Sugar Efflux from Maize (Zea mays L.) Pedicel Tissue

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Abstract
Sugar release from the pedicel tissue of maize kernels was studied by removing the distal portion of the kernel and the lower endosperm, followed by replacement of the endosperm with an agar solute trap. Sugars were unloaded into the apoplast of the pedicel and accumulated in the agar trap while the ear remained attached to the maize plant. The kinetics of 14C-assimilate movement into treated vs. intact kernels were comparable. The rate of unloading declined with time, but sugar efflux from the pedicel continued for at least 6 h and in most experiments the unloading rates approximated those necessary to support normal kernel growth rates. The unloading process was challenged with a variety of buffers, inhibitors and solutes to characterize sugar unloading from this tissue. Unloading was not affected by apoplastic pH or a variety of metabolic inhibitors. Although p-chloromercuribenzene sulfonic acid (PCMBS), a nonpenetrating sulfhydryl group reagent, did not affect sugar unloading, it effectively inhibited extracellular acid invertase. When the pedicel cups were pretreated with PCMBS, at least 60% of sugars unloaded from the pedicel could be identified as sucrose. Unloading was inhibited up to 70% by 10 mM CaCl2. Unloading was stimulated by 15 mM ethyleneglycol-bis(.beta.-aminoethyl ether)-N,N,N'',N''-tetraacetic acid which partially reversed the inhibitory effects of Ca2+. Apparently passive efflux of sucrose occurs from the maize pedicle symplast followed by extracellular hydrolysis to hexoses.