Nitrate reductase, phosphoenolpyruvate carboxylase, and glutamine synthetase in annual ryegrass as affected by salinity and nitrogen

Abstract

The concentration of organic acids, organic nitrogen (N), nitrate (NO 3 ), and total cations increased in annual ryegrass [Lolium multiflorum Lam.) with salinity and N concentration in the growth medium. Increasing salinity and N in the growth medium induced changes in the level of key enzymes of N assimilation and organic acids: nitrate reductase (NR, EC 1.6.6.1), phosphoenolpyruvate carboxylase (PEPc, EC 4.1.1.31), and glutamine synthetase (GS, EC 6.3.1.2). Plants grown in pots filled with sand were irrigated with nutrient solutions with an electroconductivity of 2 or 11.2 dS m -1 and N applied as ammonium nitrate (NH 4 NO 3 ), sodium nitrate (NaNO 3 ), or ammonium 707 Journal of Plant Nutrition 1998.21:707-723. 708 SAGI ET AL.applied as ammonium nitrate (NH 4 NO 3 ), sodium nitrate (NaNO 3 ), or ammonium (NH 4 ) as ammonium sulfate [(NH 4 ) 2 SO 4 ] at concentrations of 0.5, 4.5 or 9.0 mM. Nitrate reducíase, PEPc, and GS increased with salinity and N level. Shoot NR was highest in the presence of NH 4 NO 3 irrespective of salinity level, while root NR activity responded best to NO 3 . Enhancement of PEPc activity in both shoots and roots was highest with NH 4 NO 3 and lowest with NH 4 . Nitrogen source had no significant effect on GS activity in shoots or roots of ryegrass. Shoot NR activity increased with NO 3 concentration in the tissue, as calculated from regression coefficients. The PEPc activity correlated positively with total cations and NO 3 concentrations in the plants, irrespective of the salinity level, suggesting that the increase in total cations and NO 3 induced by salinity may have triggered the changes in enzyme activities. The concentration of organic acids in both shoots and roots correlated positively with PEPc activity irrespectively of the salinity level. The PEPc activity was higher in roots than in shoots, while organic acid concentration was higher in shoots. These results suggest that a significant part of the organic acids produced in the roots were used as carbon skeleton for transamination reactions. The increased activity of NR, PEPc, and GS in roots may constitute part of an adaptation strategy of the plant to increasing salinity in the medium. These enzymes have an important role in the metabolism of amino acids and the synthesis of organic N in annual ryegrass irrigated with saline water, and boosting them with suitable N fertilizers could increase the nutritional value and protein content of the crop.

Developmental regulation of photosynthate distribution in leaves of rice

“…5c) are in agreement with the fact that levels of PEPC increased with N concentration in C 3 plants including wheat, barley, tomato (Koga and Ikeda 1997), ryegrass (Sagi et al 1998), and Norway spruce (Wallenda et al 1996). Moreover, PEPC activity is positively correlated with the N assimilation rate (Vanlerberghe et al 1990).…”

Partial substitution of NO3− by NH4+ fertilization increases ammonium assimilating enzyme activities and reduces the deleterious effects of salinity on the growth of barley

“…Arnozis et al (1988) reported that GS activity increased in five different species under non-saline conditions when NH 4 + instead of NO 3 À was used as a nitrogen source. In ryegrass, however, nitrogen ionic form had no effect on GS activity in both nonsaline and saline conditions (Sagi et al, 1998). Our results indicate that in barley, the mixed nitrogen regime led to increased GS activity in both nonsaline and saline conditions compared to NO 3 À alone (Fig.…”

“…1A and B). GS activity has also been shown to increase in response to saline conditions in other species, for example, P. aureus (Misra and Dwivedi, 1990) ryegrass (Sagi et al, 1998) and tomato (Cramer et al, 1999). However, the response of GOGAT activity appears to be species-specific, in some species increasing (Misra and Dwivedi, 1990) while in others decreasing (Cramer et al, 1999).…”

“…The increased PEPC activity might be attributed to a number of factors. First, a rise in cytoplasmic pH under salinity may stimulate the PEPC activity (Sagi et al, 1998). Secondly, increased PEPC activity under salinity may reflect the need of the plant for increased production of carbon skeletons in order to regulate osmotic pressure and to maintain cell ionic neutrality by synthesis of organic acids (Cramer et al, 1993).…”

“…The response of plant growth and ammonium assimilation enzymes to nitrogen fertilization under saline and non-saline conditions varies according to whether the nitrogen is supplied as nitrate or ammonium and also depends on plant species (Arnozis et al, 1988;Misra and Dwivedi, 1990;Gulati and Jaiwal, 1996;Sagi et al, 1998;Flores et al, 2001). Growth of many plants is impaired when nitrogen is applied solely as NH 4 + (Gerendas et al, 1997;Siddiqi et al, 2002).…”