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II. Systems Employing Flowing Nutrient Solutions

II. Systems Employing Flowing Nutrient Solutions

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UPTAKE OF MINERAL NUTRIENTS AND CROP GROWTH



173



adjusting the flow rate of the solution through the roots, and uptake is

measured by analysis of the waste solution and/or plant analysis. They have

been used for experiments on nutrient uptake by wheat (Triticurn aestivurn)

seedlings (Reisenauer, 1969; Cox and Reisenauer, 1973), by barley

(Hordeurn vulgare) seedlings (Bloom and Chapin, 1981), and by different

parts of a root (Harrison-Murray and Clarkson, 1973; Russell and

Clarkson, 1976). Low-cost, nonrecirculating systems have been used in

longer-term experiments using tap water and technical-grade salts (Elliott

and Nelson, 1983) or commercial fertilizers (Ganmore-Neumann and

Kafkafi, 1980).

2. The nutrient solutions recirculate through the vessels containing plants

and concentrations are maintained by monitoring and nutrient additions. In

the simplest form of these systems, the nutrients are replenished after manual

sampling and analysis of the solution once or twice daily (Asher et al., 1965;

Asher and Edwards, 1978; Bhat, 1980; Osmond et al., 1981; Moorby and

Nye, 1983; Freijsen and Otten, 1984; Hatch and Canaway, 1984). The pH and

temperature of the solutions are sometimes controlled automatically (e.g.,

Asher et al., 1965; Asher and Edwards, 1978;Tolley-Henry and Raper, 1986).

Other systems have also incorporated the means for continuous automatic

monitoring and control of some nutrient concentrations (Clement et al.,

1974; Deane-Drummond, 1982; Barneix et al., 1984). In the system of mist

culture developed by Ingestad and Lund (1979), nutrients are replenished on

the basis of manual or automatic measurement of electrical conductivity, or

at a rate of addition proportional to plant growth rate.

In systems based on both nonrecirculating and recirculating solutions, it is

of critical importance to maintain an adequate flow rate in order to minimize

nutrient depletion as the solution flows through the roots. Edwards and

Asher (1974) showed that the relation between flow rate, F, in liters per vessel

per minute and the percentage decrease in concentration of the nutrient between the inlet and outlet, D, is given by



where R is root fresh weight (grams per vessel), u is the unit rate of nutrient

uptake (moles per gram of root fresh weight per minute), and, C is inlet concentration, moles per liter. Most recent systems based on recirculating solutions have incorporated constant, high flow rates to ensure that there is

adequate nutrient supply at the highest expected growth rates.

€3. THEHURLEY

SYSTEM



The system of Clement et a1.(1974), hereafter referred to as the “Hurley

system,” incorporates the main features of the system of Asher et al. (1965)



174



A. WILD ET AL.



but makes the advance of incorporating automatic monitoring of nutrient

concentrations, addition of nutrients to maintain prescribed concentrations

in solution, and the recording of these additions. The frequency of monitoring and replenishment may vary between 2 and 20 min. The system comprises eight plant culture units in which the temperature of each solution can

be separately controlled in the range 2.5-25°C to within +O.l"C. The

culture units are housed in an air-conditioned greenhouse with temperature

controlled in the range 15-25°C. For experiments in winter artificial lights

provide the plants with total radiation of 16 MJ/m2/day, one-half of which

is photosynthetically useful, i.e., within wavelengths 400-700 nm. The

monitoring layout is shown in Fig. 1.

