Early Vigor and Ozone Response in Horseweed (Conyza Canadensis) Biotypes Differing in Glyphosate Resistance

Abstract

Horseweed has become increasingly difficult to control in the San Joaquin Valley (SJV) of California. Resistance to glyphosate may not fully explain the quasi-invasive behavior of this native species. We contrast glyphosate-resistant (GR) and glyphosate-susceptible (GS) horseweed biotypes for vigor during the vegetative stage and for resistance to ozone (O3). The SJV is impacted by O3 air pollution, which could be a factor in competitiveness of GR vs. GS. Both biotypes were exposed during the seedling and vegetative stages of rosette development to a range of O3 concentrations in greenhouse exposure chambers. Leaf injury was evaluated visually and biomass production and allocation destructively. In O3-free air, the GR biotype exhibited fewer foliar lesions, more vigorous growth, and 40% greater biomass than the GS biotype. The slope of the response to O3 was greater in the GR than in the GS biotype, implying greater relative sensitivity to O3. This was due to greater vigor at low O3, as the biotypes performed similarly at high O3. The competitive advantage of the GR biotype may be reduced in polluted environments. There appeared to be no linkage between the evolution of resistances to O3 and to glyphosate.

Ozone Enhances Adaptive Benefit of Glyphosate Resistance in Horseweed (Conyza canadensis)

“…In contrast, at low O 3 , GS exhibited about 50% damaged leaf area at 46 DAP, increasing to about 70% by 53 DAP (Figure 3a). This contrasted with very low baseline levels (0 to 5%) of injury in both biotypes observed at low O 3 in the absence of glyphosate (Grantz et al 2008).…”

“…The resulting R/S ratio at low O 3 was much greater in GS (Figure 4b). In contrast, in the absence of glyphosate (Grantz et al 2008) R/S was considerably greater in GR.…”

Growth, phenology, and seed viability between glyphosate-resistant and glyphosate-susceptible hary fleabane

“…The same authors reported also the largest capacity of GR plants to accumulate dry mass even in conditions of water deficit. C. canadensis plants resistant to glyphosate under stress conditions of ozone also produced about 40% more biomass compared to susceptible biotypes (Grantz et al 2008). However, studies carried out by Moreira et al (2010) with GR and GS biotypes of C. canadensis and C. bonariensis demonstrated lesser amount of dry mass in any resistant biotype, indicating a negative effect on the ability of development of resistant plants.…”

“…Also, C. canadensis plants started to elongate the main stem earlier in the GR than in the GS biotype (Grantz et al 2008;Shrestha et al 2010). The growth in height is a feature related to competitiveness, where plants with fast development tend to shade the environment, reducing the availability of light and suppressing the smaller plants (Balbinot Junior et al 2003).…”

“…However, others have shown no effect (Westhoven et al 2008;Travlos and Chachalis 2013). Difference among competitive ability have been found in C. canadensis glyphosate resistant (GR) biotype with high early vigour compared to glyphosatesusceptible (GS) biotypes (Grantz et al 2008;Shrestha et al 2007). In another study, although the GR biotype had lower shoot dry mass than the GS biotype when grown alone, in mixed populations of competition, the GR weed were taller and accumulated more dry mass than the GS weeds (Shrestha et al 2010).…”