Physiological Response to Drought Stress and Water Use in Two Redbud (Cercis) Ecotypes

Drought tolerance is an important adaptation for landscape plants. Redbud is an important landscape plant with two ecotypes that respond differently to water availability. The current study was conducted at the University of Kentucky greenhouse facilities in Lexington, KY to determine key factors impacting differences of redbud cultivars with different drought tolerances. Cultivars selected include eastern redbud (Cercis canadensis canadensis ‘Appalachian Red’ and Cercis canadensis  texensis ‘Oklahoma’. Liners were grown in 7-gallon containers filled with 85% pine bark: 15% peat (vol/vol) in a completely randomized experiment design. Substrate moisture content was continuously monitored using EC5 (Decagon, IL) moisture sensors inserted into three representative containers per cultivar treatment. Physiological parameters related to plant water stress was collected on a daily basis throughout the study. Photosynthetic rate was collected using a LICOR 6400 and sap flow using a Dynamax sap flow sensor to track real time plant moisture status. The relationship between volumetric water content and sensor output was determined for both ecotypes. It was found that ‘Oklahoma’ maintained around 35% higher net photosynthesis compared to ‘Appalachian Red’ over a three-day dry down experiment. Over that same time period, ‘Oklahoma’ lost 78.2 g of water through transpiration compared to 126 g of water per plant for ‘Appalachian Red’. ‘Oklahoma’ maintained a photosynthetic rate at 90% or greater of the maximum rate even under substrate water content of 0.27 m³·m^-3, whereas ‘Appalachian Red’ showed a significantly reduced photosynthesis rate beginning at substrate water contents of 0.31 m³·m^-3. Sap flow showed a similar trend to direct transpiration measurements with ‘Oklahoma’ at 0.47 L cm^-1 day^-1 and 0.84 L cm^-1 day^-1 for ‘Appalachian Red’. ‘Oklahoma’ plants had thicker, broader, heavy leaves and higher number of stomata per unit leaf area compared to ‘Appalachian Red’ which may be related to the observe higher relative leaf water content, stomatal conductance and net photosynthesis under drought stress conditions.