CELL-WALL POROSITY AND AVAILABLE SURFACE-AREA OF WHEAT-STRAW AND WHEAT-GRAIN FRACTIONS

Interactions between polymers define many of the physical properties o f plant cell walls. The most important of these are available surface area and the related measures of pore size and distribution. Gas adsor ption and mercury porosimetry methods for the measurement of these par ameters in wheat straw, whole wheat grain and grain fractions were com pared. All of the samples examined had a very low surface area regardl ess of the means of measurement. However mercury porosimetry gave valu es 10-fold greater than gas adsorption. It is suggested that the assum ptions made about pore structure necessary for the use of mercury poro simetry do not hold for plant material and that closed pore systems we re distorted by the very high pressures involved. Gas adsorption gave a more realistic assessment of 2-8 m(2) g(-1) with grain having a lowe r available surface than vegetative walls. Pore regimes in both grain and straw were very similar and were determined in grain by the bran f raction. Little structure could be found associated with the starchy e ndosperm. Pores with radii of 1.5-3 nm predominated in wheat cell wall s. These are below the size which would allow free penetration of the wall by degrading enzymes.