Fig. 1.

Hypothesized rationales for a stomatal safety-efficiency trade-off. a Stomatal size and density: leaves with smaller, denser stomata (left) have higher maximum stomatal conductance (g_max), and stomata more sensitive to closure during drought (i.e., higher Ψ_gs50, indicated by thicker red lines) than leaves with larger, less dense stomata (right). b Osmotic concentration: Leaves with weaker cellular osmotic concentrations (i.e., higher osmotic potentials) at full turgor and turgor loss (left) are associated with higher g_max and higher Ψ_gs50 than leaves with stronger osmotic concentrations (i.e., lower osmotic potentials) (right). c Leaf economics and life history trade-off: Species selected for greater resource acquisitiveness, and with lower leaf mass per area (LMA; top row) would have higher g_max and photosynthetic rate under high water supply (left column), and more sensitive stomatal closure under low water supply (right column) than species with high LMA (bottom row), which have lower g_max and photosynthetic rate under high water supply, and can better maintain stomatal conductance and photosynthetic rate under low water supply. d Plant hydraulic design: Under high water supply (left column), species with high g_max have higher photosynthetic rate than species with low g_max and both maintain leaf turgor and xylem water column continuity; under low water supply (right column), species with high g_max must show sensitive stomatal closure (i.e., higher Ψ_gs50) and therefore strong reduction of photosynthetic rate to avoid leaf damage and xylem embolism (right column, top two schematics), whereas species with low g_max can maintain stomatal conductance and photosynthetic rate under low water availability (right column, lowest schematic)