Cryptic Seed Heteromorphism in Packera tomentosa (Asteraceae) : Differences in Seed Mass Characteristics and Germination

Abstract

Germination requirements of seeds can dictate when and where plant offspring establish. Microsites available for germination vary spatially and temporally in factors such as temperature and moisture; thus, the production of seeds with identical requirements may limit germination. When seed mass influences germination and offspring establishment, the production of seeds with a range of sizes encourages differential behavior in progeny. Seed heteromorphism, the production of two or more seed types with different forms and/or behaviors by the same plant, may be "cryptic" when seed types have different behaviors but similar morphologies. Although rarely documented, cryptic seed heteromorphism may be widespread among plant taxa. The production by a plant of seeds with variable mass or heteromorphism may increase the number of microsites favorable for germination. I investigated seed mass variation and seed heteromorphism in Packera tomentosa (Michx.) C. Jeffrey (woolly ragwort, Asteraceae), a clonal plant found in disturbed habitats in the coastal plain of the southeastern U.S. Like most members of the Asteraceae, P. tomentosa displays flowering heads that contain disc and ray florets, which produce central and peripheral seeds, respectively. Seeds were collected from 50 purported clones of P. tomentosa at East Carolina University's West Research Campus (WRC), Pitt County, North Carolina. Seed mass was compared among- and within-genetic individuals as well as between floret types of a flowering head. Amplified fragment length polymorphism (AFLP) profiles confirmed that all 50 clones were unique genetic individuals or "genets", verifying that seedling recruitment does occur in this population of P. tomentosa. I compared total mass and allocation to the embryo and pericarp between central and peripheral seeds. An initial study investigated germinability and speed of germination for both seed types in controlled conditions. I then tested the germination response of central and peripheral seeds to frequent, intermediate, and infrequent watering intervals. A final study determined whether germination speed or success of central and peripheral seeds was influenced by aging and/or cold stratification. Overall, seed mass of Packera tomentosa was highly variable among- and within-plants. Larger seeds exhibited faster, higher germination. Central and peripheral seeds did not have different morphologies, but differed in mass characteristics and germination. Central seeds were 11% heavier with 80% larger embryos than were peripheral seeds, whereas peripheral seeds allocated 17% more of total mass to the fruit coat than did central seeds. Differences in total mass between seed types appear to be driven by embryo size. Central seeds germinated at a higher proportion than did peripheral seeds (74.5 vs. 36.0%, respectively) and germinated faster than peripheral seeds (32.8 vs. 36.2 d, respectively). Differences in germination may be due to pericarp thickness or embryo size. Central seeds exhibited greater germination success in frequent and infrequent watering intervals, but not in the intermediate watering interval. Both seeds types showed lowest germination in response to infrequent watering, suggesting germination success decreases in drought conditions. Germination speed increased in central seeds following aging; thus, central seeds after-ripen. In contrast, peripheral seeds germinated faster after cold stratification, suggesting they delay germination and are stimulated by cooler temperatures. Cryptic seed heteromorphism occurs in Packera tomentosa, supporting the suggestion that this trait may be more common than is documented, particularly in the Asteraceae. In P. tomentosa, variation in germination behavior resulting from seed mass variation and seed heteromorphism may spread the risks associated with germination among many offspring phenotypes, potentially functioning as bet-hedging strategies and providing success in the unpredictable environments this species inhabits.