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Diurnal changes in phosphoenolpyruvate carboxylase and pyruvate, orthophosphate dikinase properties in the natural environment: interplay of light and temperature in a C 4 thermophile
Author(s) -
Gramnaatikopoulos George,
Manetas Yiannis
Publication year - 1990
Publication title -
physiologia plantarum
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.351
H-Index - 146
eISSN - 1399-3054
pISSN - 0031-9317
DOI - 10.1111/j.1399-3054.1990.tb05683.x
Subject(s) - phosphoenolpyruvate carboxylase , phosphoenolpyruvate carboxykinase , pyruvate carboxylase , biology , enzyme , enzyme assay , noon , carboxy lyases , botany , biochemistry , physics , astronomy
Activities of phosphoenolpyruvate (PEP) carboxylase (EC 4.1.1.31) were measured in leaf extracts of field grown Amaranthus paniculatus L. (C 4 ) during a natural diurnal irradiance and temperature pattern. Enzyme assays were run at both fixed (30°C) and the corresponding leaf temperature at the time of harvest. Light activation of PEP carboxylase (PEPCase) at fixed assay temperatures was expressed as a decrease in S 0–5 (PEP) after a threshold (> 330 μmol m –2 s –1 ) photon fluence rate was surpassed at noon. Earlier in the morning, increase in apparent enzyme affinity for PEP was observed when the assay was run at leaf temperature, indicating a physiologically meaningfull effect of temperature on S 0.5 (PEP). The 3.3‐fold increase in PEPCase activity at low PEP and fixed assay temperature between the minimal and maximal irradiance and temperature hours of the day, became 12.8‐, 11.5‐ and 7.4‐fold when assays were run at the corresponding leaf temperature during three diurnal cycles with respective temperature differences (max minus min) of 9.0, 8.3 and 7.4°C. The extent of malate inhibition was the same for both day and night forms of PEPCase assayed at 35°C, but increased considerably with night enzyme at 25°C. The results indicate that light increases the apparent affinity of PEPCase for PEP and that at lower temperatures malate becomes more inhibitory. Pyruvate orthophosphate dikinase activity started to increase immediately after sunrise and the 10‐fold increase at fixed temperature became 14.8‐, 14.2‐ and 13.1‐fold when assays were run at the above leaf temperatures. This indicates that the light effect predominates with pyruvate, orthophosphate dikinase, while with phosphoenolpyravate carboxylase, light and temperature co‐operate to increase the day enzyme activities.