In the Hurley system each plant culture unit contains 300 liters of nutrient

solution which is circulated at a rate of 1.2 liters per min through each of 24

culture vessels of 1.2-liter capacity arranged in parallel. The arrangement of

the culture vessels is such that the spatial distribution of the plants simulates



GLASSHOUSE



wall



MONITORING



AN0



CONTROL



LABORATORV



FIG. 1. The Hurley system for studying the uptake of ions by plants from flowing solutions of controlled temperature and composition. Ion-selective electrodes, an NH, probe, and

a flame photometer are linked through autotitrator controllers to stock solutions to maintain

predetermined concentrations of nutrients and pH; the colorimeter is linked through a computer control to pumps delivering phosphate continuously (P,) and intermittently (Pi). SV,

Sampling solenoid valve for delivering aliquots of solution to analysers; Trc, pumps for

delivering micro- and other nutrients; W, waste discharge; 4, electrical connection.



UPTAKE OF MINERAL NUTRIENTS AND CROP GROWTH



175



a crop canopy in the field, or a grass sward, thus enabling nutrient uptake

and dry matter yield to be calculated on an area basis if this is desired. The

four analytical systems shown in Fig. 1 are ion-selective electrodes for H'

and NO; (Clement et al., 1974), an ammonia probe for NH: (Hatch et al.,

1986), flame photometry for Na' and K' (Woodhouse et al., 1978 describe

its use for K'), and colorimetry for phosphate (Breeze et al. 1982). The

paper of Clement et al. (1974) described the control of pH by the use of a

glass/calomel electrode and autotitrator. Alternatively, microcomputer

control can be used (Hatch and Canaway, 1984).

The composition of the complete nutrient solution which is generally used

is given in Table I (Clement et al., 1978a; Breeze et al., 1984), although the

composition is varied according to the purposes of the experiment. For example, NO; was held constant in the range 1.43 pA4 to 143 mM for a study

of NO; uptake by perennial ryegrass (Lolium perenne) (Clement et al.,

1978a), phosphate was held constant in the range 0.04-32 p M in studying

phosphate uptake by the same species (Breeze e t a / . , 1984), and K' was held

constant at 1.3-102 pA4 in a study of K' uptake by perennial ryegrass,

barley, and fodder radish (Raphanus sativus cv. Slobolt) (Woodhouse et

al., 1978). The concentration of K' that was able to support the potential

growth rate of four other plant species in flowing solution culture was about

100 times less than required in sand culture (Wild et a/., 1974); the concentration ratio for all nutrients is probably about the same.

High ratios of antagonistic ions are avoided; for example, NH: is not included in solutions used to study K' uptake, as in the recent experiments of

Pettersson (1986), unless the extent of antagonism is itself under investigation. The co-ions used in the Hurley system are Ca2+in studies of the concentration of anions, e.g., NO;, and S042- for cations, e.g., K',all solutions

containing a base concentration of CaS04. The pH is held constant, usually

in the range of 4.0-7.0 f 0.1, by the automatic addition of H2S0, or

Ca(OH)2 as appropriate. The circulating solution is not sterile but counts

have shown bacterial numbers in solution to be very low.

The Hurley system has been used to investigate the conditions that determine nutrient uptake rates and the effects of nutrient solution composition

on the growth and composition of plants (see Section 111). Using the system

high growth rates have been achieved. For example, under optimum conditions of light, temperature, and nutrient supply, relative growth rates

(RGR) of 0.31, 0.40, and 0.20/day were achieved with seedlings up to 6

days old of fodder radish, barley, and perennial ryegrass, respectively, with

rates of 0.26,O. 17, and 0.19/day, respectively, when the plants were 20 days

old (Woodhouse et al., 1978). With older ryegrass plants, dry matter increases of 20 g/m2/day have been obtained (Clement et al., 1978a), which is

close to the high rate of 20-25 g/m2/day observed in the field for this

species (Alberda and Sibma, 1968).



Table 1

Typical Composition of Flowing Nutrient Solutions as Used with the Hurley System'

Nutrient



K

Ca

Mg



Concentration (pM)



Micronutrients



Concentration (pM)



13-26

230

100



Fe

B

Mn

cu

Zn

Mo



1.07

4.63

0.91

0.03

0.08

0.05



c1



1.69



"X)



35



S(s0: -)

P(H*Po;)



320

50



'Trom Clement et ul. (1978a) and Breeze et ul. (1984). The compositionwill be changed from that in the table according to the purpose of the investigation;the concentrations shown are nonlimiting for growth. See Asher and Edwards

(1983) for concentrations used in other systems.



