Interaction between benzyladenine and fusicoccin on the development of excised watermelon cotyledons. II. Correlation between growth and proton extrusion

Abstract

Treatment of excised watermelon ( Citrullus vulgaris Schrad., cv. Fairfax) cotyledons with 10 −5 M fusicoccin (FC) induces stimulation of growth and a marked decrease in pH of the external medium. Treatment of the same cotyledons with 10 −5 M benzyladenine (BA) also stimulates growth, but does not induce a decrease in pH. When BA and FC are administered together the growth rate is higher than with either of the two regulators alone, but the decrease in pH is slightly smaller than with FC alone. The accompanying paper, Fantelli et al., Plant Sci. Lett., 33 (1984) 267, reports that a 2-day preincubation in 10 -5 M BA prevents the inhibitory effect of FC on BA-induced organelle development. We have measured pH changes of the external medium under these experimental conditions. After transfer to FC the pH decreases for 8 h as observed in absence of preincubation with BA, but then increases again, while without preincubation the pH of the medium continues to decrease for an indefinite time. The results suggest that cytokinin-stimulated cotyledon expansion does not involve the continued operation of a proton pump, thus confirming the data obtained by Ross and Rayle, Plant Physiol., 70 (1982) 1470, with cucumber cotyledons. The activation of the pump by FC probably causes an imbalance in ion transport that blocks BA-stimulated organelle development. The preincubation in BA may help to keep the proton pump under control.