Seventeen elements have been identified as vital to plant growth. Three of them — carbon, hydrogen and oxygen — are non-minerals, and the other 14 are minerals. Carbon and oxygen enter plants through leaves as carbon dioxide. Oxygen also enters plants with hydrogen through roots as water. The other 14 elements must be dissolved in soil water and can enter and move about the plant via the xylem and phloem, the internal transportation system of plants.
Xylem tissue is used mostly for transporting water from roots to stems and leaves. In the process, it also transports any dissolved compounds. The xylem is unidirectional, only moving water and minerals up the plant’s stem. Phloem is responsible for transporting food produced from photosynthesis from leaves (sources) to non-photosynthesizing parts of a plant such as roots, young shoots, and developing seeds (sinks). The phloem can transport both up and down the plant’s stem, from source to sink. Sugars produced in sources are delivered to growing parts of the plant via the phloem in a process called translocation, or movement of sugar.
Movement, or mobility, of specific minerals through the plant is generally determined by their solubility in phloem sap. Phloem sap is alkaline, and positive ions, or cations, are poorly soluble at high pH. For example, iron, manganese, copper and zinc, are cations and considered largely phloem-immobile. In farming, these minerals are best delivered by soil applications rather than by foliar spray, as they are more easily transported by the xylem.
I think that nutrient mobility is also related to what compound the ion becomes incorporated into within the plant. For instance, most potassium remains in ionic form, and so is easy for the plant to move from one place to another. On the other hand, magnesium is incorporated into the chlorophyll molecule, and so cannot move unless the molecule is broken down.
Nutrient mobility, or immobility, provides us with special clues when diagnosing deficiency. If the deficiency symptom appears first in the old growth, we know that the deficient nutrient is mobile.
Based on our presentation, mobile nutrients include nitrogen, phosphorus, potassium, magnesium, and molybdenum. On the other hand, if the symptom appears in new growth, the deficient nutrient is immobile. Immobile nutrients that do not have the ability to translocate from old to new growth include calcium, sulfur, boron, copper, iron, manganese, and zinc.
Finally, under conditions of nutrient limitation, growth seems to be more sensitive than photosynthesis (Wissuwa et al., 2005; Keller, 2005). Consequently, sugars can accumulate in plants due to nutrient deficiency and, since the phloem is the path for sugar distribution, it may be centrally involved in signaling in this important context. High sugar concentrations in phloem saps indicate nutritional stress, while high nutrient concentrations in phloem saps indicate well-supplied leaves.
