Growth Inhibition by Lysine Plus Threonine: Comparative Inhibition Profiles between Floury-a and Normal Maize Embryos

Lysine and threonine, either alone or in combination, were testedfor their effects on growth of floury- mutant and DY normalembryos of maize in culture. At 1 mM of lysine, no inhibitoryeffect was observed on either type of embryo, although at 2.5mM slight inhibitions were observed, principally on the rootand shoot lengths. The inhibition profiles by threonine from0.5 to 2.5 mM were similar to those by lysine only in the floury-embryos; in the normal DY embryos, threonine produced a markedinhibition, especially on the root and shoot length. In conjunctinhibition experiments, the profile of inhibition obtained forfloury- embryos by variable levels of lysine with a constantlevel of threonine were always identical to that by variablelevels of threonine with a constant lysine level. On the otherhand, in the normal DY embryos, the extent of the inhibitoryeffect by variable levels of lysine with a Constant threoninelevel was much greater than that by variable levels of threoninewith a constant lysine level. The involvement of homoserinedehydrogenase of the floury- embryo in the lesser sensitivityfor feedback inhibition by threonine is discussed. ¹ Paper No. 112 of the Instituto Fitot?cnico de Santa Catalina,Facultad de Agronomia, Universidad Nacional de La Plata. (Received July 7, 1981; Accepted December 21, 1981)