Horseweed is traditionally considered a non-cropland weed. However, populations resistant to glyphosate have eventually become established in no-till agronomic cropping systems. Growth chamber and greenhouse experiments were conducted to compare selected biological and physiological parameters of glyphosate-resistant (GR) and -susceptible (GS) horseweed biotypes from Mississippi with a broader goal of fitness characterization in these biotypes. Vegetative growth parameters (number of leaves, rosette diameter and area, shoot and root fresh weights) were recorded weekly from 5 to 11 wk after emergence and reproductive attributes [days to bolting (production of a flowering stalk) and flowering] and senescence were measured for both GR and GS biotypes under high (24°C/20 °C) and low (18 °C/12 °C) temperature regimes, both with a 13-h light period. Physiological traits such as net photosynthesis, phenolic content, and cell membrane thermostability, all in the presence and absence of glyphosate, and leaf content of divalent cations such as Ca2+ and Mg2+ were assayed in the two biotypes under the high temperature regime. All horseweed vegetative growth parameters except root fresh weight were higher in the high temperature regime compared to that in low temperature regime in both biotypes. Number of leaves, rosette diameter and area, shoot and root fresh weight were 40 vs. 35, 9.3 vs. 8.7 cm, 51 vs. 43 cm2, 3.7 vs. 3.2 g, and 3.5 vs. 4.2 g under high and low temperature conditions, respectively, when averaged across biotypes and weekly measurements. All growth parameters listed above were higher for the GR biotype compared to the GS biotype. Number of leaves, rosette diameter and area, shoot and root fresh weight were 38 vs. 37, 9.1 vs. 8.9 cm, 50.2 vs. 44 cm2, 3.9 vs. 3.1 g, and 4.3 vs. 3.5 g for GR and GS biotypes, respectively, averaged across the temperature treatments and weekly measurements. Reproductive developmental data of these biotypes indicated that the GS biotype bolted earlier than the GR biotype. The GS biotype had more phenolic content and exhibited higher cell membrane thermostability, but less net photosynthetic rate compared to the GR biotype. At 48 h after treatment with glyphosate, there was no change in phenolic content of both GR and GS biotypes. However, glyphosate reduced cell membrane thermostability and net photosynthetic rate more in the GS biotype than that in the GR biotype. Chemical analysis of GR and GS leaf tissue did not reveal any differences in levels of divalent cations such as Ca2+ and Mg2+. Further studies are needed to determine if some of the differences between the two biotypes observed above relate to fitness variation in a natural environment.
Horseweed, also referred to as butterweed, coltstail, fleabane, or marestail, is an annual plant native to North America and Central America [
Glyphosate-resistant (GR) horseweed biotypes from Mississippi were 8- to 12-fold more resistant to glyphosate than a glyphosate-susceptible (GS) biotype [
Any inherent growth and developmental differences between GR and GS biotypes may impact management strategies such as preplant burndown, preemergence, or postemergence herbicide applications. Inadequate control of GR horseweed biotypes could cause a reduction in yield by competing for resources such as nutrients, soil moisture, and sunlight with GR soybean, cotton, or corn plants. Another consequence of inadequate control of GR horseweed biotypes could be plants going to seed and adding to the soil seed bank, thereby, reducing management options for the future. To date, there has been no information on growth and developmental variability differences between GR and GS horseweed biotypes originating from an agronomic cropping region. The objective of this study was to characterize morhpo-physiological differences between noncompetitively-grown GR and GS horseweed biotypes collected from US Midsouth area.
Seeds of GR and GS horseweed biotypes, collected from field-grown plants in Mississippi (Koger et al. 2004), were planted in 12.5 cm by 10 cm by 6 cm plastic trays containing a mixture of field soil (Bosket sandy loam, fine-loamy, mixed, thermic Mollic Hapludalfs) and potting mix (Jiffy Products of America Inc., Batavia, IL, USA) (1:1 by volume). Trays were covered with a plastic wrap and placed in a greenhouse (32˚C/25˚C ± 3˚C day/night temperature; natural light supplemented with light from sodium vapor lamps to provide a 13-h photoperiod). After emergence, seedlings in the cotyledon stage (1 wk after emergence, WAE) were transplanted to individual 10-cm-diam by 10-cm-deep plastic pots containing soil mix described above. Plants were transferred to two different growth chambers calibrated for high (24˚C/20˚C) and low (18˚C/12˚C) day/night temperature regimes. Photoperiod was set at 13 h to coincide at maximum temperature within each regime. Fluorescent and incandescent lamps were used to produce a photosynthetic photon flux density of 600 mmol∙m−2∙s−1. Two weeks after transplanting plants were fertilized with a nutrient solution (W. R. Grace and Co., Fogelsville, PA, USA) containing 200 mg∙L−1 each of N, P2O5, and K2O. The plants were sub-irrigated as needed.
