Changes in the content and form of magnesium in the first trifoliate leaf of subterranean clover under altered or constant root supply

Abstract

Little is known of the forms of magnesium (Mg) in leaf tissue and how these forms may differ with increasing and decreasing Mg concentration in the leaf. In the present study, plants of subterranean clover (Trifolium subterraneum L.) were grown in solution, with increasing, decreasing and stable root supply of Mg over the range 0-160 8m. Old leaf tissue was sampled in six harvests over 21 days. The chlorophyll was extracted from the leafwith dimethyl sulfoxide. This partitioned Mg into that in either the chlorophyll or residual fibre after extraction, and that Mg unaccounted for by these fractions ('other'). The forms of Mg in leaves could be related to the Mg concentration in the leaf and were not influenced by whether the concentration in the leaf was increasing or decreasing. Chlorophyll concentration was stable at about 1% when the concentration of Mg in the leaf was greater than 776 8g g-1. Below this Mg concentration, the concentration of chlorophyll declined rapidly. The proportion of total leaf Mg in the chlorophyll ranged from 6% at 5000 8g g-1 Mg in leaf to 35% at 700 8g g-1. Magnesium associated with the residual fibre was generally 80% of the total tissue Mg, although this appeared to be about 60% when Mg concentrations in leaves were < 1000 8g g-1. When the Mg concentration in the first trifoliate leaf was below 600 to 800 8g g-1, the leaf showed symptoms of Mg deficiency and became relatively heavier than non-deficient leaves.