**3. Factors affecting exudation**

The exudation of organic compounds by roots are influenced by either biotic (for example, soil microbial uptake) [27] or abiotic processes [28]. In some instances, our knowledge is sufficient to explain why exudation is affected by the root environment, but often our ignorance of the physiological processes involved in exudation precludes a correct explanation. Some of the factors influencing exudation are listed below.

## **3.1. Plant species**

**1. Introduction**

394 Insecticides Resistance

agents [3, 4, 5].

function as kairomones for natural enemies [12].

interactions of the soil with the roots [18].

**2. The role of root exudates in rhizosphere**

Plants have a unique role in food chains on the Earth. Like people and animals, plants also contract different diseases caused by fungi, bacteria, viruses, viroids, and phytoplasma [1]. They are also attacked by different animals (insects, mites, nematodes, snails, rodents, game) which feed on them and procreate on them [2]. Each animal species in nature has a unique role and significance. None of them is harmful per se. In natural biotopes we normally do not distinguish between harmful and useful species. This distinction is characteristic for agrarian biotopes, where animals multiply exceedingly and by feeding on cultivated plants causing economic damage [2]. Plant-damaging species are biotic factors which cause economic damage in agriculture and forestry. Useful organisms (biotic agents) are predators, parasitoids, entomopathogenic nematodes (EPNs), entomopathogenic fungi, bacteria, baculoviruses, which suppress harmful pests, and antagonistic microorganisms, which suppress disease

Plants in nature have developed many defense mechanisms to defend themselves against attacks by harmful organisms. These mechanisms are indirect and direct [2, 6]. When attacked by a harmful organism, many plant species release volatile substances that attract natural enemies of herbivores [7, 8, 9, 10]. Volatile substances have an important role in the tritrophic system consisting of a plant, a herbivore, and its natural enemy [11]. They function as a kind of chemical signal (semiochemical) which directly influences both harmful pests and their natural enemy [8, 9, 10]. Some of these substances appear on damaged as well as undamaged plants, while other substances are released in the case of mechanic damage or feeding of a particular herbivore species [10]. Volatile substances may repel a herbivore. Harmful pests have an important role in attracting natural enemies, as they also emit chemical signals that

The soil furnishes a living environment to the extremely diverse communities of macro and microorganisms. Likewise, the rhizosphere is the zone of contact in soil surrounding a plant root where biological and chemical parameters of the soil are influenced by the roots. In these niches, complex biological and ecological processes occur [13]. The rhizosphere is a densely populated area in which plant roots must compete with invading root systems of neighboring plants for space, water, and mineral nutrients, and with other soilborne organisms, including insects, bacteria, and fungi [14]. Rhizosphere interactions are based on complex exchanges that evolve around plant roots. Root-based interactions between plants and organisms in the rhizosphere are influenced by edaphic factors [14]. The below-surface biological interactions that are driven by root exudates are more complex than those that occur above the soil surface [15]. These interactions include signal traffic between the roots of competing plants [16], roots, and soil microbes [17], and one-way signals that are dependent on the chemical and physical

The amount, range, and balance of compounds in root exudates differ for different plant species. [29] found differences between wheat and barley (*Hordeum vulgare* L.) root exudates with respect to certain sugars (galactose, glucose, and rhamnose), whereas other sugars occurred in similar amounts in exudates of both plants. The specificity of root exudates from different plants in stimulating only certain groups of organisms is clearly demonstrated in the plant pathology literature, for example, the cysts of potato eelworm (*Heterodera rostochiensis*) hatched when supplied the root washings of potato (*Solanum tuberosum* L.), tomato, and some other solanaceous plants, but not the washings of beet (*Beta vulgaris* L.), rape (*Brassica napus* L.), lupin (*Lupinus lilosus* L.), mustard (*Brassica* sp.), or oats [30].

#### **3.2. Root age**

The research performed with peas and oats indicated that more number of amino acids and sugars exude during the first 10 days of growth than those during the second 10 days [31]. Another study [32] found 3-pyrazolylalanine in root exudate of cucumber (*Cucumis sativus* L.) only at the early seeding stage. In tomato and red pepper (*Capsicum anznumm* L.), they detected tyrosine in the exudate only at fruiting, but not at any other stages of growth.

#### **3.3. Temperature**

The release of amino acids, especially asparagine, from roots of tomato and subterranean clover (*Trifolium subterraneum L*.) increased with rise in temperature [31]. However, this effect is not universal, as some researchers reported more amino acids in exudates from strawberry plants (*Fragaria vesca* L.) grown at 5–10°C than that at 20–30°C; this markedly influenced the patho‐ genicity of pathogens that attack strawberries at low soil temperatures [33].

#### **3.4. Microorganisms**

Microorganisms may affect the permeability of root cells, metabolism of roots, and absorption and excretion of certain compounds in root exudates. It was reported that filtrates of cultures of some bacteria and fungi and also some antibiotics (penicillin), increased the exudation of scopoletin (6 methoxy -7 hydroxycoumarin) by oat roots [34]. It was found that certain polypeptide antibiotics, for example, polymyxin, produced by *Bacillus polymyxa* from soil, altered cell permeability and increased leakage [35]. There are two key factors in interpreting the significance of these results which show that culture filtrates or products increase the leakiness of plant roots. First, the conditions under which the organisms are grown are quite different both physically and nutritionally from those under which a rhizosphere population grows. Second, since it is not possible to calculate the concentration of biologically active substances in the rhizosphere, the concentrations used for "*in vitro*" experiments are selected rather arbitrarily. Moreover, any consideration of the significance of the rhizosphere popula‐ tion in altering exudation must involve the concept of microecology with a wide variety of organisms occupying different "niches" on the roots and only those plant cells in the immediate vicinity of "exudation-promoting" organisms are likely to be affected. Microorganisms also influenced the exudation of organic materials into soil. A supplementary study showed that the exudation from wheat roots into synthetic soil was increased at least fourfold by microor‐ ganisms [35]. The magnitude of the effects of microorganisms upon exudation no doubt will depend on the species colonizing the roots [36]. Some other plant biotic factors like develop‐ mental status, shoot herbivory, photosynthesis, supply of carbon from shoot to root, evapo‐ ration, transpiration, nutrient deficiency, root architecture, cytosolic concentration, membrane permeability, membrane electrochemical potential, release of microbial signal, allelochemical release, mychorrhizas, nodulation, and some soil biotic factors are also influenced by the root exudation.
