Nitrate Reductase Activity of Kleingrass (Panicum Coloratum L.) During Drought in the Northern Chihuahuan Desert

Abstract

Plant nitrate (NO_3-N) uptake rates are often low in desert environments because soil nitrogen levels are typically low, and mineralization and nitrification of nitrogen is moisture-dependent. During drought, leaf tissue NO_3-N levels toxic to grazing animals can result because the enzyme responsible for NO_3-N reduction (nitrate reductase; NR) is repressed during plant water stress. Seasonal leaf NR activity (in vivo), and NO_3-N, total nitrogen, and leaf water (%) content of kleingrass (Panicum coloratum L.) plants growing in situ in the northern Chihuahuan Desert were determined. Total precipitation during the April through November growing season (11.5 cm) was 40% less than the long-term average (19 cm). This drought resulted in low NR activity, repressed plant growth, and water-stressed plants through most of the growing season. Seasonal and diurnal leaf NR activities were positively correlated (P<.05) with leaf water contents (%) and leaf water potentials, respectively. The latter correlation was significant only with young leaf tissue. Young leaf tissue reduced 29.6 micromol NO_3-N gDW-1 on 14 July when leaf water potentials exceeded -3.0 MPa. On 18 May, 7.1 micromol NO_3-N gDW-1 were reduced when older leaf tissue was present and leaf water potentials did not exceed -3.0 MPa. Leaf NO_3-N accumulated to levels toxic to livestock during August, September, and October. The stem plus leaf sheath component of the aboveground biomass was the primary site for NR activity, and nitrogen and biomass allocation during 6 phenological stages (second through fifth leaf stages, and boot and immature seed stages). Immature seeds comprised only 12.3% of the aboveground biomass and possessed 29.9% of the nitrogen and 62.2% of the total capacity of NO_3-N reduction.