Germination and Photomorphogenesis

Many small seeds, such as lettuce, need light to germinate. Small seeds have few food reserves, so a seedling that germinated deep in the soil or under other plants could run out of reserves before it reached the light.Light-sensitive seeds contain phytochrome. When they absorb red light, P_r is converted into P_fr, the active form. P_fr triggers germination, partly by stimulating the synthesis of gibberellin (GA). Gibberellin switches on genes for enzymes such as amylase, which break down stored starch so the embryo can grow.Seeds under a leaf canopy receive light that has passed through leaves. Chlorophyll absorbs red light strongly but lets far-red light pass, so this light has a low red : far-red ratio. Most of the phytochrome stays as P_r, and germination is inhibited until a gap opens in the canopy. Seeds buried in the soil stay in darkness, so they do not germinate either.After germination, a seedling that is still underground grows as an etiolated shoot, elongating quickly towards the surface. As soon as it breaks through into the light, the P_fr formed triggers de-etiolation: the hook opens, stem elongation slows, the leaves expand and chlorophyll is made.So phytochrome links two stages of the plant's early life. It decides when the seed germinates and how the seedling grows once it reaches the light. In both cases the plant is using light as information about its environment, not as a source of energy.