Nitrate simultaneously enhances lipid and protein accumulation in developing yellow lupin cotyledons cultured in vitro, but not under field conditions

The research was conducted on yellow lupin (Lupinus luteus L.) mature seeds, developing cotyledons, developing pods, and seedlings. The main storage compound in yellow lupin seeds is protein, whose content may reach up to 45%. Oil content in seeds of yellow lupin is about 6%. In such protein-storing seeds there is a strong negative relationship between accumulation of storage lipid and protein. An increase in protein content causes a decrease in lipid level, and vice versa. However, simultaneous increase in lipid and protein content is possible in developing lupin cotyledons (the main storage organs of lupin seeds) cultured in vitro. Such an effect was obtained by feeding the cotyledons with nitrate (35mM). The same positive relationship in storage lipid and protein accumulation was also obtained in developing lupin pods fed with nitrate (35mM), detached from the mother plant, and maintained under quasi in vitro conditions. Fertilization of lupin plants with nitrate under field conditions (40 or 80kgNha−1 applied before sowing, at the nodulation stage or at the flowering and pod formation stage) did not cause significant changes in lipid and protein contents in mature seeds. Experiments performed on lupin seedlings cultivated hydroponically showed that nitrate added to the medium was accumulated mainly in roots, and at a remarkably lower level in shoots. We hypothesize that the lack of stimulatory effect of nitrate on storage lipid and protein accumulation in seeds under field conditions is due to inefficient transport of nitrate from the root to developing pods in lupin plants. This causes that the level of nitrate inside the developing lupin seeds is not elevated under field conditions.