E2 Radiation Capture

Topic

Photosynthetically active radiation is the portion of sunlight—approximately between 400 and 700 nanometers in wavelength—that a plant can utilize for photosynthesis; however, this radiation does not reach all the plant's leaves equally. As it passes through the canopy, the radiation is attenuated layer by layer, following the exponential attenuation pattern known as the Beer-Lambert law; consequently, leaves located deeper within the canopy receive a progressively smaller fraction of photosynthetically active radiation compared to the more exposed leaves at the top of the canopy.

The photosynthetically active radiation that actually reaches each leaf is absorbed by chloroplast pigments; the more radiation these pigments absorb, the more light energy becomes available within the chloroplast to initiate photosynthesis.

Not all the light energy absorbed by chloroplast pigments is converted into usable photochemical energy: the quantum efficiency of photochemical conversion determines what fraction of that absorbed light energy the chloroplast successfully converts into photochemical energy. Thus, the higher the quantum efficiency of this conversion, the greater the amount of photochemical energy the chloroplast derives from a given quantity of absorbed photosynthetically active radiation. By integrating the absorption of photosynthetically active radiation, the quantum efficiency of photochemical conversion, and all subsequent steps of photosynthetic metabolism, the curve relating net photosynthesis to irradiance describes the amount of net photosynthesis a leaf produces at each level of irradiance—that is, at each intensity of photosynthetically active radiation—it receives. At low irradiance levels, net photosynthesis increases almost proportionally with irradiance, whereas at high levels, it ceases to increase proportionally and becomes saturated, because the quantum efficiency of photochemical conversion can no longer utilize all the additional incoming photosynthetically active radiation.

Photosynthetically active radiation that is absorbed by the leaf but cannot be converted into photochemical energy due to the limits of photochemical conversion efficiency does not simply go unused within the chloroplast; instead, the leaf dissipates it as heat through non-photochemical thermal dissipation. This process protects the chloroplast from accumulating excess energy that would otherwise damage photosystem II.

An additional fraction of the light energy that the chloroplast fails to convert into photochemical energy or dissipate as heat via non-photochemical thermal dissipation is re-emitted as chlorophyll fluorescence. This fluorescence serves as a diagnostic signal of the state of photosystem II: the better photosystem II is functioning, the more light energy is converted into photochemical energy or dissipated as heat—and the less chlorophyll fluorescence the leaf emits—whereas the more damaged photosystem II is, the less light energy is converted or dissipated via these two pathways, and the more chlorophyll fluorescence the leaf emits from the same amount of initially received photosynthetically active radiation.

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