ABSTRACIIn vitro assimilate uptake and metabolism were evaluated in embryos of known age isolated from seeds at mid-podfilling through physiological maturity. The capacity of isolated Wye soybean embryos to take up exogenous ['4Cjsucrose dropped nearly 4-fold in less than 1 week at incipient cotyledon yellowing. This drop in rate of sucrose uptake coincided with cessation of seed growth as well as rapid decline in leaf photosynthetic rate that preceded leaf yellowing. Conversely, the rate of j3Hjglutamine uptake by cotyledons increased as they yellowed. Yellow cotyledons also rapidly converted exogenous VFHjglutamine to ethanolinsoluble components, but converted little exogenous 1'4Cjsucrose to ethanol-insoluble components, primarily because of greatly reduced sucrose uptake. Sustained import and metabolism of amino acids remobilized from senescing leaves may prolong seed growth beyond loss of photosynthetic competency and sucrose availability.Studies of grain filling in several cereal crops have indicated that the end of photosynthate import and grain growth is related to the crushing of specific cells involved in photosynthate import and the formation of an impenetrable black layer in the hilum region (4, 5). Perhaps in a similar manner, the phloem terminus in soybean seed coats may be crushed as the cotyledons mature (3). From studies of the import of '4C-labeled assimilates into developing soybean seeds, Tekroni et al. (12) concluded that transport in vivo essentially ceased at about the time that the seed coat color changed from green to yellow. Developmental studies by Konno (8) We determined the fresh weight, respiration rate, and dry weight of freshly dissected fruit tissue. Respiration rates were determined from CO2 production by known weights of pods, seed coats or embryos in sealed containers at 260C using IR gas analysis of small samples withdrawn by syringe. We also determined the uptake and metabolism of '4C-labeled sucrose or 3H-labeled glutamine in embryos freshly isolated at given periods after flowering (2, 11, 15). Seeds were frozen, lyophilized, dry weight determined, the relative '4C or 3H determined by combusting one cotyledon in a biological oxidizer (15). Photosynthetic rate of the upper, most recent fully expanded leaf was determined with "'CO2 on plants with seeds of known age as described previously (18). Other experimental details are described in the text and figure legends. This study was conducted twice, in 1982 and 1983. Minor differences between the studies are noted in the text.In other experiments, plants were grown and flowers tagged as before, but populations were seeded on dates such that, at the initiation of the experiment, a large number of 35-and 46-d-

We isolated large quantities of mesophyll protoplasts from source and sink leaves of soybean plants and examined them for amino acid uptake. Accumulation of amino acids in isolated protoplasts was linear for at least 40 minutes. Uptake kinetics revealed the presence of both saturable and linear components. Increasing external pH decreases the uptake. The uncoupler, carbonyl cyanide p-trifluoromethoxyphenylhydrazone at 15 micromolar inhibited and fusicoccin at 10 micromolar stimulated amino acid uptake. Our data are consistent with a proton-cotransport mechanism for the uptake of L-glutamine and a-amino isobutyric acid into soybean mesophyll cells.