Rhythms in glutamine synthetase activity, energy charge, and glutamine in sunflower roots [Helianthus annuus]

• Published in: Plant physiology (Bethesda) 1982-12, Vol.70 (6), p.1683-1688
• Main Authors: Knight, Thomas J., Weissman, Gerard S.
• Format: Article
• Language: English
• Subjects: Adenine nucleotides; Allosteric regulation; Amino acids; Circadian rhythm; Crop harvesting; Enzymes; Photoperiod; Plant roots; Plants; Sunflowers
• ISSN: 0032-0889; 1532-2548
• DOI: 10.1104/pp.70.6.1683

Roots of sunflower plants (Helianthus annuus L. var. Mammoth Russian) subjected to L12:D12, L18:D6, and L12:D12 followed by continuous light all display rhythms of about 12 hours for glutamine synthetase (GS) activity (transferase reaction) with one peak in the 'light phase' and one in the 'dark phase.' Root energy charge (EC = ATP+½ADP/ATP+ADP+AMP) is directly correlated with GS, but the GS rhythm is better explained as the result of a rhythmic adenine nucleotide ratio (ATP/ADP+AMP) that regulates enzyme activity through allosteric modification. When L12:D12 plants are subjected to free-running conditions in continuous darkness, only diurnal rhythms for GS and EC, with peaks in the dark phase, remain. The 12-hour root rhythms for GS and EC appear to be composed of two alternating rhythms, one a diurnal, light-dependent, incompletely circadian light phase rhythm and the other a light-independent, circadian dark phase rhythm. Only glutamine, of the root amino acids, displays cyclical changes in concentration, maintaining under all conditions a 12-hour rhythm that is consistently synchronized with, but nearly always inversely correlated with, GS and EC rhythms.