A conditions normally limit the plants from expressing their

A biological stress is defined as any change in environ­mental conditions that might reduce or adversely change a plant growth or development (Levitt, 1980). Any change in the environment that results in plant response that is less than the optimum might be considered stressful (Salis­bury and Ross, 1991). Stress conditions normally limit the plants from expressing their full genetic potential. Understanding physiological, biochemical, metabolic and molecular processes that are altered by stress and the responsive mechanisms developed by plants to tackle stress is of immense im­portance for improving agriculture and crop productivity (Hong and Vierling, 2000). In plants, water and nutritional de­ficiency, high salinity and extreme temperature are some of the most studied stress factors (Lambers and Jones, 1998). Nutrient stress can result either from the form in which the nutrient exist, the process by which they become available to the plant; content of soil solution and soil pH (Evans, 1989).

Because of considerable uptake and utilization of nitrogen nutri­ent, its deficiency frequently occurs in most soils (Ashraf and McNelly, 1994; Marschner, 1995). Contin­uous cultivation of crops in addition to adverse environ­mental factors make the arable soils deficient of nitrogen along with the other important nutrient and the crops grown on such soils exhibit very destructive deficiency symptoms such as poor growth, chlorosis, necrosis of leaves and disorders in many physiological and biochemi­cal characteristics (Bray and Bailey-Serres, 2000).

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Nitrogen deficiency is known to effectively presume metabolic process in plants. N affects all levels of plant function, from metabolism to biomass allocation, growth, and development and yield (Crawford 1995; Marschner 1995; Grindlay 1997; Stitt and Krapp 1999).

Recently, maize (Zea mays L.) is used as a model crop for a number of efforts to explore gene expression and enzyme activity involved in N metabolism in roots, leaves, stover, and cob at different periods of plant development (e.g. Foyer et al. 1998; Hirel et al. 2001; Seebauer et al. 2004; Hirel et al. 2005a, b; Uribelarrea et al. 2009). In numerous N fertilization experiments, the reduction in plant height, leaf area index, light interception, and biomass production has been recorded in N-deficient maize plants (Zhao et al. 2003; Vos et al. 2005;  Cirilo et al. 2009; Massignam et al. 2009; Peng et al. 2010). In cereals, the enhancement of stem elongation by nitrogen increases the susceptibility to lodging. This change in shoot morphology is less distinct with ammonium than with nitrate-nutrition (Sommer and Six, 1982), and is presumably related to nitrogen-induced changes in the phytochrome balance.

It is known that plants growing under N limitation conditions undergo adaptive morphological changes including increased root shoot ratio, metabolism of leaves and reserve or­gans (Roberts et al., 1992), early senescence, and early transition to flowering (Goyal et al., 2005). Also, plants exposed to N limitation exhibit lower levels of N-containing compounds such as amino acids, protins, chlorophyll, secondary metabolites, and accumulate anthocyanin  (Peng et al., 2007a,b) and starch (Scheible et al.,2004) com- pared to plants grown under optimal N environment. However, one of the mechanisms by which plants adjust to an imbalance of exogenous resources is by allocating new biomass to the organs that are involved in acquiring the resources that are scarcest (Marschner 1995; Hermans et al. 2006).

Undoubtedly, the fact that insufficient N input leads to the reduction in plant growth should be attributed primarily to reduced photosynthesis. Strong positive correlations have been observed between the photosynthetic capacity of leaves and their N content, most of which is used for synthesis of components of the photosynthetic apparatus (Ariovich and Cresswell1983; Evans and Terashima 1987; Sage and Pearcy 1987; Sugiharto et al. 1990) and for synthesis of photosynthes   enzymes (Evans 1989; Hikosaka and Terashima1995). Nitrogen deficiency leads to disruption of the fine structures of chlorophyll and instability of the pigment protein complex (Reddy and Dakora, 2007).

There is nowadays clear evidence that N deficiency induces sink limitation within the whole plant due to decreased growth (Paul and Foyer, 2001). This leads, in turn, to feedback downregulation of photosynthesis. N deficiency results in accumulation of carbohydrates (sugars and starch) in the leaves, higher levels of carbon allocated to the roots and an increase in root-toshoot biomass ratio (Hirai et al., 2004; Scheible et al., 2004; Remans et al., 2006). N deficiency therefore affects, to various extents, primary photosynthesis, sugar metabolism and/or carbohydrate partitioning between source and sink tissues (Paul and Driscoll, 1997; de Groot et al., 2003; Scheible et al., 2004).

Nitrogen supply has a dominant effect on yield production because agriculture is an extractive process, with removal of a crop also removing N from the soil, so very marked deficits can show quickly. Nitrogen supply affects all the components of grain yield, as discussed, via the effects on the number of tillers, ears and grains formed (Lawlor 1995; Grindlay 1997; Jeuffroy and Meynard 1997). The number of ears is largely determined during early growth, with grain number and potential size determined later via carbon and nitrogen assimilate supply, which, of course, greatly affect the final grain size.

Nitrogen deficiency during vegetative growth can reduce GY by reducing LAI and RUE, which results in limited CGR and low KN (Uhart and Andrade, 1995a, 1995b). Nitrogen deficiency in later stages speeds leaf senescence and results in lower KW (Muchow, 1988). Some stud­ies indicated that maize appears to preferentially maintain radiation interception instead of RUE under N defi­ciency; that response pattern would tend to retain LAI at the expense of a rapid decline in SLN as N stress increases (Massignam et al., 2009).