2006 Annual Report 4d.Progress report. This report serves to document research conducted under a Specific Cooperative Agreement between ARS and North Carolina State University. Additional details of research can be found in the report for the parent CRIS 3607-21000-011-00D Develop Improved Technologies for Soilless Greenhouse Plant Production to Minimize Water, Labor, Agrochemical Inputs and Environmental Impacts. The objective of this cooperative project is to identify the environmental or genetic signal that initiates sudden pH decline (SPD) syndrome in geranium and learn to control that signal. The first of five experiments compared plants with all flowers removed to plants that were allowed to flower. Experiment 2 compared plants grown at various low light levels. Experiment 3 compared plants grown at 4 temperatures (14/10, 18/14, 22/18 and 26/22º C day/night). Experiment 4 was a repeat of experiment 1 and experiment 5 was a factorial combining the 3 highest light levels and the 3 highest temperature levels. Plants allowed to form flowers had a final substrate pH of 6.3 compared to 5.7 for plants where flowers were removed. With increasing increments of temperature, substrate pH declined from 6.8 to 4.6 and with increasing light intensity from 6.1 to 4.8. There was no effect of flower removal in experiment 4. Light and temperature had no consistent effects in Experiment 5 throughout 46 days after planting with most pH values remaining in the acceptable range of 5.6 to 6.1. By 60 days, temperature treatments began to segregate with pH being highest in the low temperature treatments and lowest, down to 5.5, in the highest temperature treatments. High temperature stimulated geranium acidification in both experiments, with the effect more severe in the first experiment. The flowering and high light effects were not duplicated in the second trial. This indicates that an additional factor is involved in expression of the light, temperature, and flowering control of acidification. Radio-isotopic studies indicate that P level may be critical in determining pH decline and that environmental factors of light and temperature, as well as seedling or cutting quality may determine susceptibility of the plant to pH decline in the growth environment. Current and future studies will determine the importance of interactions between nutrient and environmental stimuli as well as the extent of this phenomenon in varied Pelargonium species.