UPTAKE OF MINERAL NUTRIENTS AND CROP GROWTH



Ill.

A.



177



NUTRIENT UPTAKE



INTERRELATIONSHIPS WITH PLANT GROWTH



It is now generally accepted that nutrient uptake and plant growth are interdependent (Moorby and Besford, 1983). The relation between these processes can be particularly well studied in flowing culture systems where continuous monitoring of nutrient uptake can be performed in combination

with sequential harvesting of plants.

A clear demonstration of the rapidity with which cessation of growth affects nutrient uptake is illustrated in Fig. 2 for a sward of perennial ryegrass

growing in flowing nutrient solution (Clement et al., 1978b). Within a few

minutes of the grass being cut, the uptake of NO; began to decrease and

after 2 hr it had fallen to 35% of the rate at the time of cutting. After cutting it was also observed that the CO, flux quickly fell to negative values as

respiration exceeded assimilation. The fall in NO; uptake was more gradual

than the fall in CO, flux, and the recovery of NO; uptake was also slower

than the recovery of COz assimilation. In the same work a close relationship

was shown to exist between NO; uptake and CO, flux. The assimilation of

CO, was greatest at about 1200 GMT with the maximum rate of NO; uptake

commonly occurring 5 or 6 hr later (Fig. 3). This demonstration of the close

relation betwen CO, assimilation, level of radiation, and NO; uptake suggests that CO, assimilation, and hence dry matter increase, may stimulate

the increase of NO; uptake. It is unclear whether the stimulation is a function of the growth increment, the plant reacting by taking up more



Time (GMT)



FIG. 2. Effect of defoliation on rate of NO; uptake by simulated sward of perennial

ryegrass growing under artificial light with a 9-hr photoperiod. Plants were grown in flowing

nutrient solution with NO; at 7 p h f and cut at 3 hr after the start of the photoperiod at 5 cm

above the base of the shoots. Data from Clement el a/. (1978b).



178



A. WILD E T A L .

.

U



9.E 20 t



.



- "



I



OL

6r



5



.'..'..



..



0..



0.



..-



FIG. 3. Rates of NO; uptake and of CO, flux by simulated swards of perennial ryegrass

during periods of 72 hr in June (experiment 1 ) and in July (experiment 2). Plants were grown in

flowing nutrient solutions with NO; at 7 p M , and measurements were made on plants growing

in nine adjoining vessels of a plant culture unit. A , 1200 GMT. Data from Clement ef a/.

(1 978b).



NO;, or is more a consequence of an increased supply of energy sources in

the root which allows increased active uptake.

The diurnal fluctuation of NH: and K' uptake and H efflux has also been

investigated with perennial ryegrass (Hatch et d.,1986). The lag periods for

the peak uptake rates of NH: and K' were about the same as for NO;. The

best fit with measured radiation was for a lag period of 5 hr with NH:, 6 hr

with K', and 7 hr for H efflux (Fig. 4). The similarity in the lag periods for

the uptake of NO;, NH:, and K', and for H' efflux suggests a common

mechanism controlling the uptake of the three nutrient ions. The control

might be through the supply of a photosynthetic metabolite that is required

for the active transport of the ions into roots or into the xylem.



179



UPTAKE OF MINERAL NUTRIENTS AND CROP GROWTH



b



4



I



I



8



12



t



I



I



J



I



1



20



24



4



4



8



12



I



16



16



20



24



Time (GMT)



701



c



24



I1



24



I2



14



I2



24



Time ( G M T )



FIG.4. Diurnal variation of uptake of K' and NH: and efflux of H' by simulated swards

of perennial ryegrass growins in flowing nutrient solution under natural light: (a and b) uptake

of K' and efflux of H' on 27 June; (c and d) uptake of NH: and solar radiation on 27,28,29

June. Values under peaks are as follows: uptake of N and K as mg/m' for a 24-hr period; H'

efflux as mg/m'/day; radiation as MJ/m'/day. Data from Hatch el al. (1986).



