Factors Altering Effectiveness 143 



above the treated leaf. This is a technique similar to that described 

 for the leaf repression test in chapter II. As the auxin is translocated to 

 the tip, growth is inhibited. The extent of inhibition of leaf size is 

 taken as a measure of the extent of translocation. It can be seen in the 

 figure that the inhibition was much greater in the high light intensity 

 than in the low. This is in spite of the fact that less auxin had been 

 absorbed in the light (cf. figure 64.) 



It should be recognized that the acropetal or upward type of 

 translocation which is being measured is not the same as the polai 

 transport system by which normal physiological concentrations of 

 auxins are translocated in stems and leaves. 



Role of Carbohydrates 



The translocation of 2,4-D has some relationship to carbohydrates, 

 for it takes place more readily from well-lighted leaves than from 

 shaded ones, and also more readily from leaf bases than from leaf 

 tips (Weaver and DeRose, 1946). 



Rice has demonstrated that 2.4-D applied in darkness to lower 

 leaves did not alter the subsequent growth of the expanding leaf, even 

 though it was clear that large amounts of 2,4-D had been absorbed at 

 the point of treatment (cf. figure 64). The fact that translocation was 

 achieved only in lighted plants, and with increasing effectiveness as 

 the light intensity was increased, suggests that light may be affecting 

 translocation by production of photosynthetic products. Plants which 

 had been treated in the dark and subsequently moved into the light 

 became capable of translocating the 2,4-D very well (Weintraub and 

 Brown, 1950). 



The effect of light in altering translocation through the formation 

 of sugars was tested directly by Rohrbaugh and Rice (1949). They 

 found that the application of sugar to darkened leaves treated with 

 2,4-D would substitute for light in bringing about translocation 

 of 2,4-D from the treated leaf. This has been confirmed by Weintraub 

 and Brown (1950) who proved that any one of a wide variety of sugars 

 will work equally well in bringing about translocation of 2,4-D. 



Because the transport of sugars is prerequisite to 2,4-D trans- 

 location, factors altering the former may be expected to alter the 

 latter as well. For example, boron is known to be essential for sugar 

 transport in some circumstances (Gauch and Duggar, 1953), and its 

 importance in 2,4-D translocation has likewise been established 

 (Mitchell et al, 1953). A beneficial effect of phosphorus on 2,4-D trans- 

 location in deficient plants has also been reported (Rohrbaugh and 

 Rice, 1954), possibly consequent to facilitating sugar synthesis. 



