Source of Respiratory Carbon Evolved from the Shoots of Rice Seedlings in Darkness

Abstract

The time course change of the shoot respiration rate, the percentage of currently assimilated carbon in respiration and the amounts of current and reserved compounds in the shoots of rice plants were examined using the steady-state ¹³CO₂ assimilation technique, in an attempt to evaluate the change of respiratory substrates with time during darkness. The shoot respiration exhibited midnight rise of respiration rate (MRR), and then a sudden decrease around 13 hours after the onset of darkness. The percentage of CO₂ derived from ¹³C-labeled current assimilates in respiration was high during the first 6 hours, and followed by a continuous decrease. Therefore, current assimilates took priority over reserved compounds to support midnight respiration with MRR in the dark period, and as being consumed current photosynthates the respiration using reserved materials increased to compensate the respiration. Since the ¹³C-labeling and the content of sucrose were high during the early dark period, it was suggested that sucrose was the principal source of respiratory substrates, and thereafter, starch and other compounds gradually became the major source of CO₂. The integrated value of the percentage of ¹³C-labeled CO₂ in total respired CO₂ during the first 12 hours was 51%, suggesting respiration dependency on current assimilates was almost equivalent to that on reserved materials.