Microautoradiographic detection of CO₂ fixation in lenticel chlorenchyma of youngFraxinus excelsior L. stems in early spring

Abstract

Microautoradiography indicated that 1-year-oldFraxinus excelsior L. stem chlorenchyma assimilated external¹⁴CO₂ in mid-April, when buds were swollen, but before bud-break. The lenticel regions showed the highest amount of radioactively labeled assimilates. Labeled assimilates declined in the tangential direction with increasing distance from lenticels, suggesting that¹⁴CO₂ entered the stem through the open intercellular spaces of lenticels. In the radial direction, the amount of radioactively labeled assimilates did not constantly decline with growing distance from the lenticel entrance. It was high in all lenticel phelloderm cells, which had high chlorophyll autofluorescence and very small starch grains, highest in the adjacent 4–6 rows of chlorenchyma, which had larger starch grains that increased in size towards the interior rows, and much lower in the inner cortex chlorenchyma, which had large starch grains. We suggest that the main function of the lenticel chlorenchyma (lenticel phelloderm plus 4–6 rows of adjacent cortex chlorenchyma) is the refixation of respiratory CO₂ which could easily leave the stem intercellular spaces, rather than the fixation of external CO₂. The lenticel chlorenchyma could reduce the loss of respiratory CO₂ by its photosynthetic activity.