Nitrate Uptake and Reduction by Durum Wheat (Triticum turgidum) and Tritordeum (Hordeum chilense x Triticum turgidum)

Abstract

Summary Tritordeum is a fertile amphiploid between a wild South American barley ( Hordeum chilense Roem. et Schultz.) and durum wheat ( Triticum turgidum L. conv. durum ). An outstanding characteristic of tritordeum is its high protein content in the grain, which is substantially higher than that of its wheat parent. This study was conducted to gain information on the properties of the nitrate uptake system of two tritordeum lines (HT8 and HT31) and their wheat parent, and to find differences among genotypes in nitrate uptake and/ or the activity of the nitrate-reducing enzymes as possible factors related to the protein content. Exposure of intact tritordeum plants to environmental nitrate induced nitrate uptake by roots, and nitrate reductase (EC 1.6.6.1) and nitrite reductase (EC 1.7.7.1) activities in roots and leaves. Ammonium did not induce the nitrate uptake system. Moreover, pretreatment of the plants with nitrate plus ammonium resulted in a decreased induction as compared with nitrate alone. The apparent K s , values for net nitrate uptake in T. turgidum , tritordeum HT8 and tritordeum HT31 were 128, 112 and 90 μM, respectively, and the corresponding V max values were 6.5, 9 and 12 μmol h -1 g -1 fresh weight, indicating that tritordeum lines, and specially HT31, absorb nitrate more efficiently than the wheat parent. Nitrate reductase and nitrite reductase activities in roots after nitrate induction ranged from 0.8 to 2.2 and from 158 to 266 μmol h -1 g -1 fresh weight, respectively. Therefore, nitrate to nitrite reduction appears to be the rate-limiting step of nitrate assimilation. Tritordeum HT8, whose grain protein concentration is substantially higher than that of tritordeum HT31, exhibited higher nitrate reductase activity both in roots and leaves but lower efficiency for nitrate absorption than tritordeum HT31. Hence, protein accumulation appears to be more related to nitrate reduction than to nitrate uptake rate.