Thermodynamics of Seed and Plant Growth

Abstract

Living systems are open, irreversible systems, determined by inheritance and dependent on temperature and time. They exchange substances with the environment and they need free energy for life. Living systems transform energy and matter during metabolism, which could be described as a controlled capacity to transform energy, by the First Law of Thermodynamics. Nevertheless, energy transformation includes the loss of some free energy as heat, by the Second Law of Thermodynamics, which as a consequence increases disorder entropy. Plant cells are simultaneously characterized by two opposing types of reactions: endergonic, such as photosynthesis (occurring in green plastids) and exergonic, such as respiration (present in mitochondria). Since the metabolic reactions are controlled, they need activation energy that is provided by biological catalysts – enzymes, which lower the activation energy without its consumption. Nevertheless, the limits in the application of thermodynamics in the biochemistry of living systems are the non-existence of time and total reversibility, as a category, which could be surpassed in plant systems by introducing of temperature sums as important factor of plant development. A living system assimilates high-enthalpy, low-entropy compounds from its surroundings, transforms them into a more useful form of chemical energy and returns low-enthalpy, high-entropy compounds to environment. From this point of view, a living organism must be ordered and cannot be at equilibrium. Steady state in an open system is the analogue of equilibrium in a closed system. From the standpoint of thermodynamics, the normal functions of living systems are enabled by the concomitant presence of two opposing tendencies: the preservation of a steady state and the aspiration to spontaneously transcend a non-equilibrium state. A steady state, i.e., near-equilibrium state is maintained based on minimal energy expenditure (Taiz & Zeiger, 2010). A steady inward flow of energy is the most stable state that an open system can achieve. Furthermore, the last ten years of 20th century were marked by the application of thermodynamics to research of functional (such as erythrocytes) and reproducible (such as Methylobacterium extorquens) cell growth (Holzhutter, 2004). In higher plants, the functional and reproducible parts in seed are connected by the irreversible transfer of hydrolysed monomers from an endosperm (functional) to an embryo (reproducible). The product of seed germination a plant, consists of two reciprocally reversible segments: a root and a shoot, which grow by the simultaneous presence of two processes: cell elongation and cell division. Water plays an important role in growing processes. At the end of the 1960s, Boyer (1969) introduced the energy concept, to quantify water transport into plants. In addition, the input of water was determined as energy input in an essay with seedlings of different crops (Manz et al., 2005; Kikuchi et al., 2006).