agriculture group symposium advances in plant nutrition

J Sci Food Agric 1988, 46, 13-27

Agriculture Group Symposium Advances in Plant Nutrition

The following are summaries of papers presented at a meeting of the Agriculture Group of the Society of Chemical Industry held on 15 March 1988 at the Society of Chemical Industry, 14-15 Belgrave Square, London SWlX 8PS . The papers published here have not been refereed and do not necessarily reject the views of the Editorial Board of the Journal of the Science of Food and Agriculture.

Recent Advances in Our Understanding of the Transport of Nutrients across Vacuolar Membranes of Plant Cells

Roger A Leigh, Andrew J Pope and Lawrence J Clark

AFRC Institute of Arable Crops Research, Rothamsted Experimental Station, Harpenden, Hertfordshire AL5 UQ, UK

Although it has long been known that nutrient ions can be reversibly stored in the vacuole of plant cells, detailed studies of ion transport across the vacuolar membrane (tonoplast) were only possible, for many years, in giant algae. In higher plants, knowledge of events at this membrane was inferred from the characteristics of transport measured in whole tissues. Inevitably, such observations were difficult to interpret. In the last few years, however, our understanding of transport at the tonoplast has increased significantly through studies using isolated, transport- competent vesicles (Sze 1985). Work with such vesicles has shown that the tonoplast possesses two inwardly directed electrogenic H + pumps, one utilising ATP, the other inorganic pyrophosphate (PPi). The tonoplast appears to be unique in possessing two such pumps. These pumps transport H + from the cytoplasm into the vacuole, creating an electrical potential difference (A$, inside positive) and a pH gradient (ApH, inside acid). These gradients provide the driving forces for the transport of nutrient ions into the vacuole. This paper describes some recent studies that have characterised these gradient-driven transport systems in tonoplast vesicles.

The inside-positive A$ provides the driving force for the transport of anions into

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J Sci Food Agric (46) (1988)--0 1988 Society of Chemical Industry. Printed in Great Britain

14 Agriculture Group Symposium

the vacuole (Pope and Leigh 1987). This has been studied in uitro using fluorescent probes that are sensitive to A+. These studies have shown that the transport system is selective. In oat roots, it can transport C1-, NO; and Br- but not malate or SO:-. However, this selectivity varies with species, and in CAM plants, which diurnally accumulate large amounts of malate, malate is the preferred ion. In oat roots, the K , for C1- and NO, is about 2 mM. Attempts to characterise a NO; export system driven by the ApH (Blumwald and Poole 1985a) have shown that the evidence for this system was an artefact (Pope and Leigh 1988).

Transport of cations is linked to the ApH, the energy from the movement of H +

out of the vacuole being used to drive the uptake of cations. A Na+ transport system of this type has been demonstrated by artificially imposing a ApH in vesicles and observing its dissipation by Na (Blumwald and Poole 1985b). The K, and V,,, of this system can be manipulated by pretreating the tissue in various ways before isolating the vesicles. For instance, salt stress increases the V,,,, implying that more transporters become operative or are synthesised under this condition. A similar H'coupled transport system has also been described for Ca2 +, and this may have an important role in maintaining very low concentrations of this cation in the cytoplasm (Schumaker and Sze 1985).

The ability to study transport in isolated tonoplast vesicles has greatly increased our understanding of the mechanisms used to transport nutrient ions into the vacuole. However, the nature of export mechanisms has not been established. In addition, we do not understand how any of the transport systems are controlled so that nutrients are accumulated in the vacuole under some conditions but released for metabolism under others. There is also a need to establish the nature of transport systems at the plasma membrane, and the in-vitro techniques that have been so successful with tonoplast should be equally applicable to this membrane. Unfortunately plasma membrane does not form sealed vesicles as spontaneously as tonoplast so there has been little progress.

References

Blumwald E, Poole R J 1985a Nitrate storage and retrieval in Beta vulgaris L. Effects of nitrate and chloride on proton gradients in tonoplast vesicles. Proc Natl Acad Sci U S A 82

Blumwald E, Poole R J 1985b Na+/H+ antiport in isolated tonoplast vesicles from storage tissue of Beta vulgaris. Plant Physiol 78 163-167.

Pope A J , Leigh R A 1987 Some characteristics of anion transport at the tonoplast of oat roots, determined from the effects of anions on pyrophosphatedependent proton transport. Planta 172 91-100.

Pope A J, Leigh R A 1988 Dissipation of pH gradients in tonoplast vesicles and liposomes by mixtures of acridine orange and anions. Implications for the use of acridine orange as a pH probe. Plant Physiol86 1315-1322.

