A Possible Role for Leaf Cell Rupture in Legume Pasture Bloat1

Abstract

Leaves from seven forage crop species were crushed in a mortar and pestle to demonstrate a relationship between susceptibility to mesophyll cell rupture and the tendency to cause bloat in ruminant animals. The bloat‐causing forages were alfalfa (Medicago sativa L.), red clover (Trifolium pratense L.), and white clover (T. repens L.). The bloat‐safe species were cicer milkvetch (Astragalus cicer L.), birdsfoot trefoil (Lotus corniculatus L.), sainfoin (Onobrychis viciaefolia Scop.), and bromegrass (Bromus inermis Leyss.). Two grams of fresh leaves were crushed between two layers of 100‐mesh nylon cloth in a mortar and pestle. The leaf material which passed through the nylon cloth was suspended in 0.2 M sodium maleate buffer (pH 6.9) and allowed to separate into sediment and supernatant fractions by standing at 4 C for 45 min. The sediment fraction contained intact mesophyll cells and the supernatant fraction contained fragments of chloroplast lamellae which had been released from ruptured mesophyll cells. Compared to the bloat‐causing legumes, the bloat‐safe legumes had 10 to 20 times more whole mesophyll cells released from the leaf tissue. Conversely, the supernatant fractions from the bloat‐causing legumes contained 6 to l0 times more chlorophyll than supernatant fractions from the bloat‐safe legumes. These data showed that mesophyll cells in the bloat‐safe forage legumes were more resistant to mechanical rupture compared to mesophyll cells in the bloat‐causing forage legumes. Disruption of sainfoin leaves in buffer containing 10% polyvinylpyrolidone did not affect the number of whole mesophyll cells in the filtrate fraction. Hence the presence of tannins and leaf cell resistance to rupture are two separate and complementary explanations for the bloat‐safe nature of sainfoin. The bloat‐safe nature of cicer milkvetch, which does not contain tannins, was attributed to the resistance to rupture of the mesophyll cell walls.