The effects of root cooling and the light-dark transition on growth of primary leaves of Phaseolus vulgaris

Use of Rotary Variable Displacement Transducers attached to growing primary leaves of Phaseolus vulgaris has shown that when the root systems were rapidly cooled from 23°C to 10°C leaf extension rate fell to very low values within a few minutes. When the root systems were returned to 23°C leaf extension increased almost immediately to overshoot the control rates within 5-10 min, before declining to control values over the next 50-60 min. When lights went off at the end of the day cycle there was an immediate and very large increase in leaf extension rate in both root-cooled and control plants; the rate then slowly declined over the next 60 min. This effect was seen when the photoperiod was artificially shortened or lengthened and was reduced in magnitude when the photoperiod was ended gradually by dimming the lights. The effect was not attributable to effects on leaf temperature but appears to be the result of an endogenous rhythm interacting with the end of the photoperiod. At the beginning of the photoperiod there was a gradual reduction in leaf extension rate occurring over 30-45 min.