Soluble Carbohydrates, Concurrent Photosynthesis and Efficiency in Regrowth Following Defoliation: A Field Study with Agropyron Species

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Abstract
(1) The contribution of stored carbon to plant regrowth following defoliation is commonly thought to be large. This study quantitatively compared the amount of carbon supplied to regrowth from storage with photosynthesis of regrowth in the field. (2) Two bunchgrass species were selected for comparison because of markely different abilities to regrow following defoliation despite very similar photosynthetic morphologic and phenologic characteristics. (3) The more grazing-tolerant Agropyron desertorum (Fisch. ex Link) Schult. consistently produced more regrowth in the absence of photosynthesis than A. spicatum (Pursch) Scribn. and Smith, but a severe preclipping treatment (which has been shown to reduce carbohydrate reserves by more than 40%) did not significantly reduce etiolated regrowth in either species. (4) Differences in regrowth between and within species were not correlated with crown non-structural carbohydrate concentrations, total pools, or amounts utilized during regrowth. (5) The daily contribution of carbon from reserves exceeded measured daily photosynthetically-fixed carbon for only 2-5 days following defoliation when regrowth rate was maximal. However, when apical meristems were removed and regrowth was much slower, photosynthesis during regrowth immediately outweighed stored reserves as a source of carbon. The latter is the usual case when these grasses are subjected to managed grazing. (6) In both species, apical meristem removal was followed by sharply reduced regrowth efficiency due to a delay in and reduced rate of regrowth production. There was, however, no increase in respiration rate. (7) Meristematic limitations appear to be the dominant control on the amount of etiolated regrowth produced. These limitations also appear to be of prime importance in determining regrowth in the light and for grazing tolerance of plants.

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