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does burning disperse weed seeds

In 2014 and 2015, the impact of wind speed on HI and EBT was assessed using a Stihl BG 55 leaf blower (Stihl Holding AG & Co. KG, Waiblingen, Germany). For this experiment, an anemometer was placed within 10 cm of the windrow, and a leaf blower was positioned to create a predetermined wind speed parallel to the row. An Omega Engineering data logger was placed under the narrow windrow at the time of burning, and temperatures were recorded every 1 s until temperatures peaked. Heat index and EBT calculations were based on the data logger readings in the same manner as described in the previous field experiment.

Seed destruction of weeds in southern US crops using heat and narrow-windrow burning

Effect of Soybean Residue and Wind Speed on Narrow-windrow Burning

Jason K. Norsworthy , 1,* Jeremy K. Green, 2 Tom Barber, 3 Trent L. Roberts, 4 Michael J. Walsh 5

For the soybean harvest residue burning experiment, HI values ranged from a low of 20,800 to a high of 659,000 in soybean. These HI values are generally an order of magnitude higher than those reported for wheat (30.6 × 10 3 ) in Australia (Walsh and Newman 2007). As expected, the amount of soybean residue present at the time of burning did have an effect on HI and EBT (Table 3). The greater the amount of residue, the greater the HI and EBT. Residue amounts following harvest ranged from 1.95 kg m –2 for the 10-row harvested width to 1.78 kg m –2 , 1.63 kg m –2 , 1.40 kg m –2 , 1.26 kg m –2 , and 1.08 kg m –2 for the 9, 8, 7, 6, and 5-row harvested widths, respectively. At the highest residue amount of 1.95 kg m –2 , the EBT 200 was predicted to be 846 s (Table 3), which is considerably longer than the EBT 200 of 264 s for wheat stubble at 2.8 kg m –2 in Australia (Walsh and Newman 2007). It is likely that the wheat stubble in the Australian study had lower moisture content than the soybean residue, hence the longer burn of the soybean residue.

where y = response (HI, EBT 200), B0 = intercept, B1 = regression coefficient for soybean residues (kg m –2 ), and B2 = regression coefficient for wind speed (m s –1 ).

Harvest weed seed control can be implemented by using various tactics, including narrow-windrow burning, chaff carts, the bale-direct system, or impact mills such as the integrated Harrington Seed Destructor (Walsh et al. Reference Walsh, Newman and Powles 2013). The low cost of implementing narrow-windrow burning makes this strategy an attractive option; however, the efficacy of narrow-windrow burning on various weed seeds that may pass through the combine at harvest is unknown and expected to be different for weed species that differ in size. In previous research by Walsh and Newman ( Reference Walsh and Newman 2007), the destruction of rigid ryegrass and wild radish differed with temperature and duration of temperature. The objective of this research was to examine the specific temperature and duration requirements needed to kill the seed of problematic weeds of southern U.S. cropping systems. This research is crucial for estimating the potential efficacy of narrow-windrow burning on weeds common to soybean production systems. Additionally, the efficacy of narrow-windrow burning following soybean grain harvest on Palmer amaranth, barnyardgrass, johnsongrass, and pitted morningglory was evaluated to assess the effectiveness of the tactic in killing seed of these weeds prior to entry into the soil seedbank. It was hypothesized that narrow-windrow burning of soybean harvest residues produced during the harvest of a typical irrigated soybean crop will be successful in destroying seed of major weed species of southern U.S. crops.

A field experiment was conducted at the University of Arkansas Northeast Research and Extension Center (35.6720 N, 90.0844 W; 70 m elev) in Keiser, AR, in 2014 and 2015 in a production field of Credenz 4950LL (Bayer CropScience, St. Louis, MO) soybean grown under irrigated conditions to assess the heat intensity and efficacy of killing the seeds of Palmer amaranth, barnyardgrass, johnsongrass, and pitted morningglory. Because the amount of soybean residue will probably affect the heat intensity of burning, narrow windrows with increasing levels of residue were created by harvesting increasingly wider soybean plots (4.8 to 9.6 m) with a Case 2388 combine (Case IH, Mount Pleasant, WI) fitted with a 9.1-m-wide header. This range in plot widths was equivalent to 5 to 10 soybean rows, where one soybean row was added (0.96 m width) for each increase in plot width. The 5 rows harvested represented a low-yielding environment, and the 10 rows represented a normal yield for a typical irrigated, high-yielding soybean, which was approximately 4,700 kg ha −1 each year. The length of row was in excess of 10 m for each narrow-windrow burn that was evaluated. After harvest, 1 m of row was collected from each narrow-windrow treatment near the end of the 10-m row. Samples were weighed in the field and were returned to the Altheimer Laboratory in Fayetteville to be dried. Just prior to burning, 100 seeds each of Palmer amaranth, barnyardgrass, johnsongrass, and pitted morningglory were placed beneath the windrow on the soil surface in separate 5-cm-diam aluminum tins to assess weed seed kill of the burn treatments. The temperature at the location of the weed seed was recorded every second throughout the burn using an Omega Engineering Type K thermocouple and data logger (Omega® Engineering Inc., Stamford, CT).

