Biology Reference
In-Depth Information
The lignin content and composition are closely related to plant cell wall
digestibility of (forage) plant cell walls. Unravelling the lignin biosynthetic
pathway will lead to a search for strategies to modify the lignin content and
composition to improve the digestibility of forage and for biofuel production.
A. LAND COLONIZATION AND APPEARANCE OF A LIGNIFIED
VASCULAR SYSTEM
One of the most important biological events in the history of life was the
colonization of land by plants and their rapid diversification during the early
Paleozoic period (
Niklas, 1997; Raven and Edwards, 2001
).
Land colonization meant plants had to cope with a new stress in addition
to adaptation to a gravitropic environment as, when taking CO
2
from the
atmosphere, an inevitable loss of water occurred. To avoid such water loss,
plants developed different strategies to accumulate water in their tissues, to
supply it or to minimize its loss. In fact, cuticle and stomata were some of the
very first innovations in the first terrestrial plants (
Fig. 2
). When CO
2
levels
were high (during the Silurian period), plants would lose less water molecules
than when CO
2
levels decreased (late Carboniferous): water loss would have
been as much as 17 times higher for a Carboniferous plant (
Sperry, 2003
).
Thus, the adaptive radiation of plants by the early Carboniferous period was
probably due to very high atmospheric CO
2
levels (
Sperry, 2003
). High CO
2
concentrations encouraged plants with high capacitance and low hydraulic
conductance. In fact, the CO
2
concentration seems to be an important factor
during plant evolution, not only in the diversification of a vascular system
but also in the origin of leaves. Until the CO
2
levels decreased, plants were
not able to develop bladed photosynthetic structures because of lethal over-
heating (
Beerling et al., 2001; Osborne et al., 2004
). A second drop in CO
2
took place during the Cretaceous period, and this was associated with
angiosperms diversification (
McElwain et al., 2004
).
However, a water supply system based on high capacitance prevents plants
from being bigger than a thin tallus. The first terrestrial plants had a cuticle
that avoided water loss and lacked a water-conducting system, and were
also small in size. These first land plants were poikilohydric, in which the
water potential was equilibrated with surrounding water sources (
Raven,
1987
). Those plants had high capacitance to accumulate water when the
environment was dry.
The first evolutionary radiation among land plants is related to the diver-
sification of tracheids (
Kenrick and Crane, 1997; Knoll and Niklas, 1987
).
This diversification originated three major clades, each with a specific type of
tracheid (
Kenrick and Crane, 1997
). True tracheids have been defined as
