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Trends in the Inorganic Nutrition of Plants

F . C. S T E W A R D

Autotrophic plants can survive i n a n inorganic world. Indeed, the ability of plants to subsist on inorganic sources of nitrogen ( n i t r a t e or a m m o n i a ) is at least one of their distinguishing characteristics. T h e ability to utilize even e l e m e n t a r y nitrogen, b y biological nitrogen fixa- tion ( w h i c h is s o m e w h a t m o r e w i d e l y dispersed t h r o u g h t h e p l a n t kingdom t h a n used to be t h o u g h t ) represents t h e highest degree of auto- t r o p h y for nitrogen. F r o m this point of view certain blue-green algae t h a t can both fix nitrogen a n d c a r r y on photosynthesis a r e p e r h a p s t h e most autotrophic organisms w h i c h are k n o w n . This m a y be p a r t of their survival value a n d their role as e a r l y colonists of n a k e d surfaces, w h i c h a r e otherwise free of organic m a t t e r , surfaces w h i c h r a n g e from volcanic laval slopes to t h e raised m u d s of salt marshes.

Essential and Dispensable Elements

W h e n one considers t h e surprisingly small n u m b e r of elements of t h e periodic table w i t h w h i c h n a t u r e has elaborated t h e form a n d substance of plants, t h e elements of w a t e r a n d of carbon dioxide m a y be, a n d u s u a l l y are, treated separately. T h e r e m a i n i n g elements a r e distinc- tively of m i n e r a l origin. W a t e r is n o t u s u a l l y considered to be a nutrient. However, w a t e r is b y far t h e most a b u n d a n t molecular species in cells a n d organisms. I t m a y be calculated, for example, t h a t a carrot root cell m a y contain about 1 01 7 w a t e r molecules a n d about 1 08 protein molecules of a n assumed, b u t probable, average molecular weight. A n u t r i e n t is t h a t w h i c h nourishes a n d out of w h i c h t h e substance of plants is built; i n this sense w a t e r certainly performs a n essentially nutritional role. I n fact, so high is t h e percentage of w a t e r t h a t one m a y say t h a t t h e m i n u t e a m o u n t of m i n e r a l m a t t e r a n d t h e larger, b u t still small, a m o u n t of organic m a t t e r w h i c h constitute t h e organization of plants is w h a t i m p a r t s to t h e mass of w a t e r t h e y contain t h e distinctive properties b y w h i c h t h e organisms a r e recognized. A m e d u s a i n t h e sea m a y be almost entirely composed of water, b u t its relatively m i n u t e a m o u n t of salts a n d organic m a t t e r i m p a r t to this m a s s of w a t e r t h e organization w h i c h makes it t h e distinctive creature t h a t it is.

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T h u s , although not c o m m o n l y regarded as a n inorganic n u t r i e n t , w a t e r nevertheless enters into all aspects of t h e physiology of plants as the essential m e d i u m in w h i c h biological reactions occur, the essential a n d most a b u n d a n t stuff of w h i c h plants are m a d e and, t h r o u g h h y d r o - gen bonding, w a t e r is also an essential p a r t of the architecture of the complex substances a n d large molecules so i m p o r t a n t in the microscopic a n d submicroscopic morphology of living things. W a t e r is also t h e molecule from w h i c h hydrogen is transferred to cause reduction, a n d to w h i c h it is restored in the essential step of transfer to oxygen, as in the t e r m i n a l oxidative step of respiration. T h u s w a t e r is the essential basis of so m a n y of the energy exchanges in cells. Being composed of v e r y small atoms, a n d b y reason also of its molecular a s y m m e t r y , w a t e r packs a large a m o u n t of m a t t e r in t h e m i n i m u m of space a n d its phys- ical properties (specific heat, latent heats of fusion a n d of vaporization, dielectric constant, surface tension) are u n i q u e a m o n g liquids at the prevailing temperatures of this earth. Therefore, one can as little con- ceive of life, as w e k n o w it, without t h e properties of w a t e r as one can conceive of it without the distinctive properties of carbon a n d of that element's ability to combine w i t h itself to form the rings, chains, films, fibrils, a n d lamellae a n d the large molecules out of w h i c h the form of cells a n d organisms is so largely built. T h u s , although c o m m o n l y t h e y are considered separately, the elements C, H , a n d Ο have their premier place in the list of essential elements.

