The influence of microstructural characteristics and cell wall material properties on the mechanical behaviors of different tissues of sorghum stems.

Sorghum stems comprise different tissue components, i.e., rind, pith, and vascular bundles in the rind and pith regions, of different cell morphologies and cell wall characteristics. The overall responses of stems to mechanical loadings depend on the responses of these tissues themselves. Investigating how each tissue deforms to various loading conditions will inform us of the failure mechanisms in sorghum stems when exposed to wind loadings, which can guide the development of lodging-resistant variants. To this end, numerical analyses were implemented to investigate the effects of cell morphologies and cell wall properties on the overall mechanical responses of the above four tissues under tension and compression. Microstructures of different tissues were constructed from microscopic images of the tissues using computer-aided design (CAD), which were then used for finite element (FE) analyses. Shell finite elements were used to model the cell walls, and the classical lamination model was used to determine the overall mechanical responses of cell walls having different fiber composite arrangements. The results from the numerical analyses helped explain how the loading (boundary) conditions, the cell microstructures, the mechanical properties of cell walls of different tissues, the cell wall thickness, the microfibril angle (MFA) of fiber composites of the cell walls, and the turgor pressure affected the overall mechanical responses of the tissues. Tissue stiffening or softening behaviors were attributed to different microstructural deformations, i.e., local or global buckling of cell walls, cell collapse, densifications of cells, or reorientation and rearrangement of cells. The mechanical properties and thickness of cell walls only affected the stiffness and load-bearing ability of the tissues. The turgor pressure affected the compressive responses but its effect on tensile responses was negligible. The MFA had a significant influence on the stiffness and load-bearing ability when the tissues were loaded along their longitudinal axis, but it had an insignificant effect on loading in the transverse direction. Tissues with smaller cell sizes and denser cells were stronger and stiffer than those with larger cell sizes. The numerical simulations also revealed that rind and rind vascular bundles were stiffer and had higher load-bearing ability than pith and pith vascular bundles.

Mechanical stimulation reprograms the sorghum internode transcriptome and broadly alters hormone homeostasis.

Stem structural failure, or lodging, affects many crops including sorghum, and can cause large yield losses. Lodging is typically caused by mechanical forces associated with severe weather like high winds, but exposure to sub-catastrophic forces may strengthen stems and improve lodging resistance. The responses of sorghum internodes at different developmental stages were examined at 2 and 26 h after initiating moderate mechanical stimulation with an automated apparatus. Transcriptome profiling revealed that mechanical stimulation altered the expression of over 900 genes, including transcription factors, cell wall-related and hormone signaling-related genes. IAA, GA1 and ABA abundances generally declined following mechanical stimulation, while JA increased. Weighted Gene Co-expression Network Analysis (WGCNA) identified three modules significantly enriched in GO terms associated with cell wall biology, hormone signaling and general stress responses, which were highly correlated with mechanical stimulation and with biomechanical and geometrical traits documented in a separate study. Additionally, mechanical stimulation-triggered responses were dependent on the developmental stage of the internode and the duration of stimulation. This study provides insights into the underlying mechanisms of plant hormone-regulated thigmomorphogenesis in sorghum stems. The critical biological processes and hub genes described here may offer opportunities to improve lodging resistance in sorghum and other crops.

Thigmostimulation alters anatomical and biomechanical properties of bioenergy sorghum stems.

Sorghum [Sorghum bicolor (L.) Moench] is a tropical grass that can be used as a bioenergy crop but commonly suffers from stem structural failure (lodging) when exposed to mechanical stimuli, such as rain and wind. Mechanical stimulation can trigger adaptive growth in plant stems (thigmomorphogenesis) by activating regulatory networks of hormones, proteins, transcription factors, and targeted genes, which ultimately alters their physiology, morphology, and biomechanical properties. The goals of this study are 1) to investigate differences in the morpho-anatomical-biomechanical properties of internodes from control and mechanically-stimulated plants and 2) to examine whether the changes also depend on the plant developmental stages at the time of stimulation. The sweet sorghum cultivar Della was grown in a greenhouse under two growth conditions: with and without mechanical stimulation. The mechanical stimulation involved periodic bending of the stems in one direction during a seven-week growth period. At maturity, the anatomical traits of the stimulated and non-stimulated stems were characterized, including internode lengths and diameters, and biomechanical properties, including elastic (instantaneous) modulus, flexural stiffness, strength, and time-dependent compliance under bending. The morpho-anatomical and biomechanical characteristics of two internodes of the stems that were at different stages of development at the time of mechanical stimulation were examined. Younger internodes were more responsive and experienced more pronounced changes in length due to the stimulation when compared to the older internodes. Statistical analyses showed differences between the stimulated and non-stimulated stems in terms of both their anatomical and biomechanical properties. Mechanical stimulation produced shorter internodes with slightly larger diameters, as well as softer (more compliant) and stronger stems.

Time-dependent mechanical behavior of sweet sorghum stems.

Grasses represent the most productive and widely grown crop family across the globe but are susceptible to structural failure (lodging) during growth (e.g., from wind). The mechanisms that contribute to structural failure in grass stems are poorly understood due to a lack of systematic studies of their biomechanical behavior. To this end, this study examines the biomechanical properties of sweet sorghum (Sorghum bicolor (L.) Moench), focusing on the time-dependent behavior of the stems. Specifically, we conducted uniaxial compression tests under ramp and creep loading on pith and stem specimens of the sorghum cultivar Della. The tests demonstrated significantly nonlinear and time-dependent stress-strain behavior in all samples. We surmise that this behavior arises from a combination of poroelasticity due to migration of water through the plant and viscoelasticity due to rearrangement of macromolecular networks, such as cellulose microfibrils and lignin matrices. Overall, our measurements demonstrate that sorghum is not a simple reversible elastic material. As such, a complete understanding of the conditions that lead to stem lodging will require knowledge of sorghum's time-dependent biomechanical properties. Of practical importance, the time-dependent biomechanical properties of the stem influence its mechanical stability under various loading conditions during growth in the field (e.g., different wind speeds).