Elucidation and Control of Sudden pH Decline of Geranium

TAYLOR, MATTHEW DARON. Elucidation and Control of Sudden pH Decline of Geranium. (Under the direction of Dr. Paul V. Nelson.) Sudden pH decline (SPD) describes the situation where crops growing at an appropriate pH, rapidly (within 1-2 weeks) cause the substrate pH to shift downward one to two units. The cause(s) of SPD by geranium is unknown. Studies have indicated that P deficiency will suppress NO3 uptake and cause acidification. If NO3 uptake is suppressed, the cation-anion uptake ratio may shift and if favored towards cations, net H + efflux will increase. Geraniums were grown with or without P in hydroponic solutions containing either 15 NH4 + or 15 NO3 for 24 hour periods to determine the impact of low P on NH4 + and NO3 uptake. The 15 NH4 + : 15 NO3 and the cation:anion influx ratio increased significantly when plants did not receive P, suggesting an increased acidification rate. Phosphorus stressed plants produced nearly two times the acidity per gram dry weight of plant than plants that received P, confirming increased acidification due to P deficiency. Geraniums were grown in three experiments to test the effects of temperature on SPD. Experiment one tested the effect of four day/night temperatures (14/10, 18/14, 22/18, 26/22 °C) on substrate acidification. End of crop substrate pH declined from 6.8 to 4.6 as temperature increased. Tissue P of plants grown at the highest three temperatures was deficient. It was not possible to determine if the drop in substrate pH was a temperature effect or a combination of temperature and P. To resolve this, a second experiment tested a factorial combination of the three highest temperatures from the first experiment and five pre-plant P rates (0, 0.065, 0.13, 0.26, or 0.52 g·L -1 substrate). Regardless of tissue P concentrations, which ranged from deficient to above adequate, substrate pH decreased with increasing temperature. The results indicate that temperature acted independent of tissue P and caused geraniums to acidify the substrate. In the third experiment the amount of acid produced by roots of plants grown at two day/night temperatures (22/18 and 26/22 °C) was quantified. Plants grown at high temperature produce 28% more acid produced per gram root. It is clear from this study that high temperature can cause SPD by geranium. Geraniums were grown in a second set of three experiments to test the effects of light on SPD and P uptake. The first experiment tested the effect of four light intensities (105, 210, 575, and 1020 ± 25 μmol·m -2 ·s -1 ) on substrate acidification. End of crop substrate pH declined from 6.0 to 4.8 as light intensity increased. Tissue P of plants grown at the highest two light levels were deficient. Since plants in the two lowest light treatments had adequate P, it was not possible to determine if the drop in substrate pH was a light effect or a combination of light and P. The second experiment tested a factorial combination of the three highest light levels from the first experiment and five pre-plant P rates (0, 0.065, 0.13, 0.26, or 0.52 g·L -1 substrate). When tissue P concentrations were deficient, pH decreased by 0.6 to 1 pH units within two weeks and P deficiency occurred more often with the highest light intensity. These data indicate that P deficiency caused substrate acidification and P uptake was suppressed by high light intensity. In a third experiment, cumulative P uptake per gram root and the rate of P uptake per gram root per day both decreased 20% when light intensity increased from 500 to 1100 μmol·m -2 ·s -1 . This third study confirmed that high light suppresses P uptake, which can consequently cause SPD. Elucidation and Control of Sudden pH Decline of Geranium by Matthew Daron Taylor A dissertation submitted to the Graduate Faculty of North Carolina State University in partial fulfillment of the requirements for the Degree of Doctor of Philosophy Horticultural Science Raleigh, North Carolina