Schumaker K S , Sze H 1985 A Ca2+/H+ antiport system driven by the proton electrochemical gradient ofa tonoplast H+-ATPase from oat roots. PIant Physiol79 11 11- 1117.

Sze H 1985 H+-translocating ATPases: advances using membrane vesicles. Ann Rev Plant Physiol 36 175-208.

3683-3687.

A d i m c e s in plant nutrition 15

Computer+ontrolled Nutrition of Intact Tomato Plants: the Effects of Depriving Plants of Phosphate on the Uptake of Phosphate and Other Nutrient Ions

Robert A England and Francis E Sanders

Department of Pure and Applied Biology, The University, Leeds LS2 9JT, UK

The coordination of nutrient uptake in whole plants, and the effects on nutrient uptake of ontogenetic changes and environmental factors, are of considerable importance. The study of such aspects demands an ability to measure accurately rates of nutrient uptake by whole plants over long periods of time.

Measuring rates of uptake from soil can provide much valuable information, but the method is extremely laborious, and short-term changes are difficult to discern because of plant variability. Liquid media are more tractable experimentally. Rates of uptake can be estimated from changes in concentration of individual nutrient ions. Uptakes of individual ions may vary independently of each other, and concentrations of ions in solution need therefore to be independently controlled.

In our system plants are permitted free access to nutrients except where stresses are deliberately imposed. Plants are grown in a system of troughs irrigated by nutrient solution from well-stirred reservoirs. At set times, which can be as frequent as 0-75 h, concentrations of potassium, calcium and sodium are measured by flame photometer, nitrate and phosphate by spectrophotometer and pH by electrode. All the apparatus necessary is directly interfaced with and controlled by a microcomputer. A flexible control algorithm then allows the computer to calculate the additions of solutions of salts containing nitrate, phosphate and potassium to restore concentrations of these ions to set levels. Calcium, magnesium, iron and sulphate are also added but their concentrations are not directly controlled. Additions are made using a system of pumps and valves. pH is adjusted by a titration procedure. To compensate for losses by evaporation and transpiration, water is added at the beginning of each suite of analyses to restore the volume of solution in the system to a known value.

Under free access conditions, the set concentrations of ions employed are dictated by the necessity to ensure that the falls in concentration caused by uptake are large enough to be accurately estimated but not so great as to influence rates of uptake. Depletions during the period of uptake should probably not exceed 10-15 %. When supply of an individual ion needs to be restricted, its concentration is set at a lower value so that much greater depletions can occur.

The results of experiments are presented in which short-term phosphate stress was imposed on tomato plants. Ca + M B ( N O ~ ) ~ , KH2P04 and KNO, are supplied to two sets of 40 plants in step with depletion of N, P and K from solution. H,S04 was added to restore pH to 6. When both sets of plants were taking up N, P and K and growing at similar rates (27-0+ 0.3 % day- '), the supply of P to one set of plants was stopped. Nitrate replaced phosphate addition in this treatment so that the supply of cations was not changed by the cessation of phosphate supply. Phosphate was resupplied to the stressed plants after 5 days. Plants were serially harvested


before, during and after the stress period, and uptake of N, P and K by each set of plants was recorded every 3 h throughout the 14day experiment.

Short-term P stress had a marked effect on the internal concentration of P in both shoots and roots. Concentrations of P in shoots and roots of stressed plants fell to 42% and 55 % of those in control plants by the end of the stress period. On resupply of P, concentrations of P in shoots and roots of stressed plants increased to 146% and 128 % of those in control plants as a consequence of greatly increased P uptake rates after the stress period as well as reduced growth rates of all plant parts both during and after the stress period.

During the stress period, the concentration of Mg in the roots of stressed plants rose significantly, being over twice as high as the concentration in the control roots by 6 days after the application of stress. Thereafter the concentration in the stressed roots returned to a value close to that of the control.

Three-and-a-half days after the application of P stress, the pH of the nutrient solution began to fall sharply. Within 15 h the pH fell from 6 to 4.7. The pH of the solution was then adjusted manually to pH 6 using Ca(OH),. A further manual adjustment was made 15 h later. The pH of the treatment solution increased from 4-7 to 6 within 18 h when P was resupplied and then resumed its normal diurnal variation.

