Abstract

Carbohydrate and N reserves are important for perennial grass regrowth after defoliation. The objective of this study was to quantify the effects of N fertilization on dynamics of reserve accumulation and utilization for regrowth of a C₄ perennial grass. A field study was conducted at Gainesville, FL, on established ‘Tifton 85’ bermudagrass (Cynodon spp.) in 2006 and 2007. Treatments were N rates of 0, 45, 90, and 135 kg N ha⁻¹ regrowth period⁻¹. Total nonstructural carbohydrate (TNC) and N concentrations, leaf area index (LAI), and canopy carbon exchange rate (CER) were measured weekly during 28-d regrowth periods. Stem and rhizome TNC concentrations decreased with increasing N rate, ranging from 20 to 80 mg g⁻¹ for stem and 45 to 145 mg g⁻¹ for rhizome, and followed quadratic time trends, with minima between 7 and 14 d of regrowth, suggesting reserve utilization up to 2 wk after defoliation. Leaf, stem, rhizome, and root N concentrations increased with N rate. Leaf and stem N concentrations followed quadratic time trends, with maxima between 7 and 14 d of regrowth, and ranged from 15 to 50 mg g⁻¹ for leaf and 10 to 40 mg g⁻¹ for stem. Rhizome N concentrations were constant throughout regrowth. Canopy CER and LAI followed logistic time trends within each 28-d regrowth period, with upper asymptotes raised by increased N rate. Nitrogen fertilization increased TNC reserve utilization, LAI, and canopy CER, thereby increasing shoot regrowth at rates up to 90 kg N ha⁻¹ period⁻¹.

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