Partitioning hydraulic resistance in Sorghum bicolor leaves reveals unique correlations with stomatal conductance during drought.

The hydraulic architecture of leaves represents the final path along which liquid water travels through the plant and comprises a significant resistance for water movement, especially for grasses. We partitioned leaf hydraulic resistance of six genotypes of Sorghum bicolor L. (Moench) into leaf specific hydraulic resistance within the large longitudinal veins (rLV) and outside the large veins (rOLV), and correlated these resistances with the response of stomatal conductance (gs) and photosynthesis (A) to drought. Under well-watered conditions, gs was tightly correlated with rOLV (r2=0.95), but as soil moisture decreased, gs was more closely correlated with rLV (r2=0.97). These results suggest that rOLV limits maximum rates of gas exchange, but the ability to efficiently move water long distances (low rLV) becomes more important for the maintenance of cell turgor and gas exchange as soil moisture declines. Hydraulic resistance through the leaf was negatively correlated with evapotranspiration (P<0.001) resulting in more conservative water use in genotypes with large leaf resistance. These results illustrate the functional significance of leaf resistance partitioning to declining soil moisture in a broadly-adapted cereal species.