Gravitropism describes a plant’s directed growth in response to gravity’s pull. This phenomenon ensures different plant parts grow in specific orientations relative to the Earth’s gravitational field. It is a fundamental growth mechanism, distinct from other plant tropisms like phototropism, which involves light.
How Plants Sense Gravity
Plants possess specialized cells that enable them to perceive the direction of gravity. In roots, these cells are called statocytes and are found within the root cap’s columella layer. In shoots, statocytes are located in the endodermis, a tissue surrounding the vascular bundles. These statocytes contain dense, starch-filled organelles known as amyloplasts, which function as statoliths.
Amyloplasts are heavier than the surrounding cytoplasm, causing them to settle at the lowest point within the statocyte, much like sediment sinking to the bottom of a jar of water. When a plant changes its orientation, these amyloplasts shift and settle to the new bottom of the cell. This physical settling provides the plant with continuous information about the direction of gravity, signaling which way is down.
The Hormonal Response to Gravity
The settling of amyloplasts within statocytes initiates a signaling pathway involving plant hormones. This perception of gravity triggers the redistribution of the plant growth hormone auxin. Auxin is transported directionally throughout the plant, and this transport is altered by gravitational cues, leading to a directed movement of auxin when a plant organ is reoriented.
This process results in an uneven accumulation of auxin on the lower side of both roots and shoots. Specialized transport proteins facilitate auxin movement, ensuring this asymmetrical distribution. The altered auxin gradient then dictates the direction of organ bending through differential growth.
Directional Growth in Shoots and Roots
The uneven distribution of auxin leads to different growth responses in shoots and roots. In shoots, a higher concentration of auxin on the lower side stimulates cell elongation, causing faster growth. This accelerated growth on the bottom side forces the shoot to bend upwards, away from the gravitational pull. This upward growth is termed negative gravitropism.
Conversely, in roots, high concentrations of auxin have an inhibiting effect on cell elongation. When auxin accumulates on the lower side of a horizontally positioned root, it slows down the growth of cells on that side. The cells on the upper side of the root, with lower auxin concentrations, continue to elongate at a normal rate, causing the root to bend downwards, towards gravity. This downward growth is known as positive gravitropism.
The Survival Advantage of Gravitropism
Gravitropism provides a significant advantage for plant survival. The upward growth of shoots, driven by negative gravitropism, ensures stems and leaves emerge from the soil and grow towards sunlight. This positioning is necessary for efficient photosynthesis. It also helps the plant orient itself even if a seed germinates in an unusual position.
The downward growth of roots, facilitated by positive gravitropism, ensures they penetrate the soil. This allows roots to firmly anchor the plant, providing stability against wind and other forces. Growing deeper also enables roots to access water and dissolved mineral nutrients, essential for plant hydration and metabolism.