Dr. Loay Al-Zube Scientific Contributions: Measuring Mechanical Tissue Properties of Pith-filled Plant Stems

This research centered on the development of testing protocols that provide accurate and reliable measurements of the mechanical tissue properties of maize stalks. Our study presented for the first time 2 methods for measuring the mechanical tissue properties of maize stalks under compressive loading. Two different compressive modulus values were obtained: E-overall and E-local. The mean and standard deviation values for E-overall and E-local were (10.1 ± 1.5 and 12.8 ± 1.5 GPa, respectively). These test methodologies did not require that end faces were strictly parallel, and both methods produced consistent results (mean repeatability of 4%). Each of these methods possesses unique advantages and disadvantages. The overall compressive modulus technique provides a single, average value for all rind tissue in the specimen, but can be obtained relatively quickly. In contrast, the measurement of local modulus required multiple strain measurements, thus requiring additional tests, but provided results which are likely more accurate. although these measurements were developed and tested for dry maize specimens, the methods and principles introduced in this study are likely applicable for other types of plant stems, such as sorghum, reed, bamboo, etc.

We investigated further to measure the mechanical tissue properties of maize stalks under different loading modes (bending, compression and tensile), and to determine the accuracy and reliability of each test method. The three testing modes produced comparable elastic modulus values. All methods produced modulus of elasticity values that were within the range 6–16 GPa, indicating that each of these methods is effectively measuring the same physical feature of the maize rind, and that each method is relatively accurate. All methods exhibited good test-to-test repeatability values. Bending tests were found to be the fastest and most repeatable measurement approach. However, because the bending test requires a long slender specimen, it produces

an aggregate estimate of the modulus of elasticity. Compression tests require somewhat more sample preparation and testing time, but provide modulus of elasticity values that are more localized. Finally, the tensile testing approach is the most intensive in terms of sample

preparation, but produces the most localized measurement of the modulus of elasticity. This information can be used in creating structural models of maize stalk lodging, and to guide future experiments.

In a subsequent study, we provided for the first time a derivation of bending stress equations that takes into consideration many issues that are relevant to modeling bending stresses in plant stems. Six assumptions that are typically used in the derivation of bending stress equations used to predict mechanical stresses and failure of plant stems. We evaluated the derivation process in order to provide a clearer understanding of the accuracy and limitations of bending stress equations as applied to plant stems. This work involved using finite element models of plant stems to investigate and quantify the effect of each assumption. We derived bending stress equations that takes into consideration many issues that are relevant to modeling bending stresses in plant stems. Equations for stems with variation in cross-sectional stiffness were also developed in this work. We demonstrated that relatively few assumptions were required to obtain bending stress equations that are applicable to plant stems. We derived normal stresses, shear stresses, and combined stresses and presented their accompanying assumptions and notes regarding their applications. Our investigation should allow researchers to make well-informed decisions when modeling plant stems.

• Loay Al-Zube, Wenhuan Sun, Daniel Robertson, and Douglas Cook. The Elastic Modulus of Maize Stems. Plant Methods. 2018 February; 14:11 (DOI: 10.1186/s13007-018-0279-6)
• Loay Al-Zube, Daniel Robertson, Jean Edwards, Wenhuan Sun, and Douglas Cook. Measuring the Compressive Modulus of Elasticity of Pith-filled Plant Stems. Plant Methods. 2017 November; 13:99 (DOI: 10.1186/s13007-017-0250-y).
• J. Stubbs, Navajit. S. Baban, Daniel J. Robertson, Loay Alzube and Douglas Cook. Bending Stress in Plant Stems: Models and Assumptions. In A. Geitmann and J. Grill, editors. Plant Biomechanics: From Structure to Function at Multiple Scales. 2018 Springer (ISBN 978-3-319-79099-2).

This work involved developing a robust testing protocol that provides accurate and reliable measurements of the compressive modulus of elasticity of the rind of pith-filled plant stems. Using compressional testing of dry, non-diseased maize stalk segments consisting of 2 nodes and the intervening internode tested between 2 self-aligning compression plates, we developed 2 methods for measuring the compressional modulus of elasticity of pith-filled node–node specimens. Two different compressive modulus values were obtained for each specimen in this study: E-overall and E-local. The mean and standard deviation values for E-overall and E-local were (10.1 ± 1.5 and 12.8 ± 1.5 GPa, respectively). Both methods had an average repeatability of ± 4%. The effect of sample position within the test fixture was quantified. We demonstrated that sample placement within ± 2 mm of the platen center had no significant effect on compressive modulus measurements made using our developed methods. The contribution of pith tissue to overall stiffness was assessed and was found to be approximately 4%. The two elastic modulus values were calculated using two different strain measurements. The overall compressive modulus technique provides a single, average value for all rind tissue in the specimen, but can be obtained relatively quickly. In contrast, the measurement of local modulus required multiple strain measurements, thus requiring additional tests, but provided results which are likely more accurate.

We also measured the modulus of elasticity of dry, mature maize rind tissues using different loading modes (bending, compression and tensile), and determined the accuracy and reliability of each test method. Our results demonstrated comparable elastic modulus values produced from the 3 testing modes ranged between 6 and 16 GPa. All three testing modes exhibited relatively favorable repeatability (i.e. test-to-test variation of < 5%). Five different compressive modulus values were obtained: E-Bending, E-Compression-overall, E-Compression-local, E-Tensile and E-Tensile-caliper. The mean and standard deviation values were 10.06 ± 1.51 GPa, 10.15 ± 1.47 GPa, 12.87 ± 1.56 GPa, 12.35 ± 1.51 GPa and 11.54 ± 1.37 GPa, respectively. Modulus values of internodal specimens were significantly higher than specimens consisting of both nodal and internodal tissues, indicating spatial variation in the modulus of elasticity between the nodal and internodal regions. Although these measurements were developed and tested for dry maize specimens, the methods and principles introduced in this study are likely applicable for other types of plant stems, such as sorghum, reed, bamboo, etc.

