CONTINUOUS FAR-RED LIGHT ACTIVATES PLASTID DNA-SYNTHESIS IN PEA LEAVES BUT NOT FULL CELL ENLARGEMENT OR AN INCREASE IN PLASTID NUMBER PER CELL

The development of the primary pea (Pisum sativum L.) leaf was charact erized in plants grown in darkness, white light, or continuous far-red light. Leaf growth in continuous far-red light was a high-irradiance response most likely mediated by the phytochrome PHYA. Leaf growth in darkness and far-red and white light was similar up to 4 d postimbibit ion (dpi). From 4 to 8 dpi, leaf length and leaf cell number increased more rapidly in plants exposed to white and far-red light compared to dark-grown seedlings. Furthermore, illumination of plants with contin uous far-red light from 2 to 4.5 dpi potentiated leaf growth in white light. White light, but not far-red light, was able to increase leaf c ell size and plastid number per cell during seedling development from 8 to 12 dpi. Growth of plants in continuous far-red light for 8 d foll owed by transfer to white light did not allow full leaf development. T herefore, although continuous far-red light can stimulate increases in leaf cell number from 4 to 8 dpi, other photoreceptors must be activa ted during this same period to allow full leaf growth. Plastids of dar k-grown plants contained 120 copies of the plastid DNA from 5 to 9 dpi . In white-light-grown plants, plastid DNA synthesis activity and DNA copy number increased between 5 and 6 dpi and DNA copy number reached 450 at 6 dpi. Far-red light illumination induced even higher rates of DNA synthesis, and plastids in this condition accumulated 830 copies o f the plastid DNA by 6 dpi. Plastid DNA copy number decreased with fur ther leaf and chloroplast development primarily through dilution of DN A into new plastids. This study demonstrates that different photorecep tors control specific subprograms of leaf development.