Materials and Methods

In 2014 and 2015, the impact of wind speed on HI and EBT was assessed using a Stihl BG 55 leaf blower (Stihl Holding AG & Co. KG, Waiblingen, Germany). For this experiment, an anemometer was placed within 10 cm of the windrow, and a leaf blower was positioned to create a predetermined wind speed parallel to the row. An Omega Engineering data logger was placed under the narrow windrow at the time of burning, and temperatures were recorded every 1 s until temperatures peaked. Heat index and EBT calculations were based on the data logger readings in the same manner as described in the previous field experiment.

For all experiments, data were fit in the FIT MODEL platform in JMP Pro 13 (SAS Institute Inc., Cary, NC).

Understanding the efficacy of narrow-windrow burning in soybean requires that multiple weed seeds, ranging from small to large, be evaluated for their response to combinations of burning temperatures and durations. Other notable weeds of concern would be species such as barnyardgrass (small-seeded grass), johnsongrass (large-seeded grass), and pitted morningglory (large-seeded broadleaf). Like Palmer amaranth, barnyardgrass has been shown to be resistant to multiple herbicide sites of action (Heap Reference Heap 2019). Johnsongrass is considered the most troublesome weed in grain sorghum [Sorghum bicolor (L.) Moench] and corn (Zea mays L.) (SWSS 2012). Johnsongrass has been shown to be resistant to glyphosate in the state of Arkansas (Heap Reference Heap 2019) and can cause substantial yield loss if left untreated in a field. Pitted morningglory is also ranked in the top 10 most troublesome weeds of multiple crops including soybean, corn, and grain sorghum (SWSS 2012, 2013). Pitted morningglory can cause significant yield reduction in soybean (Howe and Oliver Reference Howe and Oliver 1987; Norsworthy and Oliver Reference Norsworthy and Oliver 2002), interfere with harvest, and persist for long periods in the soil seedbank (Egley and Chandler Reference Egley and Chandler 1983).

It is commonly accepted that the balance between abscisic acid (ABA) and gibberellins (GAs) and/or sensitivity to these hormones are responsible for regulation of the dormancy state and germination of seeds in response to environmental signals (Finkelstein et al. 2008; Rodríguez-Gacio et al. 2009). ABA is considered as the most important hormone responsible for the establishment of dormancy during seed development and for maintenance of dormancy during seed imbibition. In turn, GAs have been cited as factors involved in dormancy release and/or germination. Dormancy release has been shown as involving a decline in the ABA content and an increase of the GAs level (Bewley et al. 2013). Participation of ABA and GAs in the regulation of dormancy state has been demonstrated in experiments involving their application. Exogenous GA3 is able to stimulate germination of dormant seeds of many plant species. In contrast, exogenous ABA increases the dormancy depth. Other hormones, e.g., ethylene, cytokinins, and brassinosteroids, have been reported to be involved in releasing seed dormancy and germination (for a review, see Gubler et al. 2005; Finkelstein et al. 2008; Hilhorst 2007). In addition, non-hormonal compounds such as reactive oxygen species (ROS) play an important role during the whole seed lifespan, from embryogenesis to germination (El-Maarouf-Bouteau and Bailly 2008). ROS can have a detrimental effect or can serve a key signaling function in dormancy release and germination. Germination can be possible only when endogenous concentration of ROS reaches a specific suitable level. Several ROS, such as the superoxide anion (O2 − ), hydrogen peroxide (H2O2), and hydroxyl radical (OH), are involved in regulation of dormancy release and germination of various seeds (El-Maarouf-Bouteau and Bailly 2008). Application of exogenous ROS or ROS-generating compounds can break dormancy in seeds of several plant species. An adequate level of ROS depends on their production and scavenging by the enzymatic system and non-enzymatic compounds. The enzymatic system includes superoxide dismutase (SOD), catalase (CAT), peroxidases, glutathione reductase (GR), monodehydroascorbate reductase (MDHAR), and non-enzymatic compounds such as reduced glutathione and ascorbate. ROS have been found to be able to cooperate with ABA, gibberellins, and ethylene in the control of dormancy release and seed germination (Diaz-Vivancos et al. 2013; Corbineau et al. 2014). Once dormancy has been released, the seeds can germinate if the conditions are suitable for the process. The process terminates with radicle protrusion through the external seed envelopes, which ends the sensu stricto germination (phase II) and begins growth (phase III). Radicle protrusion can be accomplished by elongation of the existing cells or may be preceded by cell division and new cell growth (Baíza et al. 1989; de Castro et al. 2000; Barrôco et al. 2005; Masubelele et al. 2005; Gendreau et al. 2008). The cell cycle requires β-tubulin, a microtubular cytoskeleton component. β-tubulin accumulation has been shown to precede, or to occur simultaneously with, DNA replication (Górnik et al. 1997; Śliwińska et al. 1999). De Castro et al. (2001) demonstrated dormant imbibed tomato seeds to remain at the G1 phase; dormancy release was accompanied by accumulation of β-tubulin and induction of DNA replication before radicle protrusion. In barley grains, expression of dormancy coupled with the cell cycle being blocked at the S phase (Gendreau et al. 2012).