The plant analyses showed that more anions than cations were taken up throughout the period of the experiment. The reduced uptake of H,PO; during stress and the increased uptake of Mgz+ by the roots of stressed plants could not have prevented alkalinisation of the external solution which would arise from excess uptake of anions. The acidification actually observed is therefore difficult to explain unless there was an active extrusion of protons and undissociated organic acids. Increased uptake of Mg2+ by stressed roots may have been to balance an increased internal level of organic acid anions. Further work is needed to settle this point.

pH Changes around the Roots of Rape (Brassica napus var Emerald) Seedlings in Relation to Phosphate and Nitrate Nutrition

Hilary Moorby

Wye College, University of London, Wye, Ashford, Kent TN25 5AH. UK

Hedley et a1 (1982) found that P solubilisation in the rhizosphere of rape plants was due to an imbalance in cation-anion uptake. To investigate this further, rape plants were deprived of either nitrate or phosphate.

The plants were studied using a nutrient film technique that allowed part or the whole of the root system to be subjected to specific nutrient treatments. The rapidity and direction of change of pH was assessed by embedding absorbing roots in a thin layer of agar containing bromocresol purple. In the case of phosphate deficiency, measurements were also made with a pH microelectrode placed next to the roots.

When nitrate-fed plants were deprived of nitrate, the release of H ions was immediate and uniformly distributed over the length of root deprived of nitrate.

Advances in plant nutrition 17

Thus, in split root experiments when only part of the root system was deprived of nitrate, the area of root supplied with nitrate continued to produce HCO, ions whereas an attached area of root deprived of nitrate would be producing H ions. This led to the conclusion that the production of H ions was in response to excess absorption of cations in the segment of root deprived of nitrate, and was not due to a change in nitrogen metabolism.

When phosphate-fed plants were deprived of phosphate, no change in the patterns of pH changes around the deprived roots were seen until P had been withheld for 3 days, when H ion efflux started in the terminal portions of the roots. Pdeprived roots produced HCO, ions from the areas of root away from the tips, but the net amount of HCO, ion produced by the Pdeficient roots fell as did nitrate uptake rates. Cation-anion balances measured at the end of the experiment showed that uptake of all anions and K decreased in the Pdeprived plants but uptake of Ca and Mg were little altered. This resulted in a smaller ratio of anions to cations absorbed, which was reflected in a reduced HCO, ion efflux.

The time taken for the result of phosphate deprivation to produce a response suggested that in this case a change in nitrate metabolism had occurred. Levels of nitrate uptake and nitrate reductase activity (NRA) were measured in shoots and roots of P-sufficient and Pdeficient rape plants, and any changes were examined in relation to the onset of H ion efflux from the roots.

The rate of nitrate uptake began to decline 3 days after P was withdrawn from the plants. At the same time a net efflux of H ions began from the terminal portions of the roots. No effect on NRA was detected until 5 days after the withdrawal of P, pointing to a direct effect of P deficiency in the roots on nitrate uptake and H ion

Reference

Hedley M J, Nye P H, White R W 1982 Plant induced changes in the rhizosphere of rape (Brassica napus var Emerald) seedlings 11. The origin of the pH change. New Phytol91 31- 44.

efflux.

The Uptake, Assimilation and Distribution of Nutrients by Intact Castor Oil Plants in Relation to the Source of Nitrogen Nutrition

Marinus L van Beusichem

Department of Soil Science and Plant Nutrition, Wageningen Agricultural University, PO Box 8005, NL 6700 EC Wageningen, The Netherlands

and

Ernest A Kirkby

Department of Pure and Applied Biology, University of Leeds, Leeds LS2 9JT, UK

It is well recognised that large quantities of H + and OH- can be produced and retained in plant cells and organelles. Intracellular pH perturbation is then


regulated largely by carboxylation or decarboxylation by so-called pH stat mechanisms, the most important of which involves malate (Davies 1986). Accumulation or degradation of organic acid anions may arise from both nutrient uptake and assimilation. The forms of N uptake and N assimilation are of special importance in the formation of intracellular H + or OH- . The assimilation of NH: leads to H + production, while the reduction of NO; (and SO:-) results in the production of equivalent amounts of OH- . In the removal of H + or OH - from the root cells to the external medium associated with these two forms of N uptake and assimilation, electroneutrality is maintained by a difference in the rates of uptake of cations and anions. In NH:-fed plants, absorbed NHf is almost exclusively assimilated in the roots. The source of H + from NH: assimilation must therefore be root cells. In considering the uptake and long-distance transport ofions in plants fed with NO; as the form of N nutrition, the site of NO; reduction is of paramount importance. In species which reduce NO; predominantly in the shoot and also take up anions much in excess of cations, a mechanism has been proposed which incorporates the recirculation of K + within the plant and provides the plant with a pH-regulating system through a spatial separation of the two halves of the biochemical pH stat (Ben Zioni et a1 1971). Detailed analytical data to provide unequivocal evidence for the occurrence of this recycling mechanism are still very scarce. In particular, the organic constituents of xylem and phloem saps from different species have not been investigated thoroughly.

