Relationship of hydraulic architecture and leaf economics traits across Helianthus
Recent research has suggested that functional traits of the leaves, stems, and roots of plant species may form a whole-plant economics spectrum. However, studies have shown conflicting results when looking for correlation specifically between stem and leaf traits, and have been inconsistent in traits sampled. Further work has provided conflicting results on the adaptation of hydraulic architecture to varying source-site water environmental conditions. Field studies have shown large covariation of source-site precipitation and xylem traits, while common gardenapproaches have shown that this may be due primarily to plasticity within these traits. This study seeks to assess the covariation of leaf and stem traits across the sunflower genus, Helianthus, and the correlation of these traits with source-site environmental characteristics. Two populations each of 14 species were grown in a common garden and assessed for leaf traits, such as photosynthetic rate and leaf mass per unit area, and stem traits, such as xylem density, xylem lumen fraction, fiber density, and stem modulus of elasticity. Using phylogenetically independent contrasts, traits were compared with each other across and with organ types, as well as with source-site environmental factors, such as potential evapotranspiration, aridity index, latitude, and mean annual temperature.
Significant correlation was observed between such stem traits as xylem density, fiber lumen fraction, and stem modulus of elasticity. These stem traits further highly correlated with leaf photosynthetic rate and leaf mass per unit area: two defining traits of the leaf economics spectrum. Hydraulic traits, such as xylem lumen fraction and theoretical stem conductance, correlated with source-site latitude and temperature, but not with any metric of precipitation. Leaf-level water use efficiency, however, did correlate well with potential evapotranspiration of the source environment. These results provide support for an inter-organ economics spectrum. Further, hydraulic traits correlatin well with latitude and temperature, but not with source-site precipitation, suggest that hydraulic trait evolution may be more highly influenced by cool temperatures in the late growing season than low summer precipitaion. This work suggests that covariation does occur across organ types and that adaptation of hydraulic traits to source-site environmental conditions may be predominantly to temperature gradients, rather than variation in source-site water availability.