RESUMO
The leaves of Acer negundo L. var. odessanum (H. Rothe), if permanently exposed to strong sunlight, do not green, but remain yellow and finally become bleached. In yellow leaves the plastids contain single thylakoids and no grana. In plastids of bleached leaves, however, only vesicles are present. The concentration of chlorophylls and photosynthetic activity are much lower in those leaves than in the green ones. If the illumination is reduced (e.g. by shading) both the yellow and the bleached leaves become greenish, and even fully green after a few days at a sufficiently low light intensity. The plastids of yellow-green leaves contain small grana. In dark green leaves the thylakoid system of the chloroplasts is normally developed forming true grana, regardless of whether the leaves were originally green, or became green by shading the yellow or bleached ones. Their pigment concentration and photosynthetic activity are also normal. If green leaves are exposed to sunlight they do not yellow or bleach. During a 3-week period the structure of the thylakoid system did not perceivably change, with the exception that large plastoglobules formed in the stroma.
