Leaf Structural and Biochemical Adjustments to Overcome Extreme Drought

Indicators of drought tolerance in plants help identify global patterns in plant strategies. For example, turgor loss point (TLP), the water potential at which cells lose water pressure and wilt, shows a strong correlation with biome wetness. On a smaller spatial and temporal scale however, such drought tolerance indicators might fluctuate seasonally, and individual plant species might develop sets of traits for drought tolerance that are not in line with the global expectation for one specific trait. A full assessment of cell-to-ecosystem hydraulic function would help predicting species responses to drought in specific ecosystems. We aimed to resolve the connections between cell structural traits and leaf hydraulic and biochemical traits, in relation with plant rooting depth, in the dry Mediterranean ecosystem in Southern California. We hypothesized that some species might be capable of adjusting TLP in the dry season, whereas others could have cell structural characteristics that allow a leaf to regain its structure when rewetted after turgor loss.

We selected six native Chaparral and Coastal Sage Scrub species that differ in their estimated water uptake depth. Plants were exposed to drought, during which gas exchange, leaf water potential, changes in osmotic potential and thermal tolerance were measured. Leaves of plants that were well-watered or had passed turgor loss were harvested and cross-sections were analyzed by scanning electron microscopy (SEM) and transmission electron microscopy. Cuticle thickness, cell wall thickness and vessel structure were measured. Morphological characteristics such as cuticular hairs, stomatal density and stomatal opening were measured by imaging the epidermis and epidermal imprints using SEM.

We found different strategies of drought avoidance and tolerance. Significant reductions in turgor loss point between well-watered and droughted plants for some species allowed gas exchange to continue longer before wilting, indicating acclimation potential of those species. Deciduous species did not change TLP strongly. Species with smaller epidermal cells had lower TLP. The relationship between cellular structures and physiological measurements enabled us to characterize species-specific drought tolerance and survival strategies in a highly diverse ecosystem.