T h e rich variety of the p l a n t kingdom, from the thallophytes to the angiosperms, a n d the r a n g e a n d complexity of t h e physiological func- tions a n d biochemical reactions of plants are achieved b y utilizing the chemical properties of only a v e r y few of t h e total chemical elements of the periodic table. T h o u g h it is a long w a y from the " E a r t h , Air, Fire, a n d W a t e r " of Aristotelian doctrine to m o d e r n knowledge of the ten essential m a c r o n u t r i e n t elements (C, Η , Ο, Ν , P , S, K, Ca, M g , F e ) , to the five well-established m i c r o n u t r i e n t elements for angiosperms (B, M n , Cu, Z n , a n d M o ) a n d to t h e m o r e recently established elements which are either generally essential or beneficial in certain situations (Cl, N a , Si, a n d V ) , it is still surprising h o w dispensable are so m a n y of t h e chemical elements, even those t h a t are most a b u n d a n t in the earth's surface.

T h e elements most utilized b y plants a r e certainly not those w h i c h are the most common. Despite the a b u n d a n c e of sodium, it is so dis- pensable a n d so u n a b l e to replace t h e essential role of potassium t h a t only recently has it been added to t h e list of elements t h a t are essential for certain l a n d plants (cf. Chapter 2 ) . However, one cannot conceive of sea w a t e r a n d of m a r i n e plants a p a r t from the properties of sodium.

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A l u m i n u m a n d silicon a r e also a m o n g t h e most a b u n d a n t chemical ele- m e n t s in the earth's crust, b u t again, a p a r t from certain special situa- tions in w h i c h t h e y m a y contribute to t h e skeletal substances of plants

(as for e x a m p l e silicon i n diatoms a n d in certain p l a n t cell w a l l s ) , these elements also a r e essentially dispensable. But a l u m i n u m a n d sili- con, like carbon, enter into chemical configurations w h i c h p e r m i t almost indefinitely repeating patterns in space. However, carbon, b y its small size a n d its ability to combine directly w i t h itself, can form such re- peating patterns alone, whereas other elements (oxygen, a l u m i n u m , boron, etc.) m u s t be interposed in the case of silicon. T h u s , while car- bon forms the essential skeleton of m a n y large molecules i m p o r t a n t in n a t u r e a n d together w i t h nitrogen a n d phosphorus forms t h e es- sential structure of proteins a n d nucleoproteins, w i t h o u t w h i c h terres- trial biology could not exist, the large molecules w h i c h a r e built from silicon, w i t h a l u m i n u m a n d oxygen, etc., provide t h e repeating patterns in space, w h i c h are t h e basis of m u c h inorganic form in m i n e r a l s and in soils. But for all practical purposes the elements silicon a n d alumi- n u m are dispensable b y plants.

It is almost essential to believe t h a t primeval life utilized t h e m i n i - m u m n u m b e r of elements and, as morphological specialization de- veloped, its r e q u i r e m e n t s became m o r e exacting, a n d life adapted to a n d utilized t h e special properties of a n increasing r a n g e of substances.

