Tissue Pressure and Cell Turgor in Axial Plant Organs: Implications for the Organismal Theory of Multicellularity

Abstract

Summary It has been postulated that the rate of growth of axial plant organs (stems, coleoptiles) is controlled by wall-loosening processes that are largely restricted to the expansion-limiting outer epidermal wall (OEW). If this hypothesis is correct it follows that the hydrostatic pressure exerted by the thin-walled, extensible inner tissues against the OEW (tissue pressure) must be similar to the average turgor of the internal cells. Experiments with 5 different plant organs (sunflower hypocotyl, Helianthus annuus L.; zucchini hypocotyl, Cucurbita pepo L.; pea epicptyl, Pisum sativum L.; oat coleoptile, Avena sativa L.; maize coleoptile, Zea mays L.) have shown that tissue pressure, estimated with an osmotic equilibrium method, and cell turgor, as measured with the pressure probe, are of very similar quantities. The osmotic pressure of the tissue sap was significantly larger than tissue pressure and cell turgor. It is concluded that the axial organ consists of a unified, turgid protoplast that is chambered into individual cells (inner tissues) and a thick, growth-controlling, supracellular organ wall (OEW) that bears the longitudinal wall stresses of the internal cells. The results support the organismal concept of multicellularity in plants.