User:Brendaakoto/sandbox

Root to shoot ratio

The root to shoot ratio of a plant is measured as the ratio of the dried biomass of its roots to the dried biomass of its shoot. A plant’s root to shoot ratio is determined by its resource allocation. When more photosynthates are transported to the roots of a plant, for instance, the root to shoot ratio increases. Genetics affect plant root to shoot ratio. Most plants possess an intrinsic mean root to shoot ratio observed across that specific species in normal environments. Larger plant species have, on average, lower root to shoot ratios than smaller plants, and plants that live in drier climates have higher root to shoot ratios than those in moister habitats. Higher seedling mass has been linked to lower root to shoot ratios, as well.

Phenotypic plasticity allows plants to adjust biomass partitioning as external conditions change. Source-sink relationships dictate the flux of photosynthates between the roots and the shoot. Carbon is transported from the source (where photosynthesis takes place) to the plant organs that are in most need of carbon and are limiting its growth the most (i.e. the sink). If the roots of a plant are the stronger sink, then more carbon will accumulate in the roots, increasing root size, length, or growth rate, and resulting in a higher root to shoot ratio. Increasing root biomass or decreasing shoot biomass ensures that the plant is allocating resources to the area that is under more stress. This phenomenon follows a resource optimization hypothesis for plants. The hypothesis by Bloom et al. states that plants reduce root size as nutrients are more accessible in the soil because less energy is needed to acquire these nutrients. Cytokinin and auxin phytohormones regulate the coordinated growth of roots and the shoot. Cytokinin is responsible for the source-to-sink transport of photosynthates throughout the plant. Auxin also mediates growth and development in plants. Interaction between the two plant hormones as well as biosynthesis, transport, and signaling are thought to be involved in controlling root to shoot ratio. Other studies state that cytokinin, which is predominantly synthesized in the roots, is a part of the “hormone message concept” as a root signal. This concept explains that unequal cytokinin distribution in plants is responsible for disproportionate flux of photosynthates to the roots, and therefore, controls root to shoot ratio.

Certain environmental conditions have been shown to influence root to shoot ratio, including nutrient, water, and light availability. Stresses can be induced by resource competition in the upper or lower organs of a plant. Roots become a stronger sink when growth is limited by water and minerals found underground. In contrast, organs of the plant found aboveground increase in sink strength under growth limiting CO2 and light conditions. Sunflower plants deprived of nitrate and phosphate increase root to shoot ratio to acclimate. Phosphorous deficiency has been shown to increase bean plant root to shoot ratio. Deficits in potassium, magnesium, and manganese are exceptions to this pattern. Water stress can also alter plant biomass distribution. Drought conditions can increase root to shoot ratio due to limited shoot growth or heightened root growth9. Similar results are observed in plants treated with abscisic acid (ABA) which is synthesized in plant roots growing in dry soil.

Less biomass is allocated to roots of plants growing in limited light, resulting in a decreased root to shoot ratio, possibly due to regulation by phytochrome or blue light photoreceptors. Some stresses can have varied effects on different plants due to unique genotypes. For example, some findings suggest that CO2 concentration increases root to shoot ratio in C3 plants but not in C4 plants. Age is another determinant of root to shoot ratio. Root biomass decreases throughout a plant’s life, reducing the ratio of roots to shoot. The ratio is at its peak during vegetative growth early in development possibly because roots contribute most to a growing plant, while shoot growth provides the plant with organs necessary for reproduction in later stages of life. Shifts in the aboveground and belowground biomass of a plant is another example of phenotypic variation observed in plants.

Plant root to shoot ratio can be used to determine the health of a plant. A shift from the average ratio can be an indication of changes in the environment that are limiting growth and, thus causing the plat to adjust its resource allocation as a coping mechanism. Increasing root surface area can improve mineral and water intake in poor soils. Larger shoot biomass (bigger leaves, longer shoot, etc.) can improve photosynthetic rates in a shaded area. The effect of increased levels of atmospheric CO2 on plants has also been discussed as it relates to root to shoot ratio.