Ecological fitness of a multiple herbicide-resistant Lolium rigidum population: dynamics of seed germination and seedling emergence of resistant and susceptible phenotypes.
Quantification of fitness differences between herbicide-resistant and herbicide-susceptible weeds permits better prediction of herbicide resistance, and the design of weed management strategies to exploit those traits that result in reduced ecological performance. Reported here is the first attempt to compare the germination and seedling emergence characteristics of one herbicide-susceptible and two herbicide-resistant phenotypes from a single weed population. A series of experiments was carried out in controlled conditions to study seed germination and emergence in Lolium rigidum phenotypes possessing target-site and non-target-site mechanisms of herbicide resistance. Lines composed of herbicide-susceptible (S), metabolism-based (P450) and target site-based (ACCase) resistant individuals, isolated from a single multiple-resistant population (SLR31), were used in the comparative experiments. No major differences in seedling emergence were found among the phenotypes when exposed to an alternating 25/15°C cycle with a 12-hourly photoperiod. However, the absence of light associated with soil burial (1-8 cm) markedly inhibited total germination and seedling emergence in the ACCase phenotype compared with the S and P450 phenotypes. Germination at constant temperatures was also inhibited in seeds of the ACCase phenotype, which showed the highest base temperature (Tb) for germination and required more time to reach 50% emergence (tE50) than the S and P450 phenotypes. Seedling emergence from deep burial (8 cm) in soil promoted significantly higher fatal germination in the S and P450 phenotypes compared with the ACCase phenotype. Despite fatal germination, the S phenotype produced greater emergence from deep burial than both herbicide-resistant phenotypes. Synthesis and applications. This study demonstrates differential germination and emergence responses to light and thermal environments between herbicide-susceptible phenotypes and phenotypes possessing target-site (ACCase) and non-target site (P450) herbicide resistance within a single L. rigidum population. Shallow seed burial (1 cm) by any cultivation tool will potentially inhibit seedling recruitment of the ACCase phenotype in contrast with the S and P450 phenotypes, which require less restricted conditions to germinate and emerge.