Physiological consequences of continuous, sublethal imazethapyr supply to pea plants

Imazethapyr (IM) is a herbicide that inhibits the branched-chain amino acid (BCAA) biosynthesis through the specific inhibition of acetolactate synthase activity. This herbicide acts very slowly and several weeks are required for complete plant death. From the BCAA biosynthesis inhibition to the growth inhibition and plant death, the processes involved are not fully understood. Starvation for BCAAs and/or starvation for carbohydrates in sinks. have been proposed as part of the death mechanisms. In this study, a permanent acetolactate synthase inhibition is used in order to (1) determine whether the growth inhibition effects can be attributed to a reduction in BCAA content and/or to starvation of carbohydrates; and (2) to analyse the physiological changes induced. Sublethal doses of IM were continuously supplied in 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, plants were not nitrogen-limited and net photosynthesis was only slightly affected at the higher herbicide concentration. Total soluble sugars and starch were accumulated in both leaves and roots following herbicide supply. These results were also found in non-nodulated, nitrate-fed plants. In relation 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 supply. It is concluded that acetolactate synthase inhibition by continuous, sublethal IM supply does not induce carbohydrate or a specific BCAA starvation in pea plants.