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Paul's Research Perspectives By Paul Hepperly, PhD, and Dave Wilson |
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August 10, 2006: Close your eyes and think of your favorite plant. What do you see? Flowers, leaves, stems, fruit? Very few people visualize the root system at all. Likewise, most agricultural research focuses on what is easy to see above the ground—not on what we can’t see below. Because of this and the tedious process of taking and measuring root samples, we conveniently forgo extensive root analysis and end up de-emphasizing the role that root growth and root physiology play in producing healthy plants. Although unseen and under-appreciated, the compelling fact remains: Without roots there are no shoots. At The Rodale Institute, we use the motto “Healthy Soil, Healthy Plants, Healthy People” to focus our mission. Healthy plants start with their connection to healthy soil in the root zone or by a connection to the soil through beneficial mycorrhizal fungi. (For more on mycorrhizae, see Fact Sheet: Mycorrhizae.) The interface between the soil and the plant is the zone immediately adjacent to the plants roots or the mycorrhizae hyphae (long, microsopic strands—up to 18-feet long—attached to the roots). This zone is known as the rhizosphere. Here, microorganisms are more numerous, and their activity improves the structure of the soil. Microorganisms in this zone interact with roots and mycorrhizal exudates, forming a community which interacts biologically with parts of the soil. The survival of naturally growing plants depends on a delicate balance between the root system and the shoot system. As the top of the plant grows larger and larger, the leaf area and water loss through transpiration also increase. The increased water loss through leaves is compensated by water absorption from an increasingly active and growing root system. The enlarging shoot system also requires greater amounts of minerals that are absorbed by the increasing root system. Simply put, everything above is supported and nurtured by everything below. The importance of roots—although not fully appreciated by humankind—is appreciated by the plant, which dedicates up to one third of all its food from photosynthesis to root support. Dr. Francis Zee, a horticulturalist and former colleague at the USDA’s Agriculture Research Service, proclaimed the balancing of root and shoot growth has the greatest ability to promote a healthy plant. The root system comprises a significant portion of the entire dry weight of any plant—about one quarter to one third, depending on the functional role or structure of the root. Roots, aided by protuberances (root hairs) and associated mycorrhizal fungi, generate a huge soil contact zone facilitating the absorption of water and minerals, as well as for anchoring and supporting the plant. Root hair cells and mycorrhizal hyphae are relatively short-lived, but are present in great numbers. They are rapidly produced as their tips continuously thrust through the soil exploring for water and nutrient resources. Work done at The Rodale Institute in collaboration with USDA-ARS research scientists show that mycorrhizal fungi are an important and overlooked part of plants root system. Together the root, root hair and mycorrhizal fungi provide a botanical web not only exploiting the soil resource but also sensing its condition. Other research shows that this biological association is extremely important in providing the catalyst (biochemical boost) for building productive soil by increasing the soil organic matter. (For more on these processes, check out Cedar Meadow Farm Field Day.) The mycorrhizal hyphal body and a group of specialized cells in the root form the xylem tissue, which becomes the main pathway for the transport of water and minerals. All mineral elements are ultimately absorbed from the soil together with water and are transported upward through the xylem. Organic acids and chelating agents from the mycorrhizal fungus allow absorption of materials that are barely absorbable by the plant alone. Since the ratio of the different elements in the xylem sap is quite different from that in the soil, it is clear that mineral uptake through root cells must be a selective process. Mycorrhizal fungi have been shown to be extremely important for mobilizing otherwise immobile nutrients in soil and for extending the web that captures water for plants in dry environments. While root hairs absorb nutrients within a 1mm zone around them, with the aid of myccorrhizal fungi this zone can extend to 10cm. Several conditions must be satisfied if the green plant is to acquire adequate quantities of minerals for its growth. First, the minerals must be in a form suitable for absorption by the cells of the root. Usually this means they must be dissolved in the soil solution, but oftentimes they may be released from soil particles through the solubilizing activity of roots. This shows the importance of root growth and the spatial distribution of roots in the soil. Second, the soil must be well aerated so root cells can carry out their oxidative process for mineral uptake. Third, the transport system in the plant must function efficiently in delivering minerals to the recipient cells. There is competition for minerals by root absorption versus leaching minerals through soil as water percolates past the root region. Hence the deeper and more developed the rooting zone, the more efficient and effective the plant is in absorbing these soluble nutrients. By contrast, shallow root zones lead to less spatial opportunity
for the plant to absorb nutrients.
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