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Water-Deficit Effects on Cell-Wall Composition and In Vitro Degradability of Structural Polysaccharides from Alfalfa Stems

Dep. of Animal Science, Univ. of Minnesota, St. Paul, MN 55108
USDA-ARS, Plant Science Research Unit and U.S. Dairy Forage Research Center Cluster, and Dep. of Animal Science, and Dep. of Agronomy and Plant Genetics, Univ. of Minnesota, St. Paul, MN 55108
USDA-ARS, Field Crops Research Unit and U.S. Dairy Forage Research Cluster, and Dep. of Agronomy, Iowa State Univ., Ames, IA 50011

^* Corresponding author (jungx002{at}maroon.tc.umn.edu).

Forage crops from drought years are often reported to be of higher quality than normal. Maturity through its effects on plant composition is recognized as a major determinant of forage quality. Our objective was to determine the effects of water deficit and maturity on cell-wall composition and structural polysaccharide degradability in alfalfa (Medicago sativa L.). Plants were grown under a mobile weather shelter and irrigated to 65, 88, and 112% of field capacity. Basal stems were harvested after 21, 35 and 49 d of regrowth. Components of the cell wall were determined (neutral sugars, uronic acids, Klason lignin, and ester- and ether-linked p-coumaric and ferulic acids). In vitro 12- and 72-h degradabilities were determined for cell-wall polysaccharide components. As maturity advanced, cell-wall and Klason-lignin concentrations increased (4.8 and 2.9%, respectively), while uronic acids declined (10.7%). Cell-wall signification was not affected by water status. Stems from plants most severely restricted in water supply were lowest in cell-wall and neutral sugars concentration, and contained progressively less glucose. Concentrations of esterified and etherified phenolic acids were low and not consistently related to maturity or irrigation level. Degradability (12 h) of both glucose and uronic acids decreased (8.9 and 4.8%, respectively) as plants matured, but only the 72-h degradability of uronic acids was decreased (6.5%) during maturation. Water status had no influence on cell-wall component degradability except for an increase (3.0%) in 12-h uronic acid degradation with increasing water-stress. Principal component analysis indicated that Kiason-lignin concentration was negatively related to both 12- and 72-h cell-wall polysaccharide degradation, p-Coumaric and ferulic acids affected 72-h polysaccharide degradation because of both their concentration and whether the phenolic acids were ester- or ether-linked to the wall. Improvement in alfalfa forage quality during periods of water deficit was not generally attributable to improved cell-wall degradability, but was likely the result of delayed maturity and decreased cell-wall concentration.

Received for publication May 26, 1994.

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