UPTAKE AND BIOCHEMICAL-ANALYSIS OF 2,4-D IN CULTURED ZYGOTIC EMBRYOS OF ZEA-MAYS L

In order to determine the influence of 2,4-D on the regeneration of zy gotic embryos of maize, the uptake and metabolism of 2,4-D by immature embryos were compared in an embryogenic inbred line (A188) and a non- embryogenic inbred line (A632), cultured on induction medium with 2 mg /L 2,4-D that was partly C-14-labelled. Uptake of 2,4-D under exhausti ve conditions, i.e. without subculture, was analyzed from the onset of culture until 14 days of culture. During the so called shock response period, i.e. the first 24 h of culture, uptake of 2,4-D was observed in both lines, with a higher uptake in the embryos of A632. The availa bility of 2,4-D in the medium became a limiting factor for uptake from 3 days of culture onwards. Then the concentration of 2,4-D per gram f resh weight was up to 125 times higher than the concentration in the i nduction medium for both inbred lines. Differences in uptake between t he two lines were observed until 5 days of culture. In this period a l ower concentration of 2,4-D per gram fresh mass was found in A188. Aft er 5 days of culture, however, the concentrations of 2,4-D per gram fr esh mass were comparable in both lines. Therefore, the difference in e mbryogenicity is likely not caused by differences in uptake of 2,4-D. Supplementing TIBA to the induction medium under exhaustive conditions caused a drop in the uptake of 2,4-D in both inbred lines, but TIBA n either inhibited the uptake completely nor prevented the induction of callus. As killed embryos only showed a residual uptake, not influence d by TIBA, it is concluded that immature embryos of the two inbred lin es do accumulate the greater part of the 2,4-D in an active manner. Th e C-14 labelled 2,4-D that had accumulated in cultured embryos was ana lyzed biochemically. Up to 70 % of the radioactive 2,4-D accumulated i n A188 embryos was present as free 2,4-D after 24 h of culture, and 37 % in A632 embryos. Conjugation of 2,4-D to sugars and amino acids sta rted after 16 h of culture. Metabolization of 2,4-D was observed from 2 h onwards and increased with ongoing culture. As compared with A188, A632 embryos showed a higher metabolization rate of 2,4-D. It: might well be that the higher levels of free 2,4-D in A188, together with th e higher metabolization of 2,4-D in A632, do cause differences in the influence of 2,4-D on the 2 inbred lines, i.e. the different response on tissue culture. Thus, it is concluded that the developmental differ ences found in cultured immature embryos of embryogenic and non-embryo genic inbred lines might be caused by differences in levels of free 2, 4-D and metabolization of 2,4-D. This could be regulated by genetic di fferences between the two inbred lines; however, other genetic factors might determine the sensitivity of the embryos to 2,4-D as well.