GROWTH-RESPONSE AND CHANGES IN STARCH FORMATION AS A RESULT OF IMAZETHAPYR TREATMENT OF SOYBEAN (GLYCINE-MAX L)

Decreased dry weight, increased fresh weight, and increased phenylalan ine ammonia-lyase (PAL; EC 4.3.1.15) and tyrosine ammonia-lyase (TAL; EC 4.3.1) activities were found in imazethapyr-treated roots and shoot s of soybean compared to untreated controls. In the treated seedlings, glucose content increased while starch decreased, giving rise to a de cline in the sum of free and allocated glucose. Moreover, ribulosebisp hosphate carboxylase activity (RUBISCO; EC 4.1.1.39) decreased in the treated shoots. The exogenous application of a mixture of valine, leuc ine, and isoleucine with the herbicide treatment prevented the effects on glucose and starch contents and on RUBISCO activity, but not on fr esh and dry weight or PAL and TAL activities. These findings suggest t hat the former are a consequence of the herbicide inhibition of the sy nthesis of the branched-chain amino acids, while the latter could be s tress symptoms.