Occurrence of physical dormancy in seeds of Australian Sapindaceae: a survey of 14 species in nine genera

Abstract

Background and aims: Sapindaceae is one of 16 angiosperm families whose seeds have physical dormancy (PY). However, the extent and nature of PY within this family is poorly known. The primary aims of this study were: (1) to evaluate seed characteristics and determine presence (or not) of PY within nine genera of Australian Sapindaceae; and (2) to compare the frequency of PY across the phylogenetic tree within Australian Sapindaceae.

Methods: Viability, imbibition and seed characteristics were assessed for 14 taxa from nine genera of Sapindaceae. For five species of Dodonaea, optimal conditions for germination and dormancy break were evaluated. An in situ burial experiment was performed on D. hackettiana seeds to identify the factor(s) responsible for overcoming PY. Classes of dormancy and of non-dormancy for 26 genera of Sapindaceae were mapped onto a phylogenetic tree for the family.

Key results: Mean seed viability across all taxa was 69.7 %. Embryos were fully developed and folded (seven genera) or bent (two genera); no endosperm was present. Seeds of all five Dodonaea spp. and of Distichostemon hispidulus had PY. Hot-water treatment released PY in these six species. Optimal germination temperature for seeds of the four Dodonaea spp. that germinated was 15-20 degrees C. Following 5 months burial in soil, 36.4 % of D. hackettiana seeds had lost PY and germinated by the beginning of the winter wet season (May). Laboratory and field data indicate that dormancy was broken by warm, moist temperatures (> or =50 degrees C) during summer.

Conclusions: PY occurs infrequently in genera of Sapindaceae native to Australia. Seeds of Dodonaea and Distichostemon had PY, whereas those of the other seven genera did not. Seeds of these two genera and of Diplopeltis (a previous study) are the only three of the 20 native Australian genera of Sapindaceae for which germination has been studied that have PY; all three belong to subfamily Dodonaeoideae.

Fig. 1.

Daily maximum and minimum temperatures (°C recorded at 10 mm soil depth) and daily rainfall (mm) from 31 January through to 23 May 2007.

Fig. 2.

Scatterplot and linear regression comparing percentage mass increase for control and scarified seeds of 12 Sapindaceae taxa (Table 1). Species with PY (circles: Distichostemon hispidulus, Dodonaea aptera, D. hackettiana, D. petiolaris, D. stenozyga, D. ptarmicaefolia) had very low percentage mass increase in control seeds and very high percentage mass increases in nicked seeds. In comparison, non-PY taxa (squares: Allophylus cobbe, Atalaya hemiglauca, Cupaniopsis parvifolia, Ganophyllum falcatum, Harpullia pendula, Synima cordierorum) have higher percentage mass increases in control seeds and lower relative percentage mass increases following scarification. Except for D. hispidulus (one sample), all points are derived from the average of three replicates (Table 1).

Fig. 3.

Imbibition (% mass increase ± s.e.) of non-treated (control), scarified and hot-water-treated [88–92 °C for 30 s or 2·5 min (D. petiolaris only)] seeds. One (Distichostemon hispidulus) to three replicates (all Dodonaea spp.) of 20 seeds were used for each experimental treatment. Different letters denote significant differences in maximum mass increase within each species (P < 0·05).

Fig. 4.

Percentage (± s.e.) of seeds that imbibed following no treatment (control) or hot-water (88–92 °C) treatment for 2·5 min (D. petiolaris only) or for 30 sec (all other Dodonaea spp.) and then incubated at room temperature on irrigated germination papers for 72 − 144 h. Different letters denote significant differences within a species (P < 0·05).

Fig. 5.

Percentage germination (± s.e.) of control and hot-water-treated seeds of four Dodonaea spp. after incubation for 23 d at three different temperatures. For each treatment combination, three replicates of 25 seeds were used. Different letters denote significant differences within each temperature and each species (P < 0·05).

Fig. 6.

Effects of wet (20 − 100 °C) or dry (control, 15/50 °C) heat on dormancy-break and subsequently percentage germination (± s.e.) in four Dodonaea spp. For each treatment combination, three replicates of 25 seeds were used. All seeds were incubated in darkness at 20 °C. Seeds were evaluated once for germination, after 3 weeks. Different letters denote significant differences within each species (P < 0·05).

Fig. 7.

Phylogenetic tree for all but three genera of Australian Sapindaceae plus Koelreuteria, Cardiospermum and Sapindus (adapted from Harrington et al., 2005), showing the proposed relatedness of Australian Sapindaceae genera and their placement within the two subfamilies Sapindoideae and Dodonaeoideae. The relative length of each branch reflects taxonomic relatedness. Genera with an asterisk (*) were evaluated for dormancy in this study. Dormancy classification (ND, non-dormant; PY, physical dormancy; PD, physiological dormancy; PY + PD, combinational dormancy) is based on: 1, Nicholson and Nicholson (1992); 2, Nicholson and Nicholson (1991a); 3, Nicholson and Nicholson (1991b); 4, Nicholson and Nicholson (1994); 5, Nicholson and Nicholson (2000); 6, Nicholson and Nicholson (2004); 7, Johnston et al. (1979); 8, Rehman and Park (2000); 9, Turner et al. (2006a); 10, Baskin et al. (2004); 11, C. Baskin (pers. com); 12, Woods (2003); 13, Munson (1984); 14, Negi and Todaria (1993). The three Australian genera Cossina, Dictyoneura and Heterodendrum have not been placed on this tree due to lack of taxonomic information about them.