untitled

neutral redwhich accumulates in the intact protoplasts,

then a solution of strong osmotic potential is used

to shrink the protoplasts (Fig. 11.16b,d). In both

methods the number and distribution of living cells can

be counted. For glasshouse-grown wheat seedlings, the

outermost root cortical cell layer dies first – typically

in root regions that are 7–10 days old – and then the

successively deeper cortical cell layers die at approxi-

mately 3– 4 day intervals, until only the innermost cor-

tical cell layer (next to the root endodermis) remains

alive. This progressive root cortical deathbehind the

extending root tips is seen when roots are grown

in soil or in aseptic conditions. It is faster in some

cereals than in others, faster in cereals than in grass

roots, and faster in laboratory than field conditions.

But in all cases the root cortex dies, and this does not

affect root function, because the root tips continuously

extend into new zones of soil, while the older root

regions serve a transport function, with a largely

redundant, senescent root cortex.

Fungal invasion of cereal and grass roots

The pattern of root cortical cell death, described

above, precisely matches the pattern of fungal invasion

in the roots of perennial ryegrass, Lolium perenne

(Fig. 11.17), originally described by Waid (1957). The

sequence is summarized below, but essentially it

involves the successive invasion of roots by parasites,

weak parasites, and saprotrophs, responding to the

progressive natural senescence of the root cortex:

1 The growing root tips are virtually free from fungal

hyphae, but an increasingly complex fungal com-

munity develops with distance (age) behind the tips.

FUNGAL ECOLOGY: SAPROTROPHS 227

4

2

–4

2 5 8 14

Plant age (weeks)

0

–2

Root length (m

)

11

Fig. 11.14Cumulative root production (white sym-
bols), cumulative root decay (black symbols below
the line), and white living roots (black symbols
above the line) of groundnut plants grown for 14
weeks (near-maturity). Nearly identical results were
obtained for five different groundnut cultivars (two
replicates of each). (From Krauss & Deacon 1994.)

(a) (b)

Fig. 11.15Representative tracings of the root system of

a single groundnut plant. Root distribution was traced

onto a transparent overlay at weekly intervals, but only

the tracings at 6 weeks (a) and 13 weeks (b) are shown.

Solid lines represent roots that were white (alive) at the

times of these tracings. Broken lines represent roots that

were present earlier but had decayed and disappeared.

Nearly identical results were found for five groundnut

cultivars (two replicates of each). (From Krauss & Deacon

1994.)