Grasslands, savannas and croplands cover about 40% of the terrestrial biosphere and grasses represent all major grain crops, yet little is known about how anatomy affects water movement through grasses. The evolution and expansion of grassland biomes 20-30 million years ago was due to an increase in aridity; therefore, grass evolution, physiology and ecology are inextricably linked to the acquisition, use and movement of water. Research into the evolution of the C4 photosynthetic pathway from the C3 photosynthetic pathway has forged collaborations across evolutionary biologists, physiologists, paleo-ecologists and plant geneticists who are attempting to engineer C4 photosynthesis into rice, yet comparatively little of this research has been focused on the hydraulic side of C4 evolution. The aim of this research is for a better understanding of how grass leaf anatomy, in concert with the evolution of C4 photosynthesis, controls water transport through a leaf. This research will provide a better physiological, theoretical and genetic understanding of hydraulic controls and limits in grasses. Targets for improving crop performance will be identified, while adding insight into the importance of leaf hydraulics in the evolution of C4 photosynthesis. Additionally, new research-related teaching modules will be developed for the University of Pennsylvania's Biology Professional Development Program. The program increases the biology knowledge of high school teachers in the Philadelphia School district, a heavily-underfunded school system. This project will also train a postdoc, a graduate student, and several undergraduates.
C4 grasses evolved from C3 grasses more than 20 times independently over tens of millions of years within the PACMAD clade of grasses. By allowing phylogeny to guide the examination of function, and using a combination of theory and experimentation, the following hypotheses will be addressed in this project: 1. The primary selective pressures for the evolution of C4 changed (e.g. CO2 vs. water availability) depending on when C4 evolved within a lineage, the effects of which are still measurable today. 2. Upon the evolution of C4 within a grass lineage, there was selection for hydraulic reorganization within the leaf, notably a decrease in leaf hydraulic conductance with no decrease in vein density. 3. By examining hydraulic and anatomical differences within a phylogenetic context, insight can be gained on the controls of water flux in the within-xylem (liquid water) and outside-xylem (both liquid and vapor) components of the flux pathway in relation to photosynthetic variation. 4. Genome-level mapping of recombinant inbred lines of a Setaria viridis x S. italica cross will (i) give mechanistic insight into how changes in hydraulic architecture affect water flux through leaves, and (ii) identify targets for crop improvement. An integrative approach using physiological measurements of hydraulic conductance in concert with leaf-water enrichment of heavy stable isotopes will be used in an iterative fashion with fine-scale, physical modeling of water flux through leaves to determine the anatomical controls on leaf hydraulics.
This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
