Is a long hygroscopic awn an advantage for Themeda triandra in drier areas?

Abstract

Themeda triandra has bigeniculate hygroscopic lemma seed awns that twist when wet and drying, thereby transporting the caryopsis across the soil surface into suitable germination microsites. The prediction that awns would be longer in drier grassland and have greater motility to enable them to move quickly and farther to find scarce germination sites was tested in KwaZulu-Natal. Awns (n = 100) were collected from 16 sites across a mean annual precipitation gradient (575–1 223 mm), ranging from 271−1 097 m asl. The daily movement of hydrated long and short awns (n = 10) across blotting paper was tracked for five days, and the rotational speed of anchored awns was measured. Awn length varied considerably (mean: 41.4–63.2 mm; SD: 3.44–8.99), but tended to increase (r = 0.426,
p = 0.099) not decline, with increasing MAP. Awn length was unrelated to elevation, temperature and aridity indices. Long awns rotated at the same rate (2 min 48 sec), but moved twice as fast (46.3 vs 22.1 mm day−1) and much farther (maximum: 82.1 vs 38.6 mm day−1) than short awns. Whether moisture limits awn development, the benefit of longer awns to negotiate densely tufted mesic grassland, and the multifunctionality of awns require investigation.

Keywords: autochory, grassland, rainfall, seed dispersal