Distribution of dry weight and of zinc and copper among the individual leaves of seedlings of subterranean clover (Trifolium subterraneum L.) grown in complete culture solution and in a culture solution deficient in zinc

Abstract

Changes in the distribution of dry weight, and of zinc and copper, between root plus stem and the leaflets and petioles of the individual leaves of young plants of Trifolium subterraneum L. var. Bacchus Marsh grown in zinc-deficient culture solution, have been traced during a period of 3 weeks. These changes were compared with those which took place in control plants grown in a culture solution which was adequately supplied with zinc. In control plants the dry weight and zinc content of each leaf exceeded that of the previous one. The dry weights of the cotyledons, the primary leaf, and the first trifoliate leaf on these plants began to decrease slowly after reaching a maximum, but there was no decrease in the dry weights of the younger leaves during the period of observation. The zinc content of each leaf on the control plants began to decrease rapidly after it had reached a maximum. Maximum zinc content was reached before maximum dry weight. These changes in dry weight and zinc content led to a decreasing concentration of zinc in each leaf as it emerged and unfolded, and they were responsible for wide differences between the concentrations of zinc in the leaves present at any one time. Symptoms of zinc deficiency were recognizable 25 days after germination in the plants grown in zinc-deficient cultures; a distinctive chlorosis appeared in the second trifoliate leaf when the leaflets began to unfold. Development of the third leaf was retarded and that of the fourth arrested in the affected plants, but the dry weight of the whole plant continued to increase at almost the same rate as that of control plants for a week after the chlorosis was first observed. The reason for this was traced to an abnormal increase in the weight of each of the oldest leaves after the onset of zinc deficiency. Zinc was transported more rapidly out of the cotyledons and primary leaf of zinc-deficient plants than out of corresponding leaves of control plants. The amounts of zinc in the trifoliate leaves of zinc-deficient plants were much less than those in corresponding leaves of the controls: a decreasing concentration of zinc which was observed in the trifoliate leaves of zinc-deficient plants was brought about by increases in dry weight without any appreciable change taking place in the amounts of zinc present. The changes that were observed in the concentrations of zinc in individual leaves of zinc-deficient and normal plants indicated that zinc was present in the leaves in highest concentration before they emerged and unfolded. Radioautographs showing the distribution of ⁶⁵Zn in young plants confirmed this. Copper was transported out of the cotyledons of control plants. It was also transported out of the primary and trifoliate leaves of these plants, but only after a period of accumulation. The leaves took longer to accumulate a maximum amount of copper than to accumulate a maximum amount of zinc. The onset of zinc deficiency did not have a marked effect on the concentration of copper in the leaves, because abnormal increases in the dry weights of the leaves were accompanied by corresponding accumulations of copper.