# Emission of Terpenoid Compounds from Rice Plants
## 4 Discussion
Our flow-through chamber measurements over two years suggest that the stimulus enhances linalool emission from rice plants, and non-contact measurement methods should be employed to obtain reliable quantitative values of monoterpene emission rates of rice plants. Therefore, we employed REA sampling based on tower flux measurements as a noncontact measurement method. REA sampling was repeated for more than 100 replicates over two years, and we determined the average \(\alpha\)-pinene flux to be 0.006 nmol m\({}^{-2}\) s\({}^{-1}\) on a land-area basis. This is two to three orders of magnitude lower than the fluxes reported for various trees [4]. No other terpenoids, including linalool, were detected in the atmosphere above the rice paddy. This is the first report to provide a quantitative value of monoterpene flux that was micrometeorologically measured above rice paddies. In the flow-through chamber measurement, including the above-ground parts of the rice plant, monoterpene alcohol linalool was detected. However, it was not detected in the atmosphere collected above the rice paddies. As shown in Figure 2, linalool emission was triggered by the touch stimulus and drastically dropped within 10 h. Under normal rice paddy conditions, wind-flutter rice leaves might be a weak stimulus. However, wind at a speed of \(<\)4 m/s recorded in our measurement period did not induce linalool production by rice leaves (data not shown). This was consistent with our flow-through chamber experiment. Instead, monoterpene species \(\alpha\)-pinene was emitted from the rice paddies. As we did not detect \(\alpha\)-pinene in the flow-through chamber including the same cultivar, the emission source could be weeds grown in the ridge between the rice fields included in the footprint of the flux tower. Further measurements are required to determine the source of the \(\alpha\)-pinene. Here, we compared this flux value with the land-area-based emission rate converted from the dry-weight-basis emission rate. [PERSON] et al. [13] reported that the monoterpene emission rate under standard conditions (30 \({}^{\circ}\)C leaf temperature and 1000 \(\upmu\)mol m\({}^{-2}\) s\({}^{-1}\) PPFD) was 0.40 \(\upmu\)g gDW\({}^{-1}\) h\({}^{-1}\). The dry matter of the above-ground parts of rice at the mature stage was reported to be \(\sim\)1760 g m\({}^{-2}\)[18], \(\sim\)1600 g m\({}^{-2}\) in Japan [13], and \(\sim\)680 g m\({}^{-2}\) in China [38]. Assuming that the representative value is \(\sim\)1500 g m\({}^{-2}\), the monoterpene emission rate reported by [PERSON] et al. [13] can be converted to 600 \(\upmu\)g m\({}^{-2}\) h\({}^{-1}\) (1.2 nmol m\({}^{-2}\) s\({}^{-1}\)) on a land-area basis. Our \(\alpha\)-pinene flux (0.006 nmol m\({}^{-2}\) s\({}^{-1}\)) was 1/200 of that value. They reported that monoterpene emissions from rice paddies in the Kinki region in Japan corresponded to 5% of the total monoterpene emissions in this region, which significantly contributed to photochemical oxidant formation in urban and suburban areas [14]. Using our flux data, the monoterpene emissions from rice paddies corresponded to \(<\)0.025% of the total monoterpene emissions in this region. This may be negligibly small. [PERSON] et al. [11] employed a static chamber method for the rice cultivar \"Cocodrie\" grown in a rice paddy and detected no emissions of monoterpenes, but a trace amount of isoprene (\(\sim\)3.7 \(\upmu\)g m\({}^{-2}\) h\({}^{-1}\)). They attributed the isoprene emissions to algae in the paddy water. The value corresponds to 0.015 nmol m\({}^{-2}\) s\({}^{-1}\), which is very low and of the same order as our results. Our previous report using a closed chamber system detected a trace amount of linalool from the rice cultivar \"Mutsuhomare\", but did not detect any terpenoids from \"Nipponbare\", \"TCM916\", \"Tangin-bodzu\", or \"Waito C\" [12]. [PERSON] et al. [10] detected no emission of isoprene or monoterpenes from a rice \"M202\" paddy in California using the flow-through chamber method.