Herbicides that inhibit branched chain amino acid biosynthesis produce a rapid carbohydrate increase in leaves of treated plants. The relationship between these processes is not known nor is the importance of carbohydrate accumulation in the growth inhibition caused by these herbicides. This work analyzes carbohydrate concentration in sources and sinks after herbicide treatments in pea (Pisum sativum L.), as well as photosynthetic carbon assimilation, using two classes of chemicals, chlorsulfuron and imazethapyr, applied to roots or leaves. The most remarkable result was that, in addition to carbohydrate accumulation in leaves, accumulation of sucrose and/or starch in roots was detected. This pattern of carbohydrate accumulation was similar for both herbicides and independent of whether the herbicides were applied to leaves or roots. This indicates that root growth inhibition was not caused by sugar starvation in sinks. Nevertheless, the results are consistent with a decrease in sink strength, leading to the inhibition of photoassimilate translocation.

Acetolactate synthase (ALS; EC 4.1.3.18) inhibition is the primary mechanism of action of imazethapyr (IM). However, the precise mechanisms that links ALS inhibition with plant death have not been elucidated. Supply of IM to pea (Pisum sativum L) plants produced an immediate cessation of growth, caused a 50% inhibition of the in vivo ALS activity within 1 day of treatment, and a remarkable accumulation (2.7-times) of free amino acids after 3 days. Carbohydrates (soluble and starch) were accumulated in both leaves and roots. Accumulation of soluble sugars in roots preceded that of starch in leaves, suggesting that the accumulation of carbohydrates in leaves is not the reason for the arrested root growth. A transient pyruvate accumulation was observed in roots, 1 day after the onset of IM supply. This was coincident with an increase in pyruvate decarboxylase (EC 4.1.1.1), and later increases in alcohol dehydrogenase (EC 1.1.1.1), lactate dehydrogenase (EC 1.1.1.27), and alanine amino transferase (EC 2.6.1.2) activities. This enhancement of fermentative activities was coincident with a slight decrease in aerobic respiration. The overall data suggest that the impairment of ALS activity may lead to a fermentative metabolism that may be involved in growth inhibition and plant death.

The adaptation of the respiratory metabolism in roots of soybean (Glycine max L. Merr. cv Ransom) treated with herbicides that inhibit the enzyme acetolactate synthase (ALS) was analyzed. A new gas phase dual-inlet mass spectrometry system for simultaneous measurement of 34 O 2 to 32 O 2 and O 2 to N 2 ratios has been developed. This system is more accurate than previously described systems, allows measurements of much smaller oxygen gradients, and, as a consequence, works with tissues that have lower respiration rates. ALS inhibition caused an increase of the alternative oxidase (AOX) protein and an accumulation of pyruvate. The combination of these two effects is likely to induce the activation of the alternative pathway and its participation in the total respiration. Moreover, the start of the alternative pathway activation and the increase of AOX protein were before the decline in the activity of cytochrome pathway. The possible role of AOX under ALS inhibition is discussed.There are four main classes of herbicides whose first mechanism of action is the inhibition of the enzyme acetolactate synthase (ALS; EC 4.1.3.18, also known as acetohydroxyacid synthase): imidazolinones, sulfonylureas, triazolopyrimidines, and pyrimidinylsalicilyc acids, with imidazolinones and sulfonylureas the first to be commercialized. ALS is the first common enzyme in the biosynthesis of branched-chain amino acids (BCAAs): Val, Leu, and Ile. This enzyme catalyzes the condensation of either two molecules of pyruvate to form acetolactate in the Leu and Val pathway or one molecule of pyruvate with one molecule of 2-ketobutyrate to form 2-aceto-2-hydroxybutyrate as the first step in the Ile biosynthesis (Singh, 1999). These herbicides cause a significant growth inhibition that is due more to a slower cell division than to an inhibition of cell expansion, although plants stay green for several weeks before death (Wittenbach and Abell, 1999). However, the precise mechanisms that link ALS inhibition with plant death have not been clarified yet. Plants respond quickly to ALS inhibitors by increasing protein turnover to renew BCAAs, and even the critical BCAA pool does not decline to a level that would affect protein synthesis (Wittenbach and Abell, 1999;Royuela et al., 2000). Carbohydrate accumulation in leaves and roots is one of the main symptoms of ALS-inhibiting herbicides in plants treated with imazethapyr (IM), an imidazolinone (Shaner, 1991;Royuela et al., 2000). Gaston et al. (2002) also demonstrated that the increase of soluble carbohydrates in roots can even precede that of starch in leaves, supporting the hypothesis that sugar accumulation in leaves can be due to a decrease in sink strength. In this context, it is surprising to notice that despite the cessation of plant growth and the accumulation of carbohydrates in roots, total root respiration rate (V t ) is unaffected or slightly affected (Ray, 1982; Gaston et al., 2002), indicating the possible occurrence of an impaired and/or regulatory mechanism of respiration in plants ...