Root Vegetables: Biology, Nutritional Value and Health Implications

*Mirela Ahmadi, Ștefan A. Hulea and Ioan Peț*

### **Abstract**

Plants served as main staple for humanity since time immemorial. Plant roots science is a fascinating domain that offers a window to the complex world of plantsmicroorganisms relationship. Plant roots were used throughout human history both as a food source particularly in times of food scarcity as well as for medicinal purposes aid in the treatment of various human disorders. Root vegetables are excellent sources of fiber and antioxidants and are low in calories and lipids—being indispensable in human diet. There is an increasing interest in the biochemical processes occurring in the rhizosphere between root tissues and the bacterial/fungal colonizers especially in soils where there is a deficiency in minerals such as iron, phosphorus and selenium or there is higher load of toxic metals such as aluminum, cadmium, nickel and lead. That interest stems from the need to improve crop yields in hostile environmental conditions such as drought and low nutrient availability in soils. In this chapter, we will focus on the typical edible plant roots as well as bulbs (are not proper roots) looking at their nutrient content as well as their use as health enhancers.

**Keywords:** edible roots vegetables, health enhances

### **1. Introduction**

As roots grow and search for nutrients they evolve in a very complex environment called rhizosphere. This is defined as the area around plant root that is populated by a variety of different microorganism species, which "cooperate" with the plant for the benefit of both. However, not all bacteria in rhizosphere are beneficial and plants have developed mechanisms to protect themselves against harmful bacteria. It has been estimated that there are over 10,000 bacterial species in the rhizosphere, not all with "good intentions" toward the plant.

The rhizosphere comprises two main compartments: ecto-rhizosphere and endorhizosphere. The former is the outermost zone that extends from the rhizoplane out into the bulk soil. The latter includes parts of the cortex and endodermis between which bacteria find a "home." As McNear wrote in 2013: "the rhizosphere is not a region of definable size and shape but instead, consists of a gradient in chemical,

biological and physical properties, which change both radially and longitudinally along the root" [1].

Roots are in constant "touch" with their surroundings seeking water and nutrients and also shedding root cap and border cells, mucilage and exudates. The latter comprises part of the carbon fixed via photosynthesis, namely inorganic carbon, i.e. HCO3 − and organic carbon, such as organic acids and polyphenols. The exchange of material is influenced by the plant species, climate, presence of insects that feed on plants, nutrient availability, soil moisture and its physicochemical properties. Out of all organic compounds released from roots the low molecular weight compounds are the most studied because they serve as nutrients for the bacteria in the rhizosphere. The organic compounds also serve as chemo-attractants for the soil microbial population. For example, the exudates of leguminous plant roots attract rhizobium bacteria such as *Rhizobium leguminosarum*. This bacterium colonizes the root and helps the plant by converting atmospheric nitrogen into NH4 + that is further used in amino acid synthesis [2]. The enzyme complex involved is called nitrogenase and it catalyzes the reaction:

$$\mathrm{N}\_{2} + 8\,\mathrm{e}^{-} + 8\,\mathrm{H}^{+} + 16\,\mathrm{ATP} \\ \,\mathrm{2}\,\mathrm{NH}\_{3} + \mathrm{H}\_{2} + 16\,\mathrm{ADP} + 16\,\mathrm{P}\_{\mathrm{H}}$$

The nitrogenase complex consists of two enzymes: dinitrogenase reductase (a dimeric Fe-protein) and dinitrogenase (a tetrameric FeMo-protein). The nitrogenase is rapidly inactivated by atmospheric oxygen. That is why the root nodules provide for a low oxygen environment, so that the enzyme is kept active.

Leguminous plants, which provide the largest simple source of vegetable protein in human diet and livestock feed have evolved signaling systems when under nitrogen deprivation. Legumes possess specific flavonoids that under nitrogen scarcity are released near the root tips, close to the emerging root hair zone that is the site of infection by rhizobium bacteria.

