Cessation of Assimilate Uptake in Maturing Soybean Seeds

Abstract

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by known weights of pods, seed coats or embryos in sealed containers at 260C using IR gas analysis of small samples withdrawn by syringe.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-

Changes in Soybean Fruit Ca2+ (Sr2+) and K+ (Rb+) Transport Ability during Development

“…Furthermore, during seed maturation, a period not covered by the present study, soybean seed coats (Laszlo, 1990), as well as seed coats of other legumes (Hocking and Pate, 1977), show very pronounced accumulations of Ca2+. Because the embryo is still competent in dry matter uptake at this stage (VerNooy et al, 1986), this observation supports the hypothesis that the seed coat acts as a control site for CaZ+ transport. A similar conclusion was reached by Mix and Marschner (1976) in their study of P. vulgaris.…”

Soybean Seed Water Relations during in Situ and in Vitro Growth and Maturation

“…Although there were differences in the amount of green color remaining at physiological maturity depending on the growing conditions, there was no further increase in embryo dry weight after total loss of green color in embryo tissues. However, in one report, yellowed cotyledons were able to take up glutamine in vitro despite a sharp decline in sucrose uptake (19). Loss of green color from immature soybean embryos could be induced by slow drying (14) indicating that Chl degradation is enhanced by desiccation.…”

“…Guldan and Brun (2) reported that rates of asparagine uptake into excised cotyledons of field grown soybean (Glycine max [L.] Merrill) were lowered when the tissue osmotic potential was reduced to -1.8 MPa and below. On the other hand, VerNooy et al (19) noted that excised yellowed cotyledon tissues from physiologically mature soybean embryos were able to take up glutamine but not sucrose. However, no measurements on tissue water relations were reported in the latter study.…”

“…Increasingly, evidence is pointing to the regulation of solute availability to developing embryos and endosperm by water relations of the embryo itself and the surrounding medium (2,13,18,19,23) (and others). Although solute unloading rates from seed coats varied considerably with the osmolality of the receiving medium in P. vulgaris L. (13), faba bean (Vicia faba L.), and pea (Pisum sativum L.) (23), sucrose unloading rates from attached soybean seed coats were reduced only at trap solution concentrations over 500 mM (18).…”

Soluble Carbohydrates in Soybean

“…Translocation of sucrose from photosynthate into the embryo ceases when the seed coat changes from green to yellow and seed respiration declines rapidly (TeKrony et al, 1979). Likewise, cotyledons cease to take up sucrose when the cotyledon color changes from green to yellow (Vernooy et al, 1986). At this time, transport of water and nutrients into the seed ceases and the seed begins to shrink as it loses water.…”