I n this w a y t h e properties of a given chemical element could be used in a given molecular situation. T h e h i g h l y specialized molecular situa- tions in w h i c h m i c r o n u t r i e n t or trace elements form p a r t of specific enzymes are obvious examples here. T h e m o r e advanced a n d specialized cells a n d organisms become, t h e m o r e prescribed a r e their nutritional requirements. (Calcium is not c o m m o n l y required b y bacteria a n d fungi, a n d there is little or no evidence of boron r e q u i r e m e n t for these organisms.) I n fact, it is still a puzzle w h y such elaborate molecules h a d to be developed to p e r m i t a n inorganic element to perform w h a t often seems to be a simple function. F o r example, the oxygen-carrying properties of iron in hemoglobin or myoglobin is b u t one of m a n y similar examples in both plants a n d animals. Despite the complexity of some of these relationships (as for e x a m p l e iron to cytochromes), it is surprising h o w generally distributed t h e y n o w are a n d h o w little evi- dence one can see of w h a t m a y be called a progressive biochemical evolution parallel to t h e morphological evidence.

Some Historical Landmarks

F r o m their e a r l y origins in Aristotelian doctrine, t h e primitive con- cepts of m i n e r a l n u t r i t i o n of plants advanced b u t slowly, or not at all,

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4 F . C. S T E W A R D

t h r o u g h the M i d d l e Ages. V a n H e l m o n t , a n d later others w h o resorted to experiment, ushered in the m o d e r n period in w h i c h t h e m i n e r a l nutrition of plants w a s to be based on a rational system of chemistry.

W i t h Théodore de Saussure's well-known book of 1804, entitled

"Recherches chimiques sur la végétation," this t r e n d w a s firmly estab- lished; a n d b y t h e end of the n i n e t e e n t h c e n t u r y t h e ideas of m i n e r a l nutrition h a d reached such a level that, although details w e r e still to be added, t h e essential structure did not need to b e changed.

T h e n i n e t e e n t h c e n t u r y saw v e r y rapid advances to knowledge of p l a n t nutrition; this is ably s u m m a r i z e d in m a n y available sources, such as E. J. Russell's "Soil Conditions a n d P l a n t G r o w t h , " Sach's " H i s t o r y of Botany to 1860" a n d its companion volume b y Reynolds G r e e n for the period to 1900. ( A w o r k w h i c h is quite different in style a n d scope b y T h . W e a v e r s also treats the first half of t h e twentieth century.) However, scientific discovery is not m a d e only at the volition of the investigator a n d the research worker, for it is also a product of the in- tellectual climate of the day, a n d it requires a setting w h i c h is necessary for successful advances to be m a d e a n d to be applied. T h e course of m i n e r a l nutrition since the seventeenth c e n t u r y is interesting in this connection.

T h e great w a v e of progress, v i r t u a l l y nonexistent t h r o u g h t h e Middle Ages, acquired a slow start w i t h V a n H e l m o n t . V a n H e l m o n t ' s classical experiment, i n t h e e a r l y seventeenth century, w i t h the growth of a willow twig m a y h a v e been anticipated b y Nicholas of Cusa in t h e fifteenth century, even as it was repeated b y Robert Boyle later in the seventeenth century. T h r o u g h the observations of J o h n W o o d w a r d

(1699) a n d others, progress gathered pace especially in the e a r l y n i n e - teenth century, a n d it has continued ever since. ( A short b u t useful account of e a r l y 18th c e n t u r y p l a n t nutrition a n d agriculture b y G. E.

Fussell is to be found in t h e Proceedings of the Chemical Society for J u n e 1960, pages 193-198.)

However, the time was especially ripe for developments in p l a n t nutrition to occur in western E u r o p e after t h e Napoleonic wars. D u r i n g t h e Napoleonic w a r s the prices of g r a i n soared so t h a t borderline lands w e r e brought into cultivation. I n t h e depression t h a t followed, the im- poverished economy a n d u n b a l a n c e d agriculture of western E u r o p e was revived b y the birth of the fertilizer i n d u s t r y a n d b y the m a r r i a g e of the science of chemistry w i t h agriculture. T h e population increase, which was to be stimulated b y t h e Industrial Revolution, placed even greater d e m a n d s on agriculture. Boussingault in F r a n c e , Liebig i n Ger- m a n y , Lawes a n d Gilbert in Britain, all w e r e influenced b y t h e n i n e - teenth c e n t u r y trend toward, a n d t h e search for, a m o r e efficient agri-

