The autumn effect: timing of physical dormancy break in seeds of two winter annual species of Geraniaceae by a stepwise process

Abstract

Background and aims: The involvement of two steps in the physical dormancy (PY)-breaking process previously has been demonstrated in seeds of Fabaceae and Convolvulaceae. Even though there is a claim for a moisture-controlled stepwise PY-breaking in some species of Geraniaceae, no study has evaluated the role of temperature in the PY-breaking process in this family. The aim of this study was to determine whether a temperature-controlled stepwise PY-breaking process occurs in seeds of the winter annuals Geranium carolinianum and G. dissectum.

Methods: Seeds of G. carolinianum and G. dissectum were stored under different temperature regimes to test the effect of storage temperature on PY-break. The role of temperature and moisture regimes in regulating PY-break was investigated by treatments simulating natural conditions. Greenhouse (non-heated) experiments on seed germination and burial experiments (outdoors) were carried out to determine the PY-breaking behaviour in the natural habitat.

Key results: Irrespective of moisture conditions, sensitivity to the PY-breaking step in seeds of G. carolinianum was induced at temperatures ≥20 °C, and exposure to temperatures ≤20 °C made the sensitive seeds permeable. Sensitivity of seeds increased with time. In G. dissectum, PY-break occurred at temperatures ≥20 °C in a single step under constant wet or dry conditions and in two steps under alternate wet-dry conditions if seeds were initially kept wet.

Conclusions: Timing of seed germination with the onset of autumn can be explained by PY-breaking processes involving (a) two temperature-dependent steps in G. carolinianum and (b) one or two moisture-dependent step(s) along with the inability to germinate under high temperatures in G. dissectum. Geraniaceae is the third of 18 families with PY in which a two-step PY-breaking process has been demonstrated.

Fig. 1.

Percentage of imbibed seeds (mean ± s.e.) of (A–F) G. carolinianum and (G–L) G. dissectum at constant temperature and at 20/10 °C after dry storage at different constant temperatures (15–40 °C) for 1 – 5 months. An asterisk indicates a significant difference in imbibition between the two incubation temperatures in each month. Different upper- and lower-case letters indicate a significant difference between different months in imbibition under a constant temperature and under 20/10 °C, respectively (P < 0·05).

Fig. 2.

Percentage of imbibed seeds (mean ± s.e.) of (A–F) G. carolinianum and (G–L) G. dissectum at alternating temperature and at 20/10 °C after dry storage at different alternating temperatures for 1–5 months. An asterisk indicates a significant difference in imbibition between the two incubation temperatures in each month. Different upper- and lower-case letters indicate a significant difference between different months in imbibition under a particular alternating temperature and under 20/10 °C, respectively (P < 0·05).

Fig. 3.

Percentage of imbibed seeds (mean ± s.e.) of G. carolinianum incubated at different constant temperatures after dry storage (A–C) at 40–25 °C and (D–F) at 30 °C. Different letters indicate significant differences in imbibition percentages between the incubation temperatures (P < 0·05).

Fig. 4.

Percentage of imbibed seeds (mean ± s.e.) of (A–C) G. carolinianum and (D–F) G. dissectum stored under dry and wet conditions (as indicated) under temperature sequences: (A, D) alternating; (B, E) daily maximum; and (C, F) daily minimum temperature simulating natural temperature conditions of each month. Different upper- and lower-case letters indicate significant differences in imbibition percentages between the incubation temperatures under dry storage and wet storage, respectively (P < 0·05).

Fig. 5.

Percentage of imbibed seeds (mean ± s.e.) of G. dissectum at 40 °C, after storage under different moisture regimes at 40 °C. The moisture conditions of each week during the storage period are indicated on the x-axis: W, wet; and D, dry. Different letters indicate significant differences between imbibition percentages within each moisture regime (P < 0·05).

Fig. 6.

PY-breaking of G. carolinianum and G. dissectum seeds under greenhouse conditions in 2010 and 2011. (A, D) Daily minimum and maximum air temperatures in the non-heated greenhouse in 2010 and 2011, respectively. Percentage (mean ± s.e.) of seeds with colour change from dark brown to brownish orange in the water-gap region of G. carolinianum and G. dissectum under dry conditions, (B) in 2010 and (E) 2011. Seed germination percentage (mean ± s.e) in G. carolinianum under the three moisture regimes in (C) 2010 and (F) 2011. (G) Seed germination percentage (mean ± s.e.) of G. dissectum in 2011 under the three moisture regimes.

Fig. 7.

PY-breaking in buried seeds of G. carolinianum in 2010/11 and G. dissectum in 2011. (A, I) Daily minimum and maximum soil temperatures at 2 cm soil depth in 2010 and 2011, respectively. Percentage (mean ± s.e.) of permeable seeds of G. carolinianum of (B) fresh seeds and after incubation at 20 °C; (C–G) at the time of exhuming and after incubation at 20 °C, in 2010. Percentage (mean ± s.e.) of permeable seeds of G. carolinianum of (J) fresh seeds and after incubation at 20 °C; (K–O) at the time of exhuming and after incubation at 20 °C, in 2011. Percentage (mean ± s.e.) of permeable seeds of G. dissectum of (P) fresh seeds and after incubation at 20 °C; (Q–U) at the time of exhuming and after incubation at 20 °C, in 2011. An asterisk indicates a significant difference between treatments.

Fig. 8.

Conceptual models for breaking seed dormancy in (A) G. carolinianum and (B) G. dissectum.