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IV. Partitioning of Photosynthate between Shoots and Roots

IV. Partitioning of Photosynthate between Shoots and Roots

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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

chapter.



213



UPTAKE OF MINERAL NUTRIENTS AND CROP GROWTH

Oilseed mDe



.-0

c



e



0.20;

0



2



6



L



Days



of



I



I



I



I



8



10



12



1L



treatment



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

(unpublished data).



214



A. WILD ET AL.



V.



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

photosynthate.

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



215



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

on barley.

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

minimized.

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

possible.



216



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



I.



INTRODUCTION



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

to linseed.

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

22 1

Copyright 0 1987 by Academic Press, Inc.

All rights of reproduction in any form reserved.



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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.,

1976).



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

Table I

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)

Oilseed



Palmitic



Stearic



Oleic



Linoleic



Linolenic



Safflower'

Sunflower'

Cottonseed'

Peanut'

Maize'

Soybean'

Rapeseed (canola)'

Linseed'

Linseed Zero mutantb



I

6

22

10

12

10

4

6



3

2

2

3

2

3

1

4

5



12

26

20

60

26

24

60

18

24



78

66

56

22

59

55

25

19

63



1

8

10

53

1



'Anonymous (1982).

bGreen (1986b).



I



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IV. Partitioning of Photosynthate between Shoots and Roots

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