Agriculture Reference
In-Depth Information
and [O
2
]
L
are the concentrations of O
2
in the gas space (mol per unit volume
gas space) and in root tissue (mol per unit volume root), respectively.
The boundary conditions for solving Equation (6.1) are: (a) at the root base,
[O
2
]
G
is the ambient value in the atmosphere; and (b) at the root apex, [O
2
]
G
is
the minimum value required for root respiration [
mol dm
−
3
≈
30
(gas space)].
µ
The equations are solved numerically.
6.2 ARCHITECTURE OF WETLAND PLANT ROOT SYSTEMS
In dryland plants the size of the root system compared with the shoot system is
generally governed by the plant's water requirements except under quite severe
nutrient deficiency (Tinker and Nye, 2000). However, in wetland plants in sub-
merged soil, the free availability of water means that the size of the root system
is more often likely to be governed by nutrient requirements. The length densities
of wetland root systems may be comparable to those of dryland plants: length
densities of rice roots are typically 20-30 cmcm
−
3
in the topsoil (Matsuo and
Hoshikawa, 1993). A large proportion of the length may be as fine roots. In rice
in submerged soil short fine laterals, 1-2 cm long and 0.1-0.2mm in diameter,
develop as branches along the primary roots once the primary roots are a few
cm long. These are much less aerenchymatous than the primary roots (porosities
of 1-2% compared with
≤
50%) and they do not develop secondary thicken-
ings in their walls to the same extent (Matsuo and Hoshikawa, 1993). They may
themselves be branched producing up to sixth order laterals. They account for
a small part of the root mass but the bulk of the external surface, and they are
plumbed directly into the main water and solute transport vessels in the stele of
the primary root (as can be seen in Figure 6.2).
The structure of the rice root is therefore apparently dominated by the need for
internal gas transport. On the face of it, this structure may conflict with the needs
for efficient nutrient absorption (Kirk and Bouldin, 1991). The development of
gas-impermeable layers in the root wall seems likely to impair the ability of
those parts of the root to absorb nutrients, and the disintegration of the cortex
might impair transport from the apoplasm to the main solute transport vessels in
the stele, though these points are uncertain (Drew and Saker, 1986; Kronzucker
et al
., 1998a). It seems likely that the short fine lateral roots are responsible for
the bulk of the nutrient absorption by the root system and compensate for any
impairment of nutrient absorption by the primary roots as a result of adaptations
for internal aeration.
The question arises: what combination of fine laterals and aerenchymatous pri-
mary roots provides the greatest absorbing surface for a given root mass? Not
having impermeable wall layers and having a large surface area to volume ratio,
the laterals will leak O
2
more rapidly than the adjacent primary root. A related
question is therefore how the O
2
budget of the root system is affected by the
combination of primary roots and laterals. Armstrong
et al
. (1990, 1996) mod-
elled O
2
release from adventitious and lateral roots of the rhizomatous wetland
