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).