Submitted to: Journal of the American Society for Horticultural Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/2/2008
Publication Date: 10/1/2008
Publication URL: hdl.handle.net/10113/19240
Citation: Taylor, M., Nelson, P., Frantz, J. 2008. Substrate Acidification by Geranium (Pelargonium x hortorum) II: Light Effects and Phosphorus Uptake. Journal of the American Society for Horticultural Science. 133:515-520. Interpretive Summary: Some crops cause a sudden decrease in pH and the cause of this decrease is not known. Geranium is one such crop. The geranium variety ‘Designer Dark Red’ were grown in a series of three experiments to test the effects that light intensity has on pH decline and phosphorus uptake. In the first experiment, four light intensities (5%, 10%, 25% and 50% full sun) were tested. After nine weeks of growth, substrate pH declined as light intensity increased. Phosphorus concentration in leaf tissue was extremely low at the highest light intensities (0.10-0.14% of dry weight vs 0.2% or higher for “healthy” plants). Low phosphorus supply has been reported to cause acidification by itself, and therefore it was not clear if the decrease in pH was caused by light or phosphorus deficiency. The second experiment was designed to test this using the three highest light treatments at multiple phosphorus supply rates. In this experiment, pH decreased with low phosphorus supply and high light appeared to suppress phosphorus uptake. A third experiment was conducted in hydroponics (liquid root zone) to determine the effect of high light intensity on phosphorus uptake. In this experiment, the total amount of phosphorus taken up by the plants (standardized for root size) and the rate of that uptake both decreased 20% when light intensity increased from 25% to 50% full sun. Therefore, high light triggered less phosphorus uptake (phosphorus uptake suppression) and that the lower phosphorus uptake resulted in pH decreases.
Technical Abstract: Sudden pH decline (SPD) describes the situation where crops growing at an appropriate pH, suddenly (1-2 weeks) cause the substrate pH to shift downward one to two units. ‘Designer Dark Red’ geraniums (Pelargonium x hortorum Bailey) were grown in 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. At 63 d, substrate pH declined from 6.0 to 4.8 as light intensity increased. Tissue P of plants grown at the highest two light levels was extremely low (0.10-0.14% of dry weight). P stress has been reported to cause acidification. 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 singular light effect or a combination of light and P. The second experiment tested a factorial combination of the three highest light levels from experiment one 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 high light intensity. This indicated that P deficiency caused substrate acidification and P uptake was suppressed by high light intensity. A third experiment was conducted in hydroponics to determine the effect of high light intensity on P uptake. In this 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. It is clear from this study that P deficiency causes geraniums to acidify the substrate and that high light suppresses P uptake.