Plant Science Letters, 9 (1977) 153--162 153 © Elsevier/North-Holland Scientific Publishers, Ltd. A PROTON-COTRANSPORT SYSTEM IN A HIGHER PLANT: SUCROSE TRANSPORT IN RICINUS COMMUNIS EWALD KOMOR, MARIANNE ROTTER and WIDMAR TANNER Fachbereich Biologie und Vorklinische Medizin, Botanik I, Universit~t Regensburg, 8400 Regensburg (G.F.R.) (Received November 27th, 1976) (Accepted December 27th, 1976) SUMMARY Sucrose uptake into cotyledons of castor beans (Ricinus communis) proceeds partly by an active transport system and partly by passive perme- ation; a-methylglucoside (a-MG) is almost exclusively transported by the passive route. The Km for active sucrose uptake is 18 mM and the Vmax 4--5 umoles/100 mg fresh weight per 1 h. The tissue is able to accumulate sucrose more than 100-fold. Sucrose but not a-MG induces a respiratory increase, which shows a similar sucrose concentration dependence as active sucrose transport. The addition of saturating amounts of sucrose to the incubation medium leads to a transient alkalinization of the medium. A second addition does not show this effect, neither does a-MG at any time. In the presence of 100 mM K ÷, which depolarizes the membrane potential, the uptake of sucrose is strongly inhibited. The results suggest that active sucrose transport in this tissue is mediated by an electrogenic proton co- transport system. INTRODUCTION An increasing amount of evidence suggests that the main mechanism responsible for active transport of organic solutes into living cells is a co- transport with cations. Thus sugars and amino acids are accumulated in various animal tissues via sodium cotransport systems located in the cyto- plasmic membrane [1--5]. In Escherichia coli/~-galactosides are cotrans- ported with protons [6,7] and the same ion enters a number of fungal cells together with sugars [8--10]. Whereas proton cotransport has also been reported for Chlorella vulgaris [11,12] no case of cotransport of ions with Abbreviation: a-MG, ~-methylglucoside.