Eur. J. Agron., 1993, 2(3), 161-171

Abstract

The contribution of crop physiology to strategies for new arable crops for cool wet regions : a case history from Scotland

R. K. M. Hay

Scottish Agricultural Science Agency, East Craigs, Edinburgh, EH 12 BNJ, UK.

Accepted 5 May 1993

This paper reviews a range of arguments for the continued investment of research and development resources into the formulation of new crops and cropping systems for the Scottish arable area, laying particular emphasis on the high potential for dry-matter production of well-adapted crops in northwest Europe. This high potential arises out of the combination of cool temperatures and long photoperiods which favours the prolonged interception of solar radiation by extending the duration of the leaf can­opy. In practice, realisation of this potential depends primarily on soil water conditions, which, in turn, determine dates of planting and harvesting, thereby delimiting the actual, as distinct from the poten­tial, growing season. The fitting of the new crop to the actual growing season, or the development of strategies to avoid or tolerate the effects of wet soils in spring and autumn must therefore be primary aims in the selection of new varieties or species for Scotland. Four possible approaches, each of which has been used with success elsewhere, are reviewed, namely: the empirical approach, agrometeoro­logical modelling, the adoption of perennial crops, and the use of crops yielding products of high value. Although at least three of these strategies have received attention over the last decade, only the empirical approach has had a significant influence on the proportions of species grown in Scotland (changes in areas of winter cereals, oilseeds and peas). It is concluded that, to date, crop physiology has contributed very little to the formulation of new cropping systems. However, crop physiology has made an important contribution in the interpretation of high yields at higher latitudes, pointing to a new set of arable energy crops for the next century, when supplies of fossil fuels will dwindle. In the meantime, investment in the development of such crops will depend upon policies for the pricing of fossil fuels and the use of surplus arable land in the EC.

Key-words: Alternative crops; crop physiology; day length ; Scotland; soil water relations; temperature.

INTRODUCTION grassland (14 per cent), rough grazing and mountains (SOAFD, 1992). In view of the continuous broad acres of arable land which are available elsewhere in the UK and the Community, it could be asked whether there is any point in devoting research and development resources to the formulation of alterna­tive arable cropping systems for this relatively remote and discontinuous area; observers from continental Europe, where the wilderness has largely disappeared under intensive land management, might be forgiven for suggesting that Scotland would be better to con­centrate its resources on developing leisure facilities for the benefit of more populous areas.

Out of a total of 628,000 hectares of land in Scotland cultivated in 1991 for arable crops, over 52 per cent were devoted to barley, predominantly spring sown, with the remaining area being occupied by wheat (18 per cent), oats (4.5 per cent), seed and maincrop potatoes (4.1 per cent), and a range of minor crops (SOAFD, 1992). All of the major prod­ucts of these enterprises, including high quality malt­ing barley, are in surplus in the European Community (EC) at present. This area of arable land, much of it confined to an east coastal strip stretching from the English Border (55 0 45' N) to the Kyle of Sutherland (57 0 50' N) (Figure 1), represents a small fraction (8 per cent) of the total land area of Scotland, the remainder being occupied principally by managed

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Whether serious or jocular, such arguments fail on several counts. Firstly, EC surpluses are an illusion: even on a Community basis, they are variable and normally represent only a few weeks of consumption,

162

and on a global scale, they have become strategically more important as the grain reserves in North America vary and world hunger increases. There is a clear biblical precedent for preparing for lean years. Secondly, within the global context of soil degrada­tion and loss, mature productive soils of this kind need to be husbanded carefully. This could become an even more pressing need if the ' breadbasket ' steppes of N. America and Eurasia dry up under cli­matic change. Thirdly , as the supplies of fossil fuels, especially petroleum, decline, it may become neces ­sary to reserve the remaining stocks as raw materials for chemical synthesis, and to rely more on energy crops (e.g. rape oil as a direct substitute for diesel fuel). These reasons alone provide good strategic arguments for maintaining a c apacity for arable crop­ping, without invoking economic arguments such as the desirability of securing rural employment by growing malting barley near whisky distilleries.

