2017 ASHS Annual Conference

In recent years, phosphorus runoff from agriculture has been implicated in water quality issues in the Great Lakes region. Past research on fertilization in ornamental nurseries has largely focused on nitrogen additions. Studies that have been conducted on phosphorous fertilization suggest that operational application rates may be reduced but the response is species specific. Therefore, our work focused on optimizing the rate of phosphorus for commonly grown woody ornamental plants namely, Hydrangea quercifolia (Queen of hearts), Cornus obliqua (Redtwig dogwood) and Physocarpus opulifolius (Seward). We planted 36 plants of each taxa in #3 (12L) containers. They were then transferred to the greenhouse, randomized and were subjected to temperature of 20°C and 16 hours of photoperiod. There were total of six replications per treatment and each treatment consisted of 100 mg L^-1 N and 60 mg L^-1 K 80 mg L^-1 of micronutrient (Micromax) and one of 6 doses of P (0, 1, 2, 4, 6, 8 mg L^-1) mixed in irrigation water. Plants were watered regularly using a target leaching fraction of 15%. Plant physiological parameters were measured three times over the course of the study. Quantum yield efficiency of photosystem II (Φ_PSII), photochemical quenching (qp), maximum carboxylation rate of rubisco (Vcmax), maximum rate of electron transport (J), maximum rate of triose phosphate use (TPU), dark respiration (Rd), mesophyll conductance (gm) were estimated based on gas exchange and chlorophyll fluorescence measurements. Shoot growth and chlorophyll content (SPAD index) were also assessed periodically during the study. Plant shoot and root mass, specific leaf mass and leaf area were measured following destructive harvest at the end of the trial. We developed phosphorous budgets for each plant by calculating the total amount of phosphorus applied via irrigation and the amount lost via leaching. Total plant phosphorous uptake was estimated from destructive plant harvests at the end of the study and harvests of a subsample of plants at the start of the trial. Likewise, changes in container substrate phosphorous content were estimated from pre- and post-study samples. Rate of photosynthesis, quantum yield efficiency, plant height and root to shoot ratio differed among the treatments. Total phosphorus uptake was largely driven by plant growth stages and high phosphorus application caused luxury consumption in all the species. Finally, optimum phosphorus dose was found to be species specific.