Light-enhanced dark respiration in leaves and mesophyll protoplasts of pea in relation to photorespiration,respiration and some metabolites content

The respiration rate of leaves and mesophyll protoplasts of pea (Pisum sativum L.), from plants which were previously kept in darkness for 24 h was doubled following a period of photosynthesis at ambient level of O₂ (21 %), whereas the low level of O₂ (1 % and 4 % for leaves and protoplasts, respectively) reduced this light-enhanced dark respiration (LEDR) to the rate as noted before the illumination. Similarly to respiration rate, the oxygen at used concentrations had no effect on the ATP/ADP ratio in the dark-treated leaves. However, the ATP/ADP ratio in leaves photosynthesizing at 21 % O₂ was higher (up to 40 %, dependence on CO₂ concentration in the range 40–1600 1 dm⁻³) than in those photosynthesizing at 1 % O₂ or darkened at air (21 % O₂). Also, at 1 % O₂ the accumulation of malate was suppressed (by about 40 %), to a value noted for leaves darkened at 21 % O₂. The dark-treatment of leaves reduced the ability of isolated mitochondria to oxidize glycine (by about twofold) and succinate, but not malate. Mitochondria from both the light- and dark-treated leaves did not differ in qualitative composition of free amino acids, however, there were significant quantitative differences especially with respect to aspartate, alanine, glutamate and major intermediates of the photorespiratory pathway (glycine, serine). Our results suggest that accumulation of photorespiratory and respiratory metabolites in pea leaves during photosynthesis at 1 % O₂ is reduced, hence the suppression of postillumination respiration rate.