# Emission of Terpenoid Compounds from Rice Plants
## 4 Discussion  
Several species of monoterenes and sesquiterpenes have been reported to be produced in rice seedlings under biotic and oxidative stress [23; 24; 25; 26; 27], and by the induc  
Figure 4: Interquartile ranges of upward fluxes of \(\alpha\)-pinene and methyl salicate measured above a rice paddy using the REA method. Figure 5: Relationship between the fluxes of two compounds and environmental factors. The data used here include accumulated flux data over two years. (**a**): \(\alpha\)-pinene flux vs air temperature; (**b**): \(\alpha\)-pinene flux vs. PPFD; (**c**): methyl salicate flux vs. air temperature; (**d**): methyl salicate flux vs. PPFD. tion of chemicals [28; 29]. The major genes that encode terpenoids include OsTPS3 [28], OsTPS19 [30], OsTPS20 [26], and OsTPS24 [29]. Linalool burst emission was observed in rice seedlings after wounding damage [31; 32], with the highest emission rate found 6 h after wounding [32]. These biochemical studies demonstrated the existence of genes involved in terpenoid production and reported that the emission of some terpenoids occasionally occurs after several stresses. However, the above-cited studies used young rice seedlings at the \(\leq\)6 leaf stage or \(\leq\)3 weeks old because of their suitability for laboratory experiments. From an atmospheric chemistry point of view, a quantitative study of the terpenoid fluxes of adult rice plants at the vegetative and reproductive growth stages under normal growing conditions is important to evaluate the contribution of terpenoid emissions from rice to regional and global terpenoid emission inventories and atmospheric chemistry, including photochemical ozone formation. In our flow-through chamber experiments, linalool emission was enhanced 2-3 h after enclosing the above-ground parts. Linalool is a monoterpene alcohol produced from geraniol diphosphate (GPP) via the action of linalool synthases [33]. Mechanical wounding and insect attacks can induce the gene expression of linalool synthases and enhance linalool emission [17; 33; 34]. As there was no visible injury to the leaves and stems, we could not estimate the degree of the stimulus or damage caused by touching rice leaves and stems. However, the results clearly showed that this stimulus enhanced linalool emission. These results are consistent with those of the aforementioned studies using young seedlings. The linalool emission rate was \(\leq\)0.02 ug gDW\({}^{-1}\) h\({}^{-1}\) on a dry weight basis on the second day. This value is approximately 1/20 of the monoterpene emission rate of the rice plants reported by [PERSON] et al. [13]. However, we could not confirm that linalool was emitted under normal growth conditions because of the short-term experiment (two days). It is reported that intense winds may cause mechanical damage to leaves and branches of trees, resulting in an increase in monoterpene emissions [35]. Therefore, we suspected that leaf rubbing by the wind influenced linalool emissions. As the enclosing stimulus enhanced linalool emission 2-3 h later, gas sampling was initiated within 20 min after the pots were moved to the laboratory and enclosed with the bag to eliminate the effect of the handling stimulus on the linalool burst. Up to a wind velocity of 4 ms\({}^{-1}\) recorded in the period 2-4 h before the gas sampling, no linalool burst was observed, suggesting that leaf rubbing by gentle wind does not enhance the emission of linalool. As we did not consider strong wind stimuli mostly caused by typhoons, we cannot conclude that a wide range of wind velocities has no effect on linalool emission. However, we conclude that gentle winds have no effect on the emission. In our laboratory experiment, the second stimulus applied during the dark period did not enhance the linalool emission. This means that compound production occurs only during the light period, probably because a series of reactions requires light energy [36; 37].