 Lettuce red and far-red germination experiment

The 1952 experiment used Grand Rapids lettuce seeds on wetted blotters in covered dishes. After 16 hours in darkness, seven lots of 200 received successive alternating exposures: one minute of red light at $580$–$680\mathrm{nm}$, then four minutes above $700\mathrm{nm}$, with one lot removed after each exposure. Treated lots and a dark scientific control remained at $20{}^{\circ }\mathrm{C}$ for two days; germinated seeds were counted. Separate wavelength tests used a two-glass-prism spectrograph.^[ref]

Table 1 shows alternating promotion and suppression according to the final exposure. Red light could restore promotion after far-red light; suppression was incomplete. This reversibility supports a reversible sensing state rather than simple cumulative illumination. The paper did not isolate or identify the pigment; the response concerns this lettuce preparation and timing. Its readout was a two-day germination count, not subsequent seedling performance.^[ref]

The modern phytochrome description distinguishes forms $P_r$ and $P_{fr}$ that interconvert under red and far-red light. This provides a concrete example of signal transduction: light changes a sensing state that affects development. The response should be distinguished from photosynthesis, which supplies chemical resources.^[ref]