Elucidation and Control of Sudden pH Decline of Geranium
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2008-03-04
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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. The cause(s) of SPD by geranium is unknown.
Studies have shown 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. This experiment compared geraniums grown with or without P. Plants were placed into solutions containing either 15NH4+ or 15NO3- for 24 hours. The 15NH4+:15NO3- and the cation:anion influx ratio increased significantly when plants did not receive P, suggesting an increased acidification rate. Plants receiving P consumed less than half the meq of titrating base per gram dry weight than plants that were devoid of 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 temperatures 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 3 temperatures was extremely low. 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 with factorial combinations of three temperatures and five pre-plant P rates was conducted. 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 acidity produced by roots of plants grown at two temperatures day/night was quantified. Plants grown at high temperature had a 28% increase in the meq of acidity 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 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 extremely low. 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 three light levels and five pre-plant P rates. 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. 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. It is clear from this study that high light suppresses P uptake and may consequently cause SPD.
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Keywords
temperature, light, nitrogen, phosphorus, acidification, pelargonium x hortorum
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Degree
PhD
Discipline
Horticultural Science
