The changes in the mechanical properties and compositions of cell walls in outer and inner tissues were investigated along the hypocotyls of squash ( Cucurbita maxima Duch.) seedlings. The endogenous growth capacity decreased and the minimum stress-relaxation time (T O ) of cell walls in outer tissues increased from the apical to the basal region of hypocotyls. A high correlation was observed between values of To in outer tissues and endogenous growth (r=–0.99). The values of T O in inner tissues did not change from the apical to the basal region of hypocotyls. In outer tissues, the levels of neutral sugars in pectin decreased considerably from the apical to the basal region of hypocotyls. However, relative amounts of hemicellulose showed little difference along the hypocotyls. Levels and molecular weights of hemicellulosic xyloglucans in outer tissues were about 2-3 times greater than those in inner tissues. The amount of xyloglucans in outer tissues increased in the middle region of hypocotyls, and xyloglucans in upper and basal regions had similar molecular weights. By contrast, in inner tissues, amounts of cell-wall material decreased toward the basal region. Amounts and molecular weights of hemicellulosic xyloglucans also decreased along the hypocotyls. These results clearly show that cell-wall metabolism during aging of intact growing stem tissues differs markedly between outer and inner tissues, and the absence of a simple relationship between the molecular weights of xyloglucans and the mechanical properties of the cell walls in outer tissues indicates that the changes in the mechanical properties of the cell walls in intact growing tissues cannot be explained only by the molecular weights of xyloglucans. Thus, the regulation of the mechanical properties of cell walls in intact growing stems may be somewhat different from that in auxin-treated stem sections, in which auxin promotes the depolymerization of xyloglucan molecules.
Copyright (C) 1993, The Japanese Society of Plant Physiologists
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