The growth physics and water relations of red-light-induced germination in lettuce seeds : II. Embryos germinating in water.

A study of photodormant lettuce embryos germinating in water showed that red light induces an increased rate of water uptake. Determinations of the water potential, carried out by a modified gravimetric technique which eliminates errors introduced by solute penetration into cellular osmotic space, showed that the water potential of embryos germinating in water after dark and red light treatment was equivalent and equal to the osmotic potential of a 0.0 to 0.1 molal mannitol solution. Osmotic potentials of the embryos were determined using two new methods. One of the methods utilizes penetration of deuterated water; the other, penetration of a labeled osmoticum into the tissue. For both light- and dark-treated embryos in water, the osmotic potential was equivalent to that of a 0.34 to 0.41 molal mannitol solution. Lettuce embryos thus require that turgor pressure reach a threshold considerably above zero before growth can occur.