However, perhaps the most persuasive reason for developing alternative crops for the Scottish arable area is its outstanding productivity . Surveys over the last decade have revealed that farm yields of cereal crops in the south-east of Scotland can approach the calculated maximum potential yield for the area (Hay et al. 1986), and very high yields of potato tubers have also been recorded (Hay and Galashan, 1989) The Scottish record yield for a field crop of winter wheat (as opposed to that from experimental plots), which stands at 14 t ha- ' at 15 per cent mc (Newbridge, Midlothian ; Anon, 1990), is probably also the world record for wheat. Furthermore, in many respects, crops grown under such cool condi­tions are relatively free of di sease and pest problems Clearly, arable areas of this kind yielding products of high quality are an irreplaceable resource for the Community and the planet.

This paper reviews the contribution of crop phys­iology and agrometeorology to the identification and

R. K. M. Hay

deployment of new arable crops in Scotland. Although the profitability of candidate crops, and related price support schemes, determine ultimately which species or varieties will be successful, it should be emphasised that each of the examples explored here involves products for which there is an estab­lished demand. 'Supply-led ' commodities are not con­sidered, although the future importance of biomass and industrial feedstock crops is considered in the Discussion.

THE PHYSIOLOGICAL BASIS OF HIGH YIELD AT HIGHER LATITUDES

Claims of very high yields of cereals in certain arable areas of Scotland were finally tested in a systematic way in 1976 and 1977 by comparing the performance of two cultivars of spring barley grown under very similar management at Cambridge and Edinburgh (Ellis and Kirby , 1980 ; Kirby and Elli s, 1980 ; see also Cottrell et al., 1985). As shown in Table 1, grain yield and each of its components were higher at the northern site, both in the drought year of 1976 and under the more normal conditions of 1977. Results of this kind can surprise those with a preconception that crop yields will tend to be higher in warmer zones, but the explanation of this phenom­enon can be illustrated by Figure 2 which compares the solar radiation and temperature regimes at arable sites in south-eastern England and eastern Scotland during the 1980s. From mid-May to mid-August in the mid-latitudes of the Northern Hemisphere (i .e. the most productive months of the growing season), the daily receipt of solar radiation is relatively constant (Lamb, 1972) whereas photoperiod increases with lat­itude. Consequently, approximately the same amount of radiation (within 5 per cent in July and August, otherwise identical; Figure 2) was spread over a longer day at Dundee (560 28' N) than at Rothamsted

Table 1. Grain yield and its components for spring barley crops grown under the same management al Cambridge (lat. 52" 11 ' N) and near Edinburgh (550 53' N) (from Ellis and Kirby, 1980 ).

cv Golden Promise

cv M aris Mink

Grain yield" t ha - I

Ears m-2

Grains per ear Individual grain we ight , mg Grain yield " t ha - I

Ears m-2

Grains per ear Individual grain weight , mg

" yield scaled up from small plots

1976

Cambridge

4.7 764 23.2

25.0 5.7 934 20.0

29.3

Edinburgh

6.8 1022 23.7

27.9 6.5 986 21.6

30.5

Cambridge

4.8 748 20.3

32.1 4.5 722 18.1

34.9

1977

Edinburgh

10.5 1182 22.9

37.9 10.6 1187 2 1.2

41.8

Eur. 1. A gron.

The contribution of crop physiology

(51 ° 48'), so that temperatures in midsummer were up to 2 °C lower at the Scottish site (Figure 2). Hegarty (1973) recorded greater differences in daily mean temperature (up to 2.7 0c) in comparisons of Dundee with Bracknell (51 ° 24' N). For well-adapted species such as spring barley , lower temperatures prolong development, and delay crop senescence, resulting in increased yield by the interception of more radiation and the prolongation of grain filling (Hay and Walker, 1989 ; Table 1). The longer duration of organ (tiller, ear, spikelet, floret) development, and/or reduc­tion of organ mortality under lower temperatures are reflected in greater ear populations and larger ears (Table 1). However, where water relations differ markedly between sites, relative rankings of yield potential can be upset, as with the higher yields recorded in England by Ellis and Brown (1986) .

