Prior hydration of Brassica tournefortii seeds reduces the stimulatory effect of karrikinolide on germination and increases seed sensitivity to abscisic acid

Abstract

Background and aims: The smoke-derived compound karrikinolide (KAR(1)) shows significant potential as a trigger for the synchronous germination of seeds in a variety of plant-management contexts, from weed seeds in paddocks, to native seeds when restoring degraded lands. Understanding how KAR(1) interacts with seed physiology is a necessary precursor to the development of the compound as an efficient and effective management tool. This study tested the ability of KAR(1) to stimulate germination of seeds of the global agronomic weed Brassica tournefortii, at different hydration states, to gain insight into how the timing of KAR(1) applications in the field should be managed relative to rain events.

Methods: Seeds of B. tournefortii were brought to five different hydration states [equilibrated at 15 % relative humidity (RH), 47 % RH, 96 % RH, fully imbibed, or re-dried to 15 % RH following maximum imbibition] then exposed to 1 nm or 1 microm KAR(1) for one of five durations (3 min, 1 h, 24 h, 14 d or no exposure).

Key results: Dry seeds with no history of imbibition were the most sensitive to KAR(1); sensitivity was lower in seeds that were fully imbibed or fully imbibed then re-dried. In addition, reduced sensitivity to KAR(1) was associated with an increased sensitivity to exogenously applied abscisic acid (ABA).

Conclusions: Seed water content and history of imbibition were found to significantly influence whether seeds germinate in response to KAR(1). To optimize the germination response of seeds, KAR(1) should be applied to dry seeds, when sensitivity to ABA is minimized.

Fig. 1.

Germination of Brassica tournefortii seeds following exposure to 1 nm (closed circles) and 1 µm (open circles) KAR₁. Seeds were equilibrated to 15 % RH, 47 % RH, 96 % RH, were fully imbibed, or were fully imbibed then dried to 15 % (‘re-dried’) prior to exposure to KAR₁. Exposure durations were 0, 3 min, 1 h, 24 h or 14 d. Data are presented on a log scale as binomial estimates with asymmetrical error bars representing a 95 % confidence interval (n = 150 seeds per data point).

Fig. 2.

Imbibition curves for Brassica tournefortii seeds pre-equilibrated to five different hydration states: 15 % RH, 47 % RH, 96 % RH, fully imbibed, or fully imbibed then re-dried to 15 % RH (‘Re-dried’). For clarity, mean data (n = 3 replicates of 50 seeds) are presented with fitted exponential curves (except for fully imbibed, for which a linear curve was fitted) and ±1 s.e. bars are only shown for the final time point.

Fig. 3.

Scanning electron microscope images of Brassica tournefortii seeds at 15 % RH: seed with no history of imbibition (A, C) and seed that was re-dried following 18 h of imbibition (B, D).

Fig. 4.

The endogenous concentration (mean ± 1 s.e., n = 3) of (A) ABA, (B) GA₃ and (C) GA₄ in Brassica tournefortii seeds pre-equilibrated to the five different hydration states: 15 % RH, 47 % RH, 96 % RH, fully imbibed, or fully imbibed then re-dried to 15 % RH (‘Re-dried’).

Fig. 5.

Effect of exogenously applied ABA (A; ABA with 1 µm KAR₁), GA₃ (B) and GA₄ (C) on the germination of Brassica tournefortii seeds. Prior to plating on the relevant agar media, seeds were either dry with no history of imbibition (‘15 % RH’), soaked in water for 18 h prior to treatment (‘fully imbibed’) or soaked for 18 h then re-dried to 15 % RH (‘re-dried’). Data are presented as binomial estimates with asymmetrical error bars representing the 95 % confidence interval (n = 150 seeds per data point).