Developmental responses to P-limitation
P-limitation resulted in great changes in switchgrass growth and development, including reduction in overall growth, increased root/shoot ratio (R/S), increased in root branching at moderate P-stress, and decreased root diameter with increased density and length of root hairs at severe P-stress (Figure 2 ). An increase in R/S is a classic response of plants to mineral nutrient limitation, which enables them to explore more soil volume per unit total plant biomass (Lynch et al., 1995). Increased R/S under P-limitation was observed in many annual species (e.g. Goldstein et al., 1988; Scheible et al., 1997, Wissuwa et al., 2005), and at least one perennial, tall fescue (Ding et al., 2015). Switchgrass root development and morphology responded differently to varying levels of P-stress (Figure 2 ). Under mild P-stress, total root length and surface area doubled without an increase in primary root length or biomass (Figure 1 and 2 ), in part through development of more fine/narrow roots. In contrast, total root length and surface area declined greatly under moderate and severe P-stress, while root hair density and length increased with P-stress. Deploying more and longer root hairs is a common adaptive response to P-stress in plant species (Yuan et al., 2016). Root hairs can account for as much as 90% of root P-uptake (Lynch, 2011), with relatively small investment in carbon and energy (Bates & Lynch, 1996).