Seed germination ecology of hood canarygrass (Phalaris paradoxa L.) and herbicide options for its control

Effects of light intensity and temperature

The germination of P. paradoxa (91 to 95%) and wheat (93 to 97%) was not affected by light intensity (data not shown). Our results conform to previous studies which revealed that light intensity had little role in influencing P. paradoxa germination²⁴.

The germination of wheat and P. paradoxa was influenced by temperature regimes (Fig. 1). At temperature regimes of 15/5 °C and 20/10 °C, germination of wheat and P. paradoxa did not vary. Seed germination in wheat remained similar at temperatures ranging between 15/5 °C to 30/20 °C. However, in P. paradoxa, germination was reduced at higher temperature regimes (35/25 C) compared with lower temperature regimes (15/5 °C to 25/15 °C). At the highest temperature regime (35/25 °C), the germination of wheat was 79%, while, at this temperature regime, the germination of P. paradoxa was only 1%. This suggests that wheat can germinate at high-temperature ranges, while, germination of P. paradoxa may be reduced at high temperatures (35/25 °C). These results implied that at the time of planting wheat in Australia if the air temperature is low, the chances of emergence of P. paradoxa are very high. This suggests that efforts should be made towards early control of P. paradoxa in wheat if the air temperature in the winter season falls early. These results also suggest that early planting of wheat could reduce the emergence of P. paradoxa as the prevailing temperature conditions are relatively high in early planting (e.g., end of April). In the Indo-Gangetic Plains, better control of P. minor was observed in the early planting of wheat (high-temperature conditions) due to less emergence of P. minor²⁵.

Previous studies have also revealed that germination of P. paradoxa was highest at 10 °C and then failed to germinate at 30 °C ^24,26, however, these studies were conducted at constant temperatures and the germination response of P. paradoxa was not studied in comparison with wheat in those studies.

Effect of radiant heat

The germination of P. paradoxa seeds that were stored at room temperature (25 °C) was 97%, which reduced to 88% after exposure to the 100 °C pretreatment for 5 min and became nil at 150 °C (Fig. 2). About 88% of P. paradoxa at 100 °C suggests that it can tolerate heat stress for short periods.

Germination was nil at 150 °C and above, suggesting that burning could help in managing P. paradoxa, particularly in a no-till field where seeds are on the soil surface or at shallow depths. Exposure of seeds to fire could inhibit germination by desiccating the seed coat or by damaging the embryo^27,28,29.

Burning of residue in the fields could kill weed seeds and other pests in the topsoil layer³⁰. Windrow burning proved to be an effective tool for killing weed seeds in paddocks³¹. However, the crop residue burning may cause environmental destruction by killing microbes and polluting the air. Also, it reduces the amount of soil organic matter due to the high heat, causing soil degradation. Therefore, these aspects should also be considered while formulating weed management strategies through crop residue burning. Burning may also release the dormancy of other weed seeds present in the subsoil and thus may increase infestation; therefore, this technique should be used cautiously^32,33.

Effect of osmotic stress

Germination of P. paradoxa was highest (95%) in the control treatment and germination reduced to 75% at an osmotic potential of −0.8 MPa, and became nil at −1.6 MPa (Fig. 3). However, in wheat, germination did not reduce with an increase in water potential and it was 94% in the control treatment.

At a very high concentration of PEG, the metabolic activity of P. paradoxa might be reduced due to water stress. Seed germination is affected when seeds are not able to get critical moisture threshold levels for imbibitions^34,35. These results indicate that high water stress may inhibit the seed germination of P. paradoxa. However, under no water stress or mild water stress conditions, P. paradoxa may infest the wheat crop.

Contrary to these results, previous studies reported that germination of P. paradoxa was reduced by 90% at an osmotic potential of −0.25 MPa²⁵. Good germination of wheat at high osmotic potential indicates that the wheat variety used in this study may have water stress tolerance traits for germination. It was observed that wheat could germinate well (75%) at a high-water stress level (−1.6 MPa)³⁶. This suggests that it is possible to menace P. paradoxa by growing stress-tolerant varieties of wheat and manipulating irrigation. In a previous study, less infestation of P. paradoxa was observed in drip-irrigated wheat crops due to optimal soil moisture conditions for the crop³⁷.

Effect of salt stress

Germination of P. paradoxa was highest (93%) in the control treatment, and at a NaCl of 150 mM, germination was reduced to 76% (Fig. 4). Similarly, in wheat, germination was highest (94%) in the control treatment and at a salt concentration of 150 and 200 mM, germination was reduced to 84 and 79%, respectively. These results suggest that at a high salt concentration, P. paradoxa may infest the wheat crop owing to its ability to germinate under high salt concentrations.

Contrary to this, in Iran, it was observed that germination of P. paradoxa was reduced by 70% at a NaCl of 160 mM²⁴. Most of the Australian soils are saline; therefore, it is quite possible that P. paradoxa in Australia might have developed traits for salt tolerance³⁸. The variable response of populations of P. paradoxa to salt concentrations in Iran and Australia might be due to genetic differences between the P. paradoxa populations³⁸. These observations suggest that P. paradoxa could invade the agroecosystem under the saline conditions of Australia.

