Leaf Trait Diversification and Design in Seven Rare Taxa of the HawaiianPlantagoRadiation

Abstract

Linkages among leaf traits and environment have most often been tested across communities but infrequently within lineages. We studied seven endemic Hawaiian Plantago taxa radiated across elevations, climates, and habitats. We grew plants of six taxa in controlled conditions for 1-2 yr and collected leaves from a seventh in the field. For all taxa, we measured 46 leaf traits and tested hypotheses for trait-environment and trait-trait associations. Because of the rarity of the study plants, our study included low replication within taxa and multiple growth locations; despite these limitations, given reasonable assumptions, our analyses pointed to genetic differentiation among taxa. The leaves of bog taxa were smaller and thicker than those of woodland taxa, with higher leaf mass per area (LMA), stomatal pore area per leaf area, and carbon isotope discrimination (d 13 C). Taxa from higher elevations had thicker leaves and higher LMA, as well as lower nitrogen per mass and higher adaxial stomatal distribution. Taxa of drier sites had higher vein density and d 13 C. Many traits were allometrically related to leaf area, including stomatal density, major vein densities, and xylem conduit numbers and dimensions. Stomatal and xylem traits were correlated, indicating a matching of hydraulic supply and demand. Leaf diversification in Hawaiian Plantago involved coordinated trait shifts, generating strong, apparently adaptive trait linkages.

Developmentally based scaling of leaf venation architecture explains global ecological patterns

“…Notably, genus-by-genus analyses produced comparable patterns, as did phylogenetically controlled tests within lineages for Acer, Plantago and Viola for which highly resolved phylogenetic trees were available (unpublished data) 12 . However, we note that the scaling trends within given lineages may depart from the global across-species trends shown here, especially in lineages adapted across environments.…”

“…However, despite the strong potential importance of such trends, there have not been any previous conclusive tests for the scaling of vein traits with leaf size across species. Data from three previous studies with ten or fewer species showed a negative scaling of major vein density with leaf size, and minor vein density was independent of leaf size 8,12,13 . A further study using automated analyses of low-resolution images reported vein density to be independent of leaf size across 339 species 14 ; however, that study measured only a fraction of the vein system and did not distinguish among vein orders 14 and thus, did not allow a conclusive test of general scaling of vein traits.…”

“…All leaf clearing used the same protocol as previously published by the authors 4,8,12,54,55 . Briefly, leaves were fixed in 70% formalin-acetic acidalcohol (48% ethanol: 10% formalin: 5% glacial acetic acid: 37% water) and cleared in 2.5-5% sodium hydroxide in water or ethanol followed by sodium hypochlorite bleach, and stained with safranin and fast green.…”

“…Thus, it would be possible to achieve higher final minor vein density in a smaller leaf by limiting leaf expansion after all veins are formed; or higher final minor vein density without change in leaf size by compensating with increasing expansion before the final vein order is formed; or higher final minor vein density with a larger leaf, with still greater expansion before the final vein order is formed; or a larger leaf without change in minor vein density by increasing expansion only before the final vein order is formed, thus initiating veins in the same proportion to leaf area as in the small leaf. All of these scenarios can occur, for example, for the sun relative to shade leaves of given species, and across populations or across closely related species adapted across habitats 4,12,[30][31][32][33][34][35][36][37] . Notably, leaf expansion relative to vein formation can be influenced by modulating cell proliferation, which especially affects expansion before the final vein order is formed, and final cell size, which mainly affects expansion after all veins are formed 38,39 .…”

Evolution of C 4 plants: a new hypothesis for an interaction of CO 2 and water relations mediated by plant hydraulics

“…Previous studies of adaptation to these conditions within species or across closely related species within given lineages have shown increased vein densities, and thus shorter interveinal distances [76][77][78][79]. This higher vein density would permit greater g s and higher A during periods with high water availability, to compensate for low CO 2 .…”