Following earlier work with tomato (Kirkby and Knight 1977) and pea (Van Beusichem 1983), we used thecastor oil plant (Ricinus communis L) (Van Beusichem et a1 1985, 1988) as it enabled the separation of both xylem and phloem saps from the same plant at the same growth stage. Plants were grown under controlled climatic conditions in nutrient solutions in which N was supplied as NO; or NH:, the solutions being maintained at pH 5.5 by continuously recording pH stat equipment. After 40 days of growth, phloem and xylem saps were collected from the plants and analysed for all major ionic and nitrogenous constituents. Samples of leaves, stems + petioles, and roots were taken for determinations of in-vivo nitrate reductase activity. Measurements were made of N and S content, cation-anion balance, and weight of the shoots and roots.

The extreme difference in behaviour between NO; and NH: assimilation in intracellular OH - and H + production resulted in much higher concentrations of organic acid anions in both shoots and roots of NO;-fed plants than in NH:-fed plants. In NO,-fed plants excess nutrient anion over cation uptake (988 meq kg-' DW) was equivalent to net O H - efflux as measured by H + addition (1026 meq kg-'), and the total charge from NO; and SO:- reduction (2230meq kg-') equated to the sum of organic acid anion accumulation (1242 meq kg- ') plus net OH- efflux. In NH: -fed plants a large H + eflux was recorded (3342 meq kg-'), in close agreement with excess cation uptake (3696 meq kg- '). This net H + eflux equated to the sum of net cation (NH: minus SO:-) assimilation (3101 meq kg-') plus organic acid anion accumulation (595 meq kg- '). In-vivo nitrate reductase activity assays and other evidence revealed that the shoots may have the capacity to reduce about 55 % of the NO; that is taken up and reduced in NO;-fed plants. Since organic and total N and S followed a similar distribution pattern over shoots


and roots, organic S being of quantitative minor significance relative to organic N, a calculation could be made of the contribution of the charge derived from NO; and SO:- reduction in the shoot to OH- emux from the roots. There was only a small difference between the amount of NO; and SO:- reduced in the shoot (1227 meq kg-') and the organic acid anions accumulated in that tissue (1043 meq kg-'), allowing the conclusion that approximately 80% of the OH- elllux into the nutrient medium must have been derived from NO; and SO:- reduction in the roots. The low concentrations of organic acid anions (35-4 mM) in the phloem sap relative to cations (101.2 mM) supported this conclusion. In particular, the very low concentration of malate (8-8 mM) relative to K+ (86.6 mM) leaves no doubt that a spatial separation of the malate-linked pH stat cannot play an important quantitative role in the long-distance transport of K+. In phloem sap from NH: -fed plants, K + concentrations were not greatly different from those in the NO;-fed plants, and organic acid anion concentration was as much as 19.4 mM. Thus some recirculation appears to be occurring in NHZ-fed plants, ie in plants where phloem transport of cation-organate is not a necessary factor in the regulation of intracellular shoot or root pH. The source of H+ elllux in NHZ-fed plants was mainly from NH4, assimilation in the roots, and an additional H efflux arose from the excess uptake of cations (other than NH:) over inorganic anions, resulting in an eMux of 1.1 H + for every NHZ taken up. This may explain the higher concentration of organic acid anions in the xylem sap from the NH:-fed plants (6.5 mM) as compared with the NO;-fed plants (3.2 mM), since this additional H + efflux may arise from the dissociation of organic acids in root cells. References

Ben Zioni A, Vaadia Y, Lips S H 1971 Nitrate uptake by roots as regulated by nitrate

Davies D D 1986 The fine control of cytosolic pH. Physiol Plant 67 702-706. Kirkby E A, Knight A H 1977 Influence of the level of nitrate nutrition on ion uptake and

assimilation, organic acid accumulation, and cation-anion balance in whole tomato plants. Plant Physiol 60 349-353.

Van Beusichem M L 1983 Xylary charge distribution and nitrogen transport in Pisum satiuum L during dinitrogen fixation or nitrate nutrition. Z Pflanzenphysiol 109 449458.

Van Beusichem M L, Baas R, Kirkby E A, Nelemans J A 1985 Intracellular pH regulation during NO; assimilation in shoot and roots of Ricinus communis. Plant Physiol78 768- 773.

Van Beusichem M L, Kirkby E A, Baas R 1988 Influence of nitrate and ammonium nutrition on the uptake, assimilation, and distribution of nutrients in Ricinus communis. Plant Physiol86 914-921.

reduction products of the shoot. .Physiol Plant 24 288-290.

