*2.5. VOCs and Plants Respiration and Photosynthesis*

Previous studies showed that allelochemical can affect plant respiration by interfering in various stages of respiration, including the generation of carbon dioxide (CO2) by electron transport, oxidative phosphorylation, and the activity of ATPase, and it has the potential to inhibit plant growth and development [41,110,111]. VOCs released from the leaves of *A. tridentata* Nutt. *var. vaseyana* and *Sasa cernua* Makino. inhibited the respiration

of germinating seeds [110,112]. In fact, studies of allelopathy processes in shrubs in the 1960s found that volatile terpenes could reduce respiration in the young leaves of some plants and increase respiration in mature leaves [113]. Similarly, terpenes in eucalyptus volatile oil could affect target plants by inhibiting cellular respiration [114]. The influence mechanism of these allelochemicals on plant respiration deserves more in-depth study.

In addition, photosynthesis plays an important role in realizing the energy conversion in nature and maintains the carbon–oxygen balance in the atmosphere. The high concentration of allelochemicals involved in multiple metabolic steps may lead to the inhibition of plant photosynthesis, or even block the mechanism of photosynthesis by inhibiting electron receptors, energy coupling, or destroying photosynthetic pigments and enzymes [115]. Isoprene volatilized from the foliage of many woody species was thought to increase the rate of photosynthesis by stabilizing thylakoid membranes, so that adjacent plants could tolerate high temperatures [116]. Kaur et al. found the volatile oils α-Pinene and 1,8-cineole from *Eucalyptus tereticornis* Sm. could significantly reduce the respiration and photosynthetic pigment content of *Amaranthus viridis* Linn. Seedlings; thereby, the negative effects of the oil on photosynthetic machinery was explained [117]. These studies can confirm that olefin compounds can affect the photosynthesis of plants. In addition, Tsubo et al. found that exposure to low concentrations of the volatile chemicals released by *A. adamsii* Besser stimulated the photosynthetic rates of *Stipa krilovii* Roshev [118]. The volatile oil of *Artemisia ordosica* Krasch. inhibited the growth and photosynthesis of *Palmellococcus miniatus* through oxidative damage [119]. Zhao et al. studied the effects of eucalyptol and limonene, the main terpenoids in cyanobacteria VOCs on the photosynthetic capacity of *Chlorella vulgaris* [120]. The results showed that the compounds could induce the degradation of photosynthetic pigments and reduce the photosynthetic abilities of other algae. These studies can confirm that VOCs have the ability to affect plant photosynthesis.
