Effect of phosphate supply on switchgrass growth and development
To establish P regimes that limit switchgrass growth, plants were watered daily with nutrient solutions containing 600, 200, 60 or 20 µM Pi. Compared to P-replete plants supplied with 600 µM Pi, plant growth was mildly, moderately or severely impacted by 200, 60, and 20 µM Pi, respectively (Figure 1a ). After 28 days of growth, mild P-stress resulted in a 35% decrease in shoot biomass, compared to the control, although no significant difference in root biomass was found between these two sets of plants (Figure 1b ). Moderate or severe P-stress resulted in further large decreases in shoot (77 and 97%, respectively) and in root biomasses (60 and 93%, respectively) (Figure 1b ). Root to shoot ratio increased from 0.28 in control to 0.77 in severe P stress (data not shown). Increasing P-stress led to successively larger reductions in Pi content especially in roots, where the Pi concentration dropped to ~6% under severe P limitation compared to control roots (Figure 1c ). In shoots of severely P-stressed plants, Pi concentration also dropped significantly but was still about ~30% of that in P-replete control shoots (Figure 1c ).
Primary seminal root growth (length) was not significantly different between severely or moderately P-limited plants and control plants (Figure 2a, c, d, e ), although plants under mild P-stress displayed a small but significant (10%) decrease in primary root length (Figure 2b, e ). On the other hand, total root length (i.e. the sum of the lengths of all roots) and root surface area were substantially higher in plants subjected to mild P-stress compared to control plants or plants that received even less Pi (Figure 2f, g ). More lateral roots and greater secondary branching were observed for plants subjected to mild Pi stress (data not shown). Plants under moderate and severe P-stress had greatly reduced total root lengths (83% and 91% reduction, respectively) and root surface area (83% and 85% reduction, respectively) compared to control plants (Figure 2f, g ). Plants subjected to moderate and, especially, severe P-stress produced more and longer root hairs than control plants (Figure 2j, k ), while very few or no root hairs were observed on plants exposed to mild or no P-stress (Figure 2h, i ). In addition, average root diameter decreased in moderately and severely P-stressed roots compared to control and mildly-stressed roots (Figure 2h-k ).