Silicon uptake by a pasture grass experiencing simulated grazing is greatest under elevated precipitation

Abstract

Background: Grasses are hyper-accumulators of silicon (Si) and often up-regulate Si following herbivory. Positive correlations exist between Si and plant water content, yet the extent to which Si uptake responses can be mediated by changes in soil water availability has rarely been studied and never, to our knowledge, under field conditions. We used field-based rain-exclusion shelters to investigate how simulated grazing (shoot clipping) and altered rainfall patterns (drought and elevated precipitation, representing 50% and 150% of ambient precipitation levels, respectively) affected initial patterns of root- and shoot-Si uptake in a native Australian grass (Microlaena stipoides) in Si-supplemented and untreated soils.

Results: Si supplementation increased soil water retention under ambient and elevated precipitation but not under drought, although this had little effect on Si uptake and growth (tiller numbers or root biomass) of M. stipoides. Changes in rainfall patterns and clipping had strong individual effects on plant growth and Si uptake and storage, whereby clipping increased Si uptake by M. stipoides under all rainfall treatments but to the greatest extent under elevated precipitation. Moreover, above-ground-below-ground Si distribution only changed following elevated precipitation by decreasing the ratio of root:shoot Si concentrations.

Conclusions: Results highlight the importance of soil water availability for Si uptake and suggest a role for both active and passive Si transport mechanisms. Such manipulative field studies may provide a more realistic insight into how grasses initially respond to herbivory in terms of Si-based defence under different environmental conditions.

Keywords: Clipping; Drought; Grasses; Silicon; Simulated herbivory; Water stress.

Fig. 1

Schematic diagram of experimental plots beneath one rain-exclusion shelter (a). Significant treatment effects of rainfall, silicon application (Si) and simulated grazing (Clipping) on soil water content (b), the number of tillers (c) and root mass (d). Significance indicated by *(P < 0.05), **(P < 0.01), ***(P < 0.001). Values are means (± SE). Bars with the same letters were not significantly different (P < 0.05) according to Tukey's post hoc tests. Timeline of experimental events shown (c)

Fig. 2

The effects of rainfall, silicon application (Si) and simulated grazing (Clipping) on soil silicon concentrations (a), Microlaena stipoides root and shoot silicon (b) and carbon (c) concentrations and negative correlations (± 95% CI) between concentrations of silicon and carbon (d). Significance indicated by *(P < 0.05), **(P < 0.01), ***(P < 0.001). Values of bars are means (± SE). Bars with the same letters were not significantly different (P < 0.05) according to Tukey's  post hoc tests