How do plant stems and roots respond differently to light and gravity?

Plant stems and roots exhibit different growth responses to light (phototropism) and gravity (geotropism), primarily regulated by the distribution of the hormone auxin.12

Phototropism and Geotropism in Stems

Stems typically exhibit positive phototropism (growing toward light) and negative geotropism (growing against gravity). In response to light, auxin accumulates on the shaded side of the stem, stimulating cell elongation and causing the stem to bend toward the light source. Similarly, in response to gravity, auxin accumulates along the lower side of a horizontal stem, causing those cells to elongate and pushing the stem to grow upward.12

Response in Roots

Roots generally show the opposite response to gravity compared to stems. While stems grow upward, roots exhibit positive geotropism, growing downward into the soil. This occurs because higher concentrations of auxin accumulate in the lower halves of roots; however, unlike in stems, auxin acts as an inhibitor to root cell growth. Consequently, the lower side of the root grows more slowly than the upper side, causing the root to curve downward.312