Previous research has shown that horseweed emerges throughout the year, late summer, fall, and spring, with maximum emergence occurring in the fall (September-October) and a new flush emerging in the spring (April- May) under field conditions [
Horseweed growth was slow in the first four weeks following transplanting. Plants were left undisturbed for the four wk after transplanting to accommodate for acclimatization to conditions in the growth chamber. Beginning at four weeks after transplanting (5 WAE), four each of GR and GS plants were randomly selected from the two temperature regimes for measurement of growth parameters. Number of emerged leaves, rosette diameter (mean of horizontal and vertical diameter of rosette), rosette area, shoot fresh weight, root fresh weight, and shoot/root ratio were measured. Measurements were collected weekly from 5 to 11 WAE. Rosette area was measured using a stationary motor-driven leaf-area meter (LI-COR Biosciences, Lincoln, NE, USA). Shoot/root ratio was computed by dividing shoot fresh weight by root fresh weight to estimate proportional partitioning of resources to above ground and below ground plant parts. Collection of data on growth parameters was terminated 11 WAE due to GS plants developing necrotic symptoms, but plants were allowed to grow. Thereafter, days to earliest bolting (production of a flowering stalk), flowering, and senescence were recorded for GR and GS plants from the two temperature regimes.
All measurements were made on horseweed plants (5 to 6 WAE) that had rosettes containing 28 to 32 fully expanded leaves and were grown in the high temperature regime. Photosysnthesis, phenolics, and cell membrane thermostability data were recorded on nontreated and glyphosate-treated plants at 48 hours after treatment with glyphosate at 0.84 kg∙ae∙ha−1. Glyphosate was applied with a moving nozzle sprayer equipped with 8002E nozzles (Spraying Systems Co., Wheaton, IL 60189) delivering 140 L∙ha−1 at 280 kPa. Leaf net photosynthesis was measured using a LI-6400 portable photosynthesis system as described earlier [
All experiments were setup in a completely randomized design with four replications per treatment and were repeated. The data represent the average of the two experimental trials since no experiment by treatment interaction occurred. All data were analyzed with SAS analytical software by subjecting to ANOVA to identify significant main effects and interactions. Data on vegetative growth parameters were further analyzed by fitting regression equations to raw data (SigmaPlot® 9.0, Systat Software Inc. Point Richmond, CA, USA) and means were plotted. Treatment means from the physiological experiments were separated using Duncan’s new multiple range test at the 5% level of probability.
The two horseweed biotypes differed in all the variables evaluated. Both temperature and growth duration affected all aspects of horseweed plant growth and development. Temperature affected the horseweed biotypes differently. As growth progressed from 5 to 11 WAE, the number of leaves produced by the GR biotype was generally higher than that for the GS biotype (
The GR biotype had higher rosette area compared to the GS biotype (
fresh weight measurements, where the GR biotype had greater shoot fresh weight compared to the GS biotype (
Root fresh weight was higher in the GR biotype compared to the GS biotype (
The GS biotype bolted, flowered and began senescence earlier than the GR biotype under both low and high temperature regimes (
There were no differences in leaf net photosynthesis between nontreated GR and nontreated GS plants. However, when treated with glyphosate, photosynthesis was reduced to a greater extent in the GS biotype (78%) compared to the GR biotype (50%) (
In general, herbicide-resistant weed biotypes have been found to be equally fit as their susceptible counter- parts with the exception of triazine-resistant weeds [
Low temperature | High temperature | ||||
---|---|---|---|---|---|
Growth phase | GSa | GR | GS | GR | |
__________________________________________wk after emergence__________________________________________ | |||||
Bolting | 18 | 22 | 16 | 20 | |
Flowering | 28 | -b | 25 | - | |
Senescence | 32 | - | 29 | - |
aAbbreviations: GR, glyphosate-resistant; GS, glyphosate-susceptible; bIndicates that the GR biotype remained in the bolting stage and no further data were collected.
Net Photosynthesis | ||||
---|---|---|---|---|
Plant typeb | Nontreated | Glyphosate-treated | ||
Pnet (µmol CO2 m−2∙s−1) | ||||
GR | 21a | 11a | ||
GS | 18a | 4b | ||
Phenolics | ||||
(µg∙cm−2) | ||||
GR | 135a | 122a | ||
GS | 155a | 157b | ||
Relative injuryc | ||||
(%) | ||||
GR | 34a | 25a | ||
GS | 56b | 29b | ||
Ca | Mg | |||
(%) | ||||
GR | 0.6663a | 0.3138a | ||
GS | 0.8063b | 0.3663b |
aMeans followed by same letter within a column for each main effect are not significantly different at the 5% level of probability according to Duncan’s new multiple range test; bAbbreviations: GR, glyphosate-resistant; GS, glyphosate-susceptible; cRelative injury = 100 ? CMT (cell membrane thermostability.
tive fitness of triazine-resistant smooth pigweed (Amaranthus hybridus L.) populations was 42% to 70% of a triazine-susceptible population, for the fitness component of above-ground biomass production [
There were some differences in biological and physiological growth processes between the GR and GS biotypes.
Impact of growth differences between GR and GS horseweed biotypes reported in this study and possible fitness differences on performance of nonchemical and alternative chemical management programs need to be determined. Further research is required to determine whether interbiotypic or intracrop competitive differences exist between the GR and GS horseweed biotypes in the field. Both biotypes should be grown in replacement series under natural conditions and taken to maturity.