CHANGES IN COMPOSITION OF THE OUTER EPIDERMAL-CELL WALL OF PEA STEMS DURING AUXIN-INDUCED GROWTH

The gross composition of the outer epidermal cell wall from third inte rnodes of Pisum sativum L. cv. Alaska grown in dim red light, and the effect of auxin on that composition, was investigated using interferen ce microscopy. Pea outer epidermal walls contain as much cellulose as typical secondary walls, but the proportion of pectin to hemicellulose resembles that found in primary walls. The pectin and hemicellulose f ractions from epidermal peels, which are enriched for outer epidermal wall but contain internal tissue as well, are composed of a much highe r percentage of glucose and glucose-related sugars than has been found previously for pea primary walls, similar to non-cellulosic carbohydr ate fractions of secondary walls. The epidermal outer wall thus has a composition rather like that of secondary walls, while still being cap able of elongation. Auxin induces a massive breakdown of hemicellulose in the outer epidermal wall; nearly half the hemicellulose present is lost during 4 h of growth in the absence of exogenous sugar. The perc entage breakdown is much greater than has been seen previously for who le pea stems. It has been proposed that a breakdown of xyloglucan coul d be the basis for the mechanical loosening of the outer wall. This st udy provides the first evidence that such a breakdown could be occurri ng in the outer wall.