Geotropic responses in roots 



effect of added auxin on total root elongation will be dominated by the 

 influence of the auxin on the second phase of elongation. We should, there- 

 fore, expect that the effect, if any, which can be recorded within a few minutes 

 would be a retardation of elongation. The effect of a stimulation of the first 

 phase may not contribute materially to the rate of total elongation until much 

 later. An increase in growth rate observed several days after the addition 

 of auxin can further be in part attributed to a possible adaptation to higher 

 auxin concentrations. 



When a root is placed in the horizontal position, the auxin concentration 

 soon increases in the lower half of the root. In view of the above considera- 

 tions, one would expect that the immediate effect of the local excess of auxin 

 on the second phase of cell stretching would dominate over any effect on the 

 first phase, giving rise to an over-all retardation of growth in the lower half 

 of the root. 



It thus seems that we can still apply the classical avixin theory to the 

 geotropic responses of main roots, even to such roots which in long-term 

 experiments have shown a significant acceleration of elongation as a con- 

 sequence of external additions of suitable concentrations of auxin. It is 

 entirelv possible, and indeed most likely, that the observed acceleration of 

 elongation has been preceded by a retardation of growth. The reported 

 cases of an auxin-induced acceleration of the growth of intact roots do not 

 necessitate modifications of the classical auxin theory of geotropism until 

 such accelerations have been found to be manifest within less than 30 or 60 

 minutes (see, for example, Pilet, 1953). 



2. Another problem is the nature of the mechanism which leads to the 

 unequal distribution of auxin in geotropically stimulated roots. It is a widely 

 accepted idea that in horizontally placed roots auxin moves from the upper 

 to the lower side within the root tip itself, and thereby the supply of auxin to 

 the elongation zone increases in the lower half of the root and decreases in 

 the upper. However, it has been suggested that other mechanisms may also 

 be operative. The idea of an acropetal flow of auxin has been put forward 

 by Czaja (1935) and by Pilet (1951). If such a current exists, its role in 

 normal geotropic responses should be considered. It should be made clear 

 whether the acropetal current carries an auxin or an auxin precursor. 



3. A third point is the possibility of a synthesis of auxin as a consequence of 

 geotropic stimulation. Such a synthesis has been demonstrated by Schmitz 

 (1933) in stems of grasses rotating parallel to the horizontal axis of the 

 klinostat. Similarly rotated hypocotyls of Lupimis have been shown to 

 yield more auxin than upright ones (Brain, 1942). Van Overbeek and his 

 co-workers (1945) demonstrated a production of auxin in the so-called growth 

 ring of sugar cane after placing stem portions of this plant in the horizontal 

 position. Banning ( 1 948, p. 4 1 8) has suggested that the geotropically induced 

 synthesis of auxin in nodes of grasses may not be a phenomenon peculiar to 

 this particular organ, but may be of general occurrence. This is a point which 

 should also be studied in the case of roots. Again, however, geotropically 

 induced synthesis of auxin has been demonstrated only in experiments of 

 comparatively long duration. Such synthesis may, therefore, be of no 

 significance for the performance of normal geotropic curvatures which 

 become visible after a few minutes exposure to 1 g. 



79 



