Dr. Geiger et Jc. Servaites, DYNAMICS OF SELF-REGULATION OF PHOTOSYNTHETIC CARBON METABOLISM, Plant physiology and biochemistry, 32(2), 1994, pp. 173-183
Studying photosynthesis in intact plants under a diurnal cycle reveale
d important emergent properties of the intact system that would not be
seen from the study of individual components alone. The plant achieve
s acclimation during the changing diurnal light cycle through a combin
ation of light-mediated and self-regulating mechanisms. Carbon flux th
rough the assimilatory segment achieves a general correspondence with
the course of diurnal irradiance by altering the activation state of t
he three light-activated enzymes, phosphoribulokinase (EC 2.7.1.19), r
ibulose 1,5-bisphosphate carboxylase (EC 4.1.1.39) and glyceraldehyde
3-phosphate dehydrogenase (EC 1.2.1.13), in response to changing light
. Because individual enzymes are activated somewhat independently and
do not respond precisely to increasing irradiance level, momentary imb
alances in flux occur. One emergent mechanism of the intact plant, whi
ch integrates these complex interactions and thus refines regulation,
is self-regulation. This type of mechanism provides a means of restori
ng and maintaining balance within the Calvin-cycle to adjust fluxes pr
ecisely among the enzymes within the segment. Simple fluid transfer mo
dels allow one to see how environmental control and self-regulation in
teract to regulate the Calvin-cycle in a precise, responsive and flexi
ble manner. Enzyme activities are influenced by a variety of regulator
y elements which confer a degree of flexibility to regulation. The pat
h to stability can be attained through any one of many combinations of
enzyme activation states and metabolite levels. Consequently, there i
s no unique solution to achieving a given flux rate. Understanding reg
ulation involves the study of the time-dependent process by which stab
ility is achieved. As a result of metabolic flexibility, patterns of m
etabolite levels and enzyme activities will display some basic similar
ities, but will also show marked differences between experiments.