Plant flavonoids are secondary metabolites derived from the phenylpropanoid pathway and include chalcones, flavonols, flavones, anthocianins among others [3]. Flavonoids accumulate in the dividing cells of roots and some of them act as chemo-attractants for the rhizobium bacteria. The rhizobial signaling molecules are called nodulation factors and include lipo-chitooligosaccharides having a N-acetylglucosamine backbone, N-acetylated on the terminal non-reducing sugar. The substitutions on the oligosaccharide moiety determine the specificity of the symbiosis. Some plant flavonoids such as luteolin-7-O-glucoside and quercetin-3-Ogalactoside can act as growth regulators of rhizobium bacteria.

One of the bacterial phylum present in the rhizosphere of legumes is *Firmicutes*. These are beneficial bacterial that colonize human gastrointestinal (GI) tract and as such they produce butyric acid that lowers gut pH and limits the growth of harmful bacteria. Many microbes in the rhizosphere, including *Firmicutes* have developed mechanisms for physically interacting with plant roots and through complex processes can reach other parts of the plant, e.g. stem and leaves. The ingested part plants housing colonizing bacteria help these microbes settle in the colon and in so doing help keep in check pathogens.

The symbiosis between nitrogen-fixing bacteria and leguminous plants is one way by which plants cope with limited availability of nitrogen in the soil. Besides this root exudates promote nutrient acquisition by changing the pH within the rhizosphere or

#### *Root Vegetables: Biology, Nutritional Value and Health Implications DOI: http://dx.doi.org/10.5772/intechopen.106240*

chelating ions in soil solution. The root exudates contain organic acids such as malic and citric acids that acidify the soil and solubilize phosphate bound in soil minerals. Moreover, in case of chemical fertilizers plants respond differently depending on the chemical form of nitrogen in the soil. An excess of ammonium ion (NH4 + ) leads to a more alkaline environment whereas an excess of nitrate results in a lower pH in the rhizosphere. The pH fluctuations influences the availability of minerals such as zinc, calcium and magnesium. In addition, plant-bacteria cooperation can broaden immune functions of the plant host [4]. Accumulating evidence suggests that the chemical composition of root exudates is of paramount importance in selecting beneficial bacteria, which in turn leads to healthier and more productive plants [5].

Iron (Fe) is an essential mineral for plant growth and development. It is well known that in alkaline soils (representing some 30% of the world's arable land) plants do not grow well because at higher pH, Fe is trapped in Fe oxides (Fe2O3). So plants have developed strategies for getting hold of iron. Thus, the root exudates contain a mixture of organic acids and phenols that reduce the pH in the rhizosphere, hence allowing for the reduction of Fe(III) to Fe(II), which is then taken up by the root epidermal cells (strategy I). Another strategy for Fe uptake is based on the solubilization of Fe2O3 by strong Fe(III)-chelating agents called phytosiderophores. They belong to the mugineic acid family and are released into rhizosphere by efflux transporters. The mechanisms of Fe uptake by plant roots have been extensively studied in the weed *Arabidopsis thaliana*. Its habitat includes side roads, railway tracks and disturbed habitats. It has been shown that secretion of phenolic compounds by this plant is critical for Fe acquisition from soils with low Fe availability. In an elegant study on the mechanism of Fe mobilization by *A. thaliana*, Schmidt and colleagues demonstrated that polyphenols such as coumarins are essential for Fe uptake by the plant [6]. Coumarins act both as reductants of Fe(III) and as ligands of Fe(II).

Two other minerals are in the attention of plant scientists, namely inorganic phosphorus (Pi) and aluminum (Al). In acidic soils (that occupy a sizable portion of arable lands worldwide) low Pi availability and high Al toxicity limit plant growth and productivity. Work on *A. thaliana* revealed that organic acid, phytohormones and Fe homeostasis are critical factors in plant's response to nutritional stressors such as low Pi and Al toxicity [7]. In acidic soils with high concentrations of Fe, Al, Mn, Pi is easily fixed in the form of insoluble salts. Moreover, when pH is below 5.5, Al becomes soluble and toxic to plant roots, impairing the absorption of water and nutrients. Recent studies suggested that Al-tolerant phosphobacteria isolated from ryegrass could assist plants to deal with Pi shortage and Al toxicity [8]. An active area of research deals with ways to activate genes involved in changing root system architecture (RSA) in conditions dictated by limited nutrient availability and metal toxicity. Changing RSA involves the inhibition of primary root growth and promoting the growth of lateral roots and hair.
