Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


Plant, Enviromental and Soil Sciences

First Advisor

Donald L. Robinson


Aluminum impairs uptake of Mg$\sp{2+}$ by plants grown in soil or in nutrient solution, but mechanisms of this inhibition are not understood. Experiments were prepared to elucidate the effects of pH (6.0 vs. 4.2) and Al$\sp{3+}$ activity (0, 6.6, and 26 umol L$\sp{-1}$) in nutrient solution on the kinetics of net Mg$\sp{2+}$ uptake by 23-day-old plants of ryegrass (Lolium multiflorum Lam., cultivars Gulf and Wilo). The kinetics of Mg$\sp{2+}$ uptake was resolved in: (1) a transpiration-dependent uptake component, (2) a metabolically-mediated, discontinuous saturable component that is Al$\sp{3+}$-sensitive and 4-(chloromercuri)benzenesulfonic acid (PCMBS)-resistant, and (3) a linear, carbonyl cyanide 3-chlorophenylhydrazone (CCCP)-resistant, Al$\sp{3+}$-sensitive component that might be a type of facilitated diffusion. Aluminum (Al$\sp{3+}$) exerted competitive, and pH non-competitive inhibition of net Mg$\sp{2+}$ uptake. Cultivar-specific ability to retain a higher affinity for Mg$\sp{2+}$ by postulated transport proteins, in the presence of Al$\sp{3+}$, might be one of the mechanisms of differential Al tolerance among ryegrass cultivars. An acid Stough soil (thermic Aquic Fragiudult), amended with 3 rates each of CaO and Mg, was used to grow the same ryegrass cultivars in pots in the greenhouse. Net Mg$\sp{2+}$ uptake increased with increased rates of Mg and lime application, but decreased with plant age. Linear relationships between exchangeable and soil solution Mg$\sp{2+}$ had sloped (representing Mg$\sp{2+}$ buffer power) of 0.68, 1.31, and 1.21 at lime rates 0, 2.24, and 6.72 Mg ha$\sp{-1}$, respectively. Exponential regression equations of the form Y = a + be$\sp{\rm cX}$ described the relationship between root surface area or shoot dry weight and soil solution Al$\sp{3+}$ activity. Multiple regression equations employing calculated concentrations of Mg$\sp{2+}$, Ca$\sp{2+}$, K$\sp+$, and monomeric hydroxy-Al species at the root surface were developed to explain variations in net Mg$\sp{2+}$ uptake. Observed Mg$\sp{2+}$ uptake was consistently overpredicted (1.15 to 1.70 times at decreasing Mg application rates) by the Barber-Cushman nutrient uptake model. Sensitivity analysis revealed that large values for maximal net Mg$\sp{2+}$ influx caused the overprediction. Using input parameters based on Mg$\sp{2+}$ activities decreased overprediction of Mg$\sp{2+}$ uptake.