180



A. WILD ET AL.



Just as long-term changes in nutrient uptake respond to plant growth, so

does plant growth respond to nutrient uptake, or the nutrient status of the

plant. The rapidity of growth response to interruption of nutrient supply

depends, however, on the period of interruption and on the nutrient concentration within the plant. Clement et al. (1979) found that perennial

ryegrass which had been grown for 6 weeks from sowing with NO; at 7 pM

showed no reduction in growth rate of their shoots during the following

9-day period when no nitrogen was supplied. During this period the shoots

of plants grown with a terminated supply increased in dry weight by 247 f

28 g/m2 and those grown with a continued supply increased by 250 f 28

g/m2. During the 9-day period the dry weight of roots grown with a terminated supply increased by 80k 6.2 g/m2 whereas those with the continued supply increased by only 39 k 6.2 g/m2. A partitioning response of

this kind is consistent with early work of Brouwer (1962a,b) and can be

predicted from models that define the priorities for shoot and root growth

in terms of carbon and nitrogen substrate levels (e.g., Johnson, 1985).

The effects of intermittent nutrient supply on growth have been reported

in a second experiment by Clement et al. (1979). Perennial ryegrass was

grown from sowing for 28 days with NO; at 7 pM. For one set of plants the

supply was continued for a further 42 days and for another set it was switched off and on at 3-day intervals during this period. In this experiment an intermittent supply of NO; induced greater transport of photosynthate to the

roots (Fig. 5 ) , as in the example above with a terminated supply. Although

the total uptake of N was less by plants receiving an intermittent supply, the

rate of uptake was about 40% greater during the 3-day periods of supply

than by plants on a continuous supply.

The explanation suggested for these effects of N supply on plant growth is

that for the 6-week-old plants in the first experiment the amount of NO; in

the plants provided a sufficient source of N for protein synthesis to continue

for at least 9 days without a reduction of growth. In the younger plants in the

second experiment, the amount of NO; within the plants was insufficient to

maintain growth for 3 days. In both experiments (1) NO; concentrations in

roots and shoots decreased when the supply was terminated or intermittent,

but did not fall to zero, and (2) NO; reduction was quickly and strongly suppressed when the supply was withdrawn. These observations support the view

of Ferrari et al. (1973) that much of the NO; stored in plants, probably in the

vacuole, is not readily available for reduction and that the relatively small

amount present in the “metabolic pool” requires continuous replenishment

via uptake by the roots if maximum rates of reduction are to be maintained.



B. QUANTITATIVE

DESCRIPTION

IN RELATION

TO REQUIREMENTS

OF PLANTS

Quantitative approaches to the relationships between nutrient uptake,

growth rate, and properties of root systems have been described by



UPTAKE OF MINERAL NUTRIENTS AND CROP GROWTH



Aj



5-



OL



181



30



40



50



Days after



60



70



sowing



FIG.5. Cumulative amounts of NO; supplied by nutrient pumps to replace that taken up

by simulated swards of perennial ryegrass from flowing nutrient solution with a continuous

(0-0)

or an intermittent ( 0 - 0 ) supply of NO;. Dry weights of shoots and roots at the end of

the experimental period are shown in the insert histograms. Data from Clement ef at. (1979).



Williams (1948), Loneragan (1968), and Brewster and Tinker (1972). These

relationships are well discussed in the book by Nye and Tinker (1977), to

which the reader is referred for further information.