We analyzed, in a subsequent study and for the first time, six assumptions that are typically used in the derivation of bending stress equations used to predict mechanical stresses and failure of plant stems; orthotropic linear elasticity, normal stresses in z-direction are much larger than the normal stresses in x and y directions, bending is locally approximated by an arc, homogenous cross-sectional stiffness, shear stress (τ-yz) is much larger than shear stresses (τ-xz) & (τ-xy), and shear stress is a function of y only. We evaluated the derivation process in order to provide a clearer understanding of the accuracy and limitations of bending stress equations as applied to plant stems. This work involved using finite element models of plant stems to investigate and quantify the effect of each assumption. We derived bending stress equations that takes into consideration many issues that are relevant to modeling bending stresses in plant stems. Equations for stems with variation in cross-sectional stiffness were also developed in this work. We demonstrated that relatively few assumptions were required to obtain bending stress equations that are applicable to plant stems. We derived normal stresses, shear stresses, and combined stresses and presented their accompanying assumptions and notes regarding their applications. Our investigation should allow researchers to make well-informed decisions when modeling plant stems.

Corn (Zea mays L.) is the leading grain crop globally. In the U.S. alone, more than 250 billion tons are harvested annually, generating twice as much revenue as any other crop. Corn is deeply rooted in the global economy, being used in over 42,000 different applications, including the production of consumer goods, foods, pharmaceuticals, livestock feed, and fuel. As new uses and applications are discovered each year, the demand for corn continues to rise. However, 10-20% of this valuable crop is lost every year due to late season crop failure (referred to as stalk lodging).  These losses significantly reduce productivity of farms, negatively impact individual farmers, and affect society at large by creating instability in the overall crop supply which directly affects the cost of consumer goods, commodities trading, fuel, food, and the broader economy. Reasons for crop failure have been challenging for plant geneticists to identify, measure, and control, largely because most crop research focuses on genetic, biological, and agronomic factors, not structural biomechanics. Over the past century, this focus on genetics has dramatically increased the economic importance of corn, increasing yield by over 400%. However, current high-yielding varieties of corn are very susceptible to stalk lodging. This is because yield and stalk size are often related. An increase in stalk size (i.e., a larger diameter) often causes a reduction in yield since the plant is utilizing energy to build a large stalk rather than grain. The solution to this problem lies in our ability to “design” a stalk that provides optimal strength, with minimal biological expense. Thus, our research is focused on identifying, quantifying and ranking key geometric and material features of corn stalk that are highly related to stalk strength but whose modification would only require minimal biological expense.

The many uses of corn, (biofuel, food products, pharmaceuticals, etc.), have triggered a drastically increased demand for corn production. However, yield increases in corn are currently limited by mechanical features and behavior which are not understood by plant scientists. The degree to which our society depends on corn is truly difficult to conceive. It feeds our livestock, fuels our cars and its byproducts are consumed by millions of Americans every day. In addition, its role as renewable resource has led to corn being used in an ever-increasing number of products, thereby increasing demand for worldwide corn production. Of particular importance is corn based ethanol production, which has increased by 3200% since 1982. Because of its ubiquitous use, even marginal improvements to the corn plant have the potential to result in billions of dollars of benefit to society. Stronger stalks will not only offset losses due to drought, disease and climate change, but will support the increased use of corn as a source of renewable energy. The merging of agronomy and biomechanics fields will lead to many scientific advances. While it is impossible to predict exactly what such advances will be it is likely that future collaborative efforts will result in stronger crops, fruit trees, and timber as well as an increased understanding of how genetic and environmental factors interact to determine the structural response of biological tissue.

In 2003, the Energy Biosciences Program (US Dept. of Energy) sponsored a workshop on Plant Systems Biology. Twenty top academic researchers discussed pressing grand challenges such as potential shortages in food and energy production. The primary recommendation of this workshop was a call for closer interactions between biologists and engineers toward the development of in silico (computational) models of plant physiology. Computational plant models provide numerous benefits, including (1) quantitative understanding of physilogical mechanisms; (2) understanding of interplay between various aspects of development; and (3) identifying areas of ignorance and guiding future research. The development of biomechanical models of crops has transformative potential, and is highly aligned with the recommendations and aspirations of top experts. Many important problems in plant physiology (form/function relationships, mechanical adaptation, etc.) can be solved only through the application of biomechanical principles. This project also has translational potential, as evidenced by our close collaborations with Monsanto scientists, which will insure the immediate application of research findings. The study of plant biomechanics provides an exceptional opportunity to obtain data that is appropriate for addressing many questions in human biomechanics. Like humans, plants exhibit wide variations in tissue properties and geometry. However, unlike humans both the genetic makeup and external environment can be directly controlled and manipulated in plants. In addition, plant tissues possess slower decay rates and are drastically reduced in cost as compared to human or animal studies. Thus, they provide an ideal opportunity to obtain extensive, correlated data using a biological system exhibiting many of the same features of human biomechanics but that is drastically reduced in cost, and allows more direct control over experimental variables. Data from our research will serve as a powerful and valuable resource for investigating the effects of biological variability on biomechanical models. This is a matter of pressing importance as data currently used in the creation of biomedical/biomechanical models and simulations is almost always uncorrelated.

Overall, our study promotes the use engineering principles, methodologies, and technologies to promote a closer interactions between biologists and engineers toward the development of in silico (computational) models of plant physiology.