Nitrate Accumulation In Roots Research Articles

Loquat fruit ripening is associated with root depletion. Nutritional and hormonal control

In woody species, it is known that there is a competition for nutrients, water and carbohydrates between root and fruit-shoot systems, however the influence of root development on fruit quality has received little attention. This research aims to identify the network of mechanisms involved in loquat (Eriobotrya japonica Lindl.) fruit ripening in connection with root activity. The study includes root growth rate measurements paralleling the ongoing fruit developmental stages, photosynthate translocation to the root by using 13CO2 tracing, and nitrogen fractions (NNH4+, NNO3−, and N-proteinaceous) as well as their upward translocation to the fruit. The role of hormones (IAA, zeatin and ABA) in regulating the responses is also addressed. The experiment was conducted during two consecutive years on adult and 3-year-old loquat trees from early fruit developmental stage (10% of final size, 701 BBCH scale) to fully developed fruit colour (809 BBCH scale). This approach revealed that root development depends on the growing fruit sink strength, which reduces carbohydrates translocation to the roots and prevents them for further elongation. A nitrate accumulation in roots during the active fruit growth period takes place, which also contributes to slowing elongation and paralleled reduced ammonium and proteinaceous nitrogen concentrations. Concomitantly, the concentration of IAA and zeatin were lowest while that of ABA was highest when root exhibited minimum elongation. The depletion in zeatin and nitrogen supply by the roots paralleling the high ABA transport to the fruit allowed for colour break. These results suggest that loquat fruit changes colour by reducing root growth, as fruit increases sugars and ABA concentrations and reduces nitrogen and zeatin concentrations.

Ammonium Enables Aluminum-Induced Stimulation of Nitrogen Assimilation in Roots of Al-Tolerant Maize Genotypes

One aluminum (Al)-sensitive (B-73) and two Al-tolerant (F-2 and L-2039) maize genotypes were subjected to Al stress (100 μM Al) under two nitrogen (N) treatments [13.2 mM nitrate (NO3−) and 8.3 mM NO3− + 4.9 mM ammonium (NH4+)]. Growth parameters, chlorophyll, root N and NO3− contents, root nicotinamide adenine dinucleotide (NADH-) and nicotinamide adenine dinucleotide phosphate (NADPH)-nitrate reductase, glutamine synthetase, and glutamate dehydrogenase activities were determined. Aluminum significantly decreased growth and chlorophyll content in Al-sensitive genotype. Nitrate accumulation in roots was increased by Al in tolerant plants. In the sensitive genotype, Al suppressed all enzymes in NO3− plants, while in NO3−/NH4+ plants the suppression was less severe, and NADPH-nitrate reductase was even stimulated. In tolerant NO3−plants, glutamate dehydrogenase was stimulated in F-2 and glutamine synthetase suppressed in L-2039 genotype. In tolerant NO3−/NH4+- plants, all enzymes were stimulated by Al, which may be attributed to their participation in defense mechanisms.

EFFECTS OF DIFFERENT NITROGEN CONCENTRATION AND FORM ON NITRATE ACCUMULATION IN LEAF RADISH

Biomass, nitrate content, and nitrate reductase activity (NRA) of hydroponically cultured radish, 'Dahongyanzhi', 'Maer', 'Qingyedongchunwang', and 'Liushizao' were studied in relation to N level (1, 2, 3, 4, 5 mM), N supply method (continual or reduced N supply), and N source (nitrate, urea, or both but in different ratios). Typically, yield increased with N concentration to gradually reach a plateau and then, sometimes, decrease. In contrast, nitrate content increased continuously with N rate. Different nitrate contents were found in different organs: contents in roots and petioles were considerably higher than those in leaves. Nitrate accumulation in roots was lower than in leaves or petioles. With the increase of nitrogen rate, the nitrate storage function of roots was gradually transferred to leaves with no change in petioles. The treatments being deprived of N for 3 days before harvest showed significant decrease of nitrate content, especially on the first day. Reductions in leaves and roots were more important than in shoots and petioles. With the decrease of nitrate to urea ratio, nitrate content, activity of nitrate reductase and biomass production all declined. The greatest reduction of nitrate accumulation was observed when the ratio of nitrate to urea in the nutrient solution changed from 25:75 to 0:100.

Nitrate uptake, xylem NO3- flux, and nitrate assimilation in cowpea exposed to salinity

This work was carried out to evaluate what is the nitrate acquisition stage (nitrate uptake, xylem nitrate flux or assimilatory reduction) most influenced by the presence of NaCl in cowpea. Twelve day-old seedlings were treated with 50 mM of NaCl in nutrient solution during four days and measurements carried out under two contrasting environmental conditions: typical day (full sun) and completely cloudy day (cloudiness). The salinity affected more intensely the xylem sap flux and nitrate flux than transpiration. Plants treated with NaCl showed a strong decrease in both nitrate uptake rate and leaf nitrate reductase activity as in the full sun as in cloudy day. Transpiration was reduced by the cloudiness while xylem sap flux and nitrate flux remained unchanged, in both salt-treated and control. Moreover, nitrate uptake and nitrate reductase activity were less affected by cloudiness than the transpiration. In addition, NaCl negatively affected nitrate accumulation in roots, stems and leaves while the cloudiness affected only the leaf nitrate accumulation, both in control and stressed plants. Salinity affects more negatively the nitrate xylem flux, as compared with the nitrate uptake and nitrate assimilatory reduction in cowpea leaves.

Nitrate uptake by bean (Phaseolus vulgaris L.) roots under phosphate deficiency.

Bean (Phaseolus vulgaris L.) plants were cultured for 19 d on complete or on phosphate deficient culture media. Low inorganic phosphate concentration in the roots decreased ATP level and nitrate uptake rate. The mechanisms which may control nitrate uptake rate during phosphate deficiency were examined. Plasma membrane enriched fractions from phosphate sufficient and phosphate deficient plants were isolated and compared. The decrease in total phospholipid content was observed in plasma membranes from phosphate deficient roots, but phospholipid composition was similar. No changes in ATPase and proton pumping activities measured in isolated plasma membrane of phosphate sufficient and phosphate deficient bean roots were noted. The electron microscope observations carried out on cortical meristematic cells of the roots showed that active ATPases were found in plasma membrane of both phosphate sufficient and phosphate deficient plants. The decrease in inorganic phosphate concentration in roots led to increased nitrate accumulation in roots, accompanied by a corresponding alterations in NO3 distribution between shoots and roots. Nitrate reductase activity in roots of phosphate deficient plants estimated in vivo and in vitro was reduced to 50–60% of the control. The increased NO3 concentration in root tissue may be explained by decreased NR activity and lower transport of nitrate from roots to shoots. Therefore, the reduction of nitrate uptake during phosphate starvation is mainly a consequence of nitrate accumulation in the roots.