Causes of Local Calcium Deficiency in Tomato Plants

Lim C Ho

ARFC Institute of Horticultural Research, Littlehampton, West Sussex BN17 6LP, UK

Physiological disorders in tomato plants such as blossom-end rot (BER) in fruit or tip yellowing in expanding apical leaves are caused by local calcium deficiency in


these organs. High incidences of these symptoms occur when either the salinity of the nutrient solution or the relative humidity of the glasshouse is high. Lack of calcium supply is not commonly the cause of local calcium deficiency in tomato as these symptoms often appear while the supply of calcium is abundant.

Tomato plants were grown in recirculating nutrient solution (NFT) with a range of salinities (ie electrical conductivity, 2 to 17 mS cm- '. The accumulation of calcium and dry matter in individual fruit was measured throughout fruit development. Effects of salinity on the calcium concentration in the root exudate, xylem water flow inside the fruit and the xylem vessel cross-sectional area in the fruit pedicel were investigated. The accumulation of calcium in tomato was investigated by measuring the 45Ca accumulation in fruit in relation to 45Ca uptake by roots, and 45Ca transport in the shoot under contrasting salinities (2 or 17 mS cm- ') and humidities (60 or 90% rh) (Ehret and Ho 1986). The diurnal accumulation of calcium by fruit was investigated by supplying 45Ca in either the light or dark periods (Ho 1987).

The accumulation of calcium by fruit was reduced by high salinity, and the calcium content as % of fruit dry matter was greatly reduced, particularly during rapid fruit growth. As the minimal calcium content of fruit in the first two trusses is lower than that in the subsequent trusses (El-Gizawy and Adams 1986), fruit of the early trusses is more susceptible to BER. The accumulation of 4sCa by fruit was disproportionately low in comparison to that in other organs. High salinity reduced water as well as 45Ca uptake, and the transport of 45Ca to the shoot and particularly to the fruit. However, high humidity did not greatly affect the accumulation of 45Ca by fruit but reduced markedly the accumulation of 45Ca by expanding leaves (Ehret and Ho 1986). Thus, high salinity suppressing root pressure, and humidity suppressing transpiration, are the main causes for BER and leaf-tip yellowing, respectively (Bradfield and Guttridge 1984; Adams 1987). Although the uptake of 45Ca by tomato plants was higher in the light than in the dark, a greater amount of 45Ca was accumulated by fruit in the dark. High humidity reduced the accumulation of 45Ca by apical expanding leaves, mainly during the dark period (Ho 1987).

The accumulation of calcium by tomato fruit was also affected by the anatomical features of the fruit grown under different salinities. Fruit developed under high salinity had a smaller xylem cross-sectional area and a higher xylem water transport resistance inside the berry. Thus, more of the 4sCa taken up by such detached fruit moved to the calyx than to the berry, and only a small portion of the 45Ca moved into the distal half of the berry. However, the incidence of BER, increased by high salinity, was even higher when the calyx was removed during early fruit development (Ehret and Ho 1986).

There are short- and long-term effects of salinity and humidity on the accumulation of calcium in rapidly growing fruit and leaves. The critical period for calcium deficiency in these organs occurs when the growth rate exceeds the rate of calcium import.

References

Adams P 1987 Some effects of environment on calcium status of tomato leaves. CEC


Workshop: Effects of high humidity on plant growth in energy saving greenhouses, Littlehampton, March.

Bradfield E G, Guttridge C G 1984 Effects of night-time humidity and nutrient solution concentration on the calcium of tomato fruit. Sci Hort 22 207-217.

Ehret D L, Ho L C 1986 Translocation ofcalcium in relation to tomato fruit growth. Ann Bot 58 679-688.

El-Gizawy A M, Adams P 1986 Accumulation of calcium by tomatoes in relation to fruit age. Acta Hort 178 3 7 4 3 .

Ho L C 1987 Effects of high humidity and high salinity on calcium distribution in tomato plants. CEC Workshop: Effects of high humidity on plant growth in energy saving greenhouses, Littlehampton, March.

The Use of Starter Fertiliser for Vegetable Crops

Peter A Costigan*

Institute of Horticultural Research, Wellesbourne, Warwickshire CV35 9EF, UK

The study of site-to-site variations in yield has shown that during the early parts of their lives crops frequently do not contain the minimum levels of nutrients required for optimum growth. It is apparent that, even on fertile soils which have received plenty of fresh fertiliser, nutrient deficiency can still occur if the soil/root combination cannot supply sufficient nutrients to satisfy the growth demands of the shoot. This is especially likely to happen early in the season when the soil is cold but the air temperatures are relatively high. For drilled crops a simple growth model (Costigan 1987) can demonstrate that some vegetable crops are unable to take advantage of freshly applied broadcast fertiliser until several weeks after germination because of their slow initial rate of root growth. For transplanted crops, there is a relatively large shoot demand for nutrients which has to be supplied by a spatially restricted and sometimes damaged root system. Experiments with drilled and transplanted vegetables have shown that temporary nutrient deficiencies, principally of phosphorus and nitrogen, do occur and can be ameliorated by applying small amounts of nutrients near to the seeds of drilled crops or to the roots of transplanted crops. The response to starter fertilisers varies from soil to soil and to some extent can be predicted from the soil analysis.

