Diurnal Changes in Asparaginase Activity in Pea Leaves

During the growth of leaves of Pisum sativum L., levels of asparaginase (E.C. 3.5.1.1) showed a diurnal variation during a 3 d period of leaf expansion, increasing in the light and decreasing in the dark period; the greatest diurnal variation being found in half-expanded leaves. Asparaginase activity in half-expanded leaves reached a maximum after 4 h exposure to light and this activity was maintained over the rest of the light period. Changes in asparaginase activity were not influenced by diurnal temperature changes. The increase in asparaginase activity during the light period was directly proportional to the photon flux density over the range 0–285 μmol m-2 s-1 PAR. The increase of asparaginase activity during illumination of detached leaves was inhibited by the photosynthetic electron transport inhibitors 3-(3′, 4′-dichlorophenyl)-1, 1-dimethylurea (DCMU) and atrazine. These observations indicate that the increase in asparaginase activity in half-expanded leaves is dependent upon non-cyclic electron transport.