D2 Chlorophyll Radiation and Fluorescence Analysis

Topic

Photosynthetically active radiation is the primary parameter to quantify in order to measure how much available light energy a leaf captures and utilizes; quantum sensors measuring this radiation determine, at the leaf scale, exactly how much of it actually reaches the photosynthetic tissue.

Based on the photosynthetically active radiation received by the leaf, a fluorometer—which measures chlorophyll fluorescence using modulated light pulses—allows for the assessment of the leaf's Photosystem II status: the more photosynthetically active radiation the leaf fails to utilize in Photosystem II photochemistry, the more chlorophyll fluorescence it emits; the fluorometer records this fluorescence to diagnose the state of Photosystem II.

In conjunction with the modulated light pulse fluorometer, an infrared gas analyzer measures—also at the leaf scale—the amount of carbon dioxide absorbed or released by the leaf under varying levels of photosynthetically active radiation; these measurements are used to construct a curve relating net photosynthesis to leaf irradiance across the full range of photosynthetically active radiation levels, from low to high.

Scaling up from the individual leaf to the entire canopy formed by the plant or stand, hemispherical photography quantifies the canopy leaf area index—that is, the total leaf surface area available to intercept incoming photosynthetically active radiation: the higher the canopy leaf area index, the greater the amount of photosynthetically active radiation intercepted by the canopy before reaching the ground. Scaling up further—from the canopy to the entire ecosystem—satellite-based solar-induced fluorescence enables the estimation of gross primary productivity at a regional scale: the more solar-induced fluorescence emitted by a region's vegetation, the higher the gross primary productivity generated from the photosynthetically active radiation received by the canopy across that region.

Taken together, these tools—photosynthetically active radiation quantum sensors, pulse-modulated fluorometers, infrared gas analyzers, hemispherical photography, and satellite-based solar-induced fluorescence—allow for the monitoring of photosynthetic radiation capture and utilization across scales ranging from the leaf to the ecosystem. This multi-scale coverage makes it possible to detect chronic photoinhibition in leaf photosystem II, light deficits occurring when available photosynthetically active radiation is insufficient, and spatial variations in the photosynthetic status of vegetation when comparing gross primary productivity across different regions.

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