COMPARTMENTATION ANALYSIS OF PARAQUAT FLUXES IN MAIZE ROOTS AS A MEANS OF ESTIMATING THE RATE OF VACUOLAR ACCUMULATION AND TRANSLOCATION TOSHOOTS

Efflux analysis conducted after five loading periods of various length s (2, 6, 12, 18, or 24 h) was used to investigate uptake, compartmenta tion, and translocation of [C-14]paraquat in maize (Zea mays L.) seedl ings. The time course for net paraquat uptake (paraquat concentration in uptake solution = 25 mum) into maize roots was linear (56.7 nmol g- 1 root fresh weight h-1) for 24 h. Estimates of changes in paraquat co ntent in the vacuole, cytoplasm, and cell wall after 2-, 6-, 12-, 18-, and 24-h loading periods indicated that the cell wall saturated rapid ly, whereas accumulation of paraquat into the vacuole increased linear ly (1 2.4 nmol g-1 root fresh weight h-1) over 24 h. In contrast to va cuolar accumulation, cytoplasmic paraquat content appeared to approach saturation. The half-time for paraquat efflux from the cell wall (16. 6 min +/-1.2 SD) and cytoplasm (58.8 min +/- 8.9 SD) remained relative ly constant regardless of the length of the loading period, whereas th e half-time for efflux from the vacuole was considerably longer and in creased linearly with increased loading time (6.1-18.7 h). The time co urse for paraquat translocation to the shoot was linear within a 24-h exposure to radiolabeled herbicide, but translocation did not begin un til 5 h after initiation of treatment. The experimental approach used in these experiments provides a valuable method for examining the move ment of paraquat in maize seedlings. Results indicate that the herbici de slowly accumulates in the vacuole of root cells but is also translo cated to the shoot.