**3.5. Light**

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*

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

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‐

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‐

tyrosine in the exudate only at fruiting, but not at any other stages of growth.

genicity of pathogens that attack strawberries at low soil temperatures [33].

L.), lupin (*Lupinus lilosus* L.), mustard (*Brassica* sp.), or oats [30].

**3.2. Root age**

396 Insecticides Resistance

**3.3. Temperature**

**3.4. Microorganisms**

The light intensity at which plants are growing affects the amounts and balance of compounds exuded into nutrient solution by tomato and subterranean clover roots [31]. Clover grown at full daylight intensity exuded more serine, glutamic acid, and c-alanine than plants grown in 60% shade. With tomato, the levels of aspartic acid, glutamic acids, phenylalanine, and leucine in exudate were reduced by shading. Beside these abiotic factors, few others such as moisture, humidity, wind speed and light intensity, elevated CO2 pesticides, available space, atmos‐ phereic nitrogen deposition, ozone, physical disturbance, fire, irrigation, erosion, altitude, and latitude also influence the exudation [37]. Some soil abiotic factors resembling compaction, soil type, salinity, soil pH, metal toxicity, water availability, organic matter, cation and anion exchange, drainage, aeration, rooting depth, soil texture, soil structure, and redox-potential influence the release of organic chemical from plant root [38].

#### **3.6. Root-feeding insects**

Plants in nature are exposed to attacks by insects which bite and suck plants' parts and thus diminish their vitality. Root-feeding insects play an important role in both agricultural and natural ecosystems [39]. In response to attacks by herbivores, plants excrete terpenes and monoterpenes [40]. So far it has not been established if excretion of volatile substances from damaged plants is due exclusively to attacks by insects or if these substances are stored in plant cells and are excreted only when a plant is in physiological stress [41]. Plants have the so-called morphological defense mechanisms (presence of prickles, thorns, hairs, enzymes, and secondary metabolites), whose presence is not conditioned by attacks of herbivore organisms. Besides morphological defense mechanisms, there are also induced defense mechanisms, which manifest as plants' reaction to attacks of herbivores. Induced defense mechanisms can be further divided into direct defense mechanisms (secretion of secondary metabolites as a response to attacks by insects) and indirect defense mechanisms (secretion of the [VOCs] VOCs, which attracts natural enemies of herbivore organisms) [6, 42].

Plants react to different types of injuries (mechanical, herbivorous) by excreting different volatile chemical substances, which can be specific also for the insect species attacking a plant [43]. Many studies have shown differences in excretion of volatile compounds from plants which were attacked by different insect species [8, 40, 44]. Simultaneous feeding of different herbivore organisms on a host plant is a very frequent phenomenon in nature [45], which can influence the success of natural enemies in finding their prey [46].

VOCs have been commonly identified as arthropod attractants belowground. [47] highlighted different compounds that are used by herbivores to locate the food source. One of the most important signals in the soils are the emissions of CO2 by roots [48]. [48] reported that detection of CO2 seems to be dose-dependent, and soil insect are able to detect very small differences in the concentration of CO2. Besides CO2, plants emit various volatile compounds upon herbivore attack. The study of [49] investigated on-line VOC emissions by roots of *Brassica nigra* plants under attack by cabbage root fly larvae, *Delia radicum*. The investigation showed that several sulfur-containing compounds, such as methanethiol, dimethyl sulfide, dimethyl disulfide, dimethyl trisulfide and glucosinolate breakdown products such as thiocyanates and isothio‐ cyanates, were emitted by the roots in response to infestation [49]. [50] reported that fatty acids in oaks (*Quercus* sp.) and monoterpenes in carrot (*Daucus carotta* ssp. *sativus*), and potato (*Solanum tuberosum*) plants triggered the attraction of forest cockchafer larvae (*Melolontha hippocastani*) and wireworms (*Agriotes* spp.). Volatiles of fresh perennial ryegrass roots attracted larvae of *Costelytra zealandica* [51], and roots of *Medicago sativa* and *Trifolium pra‐ tense* attracted larvae of *Sitona hispidulus* [52]. Furthermore, [8] reported that maize (*Zea mays*) roots release ß-caryophyllene in response to feeding by larvae of the beetle *Diabrotica virgifera virgifera*. In a related research, [10] reported that mechanically damaged maize roots release linalool, ß-caryophyllene, and α-caryophyllene.
