Micronutrients in Plant Physiology: Functions, Uptake and Mobility

Keywords: Micronutrients, Functions, Uptake, Mobility in plants, Stress resistance, Reproductive growth, Rhizosphere management.

The plant micronutrients, which comprise Fe, Mn, Cu, Mo, Zn, B, Cl and Ni, are required by plants in very low concentrations for adequate growth and reproduction. However, despite their low concentrations within the plant tissues and organs, micronutrients are of equal important to macronutrients for the nutrition of plants. In these lower concentrations micronutrients are fundamental for the growth and development, acting as constituents of cell walls (B) and membranes (B, Zn), as constituents of enzymes (Fe, Mn, Cu, Ni), of activation of enzymes (Mn, Zn) and in photosynthesis(Fe, Cu, Mn, Cl). Plant nutritionists and agronomists are showing increasing interest in micronutrients because of their importance in crop production. Inadequate micronutrient status of crops which is growth limiting, and which may go unrecognised, has not only a direct effect on crop development but reduces the efficiency of use of the macronutrient fertilisers. Additionally micronutrients (Cu Mn, Zn, B) are particularly involved in the reproductive phase of plant growth and hence in determining the yield and quality of the harvested crop. They also provide resistance (Mn, Zn, Mo) against biotic and abiotic stress including pests and diseases. Moreover there is an increasing awareness of the importance of micronutrients in the health of the soil, crop, human and animal ecosystem.

Crop plants in farmers’ fields require optimal micronutrient status to fulfil the specific functions of these nutrients in yield formation, product quality and stress resistance. Under normal field conditions, plant requirement is met from the water soluble and easily exchangeable pools in the rhizosphere. The most limiting step in this acquisition is adequate mobilisation in the rhizosphere, considered together with an efficient ‘high affinity’ root uptake system. In most cases uptake is achieved for particular micronutrients by corresponding membrane transporters characterised physiologically and genetically by modern molecular techniques. Optimal delivery of mobilised micronutrients thus becomes critical in soils in which conditions are adverse for solubilisation of nutrients (chemical availability) and for vigorous root growth (spatial plant availability). Besides these two processes (mobilisation and uptake), internal mobility of micronutrients within the plant and in particular retranslocation at specific growth stages and periods (e.g. flowering, disease attacks) is becoming of increasing interest.

There is still a widespread lack of awareness by farmers of the importance of micronutrients in crop production. This paper thus closes by elaborating three case studies which clearly underline the importance of the micronutrients and the need to consider management strategies to improve micronutrient acquisition from the rhizosphere or by the application of micronutrient fertilisers using innovative techniques.

Ernest A Kirkby, University of Leeds, Leeds, UK.

Volker Römheld, University of Hohenheim, Stuttgart, Germany.

52 Pages, 19 Figures, 26 Tables, 119 references.