Studies on the Growth and the Osmotic Conditions of Root Hairs

Uppsala

1953

82 S.

zahlr. Lit. Ang.

Serie

400095396

The results presented in this paper form part of my studies of the growth and the function of root hairs. In the experiments presented special interest has been devoted to the osmotic conditions of growing root haris and to studies of the effects of changes of these conditions and the consequences of these changes for the growth and the elongation rate of the hairs. The results are compared with previous experiments concerning root hairs carried out at the Institute by Lundegardh (1946), Korp (1948) and Ekdahl (1948). Finally, the mechanism of root hair elongation has been discussed. 1. Root hairs are cylindrical emergences growing out from the root epidermal cells. To distinguish between the hair-forming cell proper and the hair protruding from this cell, the two parts are referred to as hair-cell and hair. 2. Most of the observations were made on hair-cells and hairs of roots of intact seedlings of spring wheat grown in nutrient solution under artifical illumination. Grown in water or in weak nutrient solution of the standard composition (S-solution, p. 9), the hairs are completely turgid and in this S-solution they reach lengths of 1 to 2 mm, but their diameters are 10 to 15 microns only. The elongation rate and some of the reactions of the root hairs have been studied on hairs grown in flowing media in an observation chamber placed on the stage of a microscope. The main progress of the experimental procedure has been earlier described in detail (lundegardh 1946; Ekdahl 1948), but some modified arrangements are mentioned. The mean elongation rate of the hairs was found very constant and in the S-solution it has been about 90 u/hour (p. 32; Ekdahl 1948, p. 119). 3. Some concepts of somotic quantities habe been defined. The terms used have been chosen for being best suited to explain the osmotic conditions of plant cells. 4. The turgor extension of mature hair-cells was found to be very low, about 1.5 per cent. Growing hair-cells at a distance of 5 mm from the root tips were exteded by the turgor pressure to about 11 per cent (table 1). The volume of mature hair-cells at the incipient plasmolysis stage has been calculated to be about 4.5 per cent lower than the initial volume (p. 11). 5. A method is decribed with which intact root hairs can be plasmolyzed in such a way that the protoplast is loosened from the cell wall at the very hair tip only, and without any loosening of the protoplast from the cell wall in the hair-cell. As plasmolysis proceeds, the protoplast contracts downward in the hair until an equilibrium stage is reached (plate I: fig. 1). The naked apical suface of the protoplast in a plasmolyzed hair retains its capacity to form wall substances (p. 21; plate I: fig. 2). With this method the earliest stage of incipient plasmolysis in the hair apex is easily noticeable. 6. The turgor extension of the cell walss in the ...