Leaf movements of bush bean plants were studied at the relatively low photon flux density of 0.2 mmol/m2 per s, and air temperatures of 25° and 35° C in a growth chamber. A beta-ray gauge system was used to monitor continuously pulvinus water status and bending. Leaf angles were below the horizontal and were linearly related to the soil water content (R>=-0.91 at 25° C and R>=-0.93 at 35° C). The beta-ray transmission maxima coincided with the stem temperature minima in darkness and vice versa when brightness prevailed as the growth chamber temperature varied with the photoperiod. Leaf angle increased linearly with increased beta-ray transmission. The Q10 temperature coefficient, a measure of the metabolic energy requirement for leaf movement between 25° and 35° C was estimated at 1.8, and the corresponding mean Arrhenius constant at 423 kJ/mol for bush bean.