engleski

Fruit initiation in Helleborus niger L. triggers chloroplast formation and photosynthesis in the perianth

The sepals of the Christmas rose (Helleborus niger L.), which are white or rose at anthesis, persist until the seeds are ripe and become intensely green (in shaded plants), or dark red (in sun-exposed plants), during that period. An essential element in this color change is the fact that the leucoplasts present at anthesis start turning into chloroplasts, after pollination. These show ultrastructural features indicating photosynthetic activity, contain high levels of a complete set of chlorophylls and carotenoids, photooxidize 3,3'-diaminobenzidine, as an indicator for the presence of photosystem I, and synthesize light-harvesting complex proteins (LHC II) attributed to photosystem II. The photosynthetic capacity ultimately attained by the sepals, expressed in micromol O2/hour/g fresh weight, is about half that of the leaves present at the same time. These emerge during the final stages of fruit ripening, fully expand after seed maturity, and will survive until next spring, to die back at anthesis of the next generation of flowers, or shortly thereafter. During fruit development, the chloroplasts in the sepals are thus a major source of assimilates. The metamorphosis of the sepals into photosynthetically active organs only occurs in fruit-bearing flowers and is arrested at a very early stage in controls in which the pistils are removed before pollination.

Helleborus niger L.; carotenoid; chlorophyll; fruit-induced morphogenesis; perianth; photosynthesis; plastid metamorphosis

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