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culture through a knowledge of t h e m a n u r i a l a n d crop rotation practices t h a t would give the best response in terms of p l a n t growth. T h e role of nitrogen in m a n u r i a l practice, t h e i m p o r t a n c e of legumes in a p l a n of crop rotation, the p a r a m o u n t importance of Ν , Ρ , Κ in artificial ferti- lizers a n d t h e foundations of soil microbiology w e r e all to be well ap- preciated b y the end of t h a t century. Boussingault's quantitative field experiments, Liebig's ill-fated artificial fertilizer, w e r e as m u c h in t u n e w i t h t h e needs of the times as L a w e s ' m o r e successful v e n t u r e into the solubilization of rock phosphate as superphosphate of lime. T h e cele- brated p a r t n e r s h i p of Lawes a n d Gilbert w a s to study the application of t h e n e w chemistry to agriculture. But it w a s Sir J o h n Lawes, using his family estate a n d t h e income from the n e w fertilizer industry, w h o far-sightedly installed, in perpetuity, t h e L a w e s plots at Rothamsted to demonstrate t h e responses of the g r o w t h of plants to specified m a n u r i a l practices.

It h a s been said, however, t h a t p a r t of the pressure t h a t prompted this development b y L a w e s a n d Gilbert w a s a n e w imbalance in a long established economy between London a n d its agricultural environs. T h i s economy stressed sheep as t h e source of m e a t a n d root crops to feed the sheep over t h e winter. F a r m produce reached t h e city in horse- d r a w n carts, a n d t h e p r e d o m i n a n t l y horse-drawn transport of t h e city furnished r e t u r n loads of stable m a n u r e to fertilize t h e fields. W i t h the rise of population i n t h e vicinity of London, this precarious balance be- came disturbed, a n d alternative m e a n s to stimulate the growth of crops needed to be sought. It was in this atmosphere t h a t the contributions of Lawes a n d Gilbert w e r e to be m a d e . W i t h the later use of sand a n d n u t r i e n t solution techniques, t h e elaboration of the ten essential ele- m e n t s , well k n o w n b y t h e t u r n of t h e century, a n d with the furnishing of these elements in the simplest m i x t u r e s of salts (calcium nitrate, potassium dihydrogen phosphate, m a g n e s i u m sulfate, with a little iron) science seemed to h a v e largely closed the book of p l a n t nutrition b y the end of t h e n i n e t e e n t h c e n t u r y a n d the first decade of t h e twentieth.

However, d u r i n g a n d after the First W o r l d W a r , p l a n t nutrition profited from the great stimulus to chemistry w h i c h t h a t scientific pe- riod fostered. W h e n G e r m a n y was cut off b y sea power from Chilean sources of nitrate, h e r agriculture was m a i n t a i n e d b y chemical fixation of atmospheric nitrogen b y t h e H a b e r process, w h i c h received its first great impetus at this time. Indeed, it was in this postwar period t h a t the knowledge of trace elements [that is n u t r i e n t elements needed in such small a m o u n t that, as foreshadowed b y M a z e ( 1 9 1 4 ) , t h e y h a d been overlooked i n t h e erstwhile list of ten essential elements] became known. I n the period after the Second W o r l d W a r p l a n t physiology

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6 F . C. S T E W A R D

responded to the stimulus from physics a n d physical chemistry, w h i c h was to be a distinctive feature of t h a t time. T h e search for sources of power a n d of e n e r g y was n o w p a r a m o u n t . W a r s a n d the needs of in- d u s t r y h a d p l u n d e r e d the fossil fuels or stored products of t h e photo- synthesis of bygone days, a n d t h e so-called population explosion called in question the ability of conventional agriculture to feed the world population. I n p l a n t physiology at this t i m e there was a h e a v y pre- occupation w i t h t h e need to u n d e r s t a n d photosynthesis as t h e m e a n s b y w h i c h plants utilize the e n e r g y of the sun a n d also to u n d e r s t a n d the w a y t h a t energy, once stored, is applied to biological work of all kinds. T h e recognized importance of the expanse of the oceans in the total fixation of solar e n e r g y led to such ideas as those of " f a r m i n g of the seas" as sources of food to meet m a n ' s needs. Also, in this produc- tive period the n o w available radioactive isotopes soon penetrated into all branches of nutritional a n d metabolic study.

