Final answer:
High concentrations of indole acetic acid (IAA) in roots inhibit cell elongation, leading to positive gravitropism (downward growth), while in stems, high IAA levels stimulate cell expansion, resulting in negative gravitropism (upward growth). These effects of IAA ensure that plants grow correctly in response to gravity.
Step-by-step explanation:
How do High Concentrations of IAA Affect Roots vs Stems?
In plants, indole acetic acid (IAA) is a naturally occurring auxin with significant effects on growth processes. In the context of gravitropism, which is the orientation of plant growth in response to gravity, IAA demonstrates contrasting effects in roots versus stems. When amyloplasts in gravity-sensing cells settle to the bottom and stimulate calcium signaling, IAA transport is polarized toward the lower side of the cells.
In roots, a high concentration of IAA inhibits cell elongation, leading to slower growth on the lower side and normal growth on the upper side. This differential growth causes roots to grow downward, exhibiting positive gravitropism. Conversely, in stems, high IAA levels promote cell expansion on the lower side, which corresponds with upward growth, showcasing negative gravitropism. Hence, stems curve upwards. Growth responses in shoots and roots, in this context, ensure that the plant can orient itself correctly in response to gravity, contributing to its overall growth and stability.
Additionally, the role of IAA extends beyond gravitropism. It is involved in various plant developmental processes such as phototropism, apical dominance, flowering, fruit setting and ripening, and many other growth responses. IAA is so pivotal that it is utilized commercially in nurseries and crop production as a rooting hormone to encourage growth and facilitate fruit development.