Evaluating the similarity of cruciferous surrogate weeds to the early development of wild radish (Raphanus raphanistrum L.)

                        Johnny Sanchez and Eric Gallandt

Graduate Research Assistant in Ecology and Environmental Sciences, University of Maine, Orono, ME, USA; Professor of Weed Ecology, University of Maine, Orono, ME, USA

Interim report: February 26th, 2021

Take-home points:

  1. On average, wild radish has a higher maximum anchorage force than the assembled surrogate weeds
  2. However, wild radish also had the smallest root surface area, the second shortest roots, the smallest diameter roots, the smallest root-to-shoot ratio and the fewest number of root tips
  3. The approach to using surrogates could benefit from increased nuance and care when selecting the surrogate species

A more thorough understanding of weed seedling early growth patterns and biomass allocation, as well as attributes such as anchorage forces, height or stem strength may lead to greater mean efficacy and lower variance of PWC (Kurstjens and Kropff 2000). Achieving stable or declining weed pressure is a complex and multi-faceted goal that includes optimizing the timing and adjustments of cultivation tools (Brown and Gallandt 2018). To realize these goals, efficiency in research operations is imperative.
Surrogate weeds are plant species that have primarily been grown as crops but are closely related to wild weed species (Rasmussen 1991) and have been widely used in studies of PWC as substitutes for naturally occurring field weeds (McCollough et al. 2020; Merfield et al. 2017; Page et al. 2012). Surrogate weeds do not share many of the traits that make naturally occurring species difficult for research, such as low and unreliable germination rates (Tricault et al. 2018) and high rates of seed dormancy (Cheam 1986). There has been some confirmation that the responses of surrogate species to PWC are similar to those of ambient weeds (McCollough et al. 2020), though explicit comparisons of biomass allocation and early growth developmental patterns have not yet been made between naturally occurring weed species and their surrogate counterparts (Melander and McCollough 2020). The objective of this study was to determine whether commonly used weed surrogates differed significantly from the naturally occurring wild species for which they are often proxies, by comparing anchorage forces and biomass allocation of root and shoot systems.

Four Brassica surrogate weed species and wild radish (Raphanus raphinstrum L.) were grown in a greenhouse (Figure 1). Before planting, seeds were germinated in an incubator. They were irrigated every day, fertilized with a general purpose 20-20-20 fertilizer (ICL Specialty Fertilizers, Summerville, SC, USA) three times a week, and destructively harvested at the 1st, 2nd, and 3rd leaf stage.
Anchorage force was destructively measured at each developmental stage using a stationary FMI-B50 force gauge (Alluris GmbH & Co., Germany). Plants were pulled vertically at a constant velocity until fully uprooted from the sand (Figure 2).

Four parameters of root architecture were measured using WinRhizo (Version 2003b, Regent Instrument, Quebec, Canada) (Bouma et al. 2000), including root length, root surface area, average root diameter, and number of root tips. Root samples were gently removed from the sand, washed with water, and placed in a 30 cm by 40 cm Plexiglas tray containing a 4 to 6 mm deep layer of water. Roots were separated from plant shoots, spread out with rubber tweezers to minimize root overlap, and scanned using an Epson large-format scanner (Epson Expression 12,000 XL) (Fang et al. 2019) (Figure 3).

For all species, anchorage force increased with each growth stage. Anchorage force did not vary between surrogate species, however wild radish was significantly more well anchored than all surrogate weeds but one (Figure 4).

Wild radish had the smallest root surface area, the second shortest roots, the smallest average diameter roots, the smallest root-to-shoot ratio and the fewest number of root tips. These results are contrary to past research that suggested more complex root systems increase anchorage force (Ennos 1991).

While, in general, the surrogate weed species varied from wild radish in anchorage force and biomass allocation, at a species level, some surrogate species more closely reflected the real weeds. Brassica napus, or canola, had a comparable anchorage force, average root diameter, shoot surface area, root-to-shoot ratio, and average seed mass. These results suggest that the approach to using surrogates could benefit from increased nuance and care when selecting the surrogate species.


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