T h u s , p l a n t physiology a n d the study of p l a n t nutrition has re- peatedly responded to t h e t r e n d of t h e time. Its progress has likewise interacted w i t h t h e fluctuating balance between agriculture a n d in- dustry, between u r b a n a n d r u r a l societies a n d w i t h t h e onset of popula- tion pressures. These m o r e general implications merit some further c o m m e n t below.

Inorganic Plant Nutrition: Its Place in the Economy of Nature and of Man P l a n t s are still t h e u l t i m a t e source of organic nitrogen for both m a n a n d beast. Agriculture—i.e., p l a n t a n d a n i m a l h u s b a n d r y — t u r n s in- organic nitrogen into usable protein. T h u s t h e inorganic nutrition of crop plants has been dominated b y nitrogen, though even today—despite the efficiency of agriculture a n d of artificial nitrogen fertilizers—much of the world's population is protein poor. Despite all m a n - m a d e m e a n s to refurnish nitrogen in forms chemically fixed from the air, t h e biological m e a n s of r e t u r n i n g p l a n t a n d a n i m a l waste t h r o u g h the nitrogen cycle a n d the biological m e a n s of nitrogen fixation are b y far t h e most im- portant. I n this respect the standards of W e s t e r n u r b a n civilization, w h i c h r e t u r n s so m u c h n u t r i t i o n a l wastes eventually to t h e sea, pre- sents a constant drain u p o n the nitrogenous reserves of the soil. Since a n acre of shallow sea m a y furnish a n n u a l l y a m o u n t s of organic m a t t e r w h i c h are the rough equivalent of t h a t produced b y a n acre of arable land and, since t h e seas occupy so m u c h of the earth's surface, thought is n o w being given to the seas as t h e solution of m a n ' s food problems.

Phosphate a n d nitrate, replenished b y t h e rising currents from great depths, are often in limitingly low concentrations in the shallow seas, a n d ideas of " f a r m i n g the sea" in landlocked shallow bays are p e r h a p s

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no m o r e visionary t h a n those of hydroponics a n d of large scale algal culture. N o n e of these ideas, w h i c h contemplate the large-scale growth of food plants in w a t e r a n d w h i c h h a v e been p r o m i n e n t l y suggested in t h e twentieth century, would h a v e seemed either feasible or necessary in a n earlier day. But t h e r e was t h e n less awareness t h a n n o w t h a t m a n ' s reproduction m a y soon tax t h e earth's resources a n d t h a t also m a n m a y shortly v e n t u r e into space. T h u s p l a n t physiology a n d the inorganic nutrition of plants is fraught w i t h intensely practical appli- cations which are closely bound u p w i t h t h e destiny of m a n .

Additional to the large area of t h e earth's surface (approximately four-fifths) w h i c h is occupied b y sea a n d to t h a t w h i c h is limited for conventional agriculture either because it is desert, or too cold or too m o u n t a i n o u s , there are still vast areas w h i c h are occupied b y forest.

Indeed, forest trees m a y compose about 8 0 % of the living m a t t e r on land. Prior to m a n ' s intervention, a large p a r t of the N o r t h T e m p e r a t e Zone was in fact occupied b y a climax forest vegetation. F r o m the early exploitation of t h e oak forests in Britain for shipbuilding (to furnish Britain's traditional "wooden-walls") a n d for t h e later smelting of iron ore, to the wholesale cutting of t h e N o r t h A m e r i c a n forests in this cen- t u r y , t h e balanced nutrition of the climax forest has been disturbed, and one m a y note t h a t the timber i n d u s t r y removes at one harvest even m o r e of the accumulated fertility of t h e forest t h a n a conventional an- n u a l agricultural crop would do. W h e r e a s p l a n n e d rotational a n d fertilizer practices in food crop production a r e ancient, t h e knowledge a n d the economical practice of the n u t r i t i o n of forest trees a r e still rel- atively i m m a t u r e . T h u s , in the full use of the energy of t h e sun to meet m a n ' s needs, the n u t r i t i o n of forest trees has a role w h i c h is still to be perfected. Indeed, the same is also t r u e of t h e full use of vast areas of tropical land. I n both these great a r e a s — t h e nutrition of forest trees a n d of tropical plants a n d vegetation—knowledge is still meager.

