Potential biomass production depends on the accumulated amount of photosynthetically active radiation (SPAR) intercepted by the crop over the course of its growth and the efficiency with which the crop is able to convert this radiation into carbohydrates, which is known as the radiation use efficiency or RUE [31]. The contribution of each of these processes to biomass production is crop specific [32]. As a C4 plant, M. x giganteus has a naturally high RUE, which has been assessed at 4.09 [21], just short of its theoretical maximum of 4.6 gDM MJ-1 [33]. Water deficit has been shown to reduce the RUE of M. x giganteus by 30-80% [27] with M. x giganteus being more sensitive to drought than Micanthus sinensis [34].

Water use efficiency (WUE) of M. x giganteus has been shown to be higher in the United Kingdom and France than in the Mediterranean environment [27], with adult stands reaching between 9.1 and 9.5 gDM l-1 in the United Kingdom [35] and between 6 and 10 gDM l-1 in France [36].

M. x giganteus exhibits high nitrogen use efficiency (NUE) with 200 g g-1 determined in February for aboveground biomass and 180 g g-1 for the total crop including the annual increase in rhizome mass [37]. Minimum nitrogen content in the belowground biomass of a mature Miscanthus x giganteus crop ranged from 70 to 370 kgN ha-1 depending on harvest date and nitrogen treatments [28]. The transfer of nitrogen from the belowground biomass to the aboveground biomass at the beginning of growth can account for as much as 79% of the total nitrogen content of the belowground biomass [28] (Figure 4.3). Nutrient accumulation in the aboveground biomass peaks during late summer.

During crop senescence in autumn, nutrients are again remobilized but this time from the aboveground biomass to the belowground biomass (i. e. autumn remobilization). Strullu et al. [28] found that 42% of the maximum nitrogen content of the aerial organs of Mis- canthus x giganteus was relocated to the belowground biomass by an October harvest compared to 71% by a February harvest. It would appear, therefore, that only a small

proportion of plant nutrients are harvested during the winter harvest with the majority of nutrients being translocated to the rhizome and recycled to the soil.