Rocky soils alter the diurnal photosynthetic behavior of xerophytic species by regulating hydraulic properties

Functional relationships between photosynthetic behavior and hydraulic properties are essential to characterize plant drought adaptation strategies. However, little is known about such relationships in response to varying rock fragment content (RFC), which could induce severe soil water deficit. We evaluated the leaf hydraulic properties and the timing of diurnal depression of photosynthesis in three xerophytic shrub species grown under different RFC levels (0, 25, 50, 75 % v v⁻¹). We found that studied species grown in 25 % RFC soil conditions had higher leaf hydraulic conductance (K_leaf) and reached maximum photosynthetic rate (A_max) in the morning, while those grown in 75 % RFC soil conditions had lower K_leaf, but reached their A_max in the afternoon. In addition, species in 75 % RFC soil conditions also exhibited low leaf hydraulic vulnerability and narrow leaf hydraulic safety margins. Our results indicate that RFC modifies the diurnal gas exchange dynamics of xerophytic species by decreasing leaf hydraulic vulnerability and hydraulic safety margins. Specifically, species surviving in 75 % RFC soils are less vulnerable to drought induced water loss, and carbon assimilation depression were later than in 25 % or 0 % RFC soil conditions. However, when faced with severe drought, these species with latter CO₂ uptake depression are at higher risk of hydraulic failure, because their safety margins are relatively narrow. Our results contribute to the knowledge of drought adaptation strategies in xerophytic species native to dry-hot rocky mountains.