The role of isoprene emission on the performance of different genera of Arundineae

Isoprene is a highly reactive volatile organic compound (VOC) affecting the oxidative capacity of atmosphere and it is emitted by many plant species. The protective role of isoprene in plants against high temperature and oxidative stresses has been suggested. Arundo donax is a drought tolerant species of the Arundineae tribe and is known as a promising biofuel crop in Mediterranean regions. To further elucidate the role of isoprene in plant protection, this work was focused on the Arundineae tribe (Poaceae) as a case study, as several members of this tribe are fast-growing species known as strong isoprene emitters (e.g., A. donax and P. australis, as well as closely related species). Through measurements of photosynthetic and isoprene emission capacity in six different species from this tribe, I provide the first comparative characterization of isoprene emission in a clade of monocotyledons, which until now received less attention than isoprene-emitting dicotyledons. Based on the results of these analyses, two isoprene emitting (A.collina, A.donax) and one non-isoprene emitting (H.macra) species were selected to further study the function and structure of photosynthetic apparatus with the respect to the possible photoprotective role of isoprene emission (Chapter 2). Then, the effect of drought on plant performance in relation to its isoprene emission capacity was investigated, as aspect of abiotic stress tolerance which is still poorly characterized. To this end, the photosynthesis and secondary metabolism (isoprenoids and phenylpropanoids) in the isoprene emitting (A.donax) and non isoprene emitting (H.macra) species were compared at control conditions, and under different levels of drought as well as recovery (Chapter 3). Finally, to better understand the effect of isoprene emission on the drought tolerance of A.donax, the physiological characteristics of two A.donax ecotypes from Italy (IT) and Bulgaria (BG) associated with habitats characterized by different xericity were examined (Chapter 4). The results of these analyses revealed that all the studied members of the Arundineae tribe except H.macra are isoprene emitters, indicating the relevance of this trait in this clade based on its evolutionary conservation. The quantum yield and efficiency of PSII and chloroplast ultra structure of the species were positively correlated to their isoprene emission capacity under control conditions. Drought negatively affected photosynthetic performance regardless of isoprene emission capacity of the species studied. However, the damage was more severe in the absence of isoprene emission. Isoprene emission remarkably enhanced the ability of the photosynthetic machinery to recover its structure and function after re-watering. Based on these results, it is suggested that isoprene emission in A.donax triggers the MEP pathway and the production of non-volatile isoprenoids (carotenoids) under drought and this effect is intensified by the higher isoprene emission during the earlier stages of drought. Additionally, it is proposed that the high phenylpropanoids content in H.macra under drought was due to the high level of oxidative damage experienced by this species. However, in A.donax the ecotype which presented higher phenylpropanoids under drought experienced less photosynthetic impairment. Therefore, it is suggested that the protective role of phenylpropanoids and/or the strength of their protection under drought could vary between different plant species. The fast adjustment of stomatal opening and the induction of isoprenoids and phenylpropanoids in response to drought and eventually the resilience of the photosynthetic apparatus were identified as the special characteristics of A.donax leading to its drought tolerance