Tunicate bulb size variation in monocots explained by temperature and phenology

Abstract

Plant bulbs are modified shoot systems comprised of short internodes with apical bud(s) surrounded by layers of leaf bases. Bulb diameters can vary greatly, with overall bulb size playing a role in flower formation and resource allocation. Despite the importance of bulb size to the overall fitness of an individual, evolutionary and ecological aspects of this trait have been almost completely neglected. Examining over 2,500 herbarium vouchers for 115 selected species, we analyzed monocot tunicate bulb size within a phylogenetic context in order to investigate its evolutionary significance. We recorded two bulb diameter optima and observed that as bulb size increases taxa inhabit warmer areas with less temperature seasonality. Furthermore, we found that hysteranthous taxa, a habit where leaves emerge separately from flowers, exhibit overall larger bulbs potentially due to reliance upon belowground stored resources to flower rather than on current environmental inputs. This work highlights the importance of including the belowground portion of plants into ecological and evolutionary studies in order to gain a more complete understanding of the evolution of plant forms and functions.

Get the shovel: morphological and evolutionary complexities of belowground organs in geophytes

“…Biological collections harbor an enormous wealth of data and are actively being used to investigate ecological and evolutionary processes (Soltis, 2017; Howard and Cellinese, 2020). Unfortunately, many geophytic belowground organs lack representation in herbaria (Table 1).…”

International Biological Flora: Nervilia nipponica

“…Hysteranthy in Nervilia has been interpreted as an adaptation to marked seasonality in rainfall (Pettersson, 1991), with initiation of the generative shoot in most Nervilia species (including N. nipponica ) generally observed to coincide with the onset of early summer rains. However, Howard and Cellinese (2020) argue that hysteranthy across a broader range of bulbous plant species is more strongly correlated with occurrences in warmer, less thermally variable climates as compared to synanthous taxa, suggesting that it is more likely to be an adaptive response to temperature rather than precipitation.…”

Do seedlings of larger geophytic species outperform smaller ones when challenged by drought?

“…If the primary function of the storage organ is to enable persistence through unfavorable periods by supplying water, carbohydrates, or both, the size of the storage organ might be expected to be correlated with the length or severity of the unfavorable season (Dafni et al, 1981; Howard and Cellinese, 2020). Consistent with this hypothesis, Fragman and Shmida (1996) reported that the bulb size of desert geophytes is larger than that of mediterranean congeners.…”

“…The multitude of selective pressures that act on storage organ size may explain the lack of clear, discernable patterns in bulb, tuber, or corm size with length and severity of drought across multiple lineages. Storage organ size has been shown to be influenced by phylogenetic lineage (Proches et al, 2006), but considerable variation exists (Howard and Cellinese, 2020).…”

“…Similarly, large inflorescences would be expected to be supported by larger tubers. There is also some indication that hysteranthous species (those that flower in the absence of leaves) are associated with larger storage organ size (Howard and Cellinese, 2020). Other ecological factors potentially influencing storage organ size that have received very little attention are temperature and temperature seasonality, predation (e.g., porcupines), and capacity for vegetative dispersal (Gutterman, 1987; Bragg et al, 2005; Proches et al, 2006; Howard and Cellinese, 2020).…”