Sunflower

Dark-grown sunflower seedlings

While in the darkness of the soil, seedlings are dependent on stored food reserves in the embryo. Plants put most of their energy into stem elongation and suppress leaf development and chlorophyll production (i.e. they do not turn green). In addition, dark-grown dicotyledonous plants keep the end of the stem hooked and their cotyledons closed together. Presumably, this growth strategy is an adaptation for rapidly emerging from the dark soil.

The movie on the left documents the growth of sunflower seedlings in darkness. Because the seedlings were imaged with infrared illumination, the movie is in black and white. However, the seedlings did not turn green like the light-grown seedlings in the other movie.

Light-grown sunflower seedlings

When a seedling emerges from the soil into the light, growth and development changes dramatically. Stem elongation is suppressed, the apical hook opens and the cotyledons separate, enlarge and turn green as chloroplast's develop for carrying out photosynthesis.

The movie on the right documents the growth of sunflower seedlings under white light.

The movie shows shows approximately 2 days of growth. By advancing the movie frame by frame, the effects of light on plant development can readily be seen.