What organic acid is stored in vacuoles of CAM plants during the night?

Malic acid is the organic acid stored in the vacuoles of CAM plants during the night. CAM, or Crassulacean acid metabolism, lets plants take in carbon dioxide at night, when cooler temperatures reduce water loss, and save it for photosynthesis during the day.

At night, CAM plants open their stomata and use PEP carboxylase to fix incoming carbon dioxide into oxaloacetate, which is converted into malate and stored as malic acid in vacuoles. During daylight, the stomata generally close. The stored acid leaves the vacuoles, and its carbon dioxide is released inside the leaf for the Calvin cycle.

This timing is a water-saving adaptation common in succulents and other plants from dry habitats. It also explains why some CAM plants taste more sour at night: malic acid accumulates overnight and is consumed during the day. Citric, succinic, and oxalic acids can occur in plants, but they are not the characteristic nocturnal storage product of CAM photosynthesis. The pathway was first recognized in members of the family Crassulaceae.