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Title: Phosphorus Deficiency in Pelargonium: Effects on Nitrate and Ammonium Uptake and Acidity Generation

Author
item TAYLOR, MATTHEW - NC STATE UNIVERSITY
item NELSON, PAUL - NC STATE UNIVERSITY
item Frantz, Jonathan
item RUFTY, THOMAS - NC STATE UNIVERSITY

Submitted to: Journal of Plant Nutrition
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/25/2009
Publication Date: 3/1/2010
Citation: Taylor, M., Nelson, P., Frantz, J., Rufty, T. 2010. Phosphorus Deficiency in Pelargonium: Effects on Nitrate and Ammonium Uptake and Acidity Generation. Journal of Plant Nutrition. 33:701-712.

Interpretive Summary: The cause of sudden substrate pH decline by geranium is unknown and may be due to a shift in the balance in uptake between positively and negatively charged ions. Nitrogen plays an important role in this balance since it accounts for over 50% of the mineral ions that a plant takes up and is the only mineral nutrient that can be absorbed as a positive ion (NH4+) or negative ion (NO3-). Studies have shown that phosphorus deficiency will suppress NO3- uptake or simultaneously suppress NO3- uptake while increasing NH4+ uptake. These changes cause a shift in the positive-negative ion uptake ratio and if favored towards NH4+ would cause pH decline in the rootzone. This experiment compared geraniums (Pelargonium x hortorum Bailey ‘Designer Dark Red’) grown at 2 temperatures (18/22 and 22/26° C night/day) and with or without phosphorus. 3, 11, and 19 days after transplanting (DAT), plants were placed fed either 15NH4+ or 15NO3- for 1 day. Uptake of 15NH4+ and 15NO3- was not affected by temperature and decreased by phosphorus starvation. The 15NH4+:15NO3- uptake ratio increased significantly from 0.326 to 0.400 when plants did not receive P, suggesting a decrease in rootzone pH by phosphorus-starved plants was possible. Plants receiving phosphorus decreased the pH at less than half the rate as plants that were devoid of phosphorus. These data indicate phosphorus-stressed geraniums suppress the uptake of nitrogen (primarily NO3-) and increase the rate of acidification, which offers one explanation for sudden substrate pH decline.

Technical Abstract: The cause of sudden substrate pH decline by geranium is unknown and may be due to a shift in cation-anion balance. Nitrogen plays a very important role in cation-anion balance since it accounts for over 50% of the mineral ions that will cross the plasma membrane and is the only mineral nutrient that can be absorbed as a cation (NH4+) or anion (NO3-). Studies have shown that P deficiency will suppress NO3- uptake or suppress NO3- uptake and increase NH4+ uptake. These changes cause a shift in the cation-anion uptake ratio and if favored towards NH4+ would cause acidification of the rhizosphere. This experiment compared geraniums (Pelargonium x hortorum Bailey ‘Designer Dark Red’) grown in 200 L hydroponic tanks at 2 temperatures (18/22 and 22/26° C night/day) and with or without P. 3, 11, and 19 days after transplanting (DAT), plants were placed into an identical solution contain either 15NH4+ or 15NO3- for 24 hours. Influx of 15NH4+ and 15NO3- was not significantly affected by temperature and was suppressed by P starvation. The 15NH4+:15NO3- influx ratio increased significantly from 0.326 to 0.400 when plants did not receive P, suggesting an increased acidification rate by P starved plants. pH was maintained at 5.8 throughout the experiment and tanks with plants receiving P consumed less than half the mEq of titrating base per gram dry weight plant than tanks with plants that were devoid of P. These data indicate P stressed geraniums suppress the uptake of N (primarily NO3-) and increase the rate of acidification, which offers one explanation for sudden substrate pH decline.