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IV. Partitioning of Photosynthate between Shoots and Roots
A. WILD ET A L .
highest at low external concentrations (1 or 8 jd4)and decreased at concentrations up to lo00 pA4 K'. With two species, however, rose clover
(Trifoliumhirtum) and veldt grass (Ehrhartalongifolia),there was no effect
on the ratio over the whole range of concentration. Similarly, Woodhouse
(1977) found no consistent effect of the external concentration of K'
(1.3-102 jd4)on the root:shoot ratios of barley, fodder radish, and perennial ryegrass. Spear et al. (1978a), in contrast, found the highest ratios with
sunflower, maize, and 12 cultivars of cassava at 0.5-2 pit4 K' and lower
ratios at higher concentrations.
Root:shoot ratios are also affected by the pH and temperature of the
nutrient solution. For example, Breeze et al. (1987) found that for white
clover supplied with NO; or dependent on symbiotically fixed nitrogen, the
ratio was higher at pH 4 than at pH 5 , 6, or 7. An influence of root
temperature has been observed in flowing solution culture by GanmoreNeumann and Kafkafi (1980), Clarkson et al. (1986), and Macduff et al.
(1987a). Results from three similar experiments with perennial ryegrass,
oilseed rape, and barley are compared in Fig. 20. Plants of each species were
acclimatized at a root temperature of 5°C for 14 days and had common
shoot temperatures of 20/15 "C to 25/15 "C dayhight. The species differed
widely in their partitioning strategy in response to change of root
temperature in the range 3-25°C. The root:shoot ratios of perennial
ryegrass, for example, were lower at 3 "C than at 25 "C, while the reverse
was true for oilseed rape.
These effects of the supply of nutrients on root:shoot ratios generally
support earlier observations on plants grown in soil or static solutions
(Brouwer, 1962a,b). The controlled conditions provided by flowing solution culture show that there are differences between species in their response
to K and root temperature, whereas consistently high root:shoot ratios have
been found in plants grown at low concentrations of NO; or phosphate.
This latter observation differs from that found for localized application of
high concentrations of phosphate and N (as NO; or NH:), but not K, which
gives vigorous growth of lateral roots in the treated region (Drew, 1975).
Except for plants which have large storage organs in their roots for
carbohydrate, root dry weight usually accounts for about 10-20% of the
total dry weight. This partitioning affects the efficiency with which the
crop converts solar radiation into a saleable product. Models have been
developed to describe it, e.g., Davidson (1969), Reynolds and Thornley
(1982), and Johnson (1985). For plants growing in soil the acquisition of
water and nutrients depends very largely on the spatial distribution of
roots, but this consideration would take us away from the subject of this
UPTAKE OF MINERAL NUTRIENTS AND CROP GROWTH
FIG.20. Effect of root temperature on root : shoot ratio (dry weight basis) of three species
grown in flowing nutrient solutions, treatment period of 14 days, following acclimatization
period of 14 days with root temperature at 5°C. Data for oilseed rape from Macduff ef al.
(1987a), for perennial ryegrass from Clarkson et af. (1986), and for barley from J. H. Macduff
A. WILD ET AL.
CONCLUSIONS AND SUMMARY
One outcome of the work on flowing nutrient solutions is an improved
understanding of the mechanism of nutrient uptake by plants. This arises
from the following observations:
1. Nutrient uptake varies diurnally, the peak rate occurring 5-6 hr after
the peak rate of CO,influx, which occurs at about 1200 GMT on a sunny
day (see Figs. 3 and 4). Nutrient uptake also decreases quickly after the loss
of photosynthetic tissue (see Fig. 2). These observations imply that nutrient
uptake responds to metabolic demand resulting from the production of
2. The rate of nutrient uptake depends on the external concentration, but
this dependence becomes less as plants age (see Figs. 10 and 11). Under
steady-state conditions the nutrient flux becomes constant, or nearly so,
down to low concentrations when plants are more than about 3-4 weeks
old. At very low concentrations the measured nutrient flux can only be accounted for if absorption occurs close to the epidermis (see Table V).
3. Under nonsteady conditions the nutrient flux changes quickly after the
external concentration is changed. There is an increased flux when the external concentration is increased even though the potential growth rate is
achieved at the low concentration (see Fig. 13).
We account for the second and third of these observations by proposing
that (1) nutrient uptake rate depends partly on the number and activity of
ion transporters in the root cortex; (2) the number of transporters receiving
nutrient depends on the external concentration and responds quickly to a
change of concentration; and (3) the activity of transporters is controlled by
the plant, probably in relation to metabolic demand and responds slowly to
a change of concentration. This proposal fits our observations if (1) the activity of transporters is high at low external concentrations, though the
number is small, and (2) the number is higher but activity is lower at high
external concentrations. There is, additionally, the effect of supply of
photosynthate, to which nutrient uptake responds, presumably through an
effect on the activity of the transporters. The ultimate control of nutrient
flux is still uncertain, as is the mechanism through which it acts.
Evidence from flowing solution culture suggests that species differ in
their response to a range of nutrient concentrations because of differences
in potential RGR and the ratio root surface area:plant weight (see Figs. 6
and 7). These factors, and also the concentration of nutrient in plant
tissues, explain why young plants (up to about 3-4 weeks old) require higher
external nutrient concentrations than older plants.
UPTAKE OF MINERAL NUTRIENTS AND CROP GROWTH
The relative response to NHf and NO; has also been investigated in flowing nutrient solutions. With oilseed rape and perennial ryegrass differences
were small, but addition of small concentrations of NH: to NO; solutions
increased RGR. The relative uptake of NH: and NO; depends on root
temperature but the effect differs between species (see Fig. 14). No adequate
explanation can yet be given. The effect of root temperature on growth rate
also differs between species. There was no effect on oilseed rape in the range
3-25 “C but a large effect on perennial ryegrass and an intermediate effect
The distribution of Cd, Cu, Pb, and Mn between roots and shoots has
been measured in plants grown in flowing nutrient solutions at controlled
metal concentrations. Lead is strongly held in roots, Cd and Cu less so, and
Mn least. There are however, differences between species in uptake of the
metals and distribution within plants. Further work is needed if the differences are to be understood.