Numerous data indicate that fires can stimulate seed germination of many plant species in the fynbos (South Africa), chaparral (Southern California), kwongan (Australia), and in Mediterranean areas (Light and Van Staden 2004). The stimulatory effect of fire may be associated with influence exerted by different physical and chemical factors. High temperature generated by fire may remove seed coat dormancy by producing coat structure scarification, or it can stimulate the embryo directly. Fire also produces plant-derived smoke which can, alone or in conjunction with heat, promote the dormancy release in seeds. De Lange and Boucher (1990) were the first to report that smoke induces seed germination. Subsequent research revealed that smoke breaks dormancy in seeds of several fynbos species which are dominant in the Cape Floristic region (South Africa). Seeds can be treated by direct exposure to smoke, by incubation of untreated seeds on sand, or by imbibition in aqueous smoke extract, the smoke water (SW). The latter is prepared by combusting plant material and bubbling the smoke through the water. A very important finding was that SW is as effective as smoke itself, and that biological activity was independent of plant material used for smoke generation. SW is highly active at very high dilutions and can be stored for long periods at room temperature without any loss to its biological activity (Brown and Van Staden 1997). Smoke or SW was found to stimulate seed germination of species from fire-prone areas. Seeds of 1200 species in more than 80 genera were demonstrated to positively respond to smoke (Dixon et al. 2009). Likewise, dormant and non-dormant seeds from fire-free environments, including agricultural weeds and even crop plants, such as lettuce, celery, tomato, and maize, turned out to be sensitive to smoke, as well. The high biological activity of SW had made it possible to use it in commercial products. African scientists developed a seed primer, marketed as the “Kirstenbosch Instant Smoke Plus Seed Primer”, which incorporates aqueous smoke extracts and a mixture of other natural germination stimulators (Brown and Van Staden 1997). Australian studies developed a commercial product named the “Seed Starter, Australian Smoky Water”. Smoke or SW treatment is widely used not only in plant propagation, but also in ecological restoration (Nelson et al. 2012).

Plant-derived smoke, its water extract—the smoke water (SW), and karrikin (KAR1) present in the smoke stimulate seed germination in plants from fire-prone and fire-free areas, including weeds and cultivated plants. There are also plants, the seeds of which can respond only to smoke, but not to KAR1, and vice versa. Smoke and/or KAR1 can be applied in horticulture, agriculture, and revegetation. This review describes effects of smoke and KAR1 on weed seed germination and focuses mainly on the recent knowledge about the physiological role of these factors in dormancy release and germination of Avena fatua caryopses. The involvement of gibberellins, ethylene, and abscisic acid (ABA) in the response to smoke or KAR1 is discussed. Effects of smoke or KAR1 on the contents of reactive oxygen species (ROS), non-enzymatic antioxidants, and activity of the enzymes participating in ROS removal are presented. Cell cycle activity in the response to SW and KAR1 is also considered. Effects of KAR1 on thermodormancy release in A. fatua caryopses are highlighted, as well.