It is particularly interesting that starter fertiliser responses measured during crop establishment do not always translate into differences in final yield. To some extent this is inevitable because of the transition from exponential growth rates in the early phase of crop growth to linear growth rates at later stages, but recent experiments have indicated that stresses occurring after the initial establishment phase may act not only in reducing average yield but also in reducing the benefits already achieved by an early growth response. Plant growth is continuous and cumulative during the life of a crop and is susceptible to limitations of nutrient supply at many stages. It is therefore often misleading to consider that plant yield is a simple function of the rate

* Present address: ICI Fertilisers, Jealott’s Hill Research Station, Bracknell, Berkshire RG12 6EY, UK.


of application of fertiliser. An improved understanding of responses to nutrients at different stages of crop growth should enable growers to manage their crops more efficiently .

Reference

Costigan P A 1987 A model to describe the pattern of availability of broadcast fertilizer during the growth of a crop. Plant and Soil 101 281-285.

Crop Nutrition-Perspectives and Opportunities

D J Greenwood

Institute of Horticultural Research, Wellesbourne, Warwickshire CV35 9EF. UK

This review is concerned solely with applied aspects of plant nutrition. Recent developments are discussed that have greatly increased prosperity and improved the environment in some countries. An attempt is also made to identify plant nutritional problems that are of much public concern throughout the world.

Attention is drawn to (a) innovative nutritional and cultural practices (Geraldson 1987) that have transformed areas in Florida with once poor barren soils supporting only scrub vegetation, so that they now supply a large proportion of all the vegetables and fruit produced in the USA; (b) the creation of international markets by introducing new horticultural products and the associated nutritional problems that have been overcome (with special reference to work in The Netherlands); (c) the tremendous advances made in restoring and improving the appearance of land that had been degraded by past industrial activities (Jackson and Bradshaw 1979).

Nutritional problems associated with trees are becoming increasingly important. One aspect is the extent to which afforestation affects soil pH and thus the extent of unwanted metal pollution of the drainage water. Most significant, however, is the damage to large areas of forest in Europecaused by aerial pollution (Huettll986). It is notable (i) that the damage is reflected in leaf deficiencies of nutrients, the type of which is largely determined by the nutrient in shortest supply within the rooting zone of soil; and (ii) that the deficiencies can, on occasion, be alleviated by application of the appropriate fertiliser (Huettl 1986).

Crop yields in much of the world are limited by soil and nutritional factors. Probably the most pressing need of all is to devise low-input systems that will enable the very large number of subsistence farmers on poor soils to obtain sustained increases in yield of the crops they need to eat and to sell and also of the wood they need for fuel. The central problem is that man has not learnt how to recycle nutrients effectively when growing the crops he requires. Thus high yields of most crops are seldom obtained without heavy annual applications of fertiliser which are too expensive for the subsistence farmers who need them most. Also, removal of crops depletes the soil of cations and increases acidity which is difficult to rectify in many soils because they are remote from any source of lime. Considerable opportunities exist, however, for devising inexpensive cultural practices which

Advuncrs in plant nutrition 23

improve nutrient recycling and also minimise soil erosion, thereby permitting much needed improvements in productivity.

References

Geraldson C M 1987 Control of the ionic composition of the rhizosphere in the transition to

Jackson M S, Bradshaw A D 1979 Ecological principles for the restoration of disturbed and

Huettl R F 1986 Forest fertilisation: results from Germany, France and the Nordic countries.

soil-based hydroponic systems. J PI Nutr 10 1205-1211.

degraded land. Appl Biol4 141-200.

Proc Fert Soc 250 140.

The Effect of Humidity and Nutrition upon the Calcium Uptake of Tomatoes

Rachel Holder

Efford EHS, Lymington, Hampshire SO4 OLZ, UK

and

Michael J Marks

ADAS Soil Science Department, Olantigh Road, Wye, Ashford, Kent TN25 5EL, UK

A purpose-built multi-factorial glasshouse at Efford EHS was used to investigate the effect of humidity and nutrition treatments upon the growth, yield, quality and nutrition of an early tomato crop grown in rockwool. The present paper concentrates on calcium uptake and the development of calcium deficiency symptoms.