The concept of a longer duration of development at northern sites in Britain is broadly supported by the more comprehensive study of wheat by Porter et al. (1987), but in the same investigation, Kirby et al. (1987) also demonstrated the promotive influence of long photoperiods on the rate of spikelet initiation.

saoN

57°N

Figure 1. Outline map of Scot land (excluding Shetland. Orkney and the Outer Hebrides) indicating the location of s ites mentio­ned in the text: 1. Kyle of Sutherland ; 2. Aberdeen; 3, Dundee ; 4, Edinburgh .. 5, Newbridge ; 6, Isle of Arran .. 7, Ayr .. 8, English Border.

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"

163

There is, as yet, no clear fi eld evidence to confirm a direct role for long photoperiods in increasing radia­tion interception (faster leaf expansion, larger leaves; Hay, 1990), even though this effect has been clearly established under controlled conditions. Photoperiodic stimulation of growth has been invoked to explain high grass yields in southern Norway (Hay and Heide, 1984).

Finally, it should be pointed out that these adapta­tions to cool temperatures and long days can operate in Scotland and Scandinav ia only because the Gulf Stream maintains mean temperatures at values which are several degrees higher than most other areas of the world at similar latitudes.

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Figure 2. Comparison of total solar radiation and screen ai r temperature at Rothamsted 0 (5 / 0 48 ' N," Alt . 128 m) and In vergowrie, Dundee . (56 0 28 ' N ; Alt. 10 m) during the 1980s (from Annual Reports of Rothamsted Experimental Station and the Scottish Crops Research Institute). The midsummer day­length at Dundee is 17.5 h and at Rothamsted 16.7 h (day­lengths from Smithsonian Meteorological Tables).

164

ENVIRONMENTAL FACTORS LIMITING ARABLE CROPPING IN SCOTLAND

Analyses of this kind, which concentrate on the influence of temperature, photoperiod and solar radi­ation, indicate that the potential yields of certain arable crops in Scotland are higher than at lower lat­itudes because the more extended potential growing season permits the interception of a higher proportion of the annual input of solar radiation. However, realistic analyses must also take account of other lim­iting factors, of which water relations is the most important in maritime northern Europe. Thus, although temperatures in early spring can be consis­tently above the threshold for barley leaf expansion, spring barley can not be sown until the soil moisture content has fallen below field capacity. At field capacity and above, soil strength is low and the mechanised traffic required for cultivation and sow­ing leads to soil structural damage (plastic deforma­tion, smearing, reduced aeration) and poor crop estab­lishment. In some areas, such risks can be avoided by employing winter cereals, although this may simply divert the problem to the autumn.

Because of the same interaction between soil strength and water content, harvesting operations are not practicable once the soil has returned to field capacity in autumn; for example, a potato crop can continue to intercept solar radiation and accumulate dry matter well into the autumn, but if the soil has returned early to field capacity, it may be impossible

Figure 3. Geographical areas in south-west Scotland bounded by J 000 mm and 1200 mm isohye ts. Shaded areas receive more precipitation. The linear distance from Ayr to Kilmarn ock is 20 km (adapted from Fran cis, 198 / reproduced by permission of the Meteorological Office).

R. K. M. Hay

to harvest. Thus the actual length of the growing sea­son is generally much shorter than the potential length, because it is demarcated by the dates of the firs t (continuing) soil moisture deficit in spring, and of the return to field capacity in autumn. The fact that very high farm yields can be achieved in parts of south east Scotland indicates that soil water condi­tions are less critical in these areas.

Unlike seasonally-dry zones, where water-limited season lengths can be determined precisely (at least in retrospect; Hay, 1981), it is difficult to determine the end, and particularly the start, of each actual growing season in northern Europe. Substantial soil moisture deficits can persist in February, to be suc­ceeded by waterlogging in April, and soil moisture characteristics vary considerably with soil type. In spite of these difficulties , the analysis carried out by Francis (1981) using those long-term data (precipita­tion and evaporation) which were available, showed that the predominantly arable soils in Scotland (i.e. those areas where arable crops are consistently prof­itable owing to high yield) shared the characteristics presented in Table 2. However, in those zones where the interquartile ranges were later (spring) and/or ear­lier (autumn) than in the major arable areas (Francis, 1981), the risk of crop yield penalties or failure rose sharply because of the increased probability of enforced late sowing or inability to harvest at crop maturity (Hay and Walker, 1989). The arable crop­ping which takes place in these zones has a lower yield potential , and is normally in direct support of livestock enterprises.

