Roots of gama grass. Photo: USDA-ARS
The secret life of roots. 1: Grasses
Roots are seldom seen and easy to forget, but they are an essential part of plants. Maintaining a healthy root system is essential for healthy plants, especially when conditions are not ideal. Meeting the needs of roots in crop management programs will help maintain plant health and performance.
Roots have four major roles:
- They anchor plants in place.
- They take up water and nutrients from the soil.
- They take up oxygen for root growth.
- They synthesise plant hormones that regulate plant growth and development.
This article explains the features of roots by focusing on grasses. A later article will focus on broadleaved plants.
Root growth
Monocots, such as grasses, bananas, palms, ginger and onions, tend to have fibrous root systems with little variation in thickness. The root system consists of one to several primary roots deriving directly from the seed; adventitious roots, which develop from the lower stem nodes (maize is a good example); and lateral roots, which branch from the primary and adventitious roots and from other laterals. In the case of grasses, the primary roots support the main stem, and the adventitious roots support the tillers. Monocot roots tend to turn over quickly (grow, live and die).
The roots (and stems) of monocots have conductive tissue scattered in bundles. This contrasts strongly with broadleaved plants (the dicots), which have their conductive tissue in a single band around the outside. Fruit growers know this band as the cambium. The presence of the cambium allows dicot roots and stems to grow in girth. Most monocots, however, are unable to grow in girth, on account of their disconnected bundles of conductive tissue. This explains why most monocot roots remain fibrous.
Roots are said to be plastic, meaning they can adapt to varying environments. Those that can adapt quickly to a change in circumstance will fare better than others.
Root distribution in soil
The soil environment determines root distribution in part. In natural conditions, water and nutrients are unevenly distributed. From the point of view of the plant, this is much better than an even distribution. Roots following a random growth pattern will eventually stumble on concentrations of water or nutrients. The plant ends up spending fewer resources on growing these roots than on sending roots into every cubic millimetre of soil. This is more efficient.
Root distribution is determined also by root initiation, growth and death, and by evolutionary adaptation to a particular environment. For example, in hot dry regions where sporadic rain wets only the soil surface, many plants have broad, shallow root systems that can take up this limited water. An alternative strategy in the same environment is the ability of some plants to put down very deep roots to tap groundwater.
In contrast, rice plants, which are adapted to flooded cultivation, have very small root systems, because they do not have to search for water. The shrinking availability of water for irrigation around the world is driving programs to breed new rice cultivars that can grow longer and thicker roots and so cope with dryland cultivation (so-called aerobic rice).
In regions with distinct wet and dry periods, plants will benefit from a deep root architecture, which can draw up deeper resources in dry periods.
Water uptake
Water uptake is a vital function of a root system. In balance with water loss through transpiration, it determines the water status of a plant. The ability of a root system to take up water depends on root length density (how many metres of roots present in a given depth or volume of soil), root distribution (whether mainly in the topsoil or subsoil), and ability to carry water, which depends on the diameter of the water-conducting vessels.
The rate of water uptake is proportional to root length density, more so when soil is moist, as it depends on how much root length the plant has invested in a particular layer of soil. As soil dries, the rate of water uptake depends more on rooting depth, as deeper roots will be able to draw the deeper moisture. Deep roots of some species have even been shown to lift water from deeper in the soil to the dry surface soil at night for the benefit of shallower roots of the same plant (called hydraulic redistribution).
As a soil dries out, roots extend more and longer root hairs, vastly increasing the total root surface area, through which the plant takes up water and nutrients. Root hairs also excrete mucilage (a slimy sugar-based chemical), which glues soil particles to the hairs, and thereby excludes air gaps as the soil dries, reducing water loss from roots into the soil.
Nutrient uptake
Plants derive most of their nutrients via the roots. Many – perhaps all – plants form a symbiotic relationship with specific soil fungi as a way to improve nutrient uptake. The fungi invade the roots (some species between cells, other species into cells) and grow outwards into the soil to forage for nutrients and moisture. The fungi, called mycorrhizae in this role, enormously expand the surface area and foraging capacity of roots. In return, they receive carbohydrates (sugars and mucilage, for example) to build their networks.
Most plant nutrients take the form of salts dissolved in the soil water. As a soil dries out, the concentrations of these salts builds up, exerting an increasing osmotic pressure on the roots. The same principle explains how jam stays fresh, because microorganisms can’t exert a strong enough pull against the osmotic pressure of the sugar. In the case of roots in dry soil, a point is reached where the roots can no longer pull water out of the soil against the osmotic pressure of the salt, and so it stops taking up nutrients.
Growth regulation
Water is carried from roots to shoots in a hydraulic system. So when water becomes scarce at the roots, the leaves respond because of the reduced hydraulic pressure. In addition, they have been shown to respond to chemical signals produced in the roots. The main chemical messenger is a plant hormone called abscisic acid (ABA). Roots in drying soil send out ABA to the leaves, which respond by closing the stomata, reducing water loss.
ABA also inhibits growth. When water is scarce, this helps to keep plants alive by reducing expenditure. The roots send ABA before water stress becomes significant, thus priming the leaves to conserve resources.
Further reading
International Rice Research Institute. Aerobic rice.
Wikipedia. Hydraulic redistribution.
Wikipedia. Monocots.
Wikipedia Mycorrhizae.