Responses of leaf photosynthetic traits, water use efficiency, and water relations in five urban shrub tree species under drought stress and recovery

Key message In urban shrub trees, the species-specific photosynthetic response and water-use properties are related to the xylem anatomy of the petiole. It is becoming essential to select urban tree species based on drought response in warm temperate regions, because water limitation is prone to occur in an urban environment, and furthermore, urban warming along with global warming intensifies drought stress even in relatively humid regions. We focused on leaf photosynthesis, water use efficiency, and leaf water relations as key factors for the evaluation of drought response in urban trees, and compared their responses to drought stress and re-watering (recovery) in five major urban shrub tree species planted in Japan. In addition, species-specific xylem anatomical traits in the leaf petiole were evaluated. The five species showed diverse responses to drought and recovery. Rhaphiolepis umbellata possessed both the highest photosynthesis (A) and highest intrinsic water use efficiency (A/g(s)) under drought, as well as full recovery in the midday leaf water potential (psi(mid)). These results suggest that R. umbellata is the most favorable species as an urban tree among the five species. In contrast, A and A/g(s) in Rhododendron obtusum were only 19% and 55%, respectively, of those in R. umbellata under drought, along with incomplete recovery in psi(mid). The responses of A, A/g(s), and psi(mid) for the other three species were intermediate between R. umbellata and R. obtusum. We found that during recovery, the species-specific coordination between photosynthesis and leaf hydraulic traits was mediated by stomatal regulation. The species with large stomatal conductance had both high photosynthesis and high leaf hydraulic conductance, along with a large vessel area in the leaf petiole. The selection of trees with consideration of the drought response, along with appropriate watering management, will improve the photosynthetic ability, and thus, will enhance CO2 absorption by urban trees.

Automated summary

The xylem anatomy of urban shrub trees' petiole is related to their species-specific photosynthetic response and water-use properties. Selecting urban tree species based on drought response is crucial in warm temperate regions to combat water limitation and intensified drought stress caused by urban warming and global warming. Rhaphiolepis umbellata showed the highest photosynthesis and water use efficiency under drought, making it the most favorable species as an urban tree. On the other hand, Rhododendron obtusum had significantly lower photosynthesis and water use efficiency under drought, indicating it is less suitable as an urban tree species. The coordination between photosynthesis and leaf hydraulic traits during recovery was mediated by stomatal regulation.