promptly.

• Diagnosing Pests, Diseases, and Stress: Before crops exhibit visible symptoms of pests, diseases, or water stress, the SIF signal may have already changed, providing a basis for early warning and precise intervention strategies.

• Optimizing Fertilization and Irrigation: SIF data can be used to assess crop demand for nutrients and water, enabling precise fertilization and irrigation, improving resource utilization efficiency, and reducing production costs.

• Yield Prediction: There is a good correlation between SIF and final crop yield. Utilizing SIF data can lead to more accurate crop yield predictions, supporting agricultural production decisions.

林業領域

• 評估森林健康：通過監測SIF，可以評估森林的光合能力和健康狀況，及時發現森林退化或病蟲害侵襲的區域。

• 監測森林火災風險：干旱和高溫會導致森林植被水分含量降低，光合作用下降，SIF信號隨之減弱。利用SIF數據可以評估森林的干旱程度，輔助進行森林火險預警。

Forestry

• Assessing Forest Health: By monitoring SIF, the photosynthetic capacity and health status of forests can be evaluated, allowing for the timely identification of areas experiencing degradation or pest and disease outbreaks.

• Monitoring Forest Fire Risk: Drought and high temperatures lead to reduced water content in forest vegetation and decreased photosynthesis, resulting in a weakening of the SIF signal. SIF data can be used to assess the severity of drought in forests and assist in forest fire risk warning.

生態研究

• 監測植被生產力：SIF數據可以用于估算區域和全球尺度的植被總初級生產力（GPP），幫助科學家理解陸地生態系統碳循環，評估氣候變化對植被的影響。

• 研究生態系統對環境變化的響應： 利用SIF監測氣候事件（如干旱、熱浪）對不同生態系統的影響，深入了解生態系統的脆弱性和恢復能力。

Ecological

• Monitoring Vegetation Productivity: SIF data can be used to estimate Gross Primary Production (GPP) at regional and global scales, helping scientists understand terrestrial ecosystem carbon cycling and evaluate the impact of climate change on vegetation.

• Studying Ecosystem Responses to Environmental Changes: SIF can be used to monitor the impact of extreme climate events (such as drought and heatwaves) on different ecosystems, providing deeper insights into ecosystem vulnerability and resilience.

預告 Preview

我們了解了SIF是什么，它如何產生。SIF微弱卻蘊含著巨大的信息量，它是植物與我們“對話”的一種特殊方式。這些數據能在實際中發揮哪些作用呢？在接下來的文章中，我們將通過具體的應用案例，展示SIF在農業、林業、生態研究等領域的巨大潛力。敬請期待！

We have learned what SIF is and how it is produced. Despite being weak, SIF carries a wealth of information. It is a special way for plants to "communicate" with us. How can this data be applied in practice? In the following articles, we will showcase the immense potential of SIF in agriculture, forestry, ecological research, and other fields through specific application examples. Stay tuned!

如果您對SIF設備或相關方面有興趣，歡迎隨時聯系我們了解咨詢相關產品信息。

If you are interested in SIF equipment or related aspects, please feel free to contact us for product information.

參考論文 / Articals

1. Ni. Zhuoya.L. et al., A Review of Retrieving in Sun-Induced Chlorophyll Fluorescence, Advances in Meteorological Science and Technology (2021).

2. L.Guanter,Y.et al., Global and time-resolved monitoring of crop photosynthesis with chlorophyll fluorescence, Proc. Natl. Acad. Sci. U.S.A. 111 (14) E1327-E1333.