Abstract
Stress-strain experiments using glycerinated hollow cylinders (GHCs) from cowpea hypocotyls indicated the elongation growth is regulated by pH-dependent changes of wall extensibility (φ) and yield threshold tension (y). Recently we found using heat treated GHCs that the φ-regulation was not heat labile but the y-regulation was. Moreover, the φ was increased by EDTA-treatment but decreased by further addition of Ca2+. These suggested the involvement of Ca2+ in the φ-regulation. Pectins were extracted from cowpea hypocotyls and examined biochemical properties. Gel filtration showed pectins consisted of two polysaccharides with molecular masses of 1,600 and 180 kDa. The 1,600 kDa component was shifted to 3,600 kDa when Ca2+ was added. EDTA disrupted the 3,600 kDa polysaccharides to 1,600 kDa, indicating the reversible change of the pectin-calcium bridges. The disruption was also promoted by the acidification of the solution. These results indicate the participation of calcium-bridged pectins in the φ-regulation.
