Even though plants cells carry out photosynthesis, they still use their mitochondria for oxidation of pyruvate. under what conditions will plant cell mitochondria be active in this process?

In all cells, all the time

Even though plants carry out photosynthesis to produce energy and "food," their cells would still need to undergo cellular respiration, as with all other living cells. Cellular respiration occurs in the mitochondria of cells and produces ATP (energy) in the process. This ATP is then used by the plants in all its other processes.