Comparison of Seasonal Response of Isoprene Emission from Understory Type Bamboo and Canopy Type Bamboo Species

Bamboos are naturally distributed in Asia, Africa and America, and intentionally introduced in Europe. It has been reported with expansion of bamboos due to abandon of management in plantations and the niche shift under climate change. Furthermore, certain canopy type bamboo species are reported with high emission of isoprene, which can impact air quality and climate change. However, research about the isoprene emission from understory type species of bamboo, such as Sasa spp and Sasaella spp, are currently absent. This may cause uncertainties when estimating the isoprene emission from forest ecosystems. Thus, this study conducted measurement on isoprene emission flux (I) from leaves of 18 species of bamboo within five genera including understory type (Sasa and Sasaella) and canopy type (Pleioblastus, Semiarundinaria and Phyllostachys) species in a specimen garden in Kyoto, Japan, to compare the isoprene emission trait of the two types. The measurements were conducted monthly in 2^nd-5^th August, 12^th-16^th September and 15^th-17^th October 2019. Isoprene emitted from leaf is collected through an adsorbent tube while measuring factors such as photosynthesis rate and leaf temperature (T_L) under a controlled intensity of photosynthetic active radiation (PAR) at 1000 μmol m^-2 s^-1 with a modified photosynthesis-measuring system equipped with a LED light-source leaf chamber. The isoprene in the adsorbents were then desorbed and quantified respectively with a preconcentrate system and a Gas chromatography - Mass spectrometer system; measured leaves were taken to laboratory for area and dry weight measurement. As the result, most of the species showed the largest I in August (18.8 nmol m^-2 s^-1), and then gradually decrease or ceased in the following two months (8.1 and 1.3 nmol m^-2 s^-1 in September and October, respectively), which was consistent with the tendency of monthly temperature; that is, positive correlations of I and T_L were found in most of the species. Meanwhile, photosynthesis rate did not show significant variance to month in any species, which can be attributed to the weak or none correlation between photosynthesis rate and T_L. On the other hand, the species within the same genus showed similar I and dependence to T_L. The understory type genera showed significantly lower I than those from canopy type genera. The dependence of I to T_L was also weaker in understory type genera than in canopy type genera. The low isoprene emission in understory type genera may attribute to the potentially lower heat stress for the understory vegetations, where the isoprene is produced in plants for enhancing heat tolerance. The significant difference in I and its seasonal variation between understory type species and canopy type species suggest that these two types of bamboo must be seemed as different groups when in the regional modelling of isoprene fluxes.