| Nitrate uptake and nitrate reductase activity in wheat
(Triticum aestivum) seedlings V. PARKASH Division of Plant Physiology, Indian Agricultural Research Institute, New Delhi-110012, India Nitrate uptake pattern is characterised by slow initial rate and a subsequent accelerated rate occurs when wheat seedlings previously grown without nitrogen are first exposed to nitrate containing solution (MINNOTI et al. 1968). Similar results have been observed in my laboratory (DINESH 1980), the presence of Mo accelerated NO3 uptake and addition of tungstate rather inhibited its absorption. JACKSON et al. (1972) observed that initial lag phase was observed only in case of NO3- uptake when corn seedlings rasied earlier without NO3- and Cl- and when exposed to both, nitrate induced nitrate reductase during the lag phase. Experiments were raised in this laboratory to understand the association of nitrate reductase activity and nitrate uptake in wheat seedlings. Material and Methods Wheat seeds of variety HD 2258 recently bred for high fertility conditions were rinsed for one min. in 5.25% NaOCl and washed in running tap water and rinsed thoroughly in deionised water. Seeds were germinated on 5 layers of germinating paper saturated with 1 mM CaSO4. After planting the seeds, the trays were covered with plastic food wrap, perforated to allow for air exchange and placed in dark chamber at about 24C and high humidity. Germinating papers were constantly kept moist. Seven days later, the seedlings were removed (at this stage the endosperm was completely exhausted in the etiolated seedlings), and fixed in series of 1-litre pyrex beakers containing 1.0 mM (NH4)2SO4, 1.1 mM K2SO4. Trace elements were kept at 1/4 concentration of Hoagland solution (1950), four seedlings were kept in each beaker. Beakers were kept under natural light conditions in the glass house. Some of the seedlings were kept under one eighth normal molybdenum concentration. At the day 10, 15 and 20, uniform seedlings were selected and transferred in 1-litre beaker containing standerd uptake solution consisted of 0.05 mM nitrate as Ca(NO3)2, 0.5 mM CaSO4 and 20 microg of chloramphenicol per ml and pH 7 was regularly maintained (at this pH maximum NO3 absoption was observed in one of four earlier experiments with various wheat genotypes). Each beaker carried two seedlings with 15 replications. Nitrate was assayed in the depleted solutions according to technique reported in the communication entitled "Soil Science" 6(1) 71-80 (1975). Nitrate reductase was assayed as described by HAGEMAN & HUCKELSBY (1971) at 12 and 24 hr after the transfer of seedlings. Results and Discussion The presence of Mo in the test solution was helpful in the uptake of NO3 (Table 1). Similarly results were obtained earlier in my laboratory with different wheat genotypes raised with and without molybdenum and tungstate. The tungstate reduced nitrate absorption thus confirming that association of molybdenum was necessary for nitrate uptake. Since molybdenum is known to be the constituent of nitrate reductase protein, experiments therefore were raised to see that if the protein was in any way involed. The data obtained in this regard as shown in Table 2, indicated that 5-flourodeoxyuridine and 5 flourourocil known to be RNA inhibitors, both of these reduced the nitrate absorption. Cyclohexamide an inhibitor of protein synthesis drastically inhibited nitrate uptake whereas puromycin which is presumed to compete with aminoacyl tRNA during polypeptide synthesis also restricted the NO3- uptake. These experiments therefore suggest that RNA and protein synthesis were also required for the process involved in the acceleration of nitrate absorption and molybdo-protein system may be working for the same. |
| --> Next |