Phloem Unloading in Soybean Seed Coats: Dynamics and Stability of Efflux into Attached `Empty Ovules'

Abstract

The time-course of sucrose efflux from attached seedcoats (having their embryos surgically removed) into aqueous traps placed in the empty ovules had three phases. The first phase lasted 10 minutes and probably was a period of apoplastic flushing. The second lasted 2 to 3 hours and is thought to be a phase of equilibration of seed coat symplast with the frequently refreshed liquid. The third phase of relatively steady efflux was postulated to reflect the continued import of sucrose from the plant, and hence to reflect the rate of sieve tube unloading. The average steady state efflux was equal under most conditions to the estimated rate of sucrose import. Efflux and import were unaffected by 150 millimolar osmoticum (mannitol or polyethylene glycol [molecular weight about 400]), by 0.5 millimolar CaCl2, or by pretreatments up to 20 minutes with p-chloromercuribenzenesulfonic acid (PCMBS); they were enhanced by 40 micromolar abscisic acid, 40 micromolar indoleacetic acid, 20 micromolar fusicoccin, and 1 millimolar dithiothreitol (DTT) and were inhibited by 100 micromolar KCN, by 0.03% H2O2 by 20 micromolar and 5 micromolar trifluoromethoxy (carbonyl cyamide) phenylhydrazone, by repeated 5 minutes per hour treatments with 5 millimolar PCMBS, and by 5 millimolar DTT. The steady state sucrose efflux was able to account for about half the rate of dry weight growth of the embryo, but stabilization of the system with < 1 millimolar DTT taken together with other considerations is likely to give good correspondence between experimental unloading rates and in vivo growth rates.