A brief note on growth physiology of plants

The concepts of physiological structure of plant axial organ and its main components are discussed. Their physiological meanings, in particular the role of the surface and the xylem proton pumps are highlighted: the former loosens the cell wall via acidification, and the latter produces the driving force for active uptake of water. Theoretical and experimental examination on the validity of the Lock-hart growth equation is reviewed. Development of a new experimental system, perfusible glycerinated hollow cylinder of cowpea hypocotyl, demonstrates the validity of the Lock-hart type mechanical equation even in such anin vitro system. The pH-dependency of both extensibility and yield threshold offer a strong support for the acid growth theory. A molecular model of cell wall extension is proposed on the basis of these results. The importance of growth regulation via control of the cell wall yield threshold is demonstrated as a very economical way, by an analogy with the performance of electron tube of triode type. Also augmentation of the classic acid growth theory is proposed on the basis of Katou's diagram and the Katou-Furumoto's model of active water uptake.