These generalised conclusions can be illustrated by the small area of arable cropping surrounding Ayr (55 0 29' N) in south west Scotland (Figures 1, 3; Francis, 1981). This area, a 'rain shadow' in the lee of the island of Arran (maximum altitude 874 m), is anomalous and restricted ; the 1000 mm isohyet encloses a small ,coastal strip suited to arable crop­ping, mainly on the basis of early sowing (first soil moisture deficit 20 February - 20 March; return to field capacity 20 Sept. - 31 Oct.), from that devoted to intensive grass for dairying (I March - 31 March ; 20 September - 20 October). The hazards of growing arable crops just outside this area are shown by records of precipitation from the agrometeorological

Table 2. Water relations of soils in the major arable areas of eastern Scotland (from calculations of Francis, 1981) 1957-75.

Interquartile range of dates Median date (i.e. spanning at least 50 %

of observations)

First soil 1- IO March 21 Feb.-20 March moisture deficit Return to 1-10 Nov. 20 Oct.-3~ November field capacity

Eur. 1. Agron.

The contribution of crop physiology

1200

1100

E E 1000 = .g ;:! ' i5.. 'u " 5.. 900

800

700

600

165

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1930 1940 1950 1960 1970 1980 1990 Figure 4. The variabil ity of to tal annual precipitation at the SAC Auchincruive agrometeorological sta tion n ear the 1000 mm isohyet.

station at Auchincruive to the west of Ayr near the 1000 mm isohyet boundary. Not only does total annual precipitation vary considerably with season (range 700 - 1200 mm per annum, over six decades; Figure 4) but the long-term seasonal trend shows a peak of rainfall at the time of cereal harvest in late August and September (Figure 5). What is more important is that, in two seasons out of the last t en, rainfall was extremely high in July (1985 ; 1988) as well as at sowing time in spring 1988. In 1985, even some forage crops failed .

Table 3. Mean harvest dates of three species of arable crops grown in the north-east of Scotland over fou r seasons (from Matthews et aI. , 1988).

Crop 1984 1985 1986 1987

Oi lseed rape 15 A ug. 29 Aug. 25 Aug. 27 Aug. Combining peas 27 Aug. Fai led 23 Sept. 29 Sept. Winter barley I I A ug. 4 Sept. 11 Aug. 19 Sept.

Vol. 2, n° 3 - 1993

250

E E 200 .£ c: 0 E 150 ... " 0..

= .S 100 :§ i5.. 'u 50 1:

c..

A M A SON 0

Figure 5. The seasonal pattern of p rec ipitation at the SAC Auchincruive a grometeorological station (e 1985 ; 0 1988; --24 year mean).

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166

All Barley

Winter Wheat

~ 100 :::> o -5 " 75 ~ " "" o '-<

-~--- Winter Barley

U 50 Oil seed

rape

25 Oats

~peas 80 81 82 83 84 85 86 87 88 89 90 91

Figure 6. Trends in areas of combining crops in Scotland, 1980-1981 (Data from the Scottish Office Agriculture and Fisheries Department, Economics and Statistics).

STRATEGIES FOR NEW ARABLE CROPS

The conclusion of these studies is that the major arable crops (spring barley, winter wheat, potatoes) yield very well only in those areas where the actual growing season, determined by soil water conditions, is long enough to permit the crop to exploit the ben­efits of slow development in cool long days. Where the growing season is cut short by unfavourabl~ wat~r relations, these benefits are reduced, resultmg m lower yield or even in total crop loss. The matching of the growing season required by a variety or crop with the actual (soil-moisture limited) growing season which is available is, therefore, the most critical fac­tor in the selection of new arable crops for Scotland; at least four strategies can be deployed in the selec­tion of suitable species and cultivars.

