## 2 Naturally Occurring Coumarins and Their Role in Plants
### Classification of Naturally Occurring Coumarins and Their Role in Plant Protection
#### 2.2.2 Furanocoumarins  
Furanocoumarins are composed of a furan ring fused to a coumarin core (Figure 3).  
Figure 2: The most common naturally occurring simple coumarins.  
They are mostly present in plant families, including _Apiaceae_, _Fabaceae_, _Moraceae_, and _Rutaceae_. Their increased level in plants is a response to stress conditions, such as exposure to pathogenic fungi or to physical damage caused by occasional lesions or insect bites. They are involved in plant defense, acting against microorganisms, nematodes, phytophagous insects, herbivores, and plant competitors [92]. Linear furanocoumarins, such as psoralen, bergapten, isopipinellin, and xanthotoxin, together with the angular dihydrofuranacoumarin athamantin, are antifeedants. Peucedanin (Figure 3) inhibits the growth of neonate larvae of _Spodoptera litils_ (Boisduval) (Lepidoptera: Noctuidae) [93]. Plants are a prominent source of novel nematicidal chemicals. Thus, furanocoumarins 8-geranyloxy poslaren, imperatorin, and heraclenin from the root extract of _Heracleum candicans_ Wall., exhibited nematicidal effects against _Bursaphelenchus xylophilus_ ([PERSON]) Nickle and _Pangrellus redivivus_ ([PERSON].) [94]. Also, bergapten and psoralen extract from _Ficus carica_ L. leaves showed strong nematicidal activity against pine wood nematode (PWN), _Bursaphelenchus xylophilus_. Their nematicidal mechanism is probably based on the inhibition of amylase, cellulase, and acetylcholinesterase [95]. Ethanol extract from Chinese medicinal herb _Notoptergytium incisium_ rhizomes possessed strong nematicidal activity against two species of nematodes, _Bursaphelenchus xylophilus_ and _Meloidogyune incognita_. The extract contained two furanocoumarins, colombienetin and isoimperatorin [96]. Essential oil and methanol extracts of parsley (_Petroselinum crispum_) [97], as well as ethanol extracts from _Angelica pubescens_ Maxim. _f. biserrata_ Shan et Yuan roots [98], exhibited promising nematicidal activity as a source of nematotoxic furanocoumarins.  
A number of furanocoumarin compounds isolated from _Semenovia transiliensis_ shoots have herbicidal activities [31]. Citrus plants produce simple coumarins and furanocoumarins to cope with herbivorous insects and pathogens [99]. [PERSON] et al. [100] studied coumarin derivatives found in citrus peels. They isolated six coumarins (5-geranyloxy-7-methoxycoumarin, bergapton, bergapten, isopipineline, citropten, and oxypeucedanin hydrate) and tested their antifungal activity against _Colletotrichum_ sp., a fungus causing fruit anthracose. Their activity was compared with the simpler coumarins, umbelliferone, scoparone, and scopoletin. The test results showed that all six coumarins inhibited the growth of _Colletotrichum_ sp. mycelia, and among them, bergapten and citropten proved to be the most effective. The research concluded that there is a synergistic effect between the individual coumarin components in the citrus peel extract.  
Figure 3: The most common naturally occurring furanocoumarins.