Frost-Induced Movement of Potassium to the Apoplast May Increase Bloat in Ruminants Grazing Alfalfa
- Jennifer W. MacAdam ,
- Jennifer Hill and
- Ralph E. Whitesides
Exposure of alfalfa (Medicago sativa L.) to frost reportedly increases the frequency of bloat in ruminants grazing the forage. Our objective was to investigate changes in leaves of the plant that occur with freezing that could be related to ruminant bloat. A high occurrence of bloat is associated with elevation in the ratio of potassium (K+) to sodium (Na+) in the rumen, which is thought to cause flocculation of chloroplast membrane particles, producing a stable foam that results in bloat. In several plant species, exposure to frost temporarily inhibits ATPase activity, resulting in the movement of water and K+ from the protoplast of plant cells into the apoplast. Our hypothesis was that an overnight frost causes movement of K+ into the apoplast of alfalfa leaves, and leaching of this extracellular K+ from the foliage would increase the ratio of K+ to Na+ in rumen fluid of livestock grazing the alfalfa. To test this hypothesis, we slowly froze detached alfalfa leaves to −2°C, and nucleated extracellular ice formation. We then thawed the leaves and leached them in distilled, deionized water. The mean increase of 1.061 g K+ kg−1 t leaf flesh weight leached from frozen-thawed alfalfa was sufficient to increase the occurrence of bloat according to evidence from other studies. Compared with control leaves, calcium and magnesium were also elevated by 1.821 and 0.425 g kg−1 leaf fresh weight, respectively, in leachate from frozen-thawed alfalfa, but relatively little Na+ was found in this leachate. A similar increase in leachate ion concentration was found when the study was repeated.
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