Flowing nutrient solutions incorporating a pH-stat have also been useful
in providing net OH- and H’ efflux measurements. A time course, though
not necessarily a cause and effect relation with K’ influx, has been
demonstrated (see Fig. 4). Studies have also been made of the effects of Al
and its interaction with phosphate on the rate of growth of white clover, its
symbiotic nitrogen fixation, and its nitrogenase activity (Jarvis and Hatch,
1985b). Flowing nutrient solution culture is particularly useful for this work
because in the dilute solutions used precipitation of Al phosphate is
This chapter has been largely devoted to the more fundamental work on
the mineral nutrition of whole plants using flowing nutrient solutions. Using plants grown under the controlled conditions that only flowing nutrient
solutions can provide, there is, for the future, much potential in examining
the mechanism of nutrient uptake, including the control systems within
plants, the interrelationship with the supply of photosynthate and the effect of environmental conditions such as pH, temperature, and nutrient
concentrations on new genotypes. There is also scope for investigating
agronomic problems such as the effects on crop plants of salinity, the
acidity “complex,” transfer of micronutrients and other elements within
the plant, and the composition of the gaseous phase around roots. Symbiotic nitrogen fixation systems and mycorrhizal associations can be included, and plants can be grown to their reproductive phase. As gene
transfer systems for crop plants are improved, gene expression in the
growth and nutrition of plants will need to be understood. Flowing
nutrient solutions provide the controlled conditions that will make this
A. WILD ET A L .
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ADVANCES IN AGRONOMY, VOL. 41
MINERAL NUTRITION OF LINSEED
AND FIBER FLAX
Peter J. Hocking, Peter J. Randall,
and Andrew Pinkerton
Division of Piant Industry, Commonwealth Scientific and
Industrial Research Organization (CSIRO)
Black Mountain, Canberra, A.C.T. 2601, Australia
Flax (Linum usitatissimum L.) is one of about 100 species in the genus
Linum (family Linaceae). Over many centuries, lines have been selected for
fiber production (fiber flax) or for the oil content of their seeds (oilseed flax
or linseed). The term linseed will be used in this review for oilseed flax.
There are instances, however, where it has not been possible to ascertain
from the literature whether linseed or fiber flax is referred to; in these cases
we have used the term flax. We have also used flax in a generic sense when
information applies to both linseed and fiber flax. It should be pointed out
that in the North American literature the term flax refers almost invariably
The origin of cultivated flax is uncertain, but the crop probably came
from the Near East, as it was grown for its fiber in Mesopotamia and Egypt
at least 4O00 years ago (Dillman, 1936; Kipps, 1970). Later, appreciation of
the high oil content of its seeds led to the selection of oilseed types. In
general, fiber flax is tall and single-stemmed with poor seed production,
while linseed is shorter and multibranched with high seed yields but poor
fiber production and quality (Blackman and Bunting, 1951; Bailey and
Soper, 1985). There are dual-purpose fiber and oilseed types, but their
yields are lower than those of cultivars selected specifically for fiber or seed
production (Martin et af., 1976).
Flax fiber has traditionally been spun into linen yarns which are used in
the manufacture of threads and twines of various kinds. The yarn is also
woven into toweling, clothing fabric, table linen, and other textiles, but the
increasing use of synthetic fabrics has eroded these applications somewhat.
Fiber flax is used also in the manufacture of high-grade papers. The fiber
from linseed is short and often harsh, so it is used in the manufacture of
Copyright 0 1987 by Academic Press, Inc.
All rights of reproduction in any form reserved.
PETER J. HOCKING ET AL.
commodities such as cigarette paper rather than for the production of linen
(Martin et al., 1976; Carter, 1984). Approximately 12% of the world's
linseed oil supply is obtained as a by-product from seeds of fiber flax
(Bailey and Soper, 1985).
Linseed is a traditional source of industrial vegetable oil, and it was the
mainstay of the oil-based paint, varnish, and linoleum industries (Curteis,
1949; Martin et al., 1976). However, competition from plastics and acrylics
has resulted in it becoming a relatively minor industrial oilseed crop
(Matheson, 1976). Linseed is an industrial oil because of its high content of
the fatty acid linolenic acid, which oxidizes rapidly and imparts a drying
characteristic to the oil. The drying quality of the oil is indicated by its
iodine value: the higher the value, the better the quality. The oxidation of
linolenic acid also produces rancidity, so the oil is generally unsuitable for
edible purposes (Green and Marshall, 1981). However, it has been used occasionally as an edible oil after expensive treatment involving conversion of
the linolenic acid to linoleic acid. The seed meal which remains after the oil
has been extracted is a valuable, high-protein stock feed (Martin et al.,
Recently, experimental lines of high-quality, edible oil linseed have been
developed in Australia (Green and Marshall, 1984; Green, 1986a,b). The oil
from these lines is high in the desirable unsaturated linoleic acid (%63vo),
but is virtually free of linolenic acid (%lVo). The polyunsaturated quality
of the oil is better than that of rapeseed and comparable to sunflower oil
(Table I). When the edible oil lines are released commercially, considerable
Fatty Acid Composition of Oil from Major Edible Oilseeds and a Standard
Linseed Cultivar Compared to a Low Linolenic Acid Linseed Mutant
Fatty acid composition (Yo)
Linseed Zero mutantb