Effect of different levels of mannitol-induced water stress on the tolerance of cultivated oat (Avena sativa L.) to didofop-methyl

The effect of degree of water stress in Avena sativa on diclofop-methyl efficacy was assessed. Within 24 h of applying stress by adding mannitol to the root medium (0 to 12.5%), the rate of leaf extension of the youngest leaves (leaves 3 and 4) decreased with increasing mannitol concentration. Without water stress, application of diclofop-methyl had little effect on extension rate of leaves 3 and 4 during the first 4 days after spraying. Subsequently, it caused a significant decrease in the extension rate of leaf 4 with more pronounced effects on later leaves. Diclofop-methyl had little effect on leaf extension rate of plants given mannitol. Shoot dry weight at harvest for unsprayed plants decreased with increased mannitol concentration and for sprayed plants it was greater without mannitol than with mannitol (all levels). However, at mannitol concentrations greater than 4%, shoot dry weight for sprayed and unsprayed plants was not significantly different. Sprayed plants exposed to 2–8.5% mannitol produced seed heads but those at zero mannitol did not. When the root medium of all treatments was flushed of mannitol one week after spraying, then main-tained without mannitol, shoot dry weight at harvest for unsprayed plants decreased slightly with increased mannitol concentration applied initially. However, shoot dry weight for sprayed plants increased with increased mannitol concentration. Without mannitol two weeks after spraying, chlorophyll concentrations of leaves 3 and 4 were greater and water saturation deficit (WSD) values were lower for unsprayed plants than sprayed plants but there were no differences at 6.2% mannitol. It is proposed that tolerance to diclofop-methyl by A. sativa as a result of water stress, is primarily due to a decreased rate of leaf expansion resulting in lower demand for membrane synthesis and less strain on membranes damaged by the herbicide.