# Biochemistry of Terpenes and Recent Advances in Plant Protection
## 3 Role of terpenes in Plant Protection
### Alleleptathic Activities of Trepenes in Agriculture  
Agronomic weeds cause the loss of estimated annual revenue of about $95 billion globally. Weeds have severe effects on crop growth, pest buildup, and nutrient depletion. For example, _Echinochla crus-galli_ is a devastating weed that depletes about 80% of soil nitrogen. It is also a favorable host for mosaic virus diseases. It grows up to 60 inches tall, producing about 40,000 seeds annually. These features make this grass a good competitor in the rice field, reducing yield to about 50% with only 25 members per m\({}^{2}\)[104; 105; 106; 107; 108]. However, synthetic herbicides are an unfavorable antidote to weed control due to their adverse effect on the environment and biodiversity. Exploiting the allebotathic potential of plants, which has been in use since ancient times, remains promising in inhibiting several weeds. Some higher plants are functionally annotated to possess allebotathic effects. _Medicago sativa_ L, buckwheat, hairy vetch, and velvet bean, among others, are described as paddy field natural herbicides due to their allebotathy effect [109; 110]. Allelochemicals are toxic organic root exudates that can negatively impact the physiological performance of neighboring plants. These chemicals stifle respiration and germination, ion uptake, and photosynthesis of weeds. Stomatal opening, transpiration, enzyme actives, and hormonal levels are also negatively impacted by allelochemicals. Allebotathic activities could also hinder cell division and differentiation, gene expression and signal transduction, and truncate cell membrane permeability [111]. Monoterpenes, sesquiterpenes, and momilactone diterpenes are allebotathic terpenes. Studies have shown that monoterpenes, citronellal, linalool, and cineole prolong the weed germination time and also reduce their development, e.g., _Cassia occidentalis_. citronellal and linalool at 110 M and 55 M concentrations, respectively, completely stifled weed germination [112]. A related study on _Raphanus sativus L_. and _Lepidium sativum_, geraniol, carvone, borneol, and citronellol conferred significant inhibitory effects on seed sprouting and radical elongation of the two species of plants [113; 114]. [PERSON] et al. also reported that 1,8-cineole, camphor, camphene beta-pinene, and alpha-pinene monoterpenes stifled plant cell proliferation and meristematic DNA synthesis. Some sesquiterpenes also exhibit allebotathy. Arbuscular A, achillin, viscidulin C, caryophyllene, bisabolone, and chamazulene play inhibitory functions. \(\beta\)-caryophyllene extract from the roots and pines of _Pinus halepensis_ inhibited herbaceous plant growth [115]. Several studies on allebotathy elicitation and its effects have been reported. In the USA and Egypt, 557 out of 17,000 and 45 out of 1000 rice accessions, respectively, evaluated, conferred inhibitory effects on weed growth. Momilactone diterpenoids were confirmed as the inhibitors. For example, _Lepidium sativum_ (barnyard grass) and _Cyperus diffiformis_ L (rice sedge) were inhibited by constitutively synthesized momilactone B in rice roots [116; 117]. Arabidopsis germination was also hampered by Momilactone A and B at concentrations higher than 30 and 10 \(\mu\)M and IC\({}_{50}\) 742 and 48.4 \(\mu\)M, respectively. At 4, 20, 20 ppm, momic lactone B completely stifled the sprouting of _Leprochla chinenesis_, _Amantallus retroflexus_, and _Cyperus diffiform_, respectively [68; 118]. At varying concentrations of momilactone B, the growth and development of several monocots and dicots plants, including _Lepidium sativum_ (cress), _Brassica rapa_ cv, Chinese cabbage, _Lactusa sativa_ cv, Santanasu (_lettuce_), _Echinochla colanum_ crabgrass, and _Paleum pretense_ (timothy), among others, were also inhibited [119; 120].