The role of dimorphic achenes in the biology of the annual weed Leontodon longirrostris

Various aspects of the reproductive biology and dimorphism of achenes from the annual heterocarpic weed Leontodon longirrostris (Finch & PD Sell) Tala vera were studied in the laboratory and field. The species is self-incompat ible, pollinated by generalist insects and produces two different types of achene in the capitula: peripheral and central achenes. Peripheral achenes are heavier (1.63 mg) and exhibit virtually no pappus and short-range dispe rsal, whereas central achenes are lighter (0.45 mg), possess a well-develop ed pappus and are produced in greater numbers. The morphological difference s between the two types of achene result in differences not only in dispers al, but also in germination, seedling vigour and the ability of seedlings t o emerge from different achene burial depths. Central achenes germinate mor e rapidly and in higher proportions under a broad spectrum of conditions. O n the other hand, germination in peripheral achenes is much more restricted , partly as a result of their thick pericarp; a high proportion of this typ e of achene does not germinate and remains in a dormant state on the ground , forming a temporary seedbank. Because of the increased size of the embryo , seedlings from peripheral achenes can emerge from greater depths and are initially more vigorous than those from central achenes. Central achenes re present a strategy that facilitates settling in new areas; peripheral achen es represent a strategy that ensures in situ persistence. These biological characteristics are advantageous for colonizing species.