Differential effects of glyphosate on germination and chlorophyll in Zea Mays plants
Abstract
We studied the glyphosate effect on the germination and chlorophyll content of two corn varieties. The first variety is local "beldi" not improved and the second is a variety selected and imported. Our analysis showed that the imported variety is more tolerant than local. The latter would have been affected by the direct effects of glyphosate by oxidative stress manifesting itself by a strong inhibition of germination and an increased degradation of chlorophyll. However, the imported variety behaved as capable of degrading glyphosate to aminomethylphosphonic acid resulting in improved herbicide resistance.
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Introduction
Since the introduction of glyphosate-resistant plants, products derived from this herbicide have become the most widely used in the world. Glyphosate was considered one of the least toxic herbicides (Williams and al., 2000, Cerdeira and Duke 2006). Aminomethylphosphonic acid (AMPA) is the main degradation product of glyphosate by microorganisms and has been detected in soil and water (Cerdeira and Duke.2006). The Glyphosate effects on the plants physiological processes have been recently examined (Gomes and al., 2014) with deleterious effect on photosynthesis and germination. These effects are thought to be related to oxidative stress factors and it is known that the decrease in photochemical efficiency is associated with a decrease in chlorophyll content (Mateos-Naranjo and al 2009, Zobiole and al.2011). Disruption of mineral nutrient uptake (Mn and Fe) by glyphosate has been shown to affect the biosynthesis of chlorophyll (Cakmak et al., 2009). However, studies on the effects of glyphosate and mineral nutrition are contradictory (Bailey and al, 2002, Rosolem and al., 2010). Reddy et al. 2004, showed that AMPA is responsible for the deleterious effects observed on the biosynthesis of chlorophyll in soybeans. It is important to note that the decrease in chlorophyll content has been observed in plants that do not degrade glyphosate to AMPA (Mateos-Naranjo et al 2009, Huang et al., 2012).Oxidative stress was observed in plants exposed to glyphosate (Ahsan et al 2008), as in many other plants exposed to herbicides. It is widely accepted that modulation of Reactive oxygen species (ROS) metabolism may affect plant physiology by inducing chlorophyll degradation and functional losses of chloroplasts (Pitzschke et al., 2006).
Due to the importance of maize crops in Morocco, its use in animal and human nutrition and the existence of an unimproved local "beldi" variety used by Doukkala'sfarmers, we studied the effects of glyphosate on germination and chlorophyll content in the local variety "beldi" and comparing it with another imported "paulina".
Conclusion
We have demonstrated that the responses of two varieties of Zea Mays with respect to glyphosate are different. The local variety "Beldi" suffered more damage than the imported variety with a sharp reduction in germination and increased degradation of chlorophyll, which would correspond to the direct oxidative effects of glyphosate. However, in the imported variety, the germination capacity and chlorophyll content are less affected, suggesting that this variety could degrade the herbicide to AMPA and tolerate the direct oxidative effects of glyphosate. Additional studies to confirm our results are the determination of content pheophytin of glyphosate treated plants. Pheophytin is one of the degradation products of chlorophyll. Indeed, the pheophytin / chlorophyll ratios inform us if there is an effect on the biosynthesis of chlorophyll and / or on its degradation. In addition, an increase in H2O2 accumulation and lipid peroxidation after exposure to glyphosate should be verified. Finally, to better understand the process involved in the metabolism and accumulation of H2O2, we will study the activities of antioxidant enzymes (SOD, CAT and APX).
