When will chloroplasts be aligned with their flat surfaces parallel to the walls in mesophyll cells? When will they be perpendicular to the incident light?

The photo-relocation movement of chloroplasts is one of the strategies that evolved to enable land plants to adapt to changing environmental light conditions.

As plants are sessile organisms, they are unable to move away from strong sunlight or weak light conditions, for example under canopies. An essential way to ensure the protection of chloroplasts from photo damage is an avoidance movement of chloroplasts whereby they move from the cell surface (the periclinal wall) to a side wall (anticlinal wall).

Similarly, an accumulation movement of chloroplasts from areas with very weak light to those with relatively strong light is necessary for efficient photosynthetic activities.

Chloroplast align parallel to the walls of mesophyll cells (mainly palisade cells) when intensity of light is minimum, to get more and more of light. This condition is called Epi-strophe.

And when the intensity of light is high, the chloroplast gets aligned perpendicular to the incident light or the mesophyll cells to avoid photo oxidation of chlorophyll a molecule. This condition is called Para-strophe.