Some Effects of Potassium and Magnesium on the Growth of Subterranean Clover (Trifolium subterraneum)

Subterranean clover plants (Trifolium subterraneum L., cv. Mt. Barker) were grown in culture solutions at optimum nutrient levels and on the 23rd day after sowing transferred to solutions without magnesium or potassium. A third group remained at the same nutrient levels as before (controls). Magnesium deficiency caused a particularly rapid decline in root growth, followed later by a net loss of root dry matter. This, and the fact that leaf expansion declined relatively more than the increase in laminae dry matter, resulted in an accumulation of assimilates per unit leaf area. Transfer of magnesium-deficient plants to complete solutions on day 35 caused a preferential distribution of dry matter to the roots, then to petioles and relatively less to the laminae. These changes caused a pronounced fall in the root: shoot ratio as the deficiency became more severe and a rise in the ratio during recovery. Plants in solutions without potassium showed no marked shifts in dry matter distribution between plant parts. The root: shoot ratio remained close to that for control plants, except during the recovery, when there was a decrease in the ratio. Net rates of CO2 uptake by laminae from potassium-deficient plants showed little change during the first 10 days of the deficiency although values were somewhat lower than those for corresponding control laminae. After transfer to complete solutions there was a marked response in photosynthesis, rising to a final value close to that for control laminae. Laminae of plants placed in solutions without magnesium showed a rapid decline in photosynthesis only 4 days later; there was little response when plants were transferred to complete solutions a week later.