The plant cell wall is believed to play an important role in resisting gravity force and supporting the plant body under 1 g on earth, as the bones and muscles in an animal body. However, no direct evidence supporting this idea has been provided by space experiments. We analyzed the cell wall properties of rice coleoptiles and Arabidopsis hypocotyls grown under microgravity conditions during the Space Shuttle STS-95 mission. In space, elongation growth of both organs was stimulated and the cell wall extensibility increased. The increase in the wall extensibility in space was almost completely attributable to the increase in the irreversible extensibility. Also, the levels and the molecular size of the certain matrix polysaccharides, such as (1→3), (1→4)-β - glucans in rice coleoptiles and xyloglucans in Arabidopsis hypocotyls, decreased in space. These modifications in the matrix polysaccharides could be involved in increasing the cell wall extensibility, leading to growth stimulation in space. The results suggest that plant seedlings modify the metabolism of the certain cell wall polysaccharides and thus regulate the cell wall extensibility and the growth rate, thereby adapting different gravity conditions.

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A plant root is organized by three consecutive zones of cell arrays, cell division, cell elongation and cell maturation zones. The gradient of cell age along the root axis may play an important role in the growth of roots intruding into the soil. Since the growth rate of plant cell is limited by cell walls, the cell-wall extensibility of elongation zone is one of the major regulators of root growth. This article overviews the analytical methods of mechanical properties of plant cell walls focusing on the root growth which is affected by pH, gibberellin, aluminum and silicon. We have developed the method to analyze the creep extension of every one-mm zone of root cell wall in vitro by using a Kelvin-Voigt-Burgers' six-element model. The gradient of cell age along root axis and the cell wall extensibility strongly correlated with viscoelastic parameters obtained by this method. The extensibility of root cell walls increased under low pH, with specific decline of viscosity coefficient(η₀). It was also affected by gibberellin, aluminum and silicon treatment. The most remarkably affected was η₀ among the six parameters by these factors. Our goal will be to identify the molecular sites of cell wall polymers that are responsible for these viscoelastic parameters.

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Humans use food that originates from biomaterial through the processes called mastication and swallowing. Because both the food and human organs relating to mastication and swallowing are studied in biorheology, we organized a symposium with four speakers on “Rheology during Food Mastication and Swallowing”. The first speaker, Prof. Yamada, presented observations using video-fluorography during mastication and swallowing. Dr. Kohyama introduced multiple-point sensing of food crushing, and Dr. Takahashi followed the force-deformation curves measured in the human mouth using an artificial tooth sensor. Our fourth speaker, Mr. Nagatoishi, showed food velocity passing through the pharynx by an ultrasound technique.

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A multiple-point sheet sensor (MSS) is a powerful new tool for describing food rheological properties. Texture is a most important factor determining the attributes of food and contributes to the quality and palatability in many foodstuffs. Mechanical and geometrical properties are two main components of texture. As food is heterogeneous material, an MSS was introduced to measure the spatial and temporal distribution of stress applied to food samples. The plane surface of MSS consists of many pressure-sensing cells that detect stress. The MSS system displays stress distribution of a surface of sample food that humans sense using many mechanoreceptors on the skin surface. Stress distribution on an interface between a food sample and a probe was measured during compression testing with a universal testing machine. Changes in bite pressure of human molars or incisors were also measured by inserting the sensor with food between the upper and lower teeth during mastication. The active stress (the ratio of total load to contact area) was a good measure for human tactile sense. The MSS could represent both mechanical and structural properties of samples for wide strain ranges.

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Physical properties of various foods in the mouth were measured during mastication. The masticatory pressure-displacement curves in the mouth reflect the physical properties of foods during mastication. The masticatory pressure-time curves were measured in the artificial first molar tooth embedded with the strain gage. While the displacement-time curves of the first molar tooth were deduced from the change in magnetic field of a small magnet fixed to the artificial molar tooth.
The masticatory pressure-displacement curves from beginning to the end of eating enable the rheological behavior of foods in the natural action of mastication including saliva. The first masticatory pressure-displacement curves were compared with the stress-strain curves measured by a compression machine with compression rate of 80% and a compression speed of 50mm/min.
The maximum pressure on the first bite of raw carrot, raw Japanese radish and cucumber almost agreed with that of the machine measurement. While, the maximum pressure on the first bite of bread, steamed fish paste were higher and that of peanut, cracker and rice cracker were smaller than that of machine measument.

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The passage velocity of various food types in the pharynx when swallowed by healthy individuals was measured using the ultrasonic pulse Doppler method in order to clarify the relationship between the rheological properties and the swallowing of food.The subjects consisted of 23 healthy men and women, and the measured food samples used were: tea, hard gel cut into small pieces, soft gel and thickened solution.
As a result, a distribution figure of the passage velocity of food was obtained and the mean and maximum velocity calculated.
The mean and maximum velocity of soft gel and thickened solution were significantly lower than that of tea and hard gel cut into small pieces (p<0.01). The results show a close relationship between the rheological properties and the passage velocity of food in the pharynx.

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