INVOLVEMENT OF PHYTOCHROME IN LIGHT CONTROL OF STEM ELONGATION IN CUCUMBER (Cucumis sativus L.) SEEDLINGS

Stem growth in cucumber (Cucumis sativus L.) is strongly affected by blue light (B). The pending problem has been to what extent the action of B is related to phytochrome action. Hypocotyl growth in the etiolated cucumber seedling responds weakly to 5 min red (R) and B pulses whereas far‐red (FR) and long wavelength far‐red (RG9) light pulses have no effect. Blue and RG9‐light pulses were given in sequence or 5 min B was applied simultaneously with high fluence rate R (phytochrome photoequilibrium, Pfr/Ptot =∼ 0.8) or RG9‐light (? 0.01) to maintain a high or low during application of the B pulse. The results show that there is no expression of a B effect if the level of Pfr is kept low. A specific effect of B probably occurs but cannot be expressed in the absence of Pfr. Hypocotyl elongation is strongly affected by continuous R, FR and B. Light ← dark transfer experiments suggest that growth in darkness of seedlings de‐etiolated by R or B is controlled by phytochrome through a threshold mechanism. The results of dichromatic experiments (long wavelength far‐red light, 756 nm, applied simultaneously with B or R) show that the action of B on axis elongation is related to the level of Pfr, even in long‐term light. When > 0.01, the action of B is largely abolished. The residual B effect may not be attributed to a phytochrome‐independent action of B on axis elongation, because a small residual effect is also observed with dichromatic R/756 nm‐light.