ABSTRACT

Herbicide bioassay methods that involve paper media are successfully used in crops like soybeans, corn, canola and cotton. The spray bioassay method is the current industry standard for herbicide trait testing in commercial sugar beets. The duration of this spray bioassay takes 30 or more days to complete, depending on greenhouse conditions. Herbicide bioassays are an important tool to determine the presence or level of herbicide traits in a seed lot. The timeliness of receiving herbicide trait purity data on a seed lot is essential for the production, manufacturing and marketing of sugar beet seed. Based on different herbicide bioassays used in the seed industry for other crops, a seed soak bioassay could be an ideal candidate for shortening bioassay duration in sugar beets. The objective of this experiment is to evaluate the accuracy, reliability and repeatability of a seed soak bioassay in assessing herbicide tolerance on sugar beets in 7 days. The seed lots used for this experiment included one susceptible (S) sugar beet seed lot and one known Roundup Ready™ (RR) sugar beet seed lot. Seeds from these two seed lots were mixed into five known ratios of herbicide tolerance and susceptibility, to create five new seed lots. The ratios used were 100% RR, 75 RR:25 S , 50 RR:50 S, 25 RR:75 S and 100% S. Four replications of each ratio were immersed for 24 hours in a 2% solution of Roundup™ Power Max formulation [48.7% active ingredient (a.i.)], for a concentration of 0.974% a.i. After 24 hours of submersion, seeds were removed from the herbicide solution, planted in pleated filter paper (PP) and moistened with water. Bioassays were placed inside a growth chamber set at a constant 20°C, with a photoperiod of 8hr light: 16hr dark, for 7 days. RR and S check samples were planted together with each replication. These check samples were exposed to the same herbicide treatments as the test samples. Water checks were also used as a comparison of normal and abnormal seedlings, as well as any growth rate symptoms. Susceptible seedlings showed severe toxicity symptoms of yellow to brown in color and extreme stunting of the hypocotyl-radicle area. Seedlings with the RR gene had normal growth and development. The results from this experiment indicated that a seed soak bioassay can be used to accurately, reliably, and quantitatively determine herbicide tolerance in sugar beet seed lots. Moreover, the seed soak bioassay can be completed in 7 days compared to the lengthier spray bioassay.