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New Effects Of Herbicides On Plants

January 22, 2004

The aim of Navarre engineer Ana Zabalza Azn'¡rez's PhD thesis - entitled "The inhibition of the biosynthesis of amino acids in ramified chain and their use as a target-site for herbicides" - was to find out what effects herbicides produce on the metabolism of plants so as to enable a more rational use of them.

According to lecturer Zabalza Azn'¡rez, herbicides have undergone considerable development since they began to be developed in the fifties of the last century in order to eliminate weeds which interfered with crop cultivation. Amongst the great variety of existing herbicides, affecting different aspects of the metabolism of plants, the authoress of the thesis has analysed those which inhibit the biosynthesis of amino acids in ramified chain.

These herbicides were discovered in the eighties and have experienced great success in the market, given their high effectiveness. Moreover, as they are applied in very small doses to crops, they have a minimum toxic effect amongst animals, given that inhibition of the biosynthesis of amino acids in ramified chain is a process which only takes place in plants and microorganisms, and is why these herbicides do not attack animals.

Nevertheless, Ms Zabalza has pointed out that, despite knowing the target-site of the herbicides that inhibit the biosynthesis of amino acids in ramified chain, its mechanism of action is still under investigation an the exact reason for the death of the treated plants is unknown. In her opinion, research on herbicides should reveal the reasons for the death of the plants treated with the compound, i.e. its mechanism of action.
This is precisely the aim of her PhD work, analysing two chemical groups of herbicides - sulphonylureas and the imidazolinones. The mechanism of action of these herbicides is one of the inhibition of activity of the first enzyme common to the biosynthesis pathway of the three existing amino acids in ramified chain - valine, leucine and isoleucine -: the acetolactate synthase enzyme (ALS).

Thus, the main aim of the PhD thesis was to investigate the mechanism of action of ALS activity-inhibiting herbicides on. To this end the pea was chosen as a target species for study, working in a growth chamber and under controlled conditions of temperature, humidity and period subjected to light. The effects of these herbicides were assessed in the first phases of the treatment, over the first seven days.

Lack of oxygen

The PhD concluded that one of the physiological effects provoked by the inhibiting activity on ALS, and which had not previously appeared in the literature, is that the plant roots thus treated with these herbicides activate a series of enzymes related to metabolism involving oxygen starvation, although the consumption of oxygen of these roots are not affected. In this way, an intensification is produced in the plant roots of the fermenting activity of the enzymes and of that of the alanine aminotransferase, an intensification triggered by a greater availability of piruvate, substrate that the ALS enzyme and these routes share, Ana Zabalza explains.

It has also been shown that plants treated with these herbicides have a greater energy load than those that are not thus treated. Likewise, the treated plants, on ceasing to produce amino acids in ramified chain, commence degrading the pre-synthesised proteins in order to obtain these amino acids and continue synthesising proteins, although this situation inhibits growth.

Another novel aspect arising out of the thesis was the study of an imidazolinone on the antioxidant systems and on the oxidative damage markers. Regarding this, the authoress concluded that a relation between oxidative stress and the mode of action of the inhibitors on ALS activity, although this oxidative stress does not appear to be involved in any major way in the lethality of the herbicides.

Sugars in the leaves

Apart from this, work was done on the more controversial effects of ALS-inhibiting herbicides: the accumulation of carbohydrates in the leaves of treated plants, an effect that is independent of whether the treatment is applied to the leaves or to the roots. The research revealed that, apart from the accumulation of carbohydrates in the leaves, these herbicides triggered the accumulation of carbohydrates in the roots of the plants, from which the authoress concluded that the inhibition of transport from leaves to roots is due to a lack of demand of the latter. Moreover, the results obtained indicate that the lack of growth of the plant is not due to a lack of respirable substrates.

Elhuyar Fundazioa