What term describes the rapid growth on the upper surface of a plant part that causes it to bend downward?

The term that describes the rapid growth on the upper surface of a plant part, which leads to the bending downward of that part, is indeed epinasty. Epinasty specifically refers to the differential growth occurring in leaves, stems, or other plant structures where the growth rate on the upper side (adaxial) exceeds that of the lower side (abaxial), resulting in a downward bending or curling of the plant part.

This phenomenon is often observed in response to various environmental factors, including hormonal changes influenced by growth regulators like auxins. In contrast to other terms, such as photosynthesis, which describes the process by which plants convert light energy into chemical energy, epinasty is focused purely on growth responses and morphology. Turgor pressure, while important for maintaining the structural integrity of plant cells, does not directly describe a growth pattern that leads to bending. Geotropism, on the other hand, is the directional growth of plants in response to gravity, focusing on vertical growth rather than the bending due to differential growth induced by environmental factors or hormones. Thus, epinasty accurately captures the specific type of growth that causes the described bending effect.