A. R. Ennos

A Novel Mechanism by which Silica Defends Grasses Against Herbivory

BACKGROUND AND AIMS Previous studies have shown that silica in grass leaves defends them against small herbivores, which avoid high-silica grasses and digest them less efficiently. This study tested the idea that silica can reduce digestibility by preventing the mechanical breakdown of chlorenchyma cells. METHODS Both the percentage of total chlorophyll liberated from high- and low-silica grass leaves by mechanical grinding and the chlorophyll content of locust faeces were measured. KEY RESULTS High-silica grasses released less chlorophyll after grinding and retained more after passing through the gut of locusts, showing that silica levels correlated with increased mechanical protection. CONCLUSIONS These results suggest that silica may defend grasses at least in part by reducing mechanical breakdown of the leaf, and that mechanical protection of resources in chlorenchyma cells is a novel and potentially important mechanism by which silica protects grasses.

A mechanical study of retting in glyphosate treated flax stems (Linum usitatissimum)

The process of retting (the dissociation of the fibre bundles from the central stele by the action of microbes) in two varieties of flax (Linum usitatissimum; cvs. Laura and Escalina) was investigated by studying the morphology and mechanics of plants after treatment with a translocated herbicide, glyphosate. The mechanical changes in the stems were measured on a weekly basis, by carrying out a series of mechanical peel and tear tests on the stem tissue of mature plants. There was a pronounced drop in the moisture content of the stem 14 days after application of the herbicide, from a moisture content of ~60 to 10%. This was consistent with the onset of senescence and subsequent dehydration of the plant tissues. The dehydration of the stem tissues corresponded with an increase in the work required to peel the fibre bundles from the secondary phloem tissue, from 212±7.9 to 539±22 J m−2 (P<0.001). However, 27 days after application of the herbicide there was a more gradual but significant drop in the work required to peel the stem, by ~45% to a mean work to peel of 297±19.8 J m−2. This is thought to be a result of the retting process. This study indicates that peel tests can be used to measure mechanical changes in the interface between the fibre bundles (primary phloem tissue) and the secondary phloem tissue. It enables the progression of retting to be monitored and allows comparisons to be made to determine the optimum harvest time for flax.

The Effects of Mechanical Stimulation on the Morphology and Mechanics of Maize Roots Grown in an Aerated Nutrient Solution

The effects of local mechanical stimulation on local root growth was investigated in maize (Zea mays L.) growing in an aerated nutrient solution. The morphology and mechanical properties of primary nodal roots were compared between those that underwent a program of flexing and those that received no mechanical stimulation. Local mechanical stimulation had limited effects on the morphology of maize nodal roots; there was no significant effect of flexing on the diameter of primary nodal roots when compared with untreated roots. However, there were significant differences in root mass between treatments; there was a large decrease of 43% in the fresh and 41% in the dry mass of the fine roots in the flexed roots compared with the untreated roots. Surprisingly, there was only a small effect on the mass of the primary nodal roots: only at the fifth node was there a significant increase of 15% in the dry mass of flexed roots compared to those that received no mechanical stimulation. The effects of root flexure on the mechanical properties of the roots were more dramatic. Roots at both nodes 4 and 5 were significantly more rigid, stronger, and composed of a stiffer material than those that received no mechanical stimulation. After flexing, the primary nodal roots at the fifth node showed increases of 75% in the rigidity, 60% in the bending strength, and 70% in the bending modulus of the flexed roots compared to unstimulated roots. This study shows that the thigmomorphogenetic response, at least in maize roots, can be localized even down to individual roots and not just to regions of the root system.