Four discrete humidity levels (0.1, 0.2, 0.4 and 0.8 kPa vapour pressure deficit (VPD) set points) were maintained continuously for a period of 28 days between midJanuary and mid-February. Within each humidity treatment four nutritional sub-treatments were applied. Two calcium concentrations (200 and 400 mg litre- Ca in the rockwool solution) and two conductivity levels (5.0 and 7.0 mS in the rockwool solution) were combined in a two-by-two factorial arrangement. The higher conductivity level was achieved by adding sodium chloride, thereby keeping all other nutrient concentrations constant.

Calcium deficiency symptoms were first seen in young emerging leaves 14 days after the start of the humidity treatments. The initial symptoms showed as a lime- green discoloration of the terminal leaflet margins and as an associated reduction in leaf area. Affected leaves expanded slowly and developed chlorotic and bleached margins. In severe cases, the disorder developed interveinally from the margins towards the midrib. The severity of the deficiency symptoms was scored visually on a scale from 0 to 6. Leaf lamina sampled from the fith leaf below the growing point was analysed for calcium.

The severity of calcium deficiency symptoms increased exponentially with the reduction in VPD whereas the calcium concentration in the leaf lamina decreased


linearly with reduction in VPD. Severe symptoms of calcium deficiency occurred when the VPD was at or lower than 0.2 kPa. The effects of electrical conductivity and calcium concentration in the rockwool solution upon the incidence of calcium deficiency were small compared with the effect of humidity. Both increased conductivity and increased calcium concentration reduced the symptom severity and increased the calcium concentration in the leaf lamina.

The relationship between leaf calcium concentration and deficiency severity score, as influenced by the nutritional sub-treatments, was complex and was probably influenced by the method of tissue sampling.

There was no consistent effect of humidity on the calcium concentration in tomato fruits. Increased solution calcium concentration and, to a lesser extent, increased conductivity increased the calcium concentration in tomato fruits.

These results indicate that the most effective method of avoiding localised calcium deficiency in developing tomato leaves is control of humidity. Very low VPD reduces transpiration and limits the transport of calcium to developing leaves. Modification of the calcium concentration and conductivity level in the nutrient solution has a relatively small influence upon calcium uptake by developing leaves.

Potassium Nutrition of Tomato Plants: Effects of Restricting Potassium Supply to Day or Night

J Le Bot and E A Kirkby

Department of Pure and Applied Biology, University of Leeds, Leeds LS2 9JT, UK

Although it is well established that plants require large amounts of potassium to maintain normal growth, little is understood of the diurnal variation in uptake of potassium by intact plants. What differences are there in the rate of K + uptake between day and night, and are plants supplied with K + during the night only able to grow as well as those supplied with K + during the day? In order to investigate these questions, an experiment was designed to examine plant growth and composition under three different regimes of K + nutrition, namely K + supplied during the day (Kd), K+ supplied during the night (Kn) and Kf supplied day and night (Kdn).

Five-week-old tomato plants were grown hydroponically for 24 days in the following solution at pH 5.5: Ca(NO,), 2 mM, MgS04 1 mM, Ca(H,PO,), 0.5 mM. Potassium was supplied as K2S04 0-5 mM and a full micronutrient solution was provided. The experiment was conducted in a growth room: theoretical light intensity 620pEinsteins rn-, s- ' , 12 h photoperiod and 23/19"C day/night temperature regime. The plants were moved to their day or night solution every 12 h, and solutions were replenished every 48 h. Four plants were harvested every 6 days and analysed for mineral composition. At the end of the experiment 12 plants were decapitated, in the middle of either the light or the dark period, and the bleeding sap was collected over a 3-h period and analysed for mineral composition.


Dry matter was not affected by the different regimes of potassium supply during the first three harvests. At the fourth, a significantly lower yield was recorded for Kn as compared with Kdn but not as compared with Kd. No significant differences were found between Kd and Kdn.

Significantly lower K + concentrations were found in the leaves and roots of the Kn treatment as compared with Kd and Kdn. However, the concentrations in the stem were similar, possibly indicating a greater pool of recirculating K + in the Kn treatment. The lower concentration of K + in the leaves of the Kn treatment was accompanied by an increase in Ca2+ concentration as compared with the two other treatments. The Mgz+ concentration of the leaves and roots followed a similar pat tern.

Sap analyses performed at the end of the experiment were in accord with the above observations. When decapitation took place during the night, the sap of the Kn treatment was high in K + and low in Ca2+, whereas, with decapitation during the day, the opposite was the case. For the Kd treatment the results were reversed and the Kdn showed an intermediate response.

