Causes of self-limited translocation of glyphosate in Beta vulgaris plants

Causes of self-induced limitation of glyphosate translocation were studied in sugar beet plants that were susceptible to or tolerant of (Roundup Ready ) glyphosate. Glyphosate was taken up in both types of plants at the same r ate and essentially stopped at the same time, around 4 h after [C-14]glypho sate was applied in 1% Roundup formulation. Tolerant plants continued to ex port glyphosate at a significant rate during the entire 30-h observation pe riod while susceptible plants stopped after only 10 h and exported only hal f the amount. Herbicide was applied under three different experimental prot ocols to help identify causes for the inhibition of carbon and glyphosate t ranslocation. Comparing the effect of 1% Roundup applied to source leaves o f susceptible plants, during either the first half of the day or the end of the day, showed that glyphosate acting directly on source leaves under hig h light essentially stopped translocation. Glyphosate nearly completely inh ibited photosynthesis and carbon export during the remaining 9 h of the lig ht period but produced no observable effects on photosynthesis, C-3-cycle c arbon metabolism, or translocation in tolerant plants. Inhibition of carbon translocation and glyphosate translocation coincided, confirming that glyp hosate export was inhibited by disruption of a process that drives carbon e xport. Applying glyphosate to all source leaves, except the two in which ph otosynthesis and export were bring measured, revealed the effects of sink t issue inhibition on source leaf processes in the absence of direct effects on the source leaves. Import into the monitored sink leaf was reduced by 50 % after about 3 h but the inhibition of import had a relatively small effec t on carbon export and photosynthesis in the two source leaves net directly exposed to the herbicide. The results show that the glyphosate-induced dis ruption of C-3 cycle metabolism under high light was a key factor in the ma rked inhibition of photosynthesis and the rapid halting of carbon and glyph osate translocation in sugar beets. (C)1999 Academic Press.