The empirical approach

This time-honoured approach of the agronomist, is analogous to the methods used by pioneers of new lands. For example, over the last decade, the costs of importing oilseeds and protein grains to meet EC deficits have led to the empirical field testing of a range of types of oilseed and legumes, with the aim of developing new combinable crops which would fit into existing farm systems and rotations, with the minimum of changes in mechanisation.

The merits of this approach can be illustrated by a programme of evaluation of oilseed and protein crops carried out by Matthews et al. (1988) near Aberdeen during the 1980s. Crops of selected winter cultivars

R. K. M. Hay

of oilseed rape, combining peas and barley were grown using the standard crop husbandry for winter barley in the area, and the phenology of the crops compared (Table 3). Of the four summers, two were 'typical' , whereas 1984 was unusually d.ry, and 1985 was exceptionally cool and wet, delaymg crop maturity and interfering with harvest. This led to clear discrimination between those species (oilseed rape, barley) which could be grown successfully and brought to maturity, albeit with relatively late harvest dates, in an exceptionally difficult year, and peas, which could not be harvested in 1985. These findings are reflected in the areas of each crop grown by farmers in Scotland during the 1980s (Figure 6) ; sow­ings of oilseed rape and winter barley decreased markedly in 1986 because the late harvest of preceed­ing crops interfered with subsequent sowing, but these were temporary reverses. However, in the case of combining peas, 1985 marked the end of the expan­sion in the area sown and the start of a longer term decline in the area devoted to the crop. Here the empirical approach, as applied by farmers, resulted in the widespread adoption of oilseed rape, whereas combining peas are now restricted to the most favoured warm dry areas. Evening primrose provides an example of a more spectacular failure; in experi­mental plantings in the south east of Scotland, the crop required at least 14 months from planting to maturity, and crops sown in 1984 and 1986 produced no harvestable grain (Russell, 1988). Elsewhere in the UK, other exotic combinable crops have been tried (e.g. quinoa; Risi and Galwey, 1991).

These examples show that the empirical aproach does work, but it is a conservative approach, consid­ering only those species and cultivars which can be fitted into existing systems with the minimum of change in farm practice and mechanisation. Experience with crops such as flax have revealed ~he problems with species which yield well but reqUIre parallel innovation in processing technology. Furthermore, the success of the approach in identify­ing unsuitable crops depends heavily upon the pre­vailing weather during the test seasons; for example, the clear decision, in most areas of eastern Scotland, not to include combining peas in rotations was founded upon the results of one extreme season (1985) during its period of introduction. This has two major implications: on the one hand, the period of evaluation would have been much longer (and more expensive) during decades of more settled weather, and, on the other hand, the extremes of climatic con­ditions experienced in the last ten years make it dif­ficult to gain a long term perspective of the suitabil­ity of any new crop. Thus it is important to ask whether major cropping opportunities are missed if only the empirical approach is taken.

Agrometeorological approaches

In principle, quantitative agrometeorological com­parisons of the requirements of possible crops (sea-

Eur. 1. Agrofl.

The contribution of crop physiology

son length in day degrees, threshold temperatures, response to frost or drought etc.) with the correspond­ing characteristics of possible growing sites, should rule out the need for the lengthy and expensive field trials of the empirical approach. However, the success of this type of approach depends upon the quality and quantity of available information on the candidate crop and potential sites.

One of the most celebrated examples of the use of agrometeorological analysis in the search for new crops is provided by Western Australia. In the knowl­edge that there was an increasing world demand for high-quality dry white wines, Gladstones (1965) car­ried out a detailed analysis of the climate (length of growing season, water relations, frost incidence, soil properties) of the southern part of the state, searching for areas which match the most successful wine pro­ducing areas of Europe and the U.S.A. Although there were problems associated with latitude and day length, Gladstones identified at least two major candidate areas (Margaret River, Mount Barker). Twenty years later, and with the benefit of a favourable economic environment for investment, the Margaret River wines have already achieved an international reputation.

Comprehensive studies of this kind have not been a feature of the search for new crops in the UK, pos­sibly because of the variability of its maritime climate and difficulties associated with the modelling of soil water relations. The analyses of season length (day degrees) which have been carried out in relation to maize (Bunting, 1976) and more recently for navy beans (Dodd, 1991), have been concerned only with potential growing season length, without reference to soil moisture deficits.

