Energy balance of a plant under normal and unfavourable conditions

Two main components of plant energy balance are analyzed--photosynthesis and dark respiration. Different plant species, growing in optimal conditions and differing in photosynthetic metabolism, productivity and potential resistance to stress, have constant ratio between total dark respiration and grossphotosynthesis. The ratio about 38-40% at the phase of active growth. This value reflect plant state, when income (assimilation) is maximized, and expense (total oxidation) is minimized. Intensities of dark respiration of plants in darkness and light are similar despite the fact that the plants have considerable differences: 1) different carbon sources--young assimilates in light and reserve ones in the dark; 2) biochemical modifications (or inhibition) of some stages of dark respiration in light; 3) intensification of alternative respiration in unfavourable conditions differing in dark and light variants. Ratio between intensity of dark respiration and photosynthesis increases in plants growing in unfavourable conditions. This increase is more considerable in plants that are less resistant to the given stress. Characters of energy balance can be used for estimation of physiological status of plants, prediction of resistance and potential productivity at seedling stage.