The Effects of Root Cooling and the Light--Dark Transition on Growth of Primary Leaves of Phaseolus vulgaris L.

Use of Rotary Variable Displacement Transducers attached togrowing primary leaves of Phaseolus vulgaris has shown thatwhen the root systems were rapidly cooled from 23°C to 10°Cleaf extension rate fell to very low values within a few minutes.When the root systems were returned to 23°C leaf extensionincreased almost immediately to overshoot the control rateswithin 5–10 min, before declining to control values overthe next 50–60 min. When lights went off at the end ofthe day cycle there was an immediate and very large increasein leaf extension rate in both root-cooled and control plants;the rate then slowly declined over the next 60 min. This effectwas seen when the photoperiod was artificially shortened orlengthened and was reduced in magnitude when the photoperiodwas ended gradually by dimming the lights. The effect was notattributable to effects on leaf temperature but appears to bethe result of an endogenous rhythm interacting with the endof the photoperiod. At the beginning of the photoperiod therewas a gradual reduction in leaf extension rate occurring over30–45 min. Key words: Phaseolus vulgaris, leaf growth, extension rate, root cooling, wall extensibility, turgor