2007 Annual Report 1a.Objectives (from AD-416) The objective of this cooperative project is. 1)identifying the signal that initiates sudden pH decline (SPD) syndrome in geranium and learn to control that signal,. 2)identify parent germplasm in order to establish the genetic relationships between susceptible cultivars, and. 3)develop linkages between SPD in geranium with SPD in marigold and the reciprocally related problem of excess pH increase in other bedding plant species. 1b.Approach (from AD-416) A list of geranium cultivars known to be susceptible and resistant to SPD will be compiled through personal communications with geranium breeders, propagators, and the Ornamental Plant Germplasm Center. Cultivars will be collected that represent the categories Seed-hybrid, Zonal-cutting, Zonal-ivy, and Zonal-Regal and tested for their susceptibility to SPD syndrome. With these lists, we will work with breeders to establish the genetic relationships of the susceptible cultivars. The signal that turns on SPD in geranium will be found by testing specific environmental stresses such as iron or phosphate deficiencies, shifts in ammonium:nitrate ratios or cation:anion balance. We will determine how to manipulate the signal to avoid turning on SPD. Literature searches will be used to identify other susceptible species. Information gathered in determining the causes of SPD will be used to evaluate similar signals for the analogous problem of excess pH increase in other bedding plant species (calibrachoa, pansy, petunia, and vinca). 3.Progress Report This report documents research conducted under a Specific Cooperative Agreement between ARS and North Carolina State University. Additional details of the research can be found in the report for the parent project 3607-21000-011-00D Develop Improved Technologies for Soilless Greenhouse Plant Production to Minimize Water, Labor, Agrochemical Inputs and Environmental Impacts. Our findings show that low phosphorus stress can bring about this sudden, large pH decline in solid root substrate and hydroponically grown geranium. Studies supplying positively charged ammonium and negatively charged nitrate forms of nitrogen, labeled with the stable isotope 15N, demonstrated that low phosphorus stress suppressed nitrogen uptake. More importantly, nitrate uptake was suppressed more than ammonium. This resulted in the extreme substrate pH depression brought about undoubtedly by an increase in the cation to anion uptake ratio. We also found high temperature causes pH depression. However, high temperature does not shift the ammonium to nitrate uptake ratio thus, a second mechanism of pH decline is involved. These accomplishments address NP-305, Component I, Problem Statement C. The ADODR monitored activities regularly through telephone conferences and an annual meeting with the PI and his graduate student.