Final answer:
Plant hormones, such as auxin and brassinosteroids, regulate cell division in root tips, causing downward curvature of root cells on Day 6. Auxin concentrations are high near the root tip and decrease with distance, while brassinosteroids decrease near the root tip. When gravity is detected, amyloplasts settle, releasing calcium ions that stimulate the polar transport of auxin to the bottom of the cell, inhibiting cell elongation on the lower side of the root.
Step-by-step explanation:
Plant hormones, specifically auxin and brassinosteroids, play a role in the downward curvature of root cells on Day 6. Auxin concentrations are higher near the root tip and decrease with distance, while brassinosteroids decrease in concentration near the root tip. The interaction between these hormones regulates cell division in root tips. When the plant detects gravity, amyloplasts settle to the bottom of gravity-sensing cells, causing the release of calcium ions that stimulate the polar transport of auxin to the bottom of the cell. This higher concentration of auxin inhibits cell elongation on the lower side of the root, resulting in the downward curvature.