Translocation auxin

Both compounds are rapidly translocated herbicides that selectively kill broad-leaved weeds. They mimic the natural auxin, indol-3-ylacetic acid, which is responsible for promoting cell elongation and hence cause unrestrained growth. This uses up all the available nutrients, leading to the death of the plant. A review of the pyridinecarboxylic acids has been published (B-75MI10702). 

Many of the same conclusions may be drawn from results with synthetic auxins as from indole-3-acetic acid water intake increased, there was a lessening of the downward translocation of photosynthate with temporary increases of soluble carbohydrates in the leaves, and alterations appeared in the metabolic ratq and direction and in enzyme activity. However, the direct site of action is not known, and many of the effects are puzzling. n-Fructose oligosaccharides in artichoke and chicory storage-tissue were diminished by 70% (calculated on the content of dry matter) in 6 days... 

Alexander"2 has tried to assess the effect of indole-3-acetic acid, (2,4-dichlorophenoxy) acetic acid, and maleic hydrazide on the soluble carbohydrates and enzyme systems in sugarcane leaves. All of the chemical treatments increased sucrose, total reducing value, D-fructose, and D-glucose in leaves (compared with the controls), with a maximum at nine days after applying about two g. per plant. The indole auxin increased sucrose most, followed by the phenoxy compound and the hydrazide D-glucose increased in the reverse order. Poor translocation from the leaves may have caused the temporary increase in leaf photosynthate. Alterations in the enzyme activities as a result of the chemical treatment are difficult to interpret, partly since so little is known about their relative importance, and partly because the activity in the controls varied by as much as 100% from one sampling period to the next. 

Figure 2. Three-phase-process of auxin herbicide action exemplified for the dicot weed Galium aparine after root-uptake and translocation in the plant (shown in autoradiographs using labelled compound).

Our improvement of this test is based on the incubation of internode sections in the inversed position (9). Under these conditions the substances undergoing test are applied to young auxin-sensitive apical internode tissues and auxin is translocated basipetally toward the zone of curvature (Figure 1). Using this simple improvement the sensitivity of the test to IAA was increased 1,000 times as little as 10 fmoles of IAA can be detected. 

TBA is readily absorbed and translocated by plants. It moves both acropetally and basipetally and causes typical auxin effects in sensitive plants. It strongly inhibits apical growth and leaf formation (Zimmermann and Hitchcock, 1951). It does not cause epinasty in the MCPA-tolerant weeds but stops their growth so that they cannot compete with the crop at the later growth stages. 

These herbicides are auxin antagonists. They inhibit growth in the roots and stems of sensitive grassy weeds. Chlorophyll contents and photosynthetic activity are reduced in sensitive plants. The translocation of the photosynthesis in the roots is also reduced, and the accumulation of sugars increases in the stem (Chow and La Barge, 1978). 

Dyar and Webb (9) presented an interesting hypothesis as a result of their study of the relationship between boron and NAA as it influences translocation of C-labeled endogenous material in bean plants. They theorized that boron plays an essential role in the biosynthesis of auxins in the meristems of the plant, translocation occurring as a result of growth rather than the reverse. ... 

These results suggest that in the short-term collection method basal and apical receivers should be applied simultaneously. By this means, the acropetal component of the transport system is accounted for and the auxin then delivered to basal receivers probably will give a more accurate measure for the density of the basipetal auxin stream. Moreover, the calculation of the acropetal/basipetal ratio may reflect the polarity of hormone translocation. 

Fig. 3.2. A lAA flux into basal agar receivers from 10-mm sections of corn coleoptiles apically supplied with a 5-min pulse of " C-IAA (5 pM in agar). At the time indicated by each column, the sections (30 per transport block) were transferred to new receivers. (Data from Hertel and Flory 1968). B Distribution of mobile auxin in oat coleoptiles apically supplied with a 10-min pulse of " C-IAA (0.7 pM in agar). The mobile auxin concentration was estimated from the radioactivity in basal agar receivers taken from samples of nine coleoptiles subdivided serially into 1-mm sections at the times measured from the start of donor application. Note the differences in translocation of the peak, indicated by arrows, in different regions of the coleoptile. (Data from Newman 1970). C Distribution of radioactivity within 20-mm sections of corn coleoptiles apically supplied with a 15-min pulse of C-IAA (10 pM in agar). The pulse donors were replaced by blocks containing unlabeled lAA at an identical concentration. The sections were cut into successive 2-mm pieces at the times indicated, and the level of tissue radioactivity was measured. Note the differences in translocation of the peak, indicated by arrows, during the two 30-min transport periods. (Data from Goldsmith 1967b)...

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