Grass leaves are different than tree leaves, but only sometimes: Scaling relationships between anatomical characteristics and leaf size within a sing grass subfamily.

Background/Question/Methods

The plant vascular system provides the pathway for water movement from the soil, up through the leaves, and the architecture of this system plays a role in maximum rates of photosynthesis as well as the ability to tolerate drought. Correlations between cell size, vein size and density, and leaf size help explain leaf function and species distributions. These universal scaling relationships help identify physical constraints and trade-offs within the vasculature system of plants. The parallel venation of monocots creates a different pathway of water movement compared to eudicots but many of the scaling relationships apply to both plant groups. Unfortunately, these patterns are typically evaluated across distantly related species to identify broad evolutionary patterns, but it is unclear if these patterns exist among closely related species. Physiological adaptions (eg. water-use efficiency) may exist, rather than changes in the vascular architecture, to meet the demands of different environmental conditions. To investigate the anatomical and physiological relationship within the leaves of grasses, we grew 5-6 species of grasses for each of 5 genera within a subfamily and measured anatomical and morphological characteristics as well as leaf-level gas-exchange.

Results/Conclusions

We examined the scaling relationships between cell size, vein size, vein density and leaf size to determine whether the same scaling relationships occur in closely-related species in the Poaceae family as is found among more distantly-related monocot and eudichot species. We found tight correlations between xylem conduit diameter, vein diameter, and vein density (r2: 0.58-0.82) that agree with previous work. However, vein density and leaf size deviated from scaling relationships previously measured across distantly related species. Further, the relationships also differed between closely-related genera in the same subfamily; specifically, species within the Poa genus had similar vein densities regardless of leaf width, but wide-leaved species in the Festuca genus had greater vein density than narrow-leaved species. Interestingly, cell size, vein size, and vein density did not correlate well with rates of photosynthesis or transpiration, but the tapering of xylem conduits along the leaf did explain rates of transpiration in grass leaves. These unique scaling relationships within individual genera suggest we need to be looking both within and across genera and families to fully understand the evolutionary adaptions of plant vascular systems.