The physicochemical nature of the cell wall was determined in the fourth internode of three isogenic brittle mutants of barley (Hordeum vulgare L.) and corresponding nonbrittle strains. Cellulose contents of the brittle culms were 17.5 to 20.3% of those of corresponding nonbrittle strains. No major difference was found in lignin and noncellulose components (except glucose) between brittle and nonbnttle strains. Maximum bending stresses of brittle culms were 38.0 to 54.2% of those of corresponding nonbrittle strains. The degree of polymerization of cellulose, measured by viscometry, was similar between the brittle and the nonbrittle strains. Mole number of cellulose molecules in a unit length of brittle culms, calculated by dividing cellulose mass by molecular weight, was 7.7 to 17.3% of those of the nonbrittle strains. These results indicate that brittleness of mutant culms is due to fewer numbers of cellulose molecules in the cell walls.Our previous study (15) of brittle culms of three barley mutants generated by a mutagen and Kamairazu mutant whose genetic background was not known revealed that the mutants contain less cellulose in the cell wall than the normal strains. Comparison between normal and isogenic brittle strains is necessary for genetic and biochemical analyses.The low cellulose content in the mutants could be due to a lower level of UDP-Glc, breakdown of cellulose-forming enzymes, disordered cellulose-forming complex (terminal complex), and low production of cellulose-forming enzymes. Determination of mol wt of cellulose produced in the mutants affords a clue to the mechanism of brittleness and decreased cellulose content. If the mol wt of cellulose in the mutants is less than that in normal strains, less cellulose content results from the lower mol wt. Alternatively, if the mol wt ofcellulose of the mutant is similar to that in the normal strains, then fewer cellulose-forming complexes is the cause. Furthermore, determination of mol wt of cellulose can afford a clue to the complex mechanism of cellulose synthesis including assembly ofcellulose-forming enzymes, interaction ofthe enzymes with plasma membrane, and movement of the enzyme assembly, '
Growth of squash (Cucurbita maxima Duch.) roots was significantly inhibited by 1 mM AlCl3 as early as 1 h after the treatment. The growth inhibition was confined to the elongating zone (1-6 mm from the root tip). Chemical analysis of cell-wall polysaccharides from roots revealed that aluminum increased pectin, hemi-cellulose, and cellulose contents after 3 h of treatment. The effect of aluminum on pectin content was found in the elongating zone including the root tip, whereas change in cellulose content was confined to only nonelongating zones. Hemicellulose content increased in all of the regions along the root axis. The increase in the pectin fraction was due to the increases in uronic acids, galactose, and arabinose constituents, whereas hemicellulose content changed due to increases in glucose, xylose, galactose, and arabinose. The results clearly indicate that aluminum rapidly reduced squash root growth by inhibiting cell elongation and altering metabolism of cell-wall polysaccharides in the nonelongating zone as well as in the elongating zone.
Grass culms are known to differ in breaking strength, but there is little physicochemical data to explain the response. The fourth intemode of four brittle and two nonbrittle barley (Hordeum vulgare L.) strains were used for physical and chemical studies of culm strength. Inner and outer culm diameters of brittle strains (3.6 ± 0.2 and 5.0 ± 0.1 millimeters) were not significantly different from those of nonbritte strains (3.9 ± 0.2 and 5.2 ± 0.2 millimeters). Maximum bending stress, at which the culm was broken, was 192 ± 34 g/mm2 for brittle and 490 ± 38 g/mm2 for nonbrittle strains. Wall thickness and cell dimensions of epidermal, sclerenchyma, and parenchyma cells were measured in culm cross sections. The area of cell wall per unit cell area for each tissue was significantly correlated with the maximum bending stress (r = 0.93 for epidermis, 0.90 for sclerenchyma, and 0.84 for parenchyma). Cell walls of brittle culms had 6 to 64% as much cellulose content as those of nonbrittle culms. Maximum bending stress correlated significantly with cellulose content of the cell walls (r = 0.93), but not with the contents of noncellulosic compounds. The lower cellulose content of the brittle culm was significantly correlated with brittleness.Brittle (fragile) culms have been investigated mainly from the genetical view-point using maize (3), rice (9,15,23), and barley (24,25). The stiff culm of barley has been studied physiologically (6, 7). The maximum bending stress and hardness of the stiff culm was twice that of normal culms (6), although the Young's modulus ofboth culms differed by only 16% (7). The chemical nature of the cell walls of stiff and brittle culms has not been studied. Nagao and Takahashi (15) suggested that there was a lower cellulose content in the cell wall of brittle rice culms without providing experimental evidence.Two hundred and forty barley mutants (OUM2 were produced by treating uzu Akashinriki, a semi-dwarf cultivar, with ethylene methane sulfonate (10). Among them, ' Supported by a Grant-in-Aid for Science Research (No. 63110007) from the Ministry of Education, Science and Culture of Japan.2Abbreviations: OUM, Okayama University mutant; chloramine-T, N-chloro-4-methylbenzensulfonamide sodium salt; mam, maximum bending stress; Pmax, maximum load. OUM 5, 77, 97, 105, 125, 131, 133, 136, 148 showed slower coleoptile elongation than Akashinriki (normal), which had isogenic genes of these mutants except for semidwarf gene. Coleoptile of the mutant strains, including uzu Akashinriki, had a lower IAA content than Akashinriki (normal). Close correlation between growth rate and endogenous IAA content (r = 0.907) suggested that dwarfism ofthese strains was caused by reduced IAA content (8).Three strains (OUM 40, OUM 41, and OUM 42) had slow rates of stem elongation and were found to have brittle culms (1 1), although hormonal studies on the dwarfism has not been completed. Experiments were performed to study the cause of culm brittleness of these three strains by comparing maximum bending st...
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