A voidance of soil moisture limitation - perennial crops

The physiological analyses of crop growth and development in north-west Europe presented at the start of this paper show that the actual growing sea­son exploited by the predominantly spring-sown arable crops can be considerably shorter than the potential growing season, with associated losses of potential yield. However, if as in the grassland farm­ing which is practised in wetter zones, arable crop­ping were based on perennial crops, then the plants could respond to environmental temperature earlier in the season, before the onset of first soil moisture def­icit. (This, of course is one of the reasons for the trend towards biennial/winter crops (Figure 6), although areas with wet springs tend also to have wet autumns.) Similarly, cropping based on perennial spe­cies could be more easily managed for earlier harvest in late summer. The net effect on radiation intercep­tion would depend on the relative magnitudes of gains in spring and losses owing to early harvest.

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There are a number of other benefits associated with perennial crops, including reduced soil erosion, improved soil structure, and a general lowering of production costs (fewer weeds, lower seed costs, reduced fuel use for cultivation) (Wagonner, 1990). The major problems associated with perennation include generally lower yields (lower harvest index since dry matter must be devoted to survival over winter), the tendency to persist as weeds of subse­quent crops, and a general lack of useful candidate species which do not pose major management prob­lems. For example, in terms of market demand for products (e.g. fuel, pulp, timber), many of the most attractive candidates are long-lived woody species which are very difficult to include in existing farm rotations (e.g. coppice willow; Dawson, 1992). Adoption of such crops requires a major revolution in lowland agriculture.

During 1988-90, the search for suitable crop species in Scotland led to the evaluation of the Jerusalem arti­choke (Helianthus tuberosus) as a perennial (Hay and Offer, 1992). H. tuberosus, which is a tall herbaceous, commonly vegetative, crop, propagated annually by tubers, has recently been evaluated in the EC and other countries as a source of tuber dry-matter for bio­mass alcohol or biogas production, or of inulin for chemical synthesis. The drawbacks of this crop include the late harvesting of tubers (mature in October/November, Figure 7), the low harvest index of the crop (less than 50 per cent of annual dry matter production recovered in the tubers), and the fact that the appropriate processing technologies are, as yet, poorly developed. H. tuberosus grown for its tubers is clearly not a good candidate for arable cropping in north-west Europe, bearing in mind the need to har­vest early in the autumn. In the potato, Scottish agri­culture already has a tuber crop which can cause severe problems if soils return to field capacity early in the autumn.

However, the species was apparently well-adapted to the cool long days of north west Europe, it appeared to be free of pests and diseases, plants read­ily established in spring from tubers left at harvest, and the work of Rawate and Hill (1985) indicated that its leaves were a source of high-quality protein. These observations, together with the fact that the crop could be harvested with existing machinery, indi­cated that H. tuberosus might be a useful perennial crop, yielding high-quality forage for livestock which could help substitute for protein grains imported into the EC. Preliminary evaluation in the field, at low levels of applied fertiliser and with no crop protec­tion, confirmed that the crop could produce very high yields of dry matter (total yields over 30 t ha- I per year) ; furthermore, because tuber filling began in late July (e.g. Figure 7) the crop could establish success­fully in the subsequent year if the tops were har­vested during August. The work did confirm that the

168

leaf blades contained a relatively high proportion of protein, but since they constitute a small fraction of above-ground dry matter, the mean protein level of the forage was lower than that of good-quality grass hay. Further work is required to show to what extent protein content can be increased by fertiliser applica­tions nearer to those which are normal for grass crops, but livestock intake of artichoke dry matter in the form of silage did prove to be higher than pre­dicted for grass silages of similar composition (Hay and Offer, 1992) As with other novel crops, it would also be important to monitor the potential build-up of pests and diseases.

In view of the EC surplus of livestock products from grass, a species producing forage of modest nutritional quality might not appear to be a particu­larly attractive candidate as an alternative arable crop However, in the longer term, its very high yield potential under lower-input cropping (especially in relation to reduced crop protection, and fuel use for cultivation), and the fact that the yield is available from a single cut well before the return to field capacity, could release areas of high yielding grass­land for high-quality timber production However, the ultimate role of species such as H. tuberosus could be as biomass or industrial feedstock crops.

