100 Influential Papers - page 22

S ECT I ON FOUR
ME AS UR EMENT OF P L ANT FORM , GROWTH
AND DECAY AND OF THE ENV I RONMENT
40
Bradshaw
et al.
experimented with six native grass species of northern Europe that are associated with different types of soil, ranging
from strongly acidic to appreciably alkaline, and varying greatly in contents of available major nutrients. In glasshouse experiments using
sand and water culture, they determined first the responses of the six species to variation in pH, and availability of Ca and P [
Journal of
Ecology,
46
, 749-757 (1958),
48
, 143-150 & 631-637 (1960)]. In the 1964 paper they reported the responses to increasing concentrations
of NaNO
3
. These experiments on an appreciable number of wild plant species, as opposed to crops, were some of the earliest precursors
of the experimental studies in ‘comparative plant ecology’ carried out with such enormous impact by Philip Grime and colleagues using
much larger numbers of species in later years. Some results were much as expected. For example,
Agrostis stolonifera
was able to respond to
increase in N supply at concentrations higher than those at which
A. tenuis
(now
A. capillaris
) was saturated. Other results were unexpected,
as when the ‘eutroph’
Agrostis stolonifera
yielded more at the lowest NO
3
concentration than the ‘oligotroph’
Nardus stricta
. Importantly the
authors’ Discussion exemplifies ecologists’ thinking in the 1960s about the significance of inherent differences in maximum growth rate.
Peter Grubb
22
Tennant, D.M. (1975)
A test of a modified line intersect method for estimating root length.
Journal of Ecology, 63, 995-1001.
39
Bradshaw, A.D., Chadwick, M.J., Jowett, D. & Snaydon, R.W. (1964)
Experimental investigations into the mineral nutrition of several grass species. IV.
Nitrogen level.
Journal of Ecology, 52, 665-676.
Papers that describe widely adopted methods can attract many
citations, as with these two. Their authors would not have claimed
great scientific novelty, but their impact has been large. In 1966
it was well understood that the length of a root system was
more important to many aspects of root function than its mass,
but the excessive time taken to measure length of anything but
a small, pot (or ideally solution)-grown plant ensured that few
bothered. The gratitude of researchers to Newman (1966) and
subsequently Tennant (1975) stems from the vast saving in time
and the consequent opportunity to study length. Newman proposed
a simple formula for measuring the length of a set of lines using
their intersections with a fixed line and Tennant simplified that and
showed how using a grid of lines was effective. Tennant’s approach
facilitated automation. The papers remain heavily cited, even though
almost everyone now uses automated methods. A good example
of the importance of measuring length is the study by Sanders and
Tinker [(1973)
Pesticide Science
,
4
, 385-395] on mycorrhizal function
in leek seedlings. The problem of phosphate uptake for plants is that
each segment of root rapidly becomes surrounded by a depletion
zone and the rate of uptake is then determined by the slow diffusion
of phosphate ions from soil to root. A plant can enhance P uptake
either by growing a longer root system that explores undepleted soil
or by forming a symbiosis with a mycorrhizal fungus that can grow
out of the depletion zone. Nye and Tinker [(1977)
Solute Movement
in the Soil-root System
. Blackwell Scientific Publications, London]
developed the concept of inflow, the rate of uptake per unit root
length. Because of diffusion limitation, there is an absolute upper
limit for inflow to a root, determined largely by moisture content and
the buffering capacity of the soil for phosphate. In the experiment
by Sanders and Tinker, that upper limit was 3.5 pmol m
-1
s
-1
, which
was approximately what they measured (3.6) for uncolonised leek
plants. However, leek seedlings colonised by a mycorrhizal fungus
had P inflow of 17 pmol m
-1
s
-1
, five times faster than the theoretical
maximum, which was possible because the mycorrhizal hyphae
spread out beyond the depletion zone and relieved the diffusion
limitation to P uptake. This seminal paper revealed the mechanism
by which arbuscular mycorrhizal fungi worked; it would not have
been possible without measurement of root length. Measurement
of root length is now routine in all functional studies, and can be
achieved quickly and easily.
Alastair Fitter
Newman, E.I. (1966)
A method of estimating the total length of root in a sample.
Journal of Applied Ecology, 3, 139-145.
38
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AD Bradshaw
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