What is the role of malic acid in C4 plants?

In C4 plants, malic acid plays a crucial role in the process of photosynthesis by serving as a temporary storage form of carbon dioxide. During the day, C4 plants capture CO2 from the atmosphere, but they do this in a specialized way that minimizes water loss. The CO2 is initially fixed in mesophyll cells by an enzyme called phosphoenolpyruvate carboxylase, forming a four-carbon compound (oxaloacetate), which is then converted into malate or malic acid.

This malic acid is stored in vacuoles within the mesophyll cells, allowing the plant to efficiently concentrate CO2 in bundle sheath cells where the Calvin cycle occurs. This adaptation is advantageous in hot and dry environments, where C4 plants can maintain high photosynthetic efficiency while reducing photorespiration. The stored malic acid is later decarboxylated in the bundle sheath cells, releasing CO2 for use in the Calvin cycle, effectively providing a mechanism for the plant to utilize CO2 even when atmospheric levels are low. Thus, malic acid serves the vital function of storing CO2 for later use in photosynthesis, which is central to the C4 metabolic pathway.