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R. K. M. Hay

A voidance of soil moisture limitation -high-value crops

The underlying physiological principle of each of the above strategies is the maximisation of radiation interception, for a given set of inputs (fertilisation, crop protection etc.). However, for crops yielding use­ful secondary metabolites (e.g. alkaloids, terpene and phenylpropene oils) rather than carbohydrate or pro­tein , such optimisation of dry-matter yield can be less critical because of the very high value of the product, if it meets the required quality standards. Furthermore, for most secondary products, there are complex interrelationships amongst ontogeny, biomass yield and product yield; most species show pro­nounced seasonal changes in chemistry (e.g. Svoboda et aI. , 1990a) and, for example, increasing fresh- or dry-matter yield by the application of nitrogen ferti­liser can cause reduction in the concentration of the required active ingredient (e.g. Hiilvii, 1987).

In the long term, it is likely that the most profit­able crops grown in Europe for their secondary prod­ucts will be those yielding valuable pharmaceutical drugs and drug-precursors. However, at present, there is considerable worldwide interest in culinary herbs, most of which which owe their properties to volatile oils (predominantly terpenes and phenylpropenes).

August

1988

x

September

0vTot"

'\ I I

October November

Figure 7. Time courses of leaf area index, total crop dry weight and tuber dry weight for a stand of Helianthus tuberosus grown al Auchincruive in 1988 (reproduced with permission of the Society of Ch emical Industry from Hay and Offer, 1992).

Eur. J. Agron.

The contribution of crop physiology

These plants have been used for centuries in tradi­tional cooking as well as in perfumery, but the world market has begun to increase sharply, for a number of reasons. First, with the spread of foreign travel, the eating habits in the developed countries have become more adventurous, and in the same countries there has been a sharp rise in the consumption of processed foods, many of which are flavoured with herbs or their oils. At the same time, research work on vola­tile oils has shown that they possess pronounced anti­microbial and antioxidant properties (Deans and Waterman, 1993), making them suitable naturally occurring food additives to substitute for synthetic additives. This trend towards more varied and more natural food is reflected in a doubling in the world demand for volatile oil products over the last decade (Veriet, 1993). Hay et al. (1986) estimated that the UK share of this trade, predominantly in imported culinary herbs, was of the order of £30 million.

In the light of this information, it was decided, in the 1980s, to investigate whether any of these vola­tile oil species could be candidates for arable crop­ping in Scotland. Similar projects, many on a much larger scale, were being initiated throughout Europe and other temperate areas. Many of the familiar crops of this kind are natives of stressful environments (e.g. the labiate herbs, thyme, rosemary, lavender, from xeric areas of the Mediterranean Basin) or of the tropics (e.g. spices such as clove, nutmeg and cinna­mon), and there is often a presumption that'quality'is associated with warmer environments and/or stress. However, in a screening experiment carried out over several years at a markedly mesic site in south-west Scotland, including more than 100 species of culinary and pharmaceutical importance, several groups of plants gave promising results (Hay et al., 1986). In particular, a number of north-temperate and Mediterranean culinary herbs (including labiates : mar­joram, mints, sage, summer savory, thyme; umbelli­fers : chervil, dill, parsley; and the composite French tarragon) grew vigorously, and appeared to be very promising candidates for arable cropping because they accumulate commercially-important terpene and phe­nylpropene oils primarily in glands or ducts in their foliage rather than in their seeds. These species would, therefore, be suitable for whole-crop harvest­ing during the summer, when the risk of unfavourable soil conditions is low, rather than by combine-harvest­ing for seed in the autumn. Furthermore, since these crops can be traded in the form of fresh herbs, dried herbs, steam-distilled volatile oils, or solvent extracts, they could confer a welcome degree of flexibility in marketing.

