_{*Synthesized image for representation purpose only.}

Vegetation photosynthetic activity, the fundamental process by which plants convert light energy into chemical energy by fixing atmospheric carbon dioxide, is critical for the global carbon cycle, ecosystem productivity, and climate regulation. It varies greatly depending on the type of ecosystem (e.g., forest, grassland, cropland), plant species, and environmental conditions. Traditional vegetation indices (like NDVI or EVI) primarily captures the vegetation “greenness” and structure (leaf area, biomass). Chlorophyll fluorescence  is a signal emitted directly from the photosynthetic machinery of the plants, hence, serves as a powerful proxy to the photosynthetic activity and physiological state of the plants. Chlorophyll fluorescence is emitted in the red and far -red infrared region (approximately 650-780 nm) of electromagnetic spectrum. The chlorophyll fluorescence signal emitted by plants in the natural light is called Sun induced fluorescence (SIF). Since Chl fluorescence is a very small fraction of energy (1 to 2%) of total energy absorbed by the leaves for photosynthesis, its detection against the strong background of reflected solar radiance is challenging. SIF is retrieved by measuring the “in-filling” of naturally dark absorption features present in the solar spectrum, such as Fraunhofer lines and atmospheric oxygen absorption bands (like the O₂-A band around 760 nm) using very high spectral resolution (<1 nm) spectroradiometers. At present, SIF retrievals are primarily conducted using instruments onboard satellite platforms originally designed for atmospheric composition monitoring (e.g., GOME, GOSAT, OCO-2/-3, and Tropomi). Several studies have consistently demonstrated a strong linear relationship between SIF and Gross Primary Production (GPP) across seasonal to annual temporal scales.

Figure 1 presents seasonally (left to right-winter, Summer, post monsoon) averaged SIF data at 0.5° spatial resolution from the Global Ozone Monitoring Instrument-2 (GOME-2) onboard the MetOp-A platform. These images reveal spatial and temporal patterns of vegetation photosynthetic activity across the India, quantified in units of mWm−2sr−1nm−1.

Figure 1– Seasonally (left to right-winter, Summer, post monsoon) averaged SIF data at 0.5° spatial resolution.

For instance, during the winter (December, January, February), significantly higher SIF emissions are observed over Punjab compared to other areas within the Indo-Gangetic Plain, suggesting higher productivity of wheat. The ‘green wave’ of seasonal phenological development of deciduous forests is also distinctly captured by the SIF signal during summer. During the post-monsoon season (October, November, December), deciduous forests in Central India exhibit notably elevated SIF levels. This enhanced signal, correlated with a period of active leaf flush and high photosynthetic capacity following the monsoon rains, indicates a high rate of photosynthetic carbon uptake during this period. Similarly, lower part of the North Eastern part of the country, display high fluorescence emission, primarily attributable to the vigorous growth of successional species like bamboo, paddy fields and the persistently high photosynthetic activity characteristic of moist deciduous forests. these observations underscore the utility of satellite-derived SIF as a powerful quantitative metric for monitoring and assessing vegetation photosynthetic activity across diverse ecosystems and land cover types. FLEX (Fluorescence Explorer), a dedicated mission planned to launch next year will provide high spatial resolution SIF images, in conjunctions with reflectance/thermal observations, to perform cause-effect studies on vegetation stress and productivity.