Physiological consequences of continuous, sublethal imazethapyr supply to pea plants

Imazethapyr (1M) is a herbicide that inhibits the branched-chain amino acid (BCAA) biosynthesis through the specific inhibition of acetolactate syntha se activity. This herbicide acts very slowly and several weeks are required for complete plant death. From the BCAA biosynthesis inhibition to the gro wth inhibition and plant death, the processes involved are not fully unders tood. Starvation for BCAAs and/or starvation for carbohydrates in sinks, ha ve been proposed as part of the death mechanisms. In this study, a permanen t acetolactate synthase inhibition is used in order to (1) determine whethe r the growth inhibition effects can be attributed to a reduction iri BCAA c ontent and/or to starvation of carbohydrates; and (2) to analyse the physio logical changes induced. Sublethal doses of IM were continuously supplied i n the nutrient solution of nodulated pea plants. These conditions led to a significant decline in plant growth The herbicide also caused a decline in nodule initiation, but had little effect on nodule development. However, pl ants were not nitrogen-limited and net photosynthesis was only slightly aff ected at the higher herbicide concentration. Total soluble sugars and starc h were accumulated in both leaves and roots following herbicide supply. The se results were also found in non-nodulated, nitrate-fed plants. in relatio n with a likely BCAA starvation, a significant increase was observed in the free amino acid pool, with a marked imbalance among different amino acids, although among BCAAs, only valine pool declined as a consequence of IM sup ply. It is concluded that acetolactate synthase inhibition by continuous, s ublethal IM supply does not induce carbohydrate or a specific BCAA starvati on in pea plants.