2.1.1. Cardoon (Cynara cardunculus)

Cardoon is a perennial and herbaceous plant with a productive life of 10 years (in some cases attaining 15 years) and an annual growing cycle. In the Mediterranean region, the crop presents a productivity of 10 t/ha dry matter (DM) in the first year and between 12 to 15 t/ha DM from the second year, being easily adapted to a wide range of climatic variations [9]. The cardoon is a species native of the Mediterranean basin, that support the drought stress, it can be grown in drylands, being defined as a multifunctional crop due to its characteristics that allow its use for several options, e.g., energy production in the form of biofuels, heat and power, cellulose and pulp and paper, phytochemicals and pharmacological products, among others [10,11]. The cardoon seeds composition presents nearly 24% (dry mass basis) of oil and 5.6% of water. Seeds have been traditionally considered a feedstock for biodiesel production [10]. Moreover, cardoon has shown a low impact on the environment, in the marginal soils of the Mediterranean region, particularly in certain parameters such as landscape diversity, cause of the flowering season, and use of water resources, due to the low water requirements [12]. In addition, cardoon can be irrigated with wastewater to avoid yield drop due to water stress, as it has been tested in a plantation located in Alcázar de San Juan in Spain [13]. In this study it was concluded that no e ffects were perceived on the energy outputs when cardoon was irrigated with wastewater.

Therefore, this crop was chosen for the study, not only because of its multiple uses but also due to its capacity to support the dry characteristics of the Portuguese summers.

## 2.1.2. Miscanthus (*Miscanthus* x *giganteus*)

Miscanthus is an herbaceous perennial C4 plant, native from East Asia, and long-lasting non-food crop. It is able to reach 3 m of height and produces between 20 to 30 t/ha of dry matter, in Portugal, performing best with a precipitation between 500 and 600 mm per year [9]. The following characteristics have promoted this crop as a sustainable energy crop:


In the last few years, miscanthus has shown a high potential concerning its implementation in marginal land and degraded and contaminated soils. A study was reported in which various miscanthus genotypes (*M. x giganteus*, *M. sinensis* and *M. floridulus*) were evaluated in a soil contaminated with 450 and 900 mg de zinc (Zn)/kg, over two years. In the contaminated soil, the *M. sinensis* and *M. floridulus* did not present changes in yields but in the case of *M. x giganteus*, the production was 20% lower. However, this last genotype presented, even in the contaminated soils, higher yields than the other two species [15]. Moreover, the deep and extensive rooting system of the plant allows this crop to be irrigated with wastewater with success, since the growth and productivity of the species were not affected and the polluting elements were removed from the wastewater by the plants, indicating that this might represent a solution for its cultivation in semi-arid regions with a high scarcity of water [16], such as those of the Mediterranean Region.

Several technologies can be applied to the miscanthus for energy production: heat, electricity or both (combined heat and power, CHP) through combustion in cogeneration systems (its most widespread application in Europe); biogas and bioethanol production [17].

Consequently, miscanthus can be considered a promising crop in mainland Portugal, not only due to the high yields but also due to the lower fertilizer and pesticide requirements. Moreover, the crop presents a good tolerance to a variety of soils, including marginal, contaminated and degraded areas. The major drawback of this crop is the annual water demand which is above 500 mm per year [18,19].
