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 = phi-sim approximately 0.8) or RG9-light (phi-sim approximately 0.01) to maintain a high or low phi 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 phi-sim < 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.