Biological activity of the plant-derived smoke

For many years, scientists have been looking for chemical(s) responsible for the stimulatory effect of plant-derived smoke. Since such smoke contains ammonia, ethylene, and nitric oxide, it was logical to expect one or more compounds in smoke to be responsible for breaking seed dormancy (Nelson et al. 2012). However, although the compounds mentioned can remove dormancy in seeds of many species responsive to smoke, there are also seeds which do not respond to these compounds, but are nevertheless sensitive to smoke. Thus, it was concluded that smoke produces a specific signal(s) inducing germination. However, as smoke contains several thousand compounds (Maga 1988), it was difficult to isolate the active component(s) from smoke. As it turned out, over 200 compounds extracted from SW did not affect seed germination. Although the role of smoke in germination of dormant and non-dormant seeds, and in seedling growth, has been extensively studied since 1990, it was only in 2004 that a germination-active compound, 3-methyl-2H-furo[2,3-c]pyran-2-one was identified in plant-derived smoke (Van Staden et al. 2004) and burnt cellulose (Flematti et al. 2004). Initially, this compound was termed butenolide; later on, to distinguish the butenolide present in smoke from other butenolides, it was named karrikinolide or karrikin-1 (KAR1) (Flematti et al. 2009; Fig. 1), derived from the word “karrik” meaning smoke in the Australian Aboriginal language. KAR1 has been demonstrated to be active at very low concentrations, in the range of 10 −10 –10 −7 M. It was found to be neither toxic nor genotoxic at 3 × 10 −10 –10 −4 M (Light et al. 2009), and thus, it is safe for animals and humans. KAR1 is produced from d -xylose during fire, in small amounts (Flematti et al. 2015; Fig. 2). Due to a huge demand for the compound in research and on account of its potential application in agriculture, several methods of synthesis using d -xylose or other compounds as substrates have been developed (Fig. 2). KAR1 was shown to stimulate seed germination in fire-prone environments (Flematti et al. 2004; Merritt et al. 2006), in hemi- and holo-parasitic seeds (Daws et al. 2007), and in several Australian Asteraceae species (Merritt et al. 2006). Likewise, seeds of some crop plants such as lettuce, tomato, okra, bean, maize, and rice responded positively to KAR1 (Kulkarni et al. 2011). The compound is able to increase both the rate and percentage of seed germination; it also improves seedling growth. Moreover, and importantly, KAR1 allows seeds to germinate at sub- and supra-optimal temperatures, and also at a low water potential (Light et al. 2009). It can be used as a priming agent, e.g., for tomato seeds. The studies on karrikin structure–activity relationship demonstrated that the methyl group at C-3 is important for biological activity: introduction of methyl at C-4 or C-7 reduces the activity, introduction at C-5 being tolerated well (Nelson et al. 2012). Subsequent studies led to detection of five KAR1 analogs, KAR2–KAR6, in smoke (Flematti et al. 2009; Fig. 1). Concentrations of these compounds were much lower than those of KAR1. Thus, KAR1 was considered to be the major factor responsible for the stimulatory effect of smoke on seed germination. The comparison of KAR1 effects on seed germination with those exerted by other karrikins revealed different, plant species-specific responses to these compounds (Waters et al. 2014).

Viable seeds of numerous plant species are not capable of germinating immediately after harvest under conditions suitable for the germination process. Such seeds are termed primarily dormant. Primary dormancy is established during seed development and maturation on the mother plant. This type of dormancy is particularly common in wild plants. Dormancy is a very important phenomenon which prevents germination on mother plants, facilitates seed dispersal, ensures plant survival of natural catastrophes, and reduces intra-specific competition (Bewley et al. 2013). The phenomenon has turned out to be non-obligatory, its expression depending on environmental factors, e.g., temperature. Seeds can be fully dormant; such seeds are not able to germinate at any temperature (Hilhorst 2007). There are also seeds which are not capable of germination only within a certain temperature range, whereas they germinate at temperatures outside that range. Thus, the expression of dormancy in such seeds depends on temperature, and dormancy release is associated with widening the range of germination temperature. Under natural conditions, primarily dormant seeds are exposed to fluctuating environmental conditions, e.g., light, temperature, moisture, and the presence of gases, which leads to dormancy state cyclicity (Finkelstein et al. 2008). Primary dormancy can be removed also by cold stratification, dry storage, light, or chemicals (Bewley et al. 2013).

Structure of karrikins, glyceronitrile, 3,4,5-trimethylfuran-2(5H)-one, and strigol