Unchanged Molecular-Weight Distribution of Xyloglucans in Outer Tissue Cell Walls along Intact Growing Hypocotyls of Squash (Cucurbita maxima Duch.) Seedlings

The changes in the mechanical properties and compositions ofcell walls in outer and inner tissues were investigated alongthe hypocotyls of squash (Cucurbita maxima Duch.) seedlings.The endogenous growth capacity decreased and the minimum stress-relaxationtime (T_O) of cell walls in outer tissues increased from theapical to the basal region of hypocotyls. A high correlationwas observed between values of To in outer tissues and endogenousgrowth (r=–0.99). The values of T_O in inner tissues didnot change from the apical to the basal region of hypocotyls. In outer tissues, the levels of neutral sugars in pectin decreasedconsiderably from the apical to the basal region of hypocotyls.However, relative amounts of hemicellulose showed little differencealong the hypocotyls. Levels and molecular weights of hemicellulosicxyloglucans in outer tissues were about 2-3 times greater thanthose in inner tissues. The amount of xyloglucans in outer tissuesincreased in the middle region of hypocotyls, and xyloglucansin upper and basal regions had similar molecular weights. Bycontrast, in inner tissues, amounts of cell-wall material decreasedtoward the basal region. Amounts and molecular weights of hemicellulosicxyloglucans also decreased along the hypocotyls. These results clearly show that cell-wall metabolism duringaging of intact growing stem tissues differs markedly betweenouter and inner tissues, and the absence of a simple relationship between the molecular weights of xyloglucans and the mechanicalproperties of the cell walls in outer tissues indicates thatthe changes in the mechanical properties of the cell walls inintact growing tissues cannot be explained only by the molecularweights of xyloglucans. Thus, the regulation of the mechanicalproperties of cell walls in intact growing stems may be somewhatdifferent from that in auxin-treated stem sections, in whichauxin promotes the depolymerization of xyloglucan molecules. (Received November 28, 1991; Accepted November 16, 1992)