WATER-DEFICIT EFFECTS ON CELL-WALL COMPOSITION AND IN-VITRO DEGRADABILITY OF STRUCTURAL POLYSACCHARIDES FROM ALFALFA STEMS

Forage crops from drought years are often reported to be of higher qua lity than normal. Maturity through its effects on plant composition is recognized as a major determinant of forage quality. Our objective wa s to determine the effects of water deficit and maturity on cell-wall composition and structural polysaccharide degradability in alfalfa (Me dicago saliva L.), Plants were grown under a mobile weather shelter an d irrigated to 65, 88, and 112% of field capacity, Basal stems were ha rvested after 21, 35 and 49 d of regrowth, Components of the cell wall were determined (neutral sugars, uronic acids, Klason lignin, and est er- and ether-linked p-coumaric and ferulic acids), In vitro 12-and 72 -h degradabilities were determined for cell-wall polysaccharide compon ents, As maturity advanced, cell-wall and Klason-lignin concentrations increased (4.8 and 2.9%, respectively), while uronic acids declined ( 10.7%). Cell-wall lignification was not affected by water status, Stem s from plants most severely restricted in water supply were lowest in cell-wall and neutral sugars concentration, and contained progressivel y less glucose, Concentrations of esterified and etherified phenolic a cids were low and not consistently related to maturity or irrigation l evel. Degradability (12 h) of both glucose and uronic acids decreased (8.9 and 4.8%, respectively) as plants matured, but only the 72-h degr adability of uronic acids was decreased (6.5%) during maturation, Wate r 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 Klason-lign in 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 w hether the phenolic acids were ester- or ether-linked to the walt, Imp rovement in alfalfa forage quality during periods of water deficit was not generally attributable to improved cell-mall degradability, but w as likely the result of delayed maturity and decreased cell-waif conce ntration.