Growth Analysis of Glyphosate-Resistant and Susceptible Amaranthus palmeri Biotypes

This study examined the growth parameters of both glyphosate-susceptible and glyphosate-resistant biotypes of Amaranthus palmeri, designated as GA2005 and GA2017, respectively. A two-year microplot field study was conducted to assess their growth characteristics. Scheduled destructive harvests on named harvest days (HD) were conducted to collect measurements for further calculation of net assimilation rate (NAR; g m^-2 day^-1), specific leaf area (SLA), leaf weight ratio (LWR), stem-to-leaf ratio (SLR), leaf area index (LAI), leaf area ratio (LAR; cm² g^-1), leaf area duration (LAD; days), relative growth rate (RGR; g.g^-1 day^-1) and plant volume (m³). In addition, stem diameter, number of leaves, and Chlorophyll content (μmol m²) were determined. The main objective was to identify growth parameters that differentiate biotypes along the plant life cycle. While certain growth parameters showed no variation among biotypes, differences in leaf area index (LAI) over HD and chlorophyll content and leaf area duration (LAD) were observed as the main effects. Glyphosate-resistant biotypes exhibited higher LAD and chlorophyll content, potentially conferring a competitive advantage, especially in heavily used glyphosate environments. The study highlights the complexity of intraspecific genetic differentiation, adaptation, and environmental factors affecting A. palmeri. It may offer insights into biotype distinction and resistance spread while advancing our comprehension of species adaptation and growth strategies for enhanced control.