Evolution of root traits in Helianthus
Bowsher, Alan Walter
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It has long been recognized that species distributions are related to their ability to acquire and use resources such as water and nutrients. However, our understanding of root system variation in relation to resource availability is extremely limited, despite the central importance of roots in resource acquisition. Using the genus Helianthus as a phylogenetic framework, we conducted a series of controlled environment studies to examine genetically-based differentiation among species in root structure and function. Contrary to expectations, we found little evidence for tradeoffs in fine root morphology, chemistry, and anatomy across populations of 26 Helianthus species. However, in comparisons of six Helianthus species chosen as phylogenetically-independent contrasts, species native to low nutrient soils consistently produced lower total root length and greater nitrogen uptake rates than species native to high nutrient soils. This suggests that a slow-growing root system, with a high capacity to exploit nutrient pulses in soil, is favored by selection in low fertility soils. We also found that species native to low nutrient soils exhibit constitutively high exudation of primary metabolites, suggesting repeated selection for high root exudation in low nutrient soils. However, all species, regardless of their native soil fertility, responded similarly to low nutrient treatments by drastically increasing exudation of carboxylic acids, which are known to increase mineral nutrient availability in soils. Taken together, these findings generally fit with the trait syndromes expected to characterize species native to infertile soils. However, we found little evidence for the adaptive value of specific root length, root tissue density, and root nitrogen concentrations in low resource environments, either in fine roots or the whole root system level, despite the general expectation that these traits summarize species’ ecological strategies. Therefore, although several traits appear to be under strong differential selection across environmental gradients, there are likely a variety of different trait combinations that are compatible with a given environment, even in closely-related species.