Dormancy and germination of the trimorphic achenes of a cold desert annual: spreading the risk over time

Abstract

Many studies have been done on the relationship between variation in morphology, degree of dormancy and dispersal ability of heterocarpic species with dimorphic diaspores. However, there are far fewer such studies on species that produce trimorphic diaspores. Our aim was to compare dormancy and germination of achenes from peripheral, intermediate and central positions in the capitulum of the diaspore-trimorphic cold desert annual Asteraceae species Heteracia szovitsii, an important component of plant communities in the cold deserts of NW China. Dormancy breaking/germination responses of the three achene morphs and of seeds isolated from the pericarp were tested in the laboratory using standard procedures, and seedling emergence phenology of the achene morphs was monitored under natural cold desert temperature conditions in an experimental garden with and without supplemental watering. Depth of dormancy of the three achene morphs was peripheral > intermediate > central. Seedlings from the three morphs emerged in spring and in autumn. Cumulative seedling emergence percentage from achenes during 47 months of burial was central > intermediate > peripheral. Central achene morphs emerged over a period of about 12 months after sowing, while intermediate and peripheral achene morphs did so for about 40 and 47 months, respectively. Thus, H. szovitsii exhibits a temporal dispersal strategy. No viable central or intermediate achene morphs were present after 16 and 40 months, respectively, but about 60 % of the nonemerged peripheral achenes morphs were viable after 47 months. Based on our results on diaspore dormancy and those of a previous study of diaspore spatial dispersal of H. szovitsii, we conclude that this species has a high-intermediate-low risk diaspore dispersal/dormancy strategy that likely increases the chances for population persistence over time and space.

Protection against insect predation during fruit development: the role of fleshy fruit wings of three species of Zygophyllum in the cold desert of Central Asia

“…1 filter paper moistened with 2.5 ml of distilled water in 9-cm-diameter plastic Petri dishes in light (12 h daily photoperiod) and constant darkness (Petri dishes with seeds wrapped with aluminum foil) at daily (12/12 h) temperature regimes of 5/2°C, 20/10°C, 25/15°C, 30/15°C, 35/20°C and 40/25°C. The criterion for germination was emergence of radicle from the seed ( Lu et al., 2020 ). Germination under light was examined daily for 28 d, and all germinated seeds were removed at each counting.…”

Seed dormancy in Asteraceae: a global vegetation zone and taxonomic/phylogenetic assessment

“…We note that some species of Chaptalia in Brazil (Pasini et al, 2014) and Mexico (Redonda-Martínez, 2018) have trimorphic florets and presumably trimorphic cypselae. In G. papposus (Sun et al, 2009), H. szovitsii (Lu et al, 2020) and H. pinnatum (Venable et al, 1987), the central cypselae are the least dormant and the peripheral cypselae the most dormant. Central, intermediate and peripheral cypselae of the cold desert annual H. szovitsii allowed to afterripen for 48 months germinated to 85% (30/15°C), 30.5% (5/2°C) and 10.5% (15/2°C), respectively.…”

“…The embryo does not have enough growth potential to overcome the mechanical restriction of the pericarp, which in some species of Asteraceae can be strong and thick (e.g. Sun et al, 2009;Lu et al, 2020). Consequently, mechanical or acid scarification, which removes the mechanical restraint of the pericarp on the Asteraceae embryo, may promote germination (e.g.…”

“…Germination of trimorphic cypselae of Asteraceae has been studied in some detail for Calendula arvensis (Ruiz de Clavijo, 2005), Garhadiolus papposus (Sun et al, 2009), Heteracia szovitsii (Cheng and Tan, 2009;Lu et al, 2020) and Heterosperma pinnatum (Venable et al, 1987). Trimorphic cypselae have been reported for Xanthocephalum spp.…”

“…Central, intermediate and peripheral cypselae of the cold desert annual H. szovitsii allowed to afterripen for 48 months germinated to 85% (30/15°C), 30.5% (5/2°C) and 10.5% (15/2°C), respectively. However, when the pericarp was removed from intermediate and peripheral cypselae they germinated to 100 and 69.3%, respectively (Lu et al, 2020) Cypsela heteromorphism translates into differences not only in degree of dormancy but also differences in dispersal and timing of germination (Baskin and Baskin, 2014). Generally, in Asteraceae, the central cypselae have low dormancy and high dispersal, and peripheral cypselae have high dormancy and low dispersal (Venable and Lawlor, 1980; see Baskin et al, 2013).…”