Role of root traits in Nutrient Use Efficiency
Authors: Gayacharan1^*, Kuldeep Tripathi¹, Harish G D^2
1ICAR-National Bureau of Plant genetic Resources, New Delhi-110012
²ICAR-National Bureau of Plant genetic Resources, RS, Umiam, Shillong - 793103
^*Corresponding author: gayabio83@gmail.com




Nutrient Use Efficiency (NUE) may be defined as yield per unit fertilizers applied. More scientifically the NUE is often expressed as fresh weight or product yield per unit of nutrient. Most recent definition of NUE given by Fageria et al. (2008) as plant efficiency that produces higher economic yield with a determined quantity of applied or absorbed nutrient compared to other or a standard plant under similar growing conditions. The important plant traits directly affecting NUE are photosynthesis efficiency, canopy size, root architecture, longevity and sink organ capacity. Here root traits are discussed.

Root architecture and its role

An ideal genotype should have high nutrient uptake efficiency as well as high nutrient utilization efficiency. Both are influenced by number of factors. Nutrient uptake efficiency is mainly influenced by root architecture and root physiological function like nutrient transport, exudation and interaction with other beneficial micro-organisms. The root architectural components like length, thickness, surface area, biomass, branching pattern decides nutrient uptake ability of any genotype. A vigorous and ideal root system is always an important target trait for a breeder to enhance yield of a crop. The root morphology also affects its ability to modify the rhizosphere pH and the nutrient uptake kinetics.

Faster root establishment

Genotype with faster root establishment has better opportunity for early soil nutrient uptake prior to fertilizer application and even if fertilizer applied at the time of sowing. This trait has major role in early plant vigour. Early plant vigour for crops particularly of short duration like mung bean, urd bean has high impact on canopy development and setting high yield potential.

Root associated traits

Root associated traits of any plant are the major attributes for efficient NUE but assessing the quantitative value for each trait has immense difficulties. These traits vary among crops, varieties and even among plants of a single variety due to large number of genes involved in it. This complicates to identify and combine set of genes governing those traits in a single genotype to develop an ideal root system for NUE. Therefore, nutrient uptake which is a function of root architecture and various physiological processes could not be dissected out to find out major genes involved.

Branching pattern

Acquisition of nutrients, especially immobile nutrients such as phosphorus, is dependent on root architecture (especially branching), associations with microorganisms (especially mycorrhizal fungi), and modifications to the bioavailability of the nutrient in the rhizosphere. Branching patter and length of roots is very important e.g. well developed shallow long roots are ideal for nutrient uptake from upper surface of soil which is nutrient rich and deeper root system is important for reaching out water in drought condition and untapped nutrients. Further type of ideal root branching varies from crop to crop and field and climatic conditions in which they are grown. A root system with well developed shallow root branching with one central tap root is ideal for short duration crop requiring one or two irrigations to complete their life cycle e.g. Mungbean, Urdbean, Cowpea, etc. For crops like wheat, rice which require lot of irrigations resulting in nutrients leach out in deeper layer of soil, fibrous long roots are ideal for better nutrient absorption. Deeper root system with regular branching is ideal for crops like long duration pigeon pea, lentil, and chickpea crops grown in rainfed conditions. In rainfed condition deeper root system is desired as water becomes limiting in upper surface of soil which make nutrients unavailable for absorption. Crops grown in flooded conditions also deep root system as many of the nutrients leaches out to deep layer of soil.

Root exudates

Root system changes its rhizosphere environment by releasing various kinds of chemical compounds like organic acid, chelators, reducing/oxidising compounds and extracellular enzymes which makes suitable environment for beneficial microbes and enhances NUE (Sauerbeck and Helal, 1990). Root exudates particularly organic acids like citrate, malate are important for mobilization soil nutrients or converting unavailable nutrient form to available form for easy uptake. Soils with high pH containing large chunk of phosphorus in Ca-P complex form citrate mobilizes significant quantity of P (Jones and Darrah, 1994). Phytosiderophores released from roots of gramineae family plants like wheat increase Fe and Zn mobilization in the rhizosphere (Awad et al 1994). Root also exudates some compounds which inhibit soil nitrogen loss by nitrification process.

References:

Fageria NK, Baligar VC, Li YC (2008) The role of nutrientefficient plants in improving crop yields in twentyfirst century. J Plant Nutr 31:1121-1157

Awad, F., Römheld, V., & Marschner, H. (1994). Effect of root exudates on mobilization in the rhizosphere and uptake of iron by wheat plants. Plant and Soil, 165(2), 213-218

Jones, D. L., & Darrah, P. R. (1994). Role of root derived organic acids in the mobilization of nutrients from the rhizosphere. Plant and soil, 166(2), 247-257.

Sauerbeck, D. R., & Helal, H. M. (1990). Factors affecting the nutrient efficiency of plants. In Genetic aspects of plant mineral nutrition (pp. 11-17). Springer Netherlands.



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Scientist (Agri-biotechnology)