Magnesium (Mg) and rubidium (Rb) absorption by tall fescue

Abstract

Kinetic experiments were conducted to examine Mg⁺² [²⁸Mg] and K⁺ [⁸⁶Rb] absorption by 2‐week‐old tall fescue (Lolium multiflorum Lam. X Festuca arundinacea Schreb.) seedlings. Magnesium absorption was linear for 180 min and was approximately 3.5 ueq Mg⁺² g dry weight^‐1.h^‐1 from a 0.0625 mM Mg⁺² solution. Shoot accumulation of Mg was reduced by increased solution K⁺. A curvilinear concentration absorption isotherm for roots was obtained for Mg⁺². The resultant hyperbolic Hofstee plot could be separated into two linear components with Km values of 0.833 and 0.0117 mM, suggesting that two systems are operable in Mg⁺² absorption. Increased solution pH increased Mg⁺² absorption both in the presence and absence of NH⁺ ₄. At pH 6.5 NH⁺ ₄ increased Mg⁺² absorption 75%; at pH 3.5 the increase was more than 6‐fold. Potassium absorption was linear through 60 min and was decreased by increased Mg²⁺ levels. Shoot accumulation of [⁸⁶Rb] was decreased by the first increment of Mg⁺², whereas further increases in Mg⁺² did not result in significant decreases. In the presence of NH⁺ ₄, K absorption was significantly decreased in the alkaline range. In the absence of NH⁺ ₄, much higher rates of Absorption were observed and less of a reduction was noted at the higher pH values.

An important factor limiting the genetic capability of Mg accumulation in tall fescue is solution K concentration. These experiments suggest that future selection of improved tall fescue cultivars should be conducted in the presence of high solution K and with the selection criteria of ≥ 0.2 % tissue Mg.

Key words:

• Kenhy
• Lolium X Festuca hybrid
• Tall fescue
• Festuca arundinacea
• Magnesium
• ²⁸Mg
• Potassium
• Rubidium
• ⁸⁶Rb