Irradiance‐dependent senescence of isolated leaves

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Abstract
Isolated leaves from pea (Pisum sativum L. cv. Alaska or the genotype L‐1107), oat (Avena sativa L. cv. Victory), and fuchsia (Fuchsia triphylla L. cv. Koralle) were retained at irradiances between 0 and 130 μmol m‐2 s‐1 PAR (photosynthetic active radiation). Irradiance‐dependent CO2 fixation was measured when the leaves were excised, and time‐dependent changes in light compensation point were determined. If the irradiance was below the light compensation point for CO2 fixation, the respiratory quotient was low, indicating that lipids were respired. The isolated leaves senesced at these low light levels. At higher light intensities the decrease in chlorophyll level was not accompainied by diminished protein level, and the respiratory quotient was close to unity. Only an irradiance equal to or slightly above the light compensation point maintained a stable chlorophyll level for a long time. This suggested that depletion of reserves in darkness or at low levels of irradiance is important for the initiation of the senescence syndrome. At high levels of irradiance, the decrement in chlorophyll level may be caused by photooxidation. Only in leaves placed under an irradiance close to the compensation point, was CO2 fixation able to prevent aging of the leaves.