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Title: Growth, photosynthesis, nitrogen partitioning and responses to CO2 enrichment in barley mutants lacking NADH-dependent nitrate reductase activity

Author
item Sicher Jr, Richard
item Bunce, James

Submitted to: Physiologia Plantarum
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/25/2008
Publication Date: 6/3/2008
Citation: Sicher Jr, R.C., Bunce, J.A. 2008. Growth, photosynthesis, nitrogen partitioning and responses to CO2 enrichment in barley mutants lacking NADH-dependent nitrate reductase activity. Physiologia Plantarum. 134:31-40.

Interpretive Summary: Atmospheric carbon dioxide levels are increasing due to fossil fuel combustion. This change is predicted to accelerate plant growth and to alter future crop yields. Enhanced plant growth rates due to elevated carbon dioxide typically result in larger plants and this increases the demand for soil nutrients, particularly nitrogen. For reasons that are not fully understood, plants grown in elevated carbon dioxide often display symptoms of nitrogen deficiency, such as increased leaf starch and low protein levels. This is an indication that something altered the nitrogen acquisition process when plants were exposed to elevated carbon dioxide. To better understand this problem we studied plant growth and related processes in mutant barley plants lacking the primary enzyme in nitrogen acquisition. Measurements indicated that rates of nitrogen acquisition were reduced by 95% in the mutants compared to the control plants. In spite of this defect the mutant plants grew almost normally in both ambient and elevated carbon dioxide. This was a clear indication that trace levels of minor enzymes involved in nitrogen acquisition can meet the growth needs of barley, even in response to enhanced carbon dioxide. This research should be of value to plant breeders, crop modelers, and government agencies interested in the effects of global climate change on plant performance.

Technical Abstract: We examined plant growth, photosynthesis and leaf constituents of both the wild type (WT) and two mutant lines of barley (Hordeum vulgare L. cv. Steptoe) with defects in NADH-dependent nitrate reductase (NADH-NAR) activity. The first mutant, nar1, had a lesion within the NAR structural gene and the second, nar2, had a defect in the molybdopterin subunit, which rendered it pleiotropic. Total biomass was reduced 17 and 43% for nar1 and nar2, respectively, in comparison to the WT when measured 8 weeks after sowing in a greenhouse experiment. Foliar soluble protein also was 22 to 23% lower in the two mutants compared to the WT. Whole plant growth results were similar whether or not nitrate was the sole source of inorganic nitrogen. All three genotypes displayed accelerated plant growth rates in response to continuous CO2 enrichment. Trace levels of NADH-NAR and NAD(P)H-NAR activity, the bispecific form of the enzyme, were present in the two mutant lines. Net rates of CO2 assimilation measured at both growth CO2 partial pressures were only slightly decreased by a defect in the NADH-NAR structural gene alone. Levels of soluble sugars and starch were similar in leaves of all three genotypes grown at 38 Pa CO2. The above results indicated that a defect in NADH-NAR primarily affected nitrogenous leaf constituents, whereas effects on growth, carbohydrate metabolism and photosynthesis were small. Differences between nar1 and nar2 were likely the result of pleiotropism. The presence of bispecific NAD(P)H-NAR in the two mutants likely compensated for the lack of NADH-NAR. Although this enzyme was present in trace amounts, its activity was sufficient to allow nearly normal growth in both the ambient and elevated CO2 treatments.