Osmotic Relations and Cell Wall Acidification as the Prerequisites of the Start of Elongation in the Seed Axial Organs

Abstract

In Vicia faba minor seeds, early germination occurs by cell elongation until the axis is 1 cm in length. It is a good model to study some processes which prepare the axial organs for elongation. These processes develop in the water content range from 60% (fr wt), at which physical water absorption is complete, to 72–73%, at which the radicles protrude. The first process is the additional accumulation of osmotically active solutes. This accumulation follows from (1) measurements of osmotic potential in cell sap from axial organs and hypocotyls, (2) estimation of main osmotic components, mostly sugars and K⁺, and (3) examination of vacuole: cytoplasm area ratio. Accumulation of osmotic solutes provides further water inflow into the vacuole necessary for the cell elongation. The second process is acidification of cell wall resulting from H⁺-extrusion due to H⁺-ATPase activation. This follows from: (1) axes begin to acidify the ambient solution 2 h prior to radicle emergence; (2) acidification is inhibited by diethylstylbestrol and stimulated by fusicoccin; (3) fusicoccin stimulates radicle emergence, water uptake and cell elongation as the acid buffer did while vanadate and diethylstylbestrol inhibited radicle emergence. Thus, acidification of cell walls providing wall loosening and acid growth, in combination with active water flow into the cells, allows them to start the elongation.