Effect of seed burial depth on emergence

Germination of P. paradoxa was very low (10%) on the soil surface, and seedling emergence was highest (74%) at a soil burial depth of 0.5 cm (Fig. 5). Seedling emergence was similar when seeds were buried in the soil at a depth ranging from 0.5 to 4 cm. Seedling emergence was 32% at a burial depth of 8 cm.

The results from this experiment suggest that a no-till production system may inhibit the germination of P. paradoxa. This study also suggests that deep tillage (> 4 cm) could reduce the emergence of P. paradoxa to some extent; therefore, inversion tillage could be a weed management strategy if the seedbank is in the shallow layer of the soil. It has been reported that the emergence of small-seeded weeds is reduced from deeper burial depths, as the soil-gas exchange is limited ²¹. However, it is important to know the seed longevity of this weed in different soil and environmental conditions when considering tillage operations³⁹.

Likewise, previous studies also reported that seed germination of P. paradoxa was lowest on the soil surface and no seedlings emerged from a soil depth of 10-cm^2,40. Contrary to this in Iran, germination of P. paradoxa was found to be > 65% on the soil surface ²⁴.

Evaluation of PRE-herbicides

Results revealed that cinmethylin, pyroxasulfone, and trifluralin provided 100% control of P. paradoxa. Atrazine, bixlozone, imazethapyr, isoxaflutole, prosulfocarb + s-metolachlor, and s-metolachlor were not found to be effective against P. paradoxa (Table 1). Pendimethalin and triallate controlled P. paradoxa by 80 and 42%, respectively, compared with the nontreated control.

In wheat, all tested herbicides performed similarly for plant survival except dimethenamid-P and prosulfocarb + s-metolachlor, which caused wheat mortality by 41 and 16%, respectively, compared with the nontreated control. These results suggest that pyroxasulfone, pendimethalin, and trifluralin can be successfully used for the management of P. paradoxa in wheat. Alternative use of these herbicides in wheat crops could provide sustainable weed control of P. paradoxa. In previous studies conducted in Australia, herbicides namely cinmethylin, pyroxasulfone, and trifluralin were found safe for wheat and provided excellent grass weed control⁴¹.

Efficacy of PRE-herbicides in relation to crop residue cover

Cinmethylin, pendimethalin, and pyroxasulfone were proven to be very effective against P. paradoxa under no residue cover conditions (Table 2). However, at the residue cover of 6 t ha^-1 (high output systems), the efficacy of these herbicides decreased and these three herbicides failed to provide effective control of P. paradoxa. At the residue cover of 2 t ha^-1 (low output systems), the efficacy of pyroxasulfone in controlling P. paradoxa was not affected; however, cinmethylin and pendimethalin at the residue load of 2 t ha^-1 did not control P. paradoxa. These results suggest that in a residue-retained, no-till system, pyroxasulfone could provide better control of P. paradoxa compared with cinmethylin and pendimethalin.

The crop residue binds some herbicides, which results in a reduced dose to target weeds and provides poor weed control⁴². A crop residue cover of 1 t ha^-1 may prevent 50% of the herbicide from reaching the target weed seeds in the soil and thus provide poor weed control⁴³.

Efficacy of POST herbicides in relation to plant size

When plants were sprayed at the 4-leaf stage, the herbicides clodinafop and propaquizafop were not effective against P. paradoxa compared with the other tested herbicides (Table 3). The efficacy of clethodim, glyphosate, haloxyfop, and paraquat in controlling P. paradoxa was not decreased even when plants were sprayed at the 10-leaf stage. In previous studies, poor control of P. paradoxa was observed with ACCase-inhibiting herbicides^44,45. These results also suggest that under noncropped or fallow situations, early and late cohorts of P. paradoxa can be controlled successfully by delaying applications of clethodim, paraquat, haloxyfop, and glyphosate.

Germination of P. paradoxa at 25/15 °C (day/night) was lower compared with 20/10 °C. This suggests that early sowing of wheat (relatively high-temperature conditions) could reduce the emergence of P. paradoxa in fields. Phalaris paradoxa did not germinate after exposure to radiant heat of 150 °C (for 5 min), which suggests that burning may be a useful tool for managing P. paradoxa, particularly when seeds are on the soil surface or at the shallow surface. A high level of tolerance of P. paradoxa to water and salt stress was observed. These observations suggest that this weed can dominate under saline and water stress conditions in Australia. Low germination of P. paradoxa was observed on the soil surface, suggesting that a no-till system could provide better control of P. paradoxa. PRE herbicides cinmethylin, pyroxasulfone, pendimethalin, and trifluralin were effective for control of P. paradoxa in wheat; however, under a conservation tillage system, pyroxasulfone provided better control of P. paradoxa compared with other herbicides. Haloxyfop and clethodim were the most effective herbicides among the ACCase-inhibiting herbicides. Under noncropped or fallow land situations, larger plants of P. paradoxa can be successfully controlled with the application of clethodim, glyphosate, and paraquat.

Source: Ecology - nature.com