Respiration and Involvement of an Alternative Pathway as Related to Age and Phenological Strategy of the Leaf

Plants of spring wheat ( Triticum aestivum L.) and winter rye ( Secale cereale L.) pursuing different phenological strategies were studied. Respiratory activity, ratio of respiratory pathways, and effect of the alterative pathway (AP) on the Y ATP/glucose coefficient, which expresses the energy efficiency of respiration (EER), were studied over the leaf ontogeny. At 20°C, the respiratory capacity of the wheat leaf was higher than that of rye due to the decrease in rye metabolism in an autumn period of vegetation. Respiration decreased with age and relative growth rate (RGR) of the leaf. In the young leaf whose area was 20–30% of the final value, respiration mainly proceeded by the cytochrome pathway because of energy expenses for de novo synthesis. In the spring wheat leaf, the AP fraction of its respiration increased from 25 to 40% with age; this indicates the AP belonging to maintenance respiration component. In the mature rye leaf, the AP contribution decreased from 35 to 15% of a total respiration that maintained EER during plant adaptation to low temperatures. A change in the direction of respiratory gradient along the leaf was also found. In leaves of different age, the meristematically active zone manifested the greatest values of such indices as rate of respiration, fraction of AP (up to 45% of a total respiration), and rate of thermogenesis; this shows participation of alternative respiration in energy dissipation and energy balance control. Altogether, the value Y ATP/glucose did not change at the level of wheat and rye leaf of different ages. On average, it was 20 mole ATP/mole glucose, which is one third lower than the theoretically assumed value. This may be interpreted so that the metabolic level corresponds to environmental conditions and is adapted to them.