Nitrogen is an essential mineral nutrient for plant growth and development and is often a limiting factor in maintaining high-quality putting greens on golf courses. Because of environmental concerns and decreased root growth at high N, low rates of N fertilization are recommended, particularly for sites that are prone to excessive leaching. However, reduced use of N fertilizers could lead to N deficiency, which would increase remobilization of N within the grass plant for the growth of young leaves as shown by Ourry et al. (1990). They demonstrated that 69% of the total N in young leaves of 14-d-N-starved perennial ryegrass (Lolium perenne) came from endogenous N of the roots and stubble. Bowman (1993) showed that N deprivation for 7 d led to redistribution of reduced N (total N excluding nitrate-N) from perennial ryegrass roots and verdure to the leaves. Few studies of turfgrass have reported activities of enzymes in organic N metabolism of young and old leaves of plants under N deficiency (Lyons et al., 1990; Ourry et al., 1990). Elucidating on this would greatly further our understanding of the mechanisms for efficient N use by turfgrass.
Organic N metabolism may be adversely affected by N deficiency because N deficiency induces leaf senescence (Lim et al., 2007). In senescing leaves, N catabolism occurs and proteins are degraded to produce NH3 and amino acids, which are used to produce glutamine (Gln) for N transport to young leaves and other sink organs for reuse (Liu et al., 2008). In actively growing leaves, N anabolism occurs and AAs are used to synthesize proteins and chlorophylls (Chl). Two key enzymes, glutamine synthetase (EC 126.96.36.199) and glutamate dehydrogenase (EC 188.8.131.52), are involved in N anabolism and catabolism (Miflin and Habash, 2002). Glutamine synthetase catalyzes an ATP-dependent reaction, in which NH3 is combined with glutamate (Glu) to form Gln, which is a major donor of amino N for the synthesis of other AAs including the regeneration of Glu by glutamate 2-oxoglutarate aminotransferase (GOGAT) in the GS–GOGAT cycle. In higher plants, two isoforms of GS exist: GS1 in the cytosol and vascular bundles, which produces Gln primarily for N transport from source to sink tissues, and GS2 in chloroplasts and root plastids, which produces Gln primarily for local use (Lam et al., 1996). In addition to assimilating NH3 from root uptake, GS1 plays a key role in N remobilization in senescing leaves by reassimilating NH3 from the catabolism of AAs (Bernard and Habash, 2009), whereas GS2 plays an important role in photosynthesizing leaves by assimilating NH3 from photorespiration (Wallsgrove et al., 1987) and nitrate reduction (Zozaya-Hinchliffe et al., 2005).
Glutamate dehydrogenase catalyzes a reversible reaction, in which glutamate is deaminated or 2-oxoglutarate is aminated (Dubois et al., 2003). In the deamination process, glutamate is converted to 2-oxoglutarate with the release of NH3 and the reduction of NAD+ to NADH. In the amination process, 2-oxoglutarate is combined with NH3 to produce Glu with the oxidation of NADH to NAD+. The GDH is localized to the mitochondria of phloem companion cells and to the cytosol of senescing leaves and may affect the translocation of assimilates during carbon (C) and N remobilization or respond to the plant's redox status and thus stress level (Dubois et al., 2003). Although GDH primarily catalyzes the catabolism of Glu (Lam et al., 1996), there is strong evidence that the anabolism of Glu by GDH also occurs in plants and that this enzyme may participate in the assimilation and detoxification of accumulated NH3 ions from photorespiration in photosynthesizing leaves and protein degradation in senescing leaves (Kwinta and Bielawski, 1998; Lasa et al., 2002; Skopelitis et al., 2007).
Information on the effects of low N supply on organic N metabolism involving GS and GDH in turfgrass is limited, particularly differential responses in growing leaves and senescing leaves to N deficiency. In red fescue (Festuca rubra) and seaside alkaligrass (Puccinellia maritime) found in a salt marsh, GS2 of the chloroplasts was dominant over GS1, comprising 70% to 80% of the total leaf GS activity (McNally et al., 1983). The GS and GDH activities in roots, stubble, or leaves of perennial ryegrass did not change significantly 2 weeks after clipping (Boucaud and Bigot, 1989). In a study on the effects of a fungal endophyte on N accumulation and metabolism in tall fescue (Festuca arundinacea), Lyons et al. (1990) found that total leaf GS activity expressed on a fresh weight basis was greater at high N supply than at low N supply, and endophyte-infected plants had greater leaf GS activity than the non-infected plants. Faure et al. (1998) found that perennial ryegrass colonized by an arbuscular mycorrhizal fungus had greater leaf GS activity than control plants, but this difference was not observed in roots. They also found that Gln was the main sink for nitrate-N immediately after nitrate was supplied to the roots. The nature of the activity of GS and GDH in growing leaves compared with senescing leaves under N deficiency has not yet been elucidated.
The objective of this study was to examine the effects of N deficiency on GS and GDH activities as well as AA and soluble protein contents in young and old leaves and roots of creeping bentgrass. This information will help us better understand the mechanisms of efficient N use in turfgrass given that these enzymes are critical in the assimilation of NH3 into AAs and in the detoxification of accumulated NH3 from nitrate reduction, photorespiration, protein degradation, and AA catabolism. The information may also benefit molecular research on N use efficiency because manipulation of GS and GDH genes affected root growth and productivity in other crops (Miflin and Habash, 2002).
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