Seeds of Brassicaceae weeds have an inherent or inducible response to the germination stimulant karrikinolide

Abstract

Background and aims: Karrikinolide (KAR(1)) is a smoke-derived chemical that can trigger seeds to germinate. A potential application for KAR(1) is for synchronizing the germination of weed seeds, thereby enhancing the efficiency of weed control efforts. Yet not all species germinate readily with KAR(1), and it is not known whether seemingly non-responsive species can be induced to respond. Here a major agronomic weed family, the Brassicaceae, is used to test the hypothesis that a stimulatory response to KAR(1) may be present in physiologically dormant seeds but may not be expressed under all circumstances.

Methods: Seeds of eight Brassicaceae weed species (Brassica tournefortii, Raphanus raphanistrum, Sisymbrium orientale, S. erysimoides, Rapistrum rugosum, Lepidium africanum, Heliophila pusilla and Carrichtera annua) were tested for their response to 1 µm KAR(1) when freshly collected and following simulated and natural dormancy alleviation, which included wet-dry cycling, dry after-ripening, cold and warm stratification and a 2 year seed burial trial.

Key results: Seven of the eight Brassicaceae species tested were stimulated to germinate with KAR(1) when the seeds were fresh, and the remaining species became responsive to KAR(1) following wet-dry cycling and dry after-ripening. Light influenced the germination response of seeds to KAR(1), with the majority of species germinating better in darkness. Germination with and without KAR(1) fluctuated seasonally throughout the seed burial trial.

Conclusions: KAR(1) responses are more complex than simply stating whether a species is responsive or non-responsive; light and temperature conditions, dormancy state and seed lot all influence the sensitivity of seeds to KAR(1), and a response to KAR(1) can be induced. Three response types for generalizing KAR(1) responses are proposed, namely inherent, inducible and undetected. Given that responses to KAR(1) were either inherent or inducible in all 15 seed lots included in this study, the Brassicaceae may be an ideal target for future application of KAR(1) in weed management.

Fig. 1.

Climate data for Perth, Northam and Merredin, where seed bags were buried for the period of December 2008 to December 2010: mean monthly temperature range and mean monthly rainfall, as indicated. ‘Total rainfall’ = cumulative rainfall for the entire trial period; ‘No. of rain days’ = total number of days on which ≥1 mm rainfall was recorded during the trial period; ‘No. of rain events’ = number of rain events during the study period, where one ‘rain event’ was defined as a day or consecutive days of ≥1 mm rainfall, bounded by ≥1 day with zero rainfall. All data were taken from the Australian Government Bureau of Meteorology (2010).

Fig. 2.

Germination of eight Brassicaceae weed species in response to 0 or 1 µm KAR₁ (as indicated), with and without 12-h alternating light, at six constant temperatures (10–35 °C) and two alternating temperatures (20/10 °C and 35/20 °C). (A–C) Brassica tournefortii, (D–F) Raphanus raphanistrum, (G–I) Sisymbrium orientale, (J–K) S. erysimoides, (L) Heliophila pusilla, (M) Rapistrum rugosum, (N) Lepidium africanum and (O) Carrichtera annua. Seeds were tested within 1 month of harvest. n = 150 seeds per treatment; vertical bars represent the 95 % confidence interval for binomial estimates.

Fig. 2.

Germination of eight Brassicaceae weed species in response to 0 or 1 µm KAR₁ (as indicated), with and without 12-h alternating light, at six constant temperatures (10–35 °C) and two alternating temperatures (20/10 °C and 35/20 °C). (A–C) Brassica tournefortii, (D–F) Raphanus raphanistrum, (G–I) Sisymbrium orientale, (J–K) S. erysimoides, (L) Heliophila pusilla, (M) Rapistrum rugosum, (N) Lepidium africanum and (O) Carrichtera annua. Seeds were tested within 1 month of harvest. n = 150 seeds per treatment; vertical bars represent the 95 % confidence interval for binomial estimates.

Fig. 3.

Germination response of six Brassicaceae weed species to combinations of 1–3 months of dry after-ripening (DAR) and wet–dry cycling (WD), germinated with or without 1 µm KAR₁ (as indicated), with and without 12-h alternating light at 20/10 °C. Germination was scored 21 d after sowing. (A) Brassica tournefortii, (B) Raphanus raphanistrum, (C) Sisymbrium orientale, (D) S. erysimoides, (E) Heliophila pusilla and (F) Rapistrum rugosum. * indicates significantly higher germination with KAR₁ than without (P < 0·001); (*) indicates significantly higher germination without KAR₁ than with (P < 0·001); numeric values above bars are P-values where P > 0·001 but < 0·05; n = 150 seeds per treatment; vertical bars represent the 95 % confidence interval for binomial estimates.

Fig. 3.

Germination response of six Brassicaceae weed species to combinations of 1–3 months of dry after-ripening (DAR) and wet–dry cycling (WD), germinated with or without 1 µm KAR₁ (as indicated), with and without 12-h alternating light at 20/10 °C. Germination was scored 21 d after sowing. (A) Brassica tournefortii, (B) Raphanus raphanistrum, (C) Sisymbrium orientale, (D) S. erysimoides, (E) Heliophila pusilla and (F) Rapistrum rugosum. * indicates significantly higher germination with KAR₁ than without (P < 0·001); (*) indicates significantly higher germination without KAR₁ than with (P < 0·001); numeric values above bars are P-values where P > 0·001 but < 0·05; n = 150 seeds per treatment; vertical bars represent the 95 % confidence interval for binomial estimates.

Fig. 4.

Germination response of six Brassicaceae species to warm (20 °C) and cold (5 °C) stratification on 1 % (w/v) water agar media for up to 12 weeks. Stratified seeds were subsequently incubated at 20/10 °C for germination, with and without 1 µm KAR₁ (as indicated). (A) Brassica tournefortii, (B) Raphanus raphanistrum, (C) Sisymbrium orientale, (D) S. erysimoides, (E) Heliophila pusilla and (F) Rapistrum rugosum. n = 150 seeds per treatment; vertical bars represent the 95 % confidence interval for binomial estimates.

Fig. 5.

Germination in 12-h alternating light of seed lots recovered from seed burial trials at Perth, Northam and Merredin over a 2 year period. Seeds were germinated with or without 1 µm KAR₁ (as indicated). Seed lots ‘A’ and ‘B’ of Brassica tournefortii (A, B) and Raphanus raphanistrum (C, D) were incubated at 15 °C constant temperature, whilst seed lots ‘A’ and ‘B’ of Sisymbrium orientale (E, F) and seed lot ‘A’ of S. erysimoides (G) were tested at 25 °C. n ≥100 seeds per treatment; vertical bars represent the 95 % confidence interval for binomial estimates.

Fig. 6.

Germination in constant darkness of seed lots recovered from seed burial trials at Perth, Northam and Merredin over a 2 year period. Seeds were germinated with or without 1 µm KAR₁ (as indicated). Seed lots ‘A’ and ‘B’ of Brassica tournefortii (A, B) and Raphanus raphanistrum (C, D) were incubated at 15 °C constant temperature, whereas seed lots ‘A’ and ‘B’ of Sisymbrium orientale (E, F) and seed lot ‘A’ of S. erysimoides (G) were tested at 25 °C. n ≥ 100 seeds per treatment; vertical bars represent the 95 % confidence interval for binomial estimates.