Seabuckthorn (Hippophae rhamnoides L.) is a small fruit tree belonging to the family Elaeagnaceae. Because seabuckthorn fruit is rich in unsaturated fatty acids and vitamins, this plant has potential as a food and medicinal crop. Here, we focused on symbiotic nitrogen fixation that could aid in the cultivation of this species. Microscopic observations showed that seabuckthorn root nodules have a standard morphology characteristic of Frankiaactinorhizal root nodules. Under nitrogen-free conditions, seabuckthorn seedlings inoculated with a homogenate of root nodules grew normally, and the fresh weight of root nodules was positively correlated with plant growth. In the field, nitrogenase activity in root nodules was high from May to September, when air temperatures were high and photosynthesis was active. Inhibition of nitrogen fixation by nitrate has been well documented in the root nodules of legumes. Therefore, we investigated the effect of nitrate on nitrogenase activity in seabuckthorn root nodules. Nitrogenase activity in seabuckthorn root nodules was not inhibited by the addition of high concentrations (up to 30 mM) of nitrate over a short term (5 days), but was apparently inhibited by long-term (20-30 days) treatments with 5 and 10 mM of nitrate.

Sorbitol-6-phosphate dehydrogenase (S6PDH) is known to be a key enzyme in the biosynthesis of sorbitol, an early product of photosynthesis, common to fruit trees of the Rosaceae family. Effects of low temperature and abscisic acid (ABA) on the expression of the S6PDH gene were investigated in apple leaves. In leaf-disc experiments, the expression of S6PDH was enhanced by an ABA treatment, as well as by low temperature and high-salinity stresses. The level of S6PDH mRNA increased 8 h after the addition of ABA; the highest level of S6PDH mRNA resulted on exposure to 10 µM ABA. The level of S6PDH mRNA in leaves of apple trees growing in an orchard increased with a decrease in temperature in the fall while ABA content increased. This induction may partly be a stress-response to low temperature, a prerequisite for freezing tolerance during the coming winter. Southern blot analysis revealed that S6PDH is a single-copy gene in the apple genome, indicating that it is a unique, multifunctional one, for sorbitol biosynthesis under various stress responses, as well as for the translocation of photosynthates.

The activity and gene expression of sorbitol-6-phosphate dehydrogenase (S6PDH), NAD + -dependent sorbitol dehydrogenase (NAD-SDH), sucrose phosphate synthase (SPS), sucrose synthase (SS) and soluble acid invertase (S-AIV), in developing 'La France' pear leaves in relation to the metabolism of sorbitol and sucrose as translocated sugars, were investigated. The levels of activity and mRNA of S6PDH were significantly higher in mature leaves than in folded and young ones. S6PDH activity was more than ten times higher than SPS activity. SPS activity remained at an almost constant level throughout the developmental stages; it was undetectable except in the very young leaves although its mRNA was present. NAD-SDH and SS activities were higher in young leaves than in mature ones; the levels of mRNAs, however, exhibited an inverse patterns to that of activities. The levels of both activity and mRNA of S-AIV were significantly higher in folded and young leaves than mature ones. The distinctly higher expression of S6PDH than that of SPS in mature leaves suggests that the former is more important than the latter in relation to CO 2 assimilation (source activity). Thus, the most important change in the transition from sink to source of the pear leaf is the increasing expression of S6PDH gene with leaf maturation.