Irreversible Thermodynamics and Biological Evolution in Spinach Leaves as Studied by NMR

Abstract

Summary Changes in molecular dynamics, which can be measured by nuclear magnetic resonance (NMR) methods, are closely related to internal energy and entropy changes and can give a valuable approximation of the total entropy evolution of a system. It is thus possible to analyse the different physiological states of a living organism in terms of its global thermodynamical properties. As an experimental system, leaves of vegetative or floral spinach plants were used, as well as leaves subjected to changes in day length or to chemical treatments which promote or inhibit the induction of the flowering process. The dynamics of the hexafluorobenzene spin-labels in lyophilized leaves indicate that the molecular movements are partially blocked in the floral state, which can thus be considered as more structured and ordered than the vegetative state. The time-dependent changes in NMR line widths of living leaves show a nearly circadian rhythm in entropy production in both the vegetative and the floral states, this latter state being characterized by a lower order of magnitude and a downward trend. This oscillatory behaviour can be interpreted in terms of non-equilibrium thermodynamics and of theory of irreversibility. The biophysical and biochemical changes occurring during the floral induction in the leaves seem to be closely related to fast energy and entropy changes, which may result in a rapid modification of the membrane permeability properties. It is supposed that a new balance in the fluxes of matter and energy may result from these changes and thus give rise to a thermodynamically more stable floral state.