Monday, November 13, 2006

Big Bluestem Growth, Photosynthesis and Hyperspectral Reflectance Properties as Affected by Nitrogen Deficiency.

Vijaya G. Kakani, University of Florida-Agronomy Dept., 117 Dorman Hall, Box 9555, Mississippi State, MS 39762 and K. R. Reddy, Mississippi State University, Mississippi State, MS 39762.

Big bluestem (Andropogon gerardii Vitman), a C4 species, is one of the dominant grasses in the tallgrass prairie, comprising up to 80% of the biomass on favorable sites. An experiment was conducted under outdoor pot-culture conditions to determine effects of nitrogen (N) deficiency on big bluestem growth, physiology, and leaf hyperspectral reflectance properties. Big bluestem (cv. Bonilla) was seeded in 120 twelve-litre pots filled with fine sand. All pots were irrigated with half-strength Hoagland’s nutrient solution from emergence to 60 days after sowing (DAS). Thereafter, pots were separated into three identical groups and the following treatments were initiated: (1) the control (100% N) continued receiving the half-strength nutrient solution; (2) reduced N to 20% of the control (20% N); and (3) withheld N from the solution (0% N). Photosynthetic rate (Pn), chlorophyll (Chl) and N concentrations, and hyperspectral reflectance of the uppermost, fully expanded leaves were determined at 10-day-interval from 60 to 120 DAS during the N treatment period. Plants were harvested 120 DAS to determine effects of N deficiency on biomass accumulation, yield and partitioning. Nitrogen deficiency significantly reduced leaf Chl content and Pn, resulting in lower biomass production. Tillers per plant, tillers with panicles and total panicles per plant were reduced with decrease in leaf N concentration thus reducing seed yield. Spectral ratio of R545/R1605 (r2 = 0.82) or R715/R1705 (r2 = 0.72) can be used to determine leaf N content, while leaf Chl concentration can be determined using the spectral ratio R555/R405 (r2 = 0.81). First derivatives of leaf reflectance (dR/Δλ) at 690 and 710 nm also showed linear relationship (r2 ~ 0.72 - 0.89) with leaf N and Chl concentrations.  Our results demonstrate the use of hyperspectral reflectance in determining and maintaining the health of rangelands.