A5 Light Deficit and Photoinhibition

Topic

Irradiance received by the leaf is the central variable in photosynthesis, and leaf productivity depends on irradiance remaining within an intermediate range: neither so low that photochemical energy is lacking, nor so high that the photosynthetic apparatus is damaged. When irradiance deviates from this intermediate range in either direction, leaf productivity declines, though the underlying mechanisms differ depending on the direction of the deviation.

When irradiance is insufficient, a light deficit occurs: the leaf fails to receive the photochemical energy needed to maintain positive net photosynthesis—that is, to ensure net photosynthesis offsets leaf respiration. The further irradiance drops below this threshold, the less photochemical energy is available and the more net photosynthesis is reduced, thereby dragging down leaf productivity.

Conversely, when irradiance is excessive, photoinhibition occurs: the excess irradiance damages Photosystem II within the leaf's photosynthetic apparatus, specifically targeting the Photosystem II D1 protein. The longer and more intense the excess irradiance, the greater the accumulation of damage to the Photosystem II D1 protein; this accumulated damage to the photosynthetic apparatus is what reduces leaf productivity in this scenario.

In response to excess irradiance, the leaf activates non-photochemical thermal dissipation, releasing light energy that cannot be utilized for photochemistry as heat. This non-photochemical thermal dissipation reduces the fraction of excess irradiance that actually damages the Photosystem II D1 protein, thereby partially mitigating the link between excess irradiance and the resulting accumulated damage to the photosynthetic apparatus. To distinguish whether a drop in productivity stems from a light deficit or photoinhibition—and to quantify the extent of existing damage versus remaining recovery potential—two tools are employed: the curve relating net photosynthesis to irradiance (which indicates the irradiance level at which the leaf ceases to respond with a proportional increase in net photosynthesis) and chlorophyll fluorescence. The latter provides data on both the maximum photochemical efficiency of photosystem II—which declines as damage to the D1 protein accumulates—and the non-photochemical thermal dissipation the leaf is currently utilizing to protect itself from excess irradiance.

Ultimately, leaf productivity is determined by the magnitude and direction of the deviation of irradiance from the intermediate range required for photosynthesis: a sustained light deficit keeps photochemical energy—and thus net photosynthesis—low, whereas sustained excess irradiance causes damage to accumulate in the D1 protein of photosystem II beyond the protective capacity of non-photochemical thermal dissipation. In both scenarios, the curve relating net photosynthesis to irradiance and the chlorophyll fluorescence measurements allow one to assess—via the maximum photochemical efficiency of photosystem II—how much productivity has been lost and how much can still be recovered.

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