FROST-INDUCED MOVEMENT OF POTASSIUM TO THE APOPLAST MAY INCREASE BLOAT IN RUMINANTS GRAZING ALFALFA

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 fre ezing that could be related to ruminant bloat. A high occurrence of bl oat is associated with elevation in the ratio of potassium (K+) to sod ium (Na+) in the rumen, which is thought to cause flocculation of chlo roplast membrane particles, producing a stable foam that results in bl oat. In several plant species, exposure to frost temporarily inhibits ATPase activity, resulting in the movement of water and K+ from the pr otoplast of plant cells into the apoplast. Our hypothesis was that an overnight frost causes movement of K+ into the apoplast of alfalfa lea ves, and leaching of this extracellular K+ from the foliage would incr ease the ratio of K+ to Na+ in rumen fluid of livestock grazing the al falfa. To test this hypothesis, we slowly froze detached alfalfa leave s to -2 degrees 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) leaf fresh weight leached from froz en-thawed alfalfa was sufficient to increase the occurrence of bloat a ccording 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 alfalf a, but relatively Little Na+ was found in this leachate. A similar inc rease in leachate ion concentration was found when the study was repea ted.