Activities of Sucrose Synthase and Acid Invertase in Pea Seedling Organs

The organ topography of sucrose synthase and soluble acid invertase in pea seedlings at heterotrophic stage (3–14 days) was studied. Sucrose synthase was most active in the roots, with the highest activity on the 6–8th days. In the leaves, its activity decreased from day 3 to day 14. In the stems, sucrose synthase activity was at an invariantly low level. The patterns of sucrose synthase activity in etiolated and green plants were similar. As distinct from sucrose synthase, invertase activity was the highest in the stem, especially in etiolated plants. The peak of its activity was observed on the 6-8th days. In the leaves, invertase activity was lower but its pattern was the same. In the roots, acid invertase activity decreased from the 3rd day and did not depend on illumination. The conclusion is that differences in sucrose synthase and acid invertase activities in roots, leaves, and stem are determined by differences in the import of hydrolytic products of stored compound from the cotyledons as well as by different demands of these organs for these products for the processes of organ expansion and for the maintenance of organ metabolism.     

Effect of nitrate on pea sucrose synthase

The in vivo and in vitro nitrate effects on pea (Pisum sativum L.) sucrose synthase (SS) were studied. At the period of plant transition from heterotrophic to autotrophic nutrition, exogenous nitrate (14.2 mM) absorbed in the form of KNO₃ and Ca(NO₃)₂ during 10–20 days activated SS in the roots by 22–100% as compared with plants grown on nitrogen-free medium. Such effect was observed only at plant growing under high light (natural illumination up to 25 klx) and thus their sufficient supplement with sucrose. Under low light (climate-controlled chamber, 2.5 klx), nitrate could not activate SS. In the in vitro experiments, nitrate activated SS exponentially by a dose-dependent mode with the plateau at 3–5 mM, where its activity was increased by 50%. It is supposed that there is a second constituent in SS activation by nitrate, and it carries information about plant carbohydrate status. Possible mechanisms of nitrate-induced SS activation are discussed.     

Nitrate Assimilation Function of Yellow Lupine Root Nodules?

Low nitrate assimilation activity of the root nodules was demonstrated by assaying the activities of nitrate reductase, glutamate synthetase, glutamate dehydrogenase, and asparagine synthetase as well as the kinetics of ¹⁴C-labeled sucrose incorporation in the amino acids and amides of the cortex and the bacteroid-containing root nodule zones. Irrespective of the exogenous nitrogen concentration (0, 11.2, or 25 mM NO^- ₃), nitrate concentration in the nodules was low as compared to the plant roots, leaves, and stems. This allowed us to propose the presence of structural and/or metabolic barriers in the nodules limiting nitrate transport and assimilation in the nodule.