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Forages

Photosynthesis and Nutritive Value in Leaves of Three Warm-Season Grasses before and after Defoliation

^a USEPA, Research Triangle Park, NC
^b USDA-ARS, JPCSNRCC, 1420 Experiment Station Road, Watkinsville, GA 30677
^c USDA-ARS and North Carolina State Univ., Raleigh, NC 27695-7620

^* Corresponding author (Dwight_Fisher{at}Scientist.com)

Received for publication March 2, 2004. Forage yields are influenced by plant response to defoliation. We examined the photosynthesis and nutritive value of first (first leaves) and third (third leaves) fully expanded leaves (numbered from the apex) in three warm-season (C4) grasses. Net photosynthetic rates at uniform temperature and light both before and after a 2-wk exposure to full sunlight and the effect of leaf position on nutritive value were determined on vegetative tillers in well-established swards of bermudagrass [Cynodon dactylon L. (Per.) cv. Tifton 44], caucasian bluestem [Bothriochloa caucasica (Trin.) C.E. Hubb.], and Atlantic coastal panicgrass [Panicum amarum var. amarulum (Hitchcock and Chase) P.G. Palmer] growing on a Cecil clay loam (clayey, kaolinitic, thermic Typic Hapludults). Bermudagrass had the greatest level of crude protein (CP), followed by panicgrass and bluestem. Fiber was greater in the first leaves than in the third leaves for bermudagrass and panicgrass but not for bluestem. Photosynthetic rates of panicgrass and bluestem third leaves estimated 2 wk after defoliation of the surrounding canopy were less than estimates made before defoliation in the first leaves but were similar to the third leaves before canopy defoliation. The third leaves of bermudagrass 2 wk after defoliation had lesser photosynthetic rates per unit chlorophyll than the first or third leaves before defoliation. Photosynthetic rates were correlated with hemicellulose across leaf classes and species (r² = 0.93). The photosynthetic decline observed in third leaves of bermudagrass compared with panicgrass and bluestem is evidence of variation in leaf response after defoliation among warm-season grasses.

Abbreviations: ADF, acid detergent fiber • CP, crude protein • IVDMD, in vitro dry matter disappearance • NDF, neutral detergent fiber • PAR, photosynthetically active radiation