Drought response strategies of blueberry shrubs: Stomatal regulation, xylem embolism, and leaf dropping protect remaining tissues for resprouting

The drought response mechanisms of shrubs such as blueberry plants remain poorly understood. Crucial gaps exist in the mechanistic understanding of shrub drought response strategies, the threshold for irreversible declines in physiological function, and their recovery potential. As drought events intensify in the northeastern U.S., it is increasingly important to understand the drought response of blueberry plants in this region. Here, we investigated the response of key plant physiological processes such as turgor loss, stomatal conductance, photosynthesis, xylem embolism, and phytochemistry in one highbush and one lowbush blueberry species. We found a coordinated decline in stomatal conductance, photosynthesis, transpiration, and plant hydraulic conductance before reaching the turgor loss point (TLP) of -2.0 MPa. At water potentials beyond the TLP, there was a progressive decline of phytochemistry as well as an increase in xylem embolism, leaf browning and leaf shedding, supporting TLP as a critical threshold beyond which drought-induced embolism and subsequent branch mortality occur in blueberries. We also revealed novel mechanisms regarding their high recovery potential: lowbush blueberries resprouted from belowground stems while highbush blueberries generated new branches from the basal stem, even following severe drought stress with 100 % loss of xylem hydraulic conductivity and total branch dieback. Our results thus provide new insights into the drought resistance and recovery mechanisms of these shrub species. The protection of belowground or stem base tissues during drought may have played a key role in enabling post-drought resprouting in blueberries and other shrubs.