A Photosynthesis-Based Irrigation Model for Woody Plants

Many techniques exist for estimating plant water use and refining irrigation requirements.  However, grower adoption of existing irrigation technology is low.  Development of an easily transferrable irrigation model could aid in grower adoption of irrigation technology.  Previously, a photosynthesis-based irrigation model was developed for Hibiscus ‘Cashmere Wind’ under controlled-environment conditions.  This model was based on the relationship between substrate moisture content and photosynthetic rate.  A sigmoidal equation best fit the curve (r₂=0.62).  Photosynthesis was maintained at or near maximum rates from 100% container capacity to 62% container capacity.  A distinct decrease in photosynthesis was apparent at 61% container capacity and this was used as an irrigation setpoint.  Subsequent experiments showed that biomass and quality could be maintained equally among irrigation treatments so long as substrate moisture content was maintained above this setpoint.   Additionally, a model was developed for two Cornus species under controlled environment conditions and exhibited similar characteristics.  In order to determine the transferability of a photosynthesis-based irrigation model from controlled environment to outdoor conditions and to determine if different setpoints would be necessary among closely related Cornus taxa, the relationship between photosynthetic rate and container water content was measured for three related Cornus taxa (C. florida ‘Cherokee Princess’, C. kousa ‘National’, and C. x Constellation) under outdoor, pot-in-pot conditions.  Photosynthesis was measured as substrate moisture content decreased from well-watered conditions (90% or greater container capacity) to water deficit conditions.  Again, a sigmoidal equation best fit the data (r₂=0.70-0.74), however, photosynthesis could not be maintained at near maximum levels over as wide range of substrate moisture contents as observed for greenhouse-grown plants.  All three field-grown dogwood taxa showed a similar photosynthetic rate (near 80% of maximum) at 88% container capacity.  Therefore, a revised model using this setpoint will be evaluated for its impact on water use and biomass accumulation in pot-in-pot dogwood production.