In following the K + depletion from the nutrient solutions it was observed that the rate of depletion was greater in the Kd and Kn treatments than when K + was present during day and night (Kdn).

(1) Satisfactory plant growth was obtained when plants were supplied with K + only during the night (Kn).

(2) Potassium uptake adjusted to plant demand depending on the period of K supply. Thus during the night the rate of uptake of K + in the Kn treatment was greater than in the Kdn treatment, and similarly during the day K uptake was higher in Kd than Kdn.

(3) When K + was not present in the nutrient medium, the plant compensated for the deficit of positive charge by greater accumulation of CaZ+ and Mgz+.

The following conclusions may be drawn from this experiment:

Osmolalit y and Salt Concentration in Single Cells

Michael Malone," Roger A Leighb and A Deri Tomosa

"Department of Biochemistry and Soil Science, University College of North Wales, Bangor, Gwynedd LL57 2UW, UK bRothamsted Experimental Station, Harpenden, Herts AL5 214, UK

Between studies of whole plant physiology and that at the 'grind and find' level of biochemistry lies a potentially important gap in our understanding of crop plant physiology. This is knowledge at the resolution of the single intact cell. Although information is available about the variation in nutritional status between different tissues in some cases (eg C , photosynthesis), this level of resolution has generally been neglected; and unequivocal information about variation between neighbouring cells within a tissue is almost totally lacking (Tomos and Wyn Jones


1988). Could it be that, in attempting to define biochemical targets for genetic manipulation, the sought-after modifications already exist but are ‘inefficiently’ distributed throughout a tissue? Such comments are clearly speculative, but this report describes initial results of work aimed at investigating the specific roles of cell solutes with regard to tissue water relations studied at single cell resolution. To study these aspects we need to analyse individual cells in situ in the intact plant.

The turgor pressure of individual cells of various tissues (eg wheat and Loliurn temulentum leaves) are measured with the pressure probe (Hiisken et a1 1978). This is a micromanometer that can be inserted into individual cells in order to both measure and manipulate their hydrostatic pressure. Samples (5S200 pl) of cell sap are then removed for analysis.

Initially these samples are loaded under paraffin on to a glass sheet which is inserted into a ‘nanolitre’ osmometer (Clifton) which allows measurement of their osmotic pressures by the freezing-point depression method (precision to within about 5 mosm kg-’ can be obtained). A similar approach has recently been described (Shackel 1987). The method used here has the significant practical advantage that several hundred samples can be handled simultaneously in the osmometer .

Subsequently subsamples can be taken, using a constriction pipette (of volume in the range of 40 pl) and loaded on to aluminium scanning electron microscope studs. These are frozen rapidly, to avoid formation of large crystals of individual salts, then freeze-dried and analysed by X-ray fluorescence. We use both energy dispersive (EDAX) and wavelength dispersive analysis (the latter being more sensitive but also more cumbersome). Standard curves of Na, K, CI and P indicate that sap concentrations down to levels that are osmotically insignificant (< 5 mM) can be measured in this way. Sensitivity for most common salts to < l o - ” g can be achieved.

Taken together this procedure will allow not only determination of sap inorganic and osmotic composition, but also cell and apoplast water potential at the resolution of the single cell. The water potential of the wall space is currently of some interest (Leigh and Tomos 1983; Nonami and Boyer 1987; Tomos 1988; Tomos and Wyn Jones 1988).

The objective of this work is to determine the extent to which metabolically expensive or potentially injurious osmotica (eg nitrate) can be replaced by alternative solutes. How important is the conflict between the need to maintain turgor pressure in a cell and the mobilisation of nutrients from the cell?

References

Hiisken D, Steudle E, Zimmermann U 1978 Pressure probe technique for measuring water relations of cells in higher plants. Plant Physiol61 158-163.

Leigh R A, Tomos A D 1983 An attempt to use isolated vacuoles to determine the distribution of sodium and potassium in cells of storage roots of red beet Beta uulgaris L. Planta 159 469475.

Nomami H, Boyer J S 1987 Origin ofgrowth induced water potential; solute concentration is low in apoplast of enlarging tissues. Plant Physiol 83 593401.

Shackel K A 1987 Direct measurement of turgor and osmotic potential in individual epidermal cells. Plant Physiol83 719-722.


Tomos A D 1988 Cellular water relations of plants. In: Water Science Reviews 3 ed Franks F, Cambridge University Press, Cambridge, pp 186-277.

Tomos A D, Wyn Jones R G 1988 Some transport properties ofcells within tissues. In: Solute Transport in Plant Cells and Tissues, eds Baker D A & Hall J L, Longman, Harlow, pp 22CL250.

agriculture group symposium advances in plant nutrition

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