Acid extension in vitro of epidermal tissues from Phaseolus vulgaris hypocotyls

Abstract

Frozen–thawed epidermal strips prepared from the first centimetre below the hypocotyl hook of 6-day-old dark-grown Phaseolus vulgaris seedlings were loosened by transfer from a bathing solution of pH 7.5 to solutions ranging from pH 6.0 to pH 2.6 (0.05 M citric acid – 0.10 M disodium phosphate). A minimum stress of between 1.6 and 7.6 g was required for the acid extension of strips that were 4.5 ± 0.5 mm wide. Tissues in acid media initially extended rapidly, but this was followed by only limited long-term extension. Substantial sustained extension rates were only achieved using extreme tension, which was judged to exceed that naturally occurring in these cell walls, or following abrasion of the epidermal surface. Although acidic β-glucosidase, β-galactosidase, β-xylosidase, and α-galactosidase activities were eliminated by boiling for 15 min in ethanol or extraction in 6 M LiCl for 8 h at 4 °C and lowered by 70–80% by extraction in 3 M NaCl for 4 h, total acid extension of strips over 1 h was less affected by these treatments. Although this result suggests that glycosidases and any other similarly soluble enzyme are not responsible for acid wall loosening, it is not considered conclusive.