Photomorphogenesis in Chenopodium album. Effects of supplementary far-red light on the kinetics of stem extension.

The effects of far-red light given against a background of white light on the stem-extension kinetics of three-week-old, light-grown Chenopodium album seedlings were investigated. Under white light alone, the stems (cotyledon-to-apex) extended almost exactly logarithmically with time. Under these conditions the increase in log [stem length in mm] per hour was approx. 3.7·10(-3), equivalent to about 1% per h during both skoto-and photoperiods. Supplementary far-red given throughout each photoperiod massively stimulated extension. The calculated logarithmic extension rate, however, slowly returned to that of the controls, following an initial large increase. This is predicted by a model in which far-red light linearly increases the extension rate of individual internodes which arise at an exponentially increasing rate. The behaviour of the model is also consistent with critical experiments in which far-red was given as a pre-treatment or transiently, as well as with other published data. Far-red stimulation of logarithmic extension rate in successive photoperiods was closely and linearly correlated with calculated phytochrome photoequilibrium. Daily short periods of supplementary far-red were especially potent in accelerating extension; the plants seemed least responsive at the end of the photoperiod.