Subsequent physiological and biochemical investi­gation of a selection of candidate herbs (summer savory Satureja hortensis, thyme Thymus vulgaris, French tarragon Artemisia dracunculus) in field plots confirmed that their phenology and ontogeny are

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suited to whole crop harvesting in late summer, that they yield well (in comparison with established areas of cultivation in Eastern Europe) under relatively low levels of input, and that the quality of the oils pro­duced is high (Hay et al., 1988; Svoboda, Hay and Waterman, 1990a,b; Jackson, 1990). There was no requirement for water-, temperature- or nutrient-stress to stimulate the production, or ensure the quality, of the required secondary products. These findings, which are consistent with results from other cool tem­perate areas (e.g. Finland, Galambosi, 1989; New Zealand, Lammerink et at., 1989) were generally sup­ported by the results of field-scale trials on Scottish farms.

In summary, the agronomic, physiological and bio­chemical aspects of these crops (high yield, high quality, mainly grown using existing systems and equipment) indicate that they could be grown profit­ably in cool wet areas. Economic analysis (e.g. Verlet, 1993) confirms that there is a substantial and expanding market, including opportunities to substi­tute for materials imported into the EC, much of which is of relatively low quality. Nevertheless, in spite of the activities of pioneering farmers and grow­ers, such opportunities have yet to be exploited in Scotland, because of poorly-developed markets and a lack of processing plant (for oil distillation or extrac­tion). This is in marked contrast to France, for exam­ple, where an expanding herb growing industry is founded upon farmers 'cooperatives.

DISCUSSION

The main arable areas of maritime northern Europe are an important world resource because of their high potential for dry-matter production; this, in turn, is a consequence of the combination of cool temperatures and long days which favours the prolonged intercep­tion of solar radiation, by adapted species and culti­vars. However, the products from these crops are largely in surplus in the Ee, and assuming that EC regulations continue to evolve towards reducing food surpluses (rather than establishing a granary for hun­gry countries), the agricultural exploitation of these resources in the near future requires the deployment of new crops and cropping systems. The alternatives of extensive cropping of existing crops, or widespread fallowing, have serious economic and ecological implications. In the more distant future, the need to feed a world population of two to three times the present level, from a depleted world soil resource, could well cause a return to traditional, high yielding, arable crops.

Of the four strategies for the deployment of new crops in Scotland outlined in this paper, only the empirical approach has been widely applied, resulting in the expansion of the area devoted to winter cereals, oilseed rape and peas (Figure 6). The crop physiolog-

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ical input to this work, by agricultural scientists and farmers, has been negligible. Research and develop­ment resources have been invested in the other strat­egies, particularly in relation to high-value culinary and medicinal species, including studies of their agronomy, physiology, biochemistry, microbiology and biotechnology as well as related food science aspects. However, in spite of the suitability of many of these species for cropping in Scotland, the high biochemical and microbiological quality of the prod­ucts, and the proven demand for the products, there has been little interest beyond that of the pioneering growers who experienced acute marketing problems. As parallel research and development programmes progress in continental Europe, it is likely that the commercial opportunity for cropping of this kind in Scotland has passed.

The important role that crop physiology has played in interpreting and verifying the high potential of north-west Europe for dry-matter production - a fea­ture which has tended to be obscured in previous analyses (e.g. Pons, 1982) - also lays the foundation for the development of a further range of novel crops which may be required in the next century as fossil fuel reserves decline. These include arable crops for energy (biomass for direct combustion or fermenta­tion, plant oils as direct fuels), for natural fibres, as well as those which can act as feedstocks for chemi­cal synthesis (starch and oils in the manufacture of plastics, simpler carbohydrates such as inulin as the starting point for complex syntheses). The high pro­ductivity of the major arable areas of Scotland (over 30 t DM ha- 1 biomass from cereals and Jerusalem artichokes) indicates that crops of this kind have a future, but exploitation of these resources depends critically upon the price of a barrel of oil (Pierce, 1991), and upon any new subsidies for energy crops which could arise out of EC set-aside regulations.

ACKNOWLEDGEMENTS

I am grateful for the helpful comments of Professor John Hillman and Dr Terry Hegarty on this paper in draft, and to Dr Andrew Walker, Dr Katya Svoboda and Janice Hampson for their valuable contributions to the alternative crops project at SAC Auchincruive.

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