Mobilization of Endosperm Reserves and Uptake of Sucrose and Valine by the Cotyledons ofEuphorbia lathyrisL.

Upon germination, the endosperm triacylglycerols and proteins were converted to sucrose and amino acids. During early postgerminative growth, the rate of sucrose and amino acid production exceeded the rate of uptake by the cotyledons. As a result, the levels of total amino acid and sucrose in the endosperm increased; maximum levels were reached at 7 d and 10 d after imbibition (DAI), respectively. Intact seedlings were used to measure the development of valine, arginine, glutamic acid, and sucrose uptake rate throughout the course of endosperm depletion. Maximum amino acid uptake rates were measured at around 9 DAI, the highest uptake rate for sucrose was obtained at 12 DAI (just before depletion of the endosperm). The daily increase of sucrose and amino acid uptake could be manipulated, by replacing the endosperm with a pre-incubation solution during 1 d. The increase in sucrose uptake in vitro was equal to that measured with intact seedlings when the cotyledons were pre-incubated in 10 mol m−3 sucrose. Higher sucrose concentrations reduced the increase of sucrose uptake; at 300 mol m−3 sucrose (corresponding to the mean endosperm sucrose concentration) sucrose uptake after pre-incubation was even lower than before. This reduction was largely counteracted when the pre-incubation solution was supplemented with minerals. The development of the valine uptake was hardly affected by sucrose, but was inhibited by several amino acids.