Physiological and leaf ultrastructural characteristics of perennial ryegrass (Lolium perenne L.) biotypes from Tunisia under sulfonylurea herbicide application

- ALS and ACCase inhibitors affected biomass, gas exchange and mesophyll ultrastructure of Tinja susceptible ryegrass.
- MSM-IDS did not affect biomass, gas exchange and mesophyll ultrastructure of Mateur resistant ryegrass.
- Resistance to sulfonylurea in “Mateur” biotype is not attributed to ALS gene target-site mutations.
- Abundant epicuticular waxes and a thick cuticle are responsible for decreased herbicide absorption in resistant ryegrass.
- Local ryegrass exclusionary resistance to MSM-IDS is due to markedly decreased herbicide absorption.

During the last decade, reduced efficacy and failure to control perennial ryegrass (Lolium perenne L.) with labeled rates of Acetolactate Synthase (ALS) inhibitor herbicide (MSM-IDS) were reported. In this context, the effects of MSM-IDS (Sulfonylurea) on suspected resistant (Mateur) and susceptible (Tinja) ryegrass biotypes, prospected in Tunisian wheat fields, were investigated in comparison with those of Pinoxaden, an Acetyl CoA Carboxylase (ACCase) inhibitor herbicide, based on physiological and ultrastructural studies. At their labeled rates, both herbicides significantly affected biomass and gas exchange of Tinja biotype. In Mateur biotype, similar responses as control were observed after ALS inhibitor application. At the molecular level, the most common mutations (P197 and W574) of the ALS gene were not identified in Mateur biotype. Microscopically, a marked collapse of Tinja biotype mesophyll cells was observed by Scanning Electron Microscopy (SEM) after both herbicide applications. However, Mateur biotype mesophyll surface exhibited abundant epicuticular wax deposits and was not affected by ALS inhibitor herbicide. By Transmission Electron Microscopy (TEM), cuticle in Mateur biotype presented a continuous thick lamellate region, while in Tinja biotype, the lamellate region was discontinuous indicating an easy diffusion of MSM-IDS through the cuticle. MSM-IDS resistant Tunisian Mateur ryegrass biotype seems to have developed efficient micromorphological and ultrastructural barriers against sulfonylurea absorption, which in turn allowed a photosynthetic capacity similar to control.