Past and prospects of forage maize breeding in europe. I. The grass cell wall as a basis of genetic variation and future improvements in feeding value.

摘要

Although maize was early recognized as an excellent forage plant soon after its introduction in Europe, during a long time it was only bred for grain traits. Conversely to agronomic value, a steady decline in the average cell wall digestibility of hybrids was observed since the 1950s, despite a greater specialization of hybrids between forage and grain uses, except for the Netherlands where varietal registration has taken into account traits related to plant digestibility for longer times. Modern hybrids had on average a 5.5% lower in vivo cell wall digestibility than the old ones, leading to an organic matter digestibility reduction by 2%, despite a tendency to a slight, but significant, increase in grain content. When compared to the well-known early hybrid LG11, hybrids with a lower cell wall digestibility were 32, 63, 75, 84, and 93% in each of the registration periods before 1981, 1989, 1994, 1999, and in or after 1999, respectively. Alle-les for highly friable and digestible cell walls were either eliminated during breeding for stalk standability and breakage resistance, or lost by genetic drift during breeding for grain yield. Breeding of highly digestible and in-gestible forage maize may depend on the reevaluation and use of old genetic resources that are not currently used, and/or specific breeding of digestibility/ingestibility resource lines. Cell wall structure and organization, and lignified tissue patterning, are the key factor explaining silage maize variation both for cell wall digestibility and intake. In maize, as in other Poaceae, p-coumaric acid and ferulic acid, along with its array of dehydrodimers, are ester and/or ether-linked to the cell wall polymers. Linkages of pCA esters to syringyl lignin units may markedly influence not only the binding mode of these lignin units, but also the spatial organization of the polymers and their interactions with polysaccharides. Similarly, ferulates cross-linking between guaiacyl residues and arabinoxylans is detrimental to cell wall digestibility, acting as a reinforcement of the wall structure, and decreasing wall component accessibility to microorganism enzymes. Moreover, diferulate cross-linking of arabinoxylan chains is also involved in cell wall digestibility decrease, and is commonly thought to play a role in stiffening cell walls. While first improvements of maize cell wall digestibility were (will still be) based on whole plant digestibility tests, understanding the biochemical and molecular basis of cell wall biogenesis, organization, and lignification, is now required to realize further progress in plant cell wall digestibility and intake.