The Effect of Prolonged Shading on the Light Saturation Curves of Apparent Photosynthesis in Sun Plants.

Abstract

Alanine_C'4 0.53 0.47 0.29 Glutamate-C14 0.85 0.76 0.57 Leucine-C"4 0.18 0.16 0.11 * Amino acid incorporation by intact leaves, attached to the plant, was measured as described previously (7) following application of 10 micromoles (1,000,000 cpm) to the leaf surface. Leaves were treated with biuret or biuret plus urea five days before application of the amino acid. urea mixture decreased markedly. These results sug-gest that biuret does not promote the rate of leaf protein decomposition in any striking manner. Conversely, biuret inhibits the incorporation of C14-alanine, C14-glutamate, and C14-leucine into the leaf proteins of intact Xanthium plants (table I). As is the case with protein level (fig 1), the greatest in-hibition of amino acid incorporation into protein is produced by a mixture of biuret and urea. It would seem, therefore, that the decrease in leaf protein elicited by biuret may be due to an inhibition of pro-tein synthesis. The exact manner in which this in-hibition occurs is not yet clear. It could be due to: (a) an interference with amino acid production; (b) a direct inhibition of protein synthesis; or (c) a sec-ondary effect evoked by the action of biuret on a completely unsuspected site. Likewise, the observa-tion that a mixture of biuret and urea is a more effective inhibitor than biuret alone is susceptible to several interpretations. The fact remains, however, that the mechanism of leaf damage by biuret is probably not a simple case of biuret affecting some obvious phase of urea metabolism. Instead, the effect is apparently either on protein synthesis or on some equally basic process of metabolism that di-rectly affects protein synthesis. SUMMARY Biuret, which elicits marked leaf injury in urea-treated Xanthium plants, has no significant effect on the following metabolic processes: absorption of urea by Xanthium leaves, hydrolysis of urea, oxidation of amino acids, decarboxylation of glutamate, hydrolysis of arginine, and breakdown of leaf protein. In con-trast, biuret produces a sharp decrease in total leaf protein, and inhibits the incorporation of amino acids into leaf proteins. It is concluded that biuret, either directly or indirectly, inhibits protein synthesis. LITERATURE CITED