Flooding negatively impacts photosynthesis primarily by reducing nutrient availability to plants, leading to decreased photosynthetic efficiency.
When soil is flooded, several changes occur that directly affect a plant's ability to perform photosynthesis:
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Oxygen Deprivation: Flooding displaces oxygen in the soil, creating anaerobic conditions. Plant roots require oxygen for respiration, which provides the energy needed for nutrient uptake. Without sufficient oxygen, root function is impaired.
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Nutrient Deficiency: Oxygen deprivation in the soil reduces the uptake of essential nutrients, especially nitrogen (N). This occurs because active transport mechanisms used by roots require energy generated via respiration. Also, the flooded conditions favor denitrification leading to losses of available nitrogen in the soil. Nitrogen is a crucial component of chlorophyll and the enzyme RuBisCO, both essential for photosynthesis. Decreased nitrogen availability leads to:
- Reduced Chlorophyll Production: Less nitrogen means less chlorophyll, the pigment responsible for capturing light energy for photosynthesis.
- Impaired RuBisCO Activity: RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) is the key enzyme that fixes carbon dioxide during the Calvin cycle. Reduced nitrogen availability affects RuBisCO synthesis, activity, or both.
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Stomatal Closure: Waterlogged soils can cause plants to close their stomata (small pores on leaves), limiting the intake of carbon dioxide, another essential ingredient for photosynthesis. This closure can be triggered by root signals communicating stress to the leaves.
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Root Damage: Prolonged flooding can damage or kill roots, further reducing the plant's ability to absorb water and nutrients. Damaged roots can also become susceptible to pathogens.
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Increased Non-Stomatal Limitations: Flooding increases the limitations to photosynthesis not directly related to stomata (LNS). These limitations usually involve factors related to impaired CO2 fixation or electron transport capabilities within the leaf mesophyll cells.
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Decreased Stomatal Limitations: Flooding decreases the stomatal limitations to photosynthesis (LS). This is often correlated with impaired root function, and reduced CO2 diffusion into the leaf.
In summary, flooding hinders photosynthesis by creating anaerobic conditions in the soil, reducing nutrient availability (especially nitrogen), impairing root function, and potentially inducing stomatal closure. These factors collectively lead to reduced chlorophyll content, impaired RuBisCO activity, and decreased carbon dioxide uptake, ultimately suppressing the rate of photosynthesis.
