Pith parenchyma plasmoptysis: another tissue tension phenomenon driven by osmotic water fluxes

Tissue tension phenomena, rapid changes in size and/or shape of plant organ segments upon their isolation, commonly are interpreted as indicators of biomechanic conditions that control growth and morphogenesis in the intact organ. Studying tissue tension in homogeneous pith parenchyma isolated from etiolated Castor bean ( Ricinus communis L.) hypocotyls, I found that excisates actually shrank upon isolation, showing ‘spontaneous’ expansion, the classical tissue tension response of isolated inner tissues, only when brought into contact with an exogenous water source. Pith parenchyma segments also were able to expand by recruiting water from other tissues if they remained hydraulically connected. Surprisingly, rapid water-induced elongation to >120% of the original length was followed by shrinkage to below the initial size. This unexpected response depended on external osmolarity, and was caused by plasmoptysis—cell wall rupture—due to excessive osmotic water uptake. While the isolation of pith parenchyma from other tissues is a necessary condition for these effects to occur, none of them can be explained as an elastic response to the release of compressive tissue stresses acting on the pith in intact organs. Instead, the essential role of water fluxes in the causation of tissue tension phenomena is confirmed.