Nutrient uptake rate expressed as a flux (F) (uptake per unit root surface

area) is related to growth rate and nutrient concentration in the plant by the

following equation (Nye and Tinker, 1969):

dx



1



where X = nutrient concentration in plants,

W = plant weight,

t = time,

r = mean root radius, and

L = total root length

If X is constant, the minimum flux, F, , required to maintain potential

growth rates is



182



A. WILD E T A L .



where X,, is the critical, or threshold, concentration required in plant

tissues (Woodhouse et a[., 1978). Equation (3) identifies three parameters,

Xcrit,

relative growth rate, (dW/dt)(l/W) and the ratio of plant weight to

root surface area, W/2nrL, each of which contributes to the required flux,

F,. It should be noted that F and F, are net fluxes and are averages for the

whole root system, as are all values for uptake reported here. Separation into efflux and influx has usually not been made in this form of analysis.

Forms of dimensional analysis equivalent to that in Eq. (3) can be derived

for minimum inflow (uptake per unit root length) and unit absoption rate

(uptake per unit fresh weight). Furthermore, the terms in Eq. (2) and (3)

may be stated on a dry weight or fresh weight basis. The latter is to be

preferred because nutrient concentrations may change much less with time

when expressed on a fresh weight basis (Leigh and Wyn Jones, 1984; Leigh

and Johnston, 1985), which gives better justification for the assumption of

in Eq. (3).

constant Xcrit

Measurements of the RGR (Fig. 6) and of the ratio root surface area :

plant weight (Fig. 7) have been reported by Woodhouse et a[. (1978) for

fodder radish, barley, and perennial ryegrass during their first 3-4 weeks of

growth from germination. The response of dry matter to the external concentration of K' in the range 1.3-102 pA4 was radish > barley > ryegrass.

For most of the experimental period radish had the highest RGR but the

lowest ratio of root surface area to plant weight and therefore required the

highest K' flux. The order of response of the three species to the range of K'

concentration in the external solution was therefore attributed to differences in potential RGR and ratio of root surface area to plant weight.

The changes in these two parameters with the age of the plant (Figs. 6 and

7), and in the internal threshold concentration, may explain why plants are

generally most sensitive to the external nutrient concentration during the

first 2-3 weeks after germination. Similar observations on the sensitivity of

young plants to external phosphate concentration have been made by

Breeze et a[. (1984). As the explanation lies in the ontogeny of plants the

observations may apply to all nutrients.

The ratio F/F, is a measure of the effectiveness of a species in meeting its

nutrient requirements (Wild and Breeze, 1982). A ratio of less that 1 indicates that F is less than is required to achieve the potential growth rate, as

found with fodder radish and barley during early growth at 1.3 and 6.4 p.M

K'. For these two species the relationship between F/F, and F,,, was the

same (Fig. 8), which implies that for any required flux, F,, under the experimental conditions, the two species were equally effective in taking up

K'. It is therefore suggested that the parameters identified in Eq. (3) can



183



UPTAKE OF MINERAL NUTRIENTS AND CROP GROWTH



0.40r



0

>



3

a



0.30-



T



-e

0,



f 0.20-



s



E01

l

aJ

.-



5

- 0.100



a



O



0



5



20

25 30

Days after germination



10



15



35



J

LO



FIG.6. Relative growth rate, calculated on fresh weight basis, in relation to age of plants

of barley (+-+), fodder radish (0-0). and perennial ryegrass (m-m) grown in flowing

nutrient solutions with K' at 102 pkf, at which the potential growth rate was achieved; the supply of all other nutrients was adequate. Data from Woodhouse et al. (1979).



10-



f01

.01

3



8-



c



9Q;



- 0



6-



F;



a



g 4-



$ €



2 2



2



3m



2-



o



L



I



I



I



FIG. 7. Change in the ratio root surface area : fresh weight of whole plants in relation to

age of three species grown in flowing nutrient solutions with K' at 102 pA4 and all other

nutrients in adequate supply. Data from Woodhouse el a/. (1978).



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