Characteristics of heteromorphic achenes of Garhadiolus papposus, an ephemeral Asteraceae species, with reference to their adaptations to desert environment
Huazhi Sun, Dunyan Tan, Rongming Qu
Biodiv Sci. 2008, 16 (4):
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Garhadiolus papposus is a common ephemeral Asteraceae species that germinates in early spring in the Junggar Desert. It produces three types of achenes in one infructescence that differ in morphology and dispersal behavior. To better understand how this species is adapted to its desert habitat, we studied morpho-logical and dispersal characteristics of its achenes. The three types of achenes differed significantly in the number, shape, color, mass, length and density of pappuses, and embryo size. Peripheral achenes, which were subtended by the inner involucral bracts (phyllaries), were smooth, weighed 118.10 ± 1.30 mg per 100 ache-nes, were columnar and arcuate, yellow or yellow whitish, 6.67 ± 0.10 mm in length, and had a scarcely-developed pappus. Central achenes with numerous conspicuous thin longitudinal ribs weighed 69.00 ± 0.60 mg per 100 achenes, were columnar and arcuate or straight, brown, elongated (9.74 ± 0.14 mm length), and had a well developed pappus. Intermediate achenes represented a range of morphologies be-tween the “pure” central and peripheral types. In a given head, the central achenes were more numerous than intermediate and peripheral ones (5.77 ± 0.13 vs 5.39 ± 0.14 vs 5.24 ± 0.05). In addition, central and interme-diate achenes had thinner and anatomically different pericarps (half as much biomass) compared to peripheral achenes (69.7 µm vs 82.1 µm vs 156.7 µm), and embryos in peripheral achenes were heavier than those in central and intermediate achenes (46.70 ± 0.50 mg vs 36.80 ± 0.30 mg vs 36.90 ± 0.30 mg per 100 embryos). The three morphs differed in dispersal potential. In the laboratory, dispersal distance was longer for central than for intermediate achenes, and longer for intermediate than peripheral achenes. In the field, central ache-nes were dispersed relatively long distances by wind, whereas peripheral achenes, which are covered by lig-nified phyllaries, remained on the capitulum after maturation and were dispersed, close to the mother plants, only when the capitulum broke apart. Dispersal features of intermediate achenes are intermediate to those of central and peripheral achenes. We discuss how the morphological and dispersal characteristics of the three achenes types in G. papposus influenced the distribution of achenes to new areas, development of seedlings, population persistence and expansion, and competition among offspring. Heterocarpy may be partly respon-sible for the success of G. papposus in the Junggar desert.