IMPACT OF DEFOLIATION AND LIGHT EXPOSURE IN LEAVES OF THREE SUBTROPICAL GRASSES

Authors: MEHAFFEY, D - US-EPA, NV; Fisher, Dwight; Burns, Joseph

Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/3/2005
Publication Date: N/A
Citation: N/A

Interpretive Summary: Leaves remaining in a grass pasture after defoliation by grazing or haymaking can become sources for new growth. However, most of the remaining leaves are older and less productive. In part, this is because prior to defoliation they were shaded by the growth and development of the forage. These older leaves may increase, remain the same, or decrease in productivity after defoliation and exposure to full sunlight. We tested three warm-season grasses for their ability to utilize these older leaves when they were exposed to full sun by defoliation. The grasses varied in their ability to maintain productivity in the older leaves. This suggests that in some grasses a more severe defoliation may be appropriate to remove a greater proportion of the older leaves since they make a relatively smaller contribution to growth of new pasture. Rotational grazing systems may be designed to take advantage of the physiology of the grass being grazed.

Technical Abstract: Forage yields are influenced by plant response to the frequency and severity of defoliation by either herbivory or mechanical harvest. Residual leaf area may contribute to regrowth following defoliation. We examined the defoliation response of third fully expanded leaves (numbered from the apex) in three subtropical (C4) grasses. Net photosynthetic rates before and after a 2-wk exposure to full sunlight, and the effect of leaf position on composition, were determined on vegetative tillers 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]. The experimental design was a randomized complete block with two replicates and multiple samples per plot collected over two years. Net photosynthesis and respiration were estimated in vitro at uniform temperature and light. In vitro dry matter disappearance and Van Soest fiber fractions were also determined to estimate forage quality. Photosynthetic rates of the third fully expanded leaves exposed to full sunlight for 2 wk (3rd-LX) of panicgrass and bluestem were lower than the first fully expanded leaves (1st-L), but similar to third fully expanded leaves prior to exposure (3rd-L). The 3rd-LX of bermudagrass had a lower photosynthetic rate per unit chlorophyll than the 1st-L or 3rd-L. Photosynthesis was positively correlated with hemicellulose across leaf classes and species (r2=0.93). The similarity of photosynthesis between 3rd-L and 3rd-LX in panicgrass and bluestem while we observed a decline in photosynthesis in these leaf classes in bermudagrass provides evidence of variation in leaf response to high light following defoliation of the surrounding canopy among subtropical grasses was found. The possible relationship of this character in the adaptation to herbivory warrants further investigation.