Aluminum Effects on Growth and Macronutrient Uptake by Annual Ryegrass

Abstract

AbstractAnnual ryegrass (Lolium multiflorum Lam.) is an important forage crop in the southeastern USA, often grown on acid soils with potentially high levels of soluble Al. Effects of Al on growth and nutrient relationships in ryegrass are poorly understood. In this study four ryegrass cultivars were grown in a growth chamber in nutrient solutions at 0, 37, 74, 148, 296, and 592 µmol L−1 Al. The pH was initially adjusted to 4.2 and subsequent changes were monitored daily. More Al‐tolerant cultivars induced higher nutrient solution pH than more sensitive cultivars. For more Al‐tolerant cultivars, these pH changes were positively correlated with root and shoot tolerance indexes and negatively correlated with percent inhibition of Ca, Mg, K, and Na uptake. Root (shoot) tolerance index was defined as dry weight (wt.) of roots (shoots) grown with Al/dry wt. of roots (shoots) grown without Al. Changes in nutrient solution pH were effects rather than causes of differential Al tolerance, but once induced, differences in pH intensified cultivar differences in Al tolerance. An Al concentration of 37 µmol L−1 in solution significantly increased root Al concentrations, but very little Al was translocated to shoots. Higher shoot Al concentrations were found more often in the less Al‐tolerant cultivars Wilo and Urbana than in the more tolerant cultivars Gulf and Marshall. Larger root and shoot tolerance indexes, and lower percentages of inhibition of total Ca and Mg uptake were observed more often for more Al‐tolerant cultivars Marshall and Gulf than for more sensitive cultivars Wilo and Urbana. The average net influx of Ca and Mg was decreased where Al was added. Total uptake of K (quantity factor) was decreased by Al, while net K influx (intensity factor) increased for plants grown in the nutrient solution containing up to 74 µmol L−1 Al. Short exposure to Al increased net influx and total Na uptake. Longer exposure to higher concentrations of Al (at least 74 µmol L−1) inhibited both total uptake and net influx of Na. The root tolerance index was positively correlated with net influx of Ca, Mg, K, and Na. Increasing the nutrient solution Al level to 148 µmol L−1 greatly increased K/(Ca+Mg) ratio of shoots, thus increasing the grass tetany potential of the ryegrass forage.