Rhythms in Glutamine Synthetase Activity, Energy Charge, and Glutamine in Sunflower Roots

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Abstract
Roots of sunflower plants (H. annuus L. cv. Mammoth Russian) subjected to L12:D12, L18:D6 and L12:D12 followed by continuous light all display rhythms of .apprx. 12 h for glutamine synthetase (GS) activity (transferase reaction) with 1 peak in the light phase and 1 in the dark phase. Root energy charge (EC = ATP plus 1/2 ADP/ATP plus ADP plus AMP) is directly correlated with GS, but the GS rhythm is better explained as the result of a rhythmic adenine nucleotide ratio (ATP/ADP plus 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-h root rhythms for GS and EC appear to be composed of 2 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-h rhythm that is consistently synchronized with, but nearly always inversely correlated with, GS and EC rhythms.