Plants exhibit various growth responses to external cues, a phenomenon known as tropism. One such response is phototropism, which describes how plants grow in relation to a light stimulus. While many plant parts grow towards light, a response called positive phototropism, some parts demonstrate negative phototropism, meaning they grow away from the light source.
Understanding Negative Phototropism
When a plant grows towards a light source, it exhibits positive phototropism, as seen in most plant stems and shoots that bend towards sunlight to maximize light capture for photosynthesis. Observing this directional growth involves noting how certain plant parts, like roots, consistently orient themselves away from illuminated regions.
Why Plants Grow Away from Light
The exhibition of negative phototropism by certain plant parts offers clear evolutionary and survival advantages. For instance, plant roots typically grow downwards into the soil, away from light, which serves several purposes. This orientation helps roots anchor the plant firmly, providing physical support against environmental forces like wind.
Growing away from light also directs roots towards sources of water and dissolved minerals deep within the soil, which are crucial for the plant’s nutrient absorption and overall health. Exposure to light can lead to water loss from roots and potentially damage them, so negative phototropism helps protect these underground structures from desiccation and other adverse effects of light. This adaptive behavior ensures roots can fulfill their functions of absorption and stability, contributing to the plant’s overall survival.
How Plants Sense and Respond to Light
Plants sense light through specialized molecules called photoreceptors, which are proteins linked to a light-absorbing pigment called a chromophore. When these photoreceptors, particularly phototropins, absorb blue light, they change shape and become active. This activation initiates a signaling pathway within the plant cells.
The signaling pathway leads to an unequal distribution of plant hormones, specifically auxins. In the case of negative phototropism, light perception causes auxins to accumulate on the side of the plant part that is more exposed to light, or to move away from the light. This differential concentration of auxins then leads to varied cell elongation; the cells on the side with less auxin or on the illuminated side elongate less, while cells on the side with more auxin or on the shaded side elongate more. This uneven growth results in the plant part curving away from the light source.
Common Examples in Nature
A primary example of negative phototropism in nature is the growth of plant roots. Roots consistently grow downwards into the soil, away from the light that penetrates the surface. This ensures they can effectively seek out moisture and nutrients, which are abundant underground, and provide stable anchorage for the plant.
Other instances of negative phototropism can be observed in certain climbing plants. Some vine shoot tips and tendrils exhibit negative phototropism, enabling them to grow towards darker, solid objects like tree trunks or walls. This behavior helps them find support structures to climb, allowing the rest of the plant to ascend towards better light conditions for photosynthesis. Additionally, some underground stems, known as rhizomes, such as those found in ginger or ferns, also demonstrate negative phototropism by growing away from light and remaining beneath the soil surface.