T h e balance between agriculture, as the source of food, a n d i n d u s t r y as the m e a n s of satisfying m a n ' s technological needs has loomed large in h u m a n affairs ever since the Middle Ages. T h i s a n d t h e prevailing standards of u r b a n a n d r u r a l civilization h a v e h a d their implications in relation to p l a n t nutrition. I n the fifteenth a n d sixteenth centuries t h e open fields gave place to inclosures and, because of the w e a l t h in wool, the landlords of Britain gave over their l a n d largely to sheep, so that measures w e r e enforced to curb the conversion of arable land to grazing for sheep. " B y the 39th y e a r of t h e reign of Elizabeth (1597) arable land m a d e pasture since 1st Elizabeth (1558) shall be again t u r n e d into tillage, a n d w h a t is arable shall not be converted into pasture." T h i s quotation shows a n early a t t e m p t to stem the inroads

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of technology u p o n t h e food of m a n a n d to balance agriculture a n d i n d u s t r y i n the economy. T h e first references to t h e fattening of sheep on t u r n i p s in w i n t e r a n d to t h e beneficial effects of a n i m a l d u n g to im- prove t h e efficiency of food production occurred t o w a r d t h e end of the seventeenth century. Clover a n d probably also turnips w e r e introduced to Britain from H o l l a n d about 1652 b y a Sir Richard W e s t o n to increase t h e efficiency of agricultural operations, a n d h e is said to h a v e described, w i t h startling accuracy in t h e light of m o d e r n knowledge, h o w to grow a stand of clover o n a light h e a t h soil after it was cleared, b u r n t , a n d lime was added to t h e ashes. After several years of cropping t h e clover, the l a n d would t h e n yield well in w h e a t for several years more! T h e balance between industrial a n d agricultural technology has n o w s w u n g far i n t h e other direction since, p a r t i c u l a r l y i n t h e U n i t e d States, effi- cient control of n u t r i e n t s u p p l y — p a r t i c u l a r l y of n i t r o g e n — a n d an efficient mechanized agriculture p e r m i t a v e r y small fraction of the population to produce food in sufficient, even excessive, quantities for the whole population.

But as Britain became ever m o r e intensely industrialized it became less a n d less self-sufficient until, prior to t h e First a n d Second W o r l d W a r s , Britain depended m o r e u p o n its p e r m a n e n t l y established grass lands t h a n u p o n its arable lands. I n such a situation t h e imported fertility from other lands, in t h e form of grain, supported both m a n a n d beast; t h e latter w e r e fattened a n d fed to convert m u c h imported plant protein, somewhat inefficiently (about 1 5 % ) , into a n i m a l protein; and, after h u m a n consumption, m u c h of this fertility was destined for the sea u n d e r W e s t e r n systems of sanitation a n d hygiene. Such a n expen- sive agricultural practice a n d imbalanced economy can be supported only b y a rich c o m m u n i t y w h i c h is able to export t h e product of its industry. However, experiments m a d e in G e r m a n y in the i m m e d i a t e postwar period showed t h a t certain p l a n t sources of protein w e r e en- tirely adequate as a substitute for milk in t h e feeding of infants, espe- cially if it is fortified b y t h e addition of methionine a n d lysine. M o r e - over, the postwar t r e n d even in Britain has been to replace m u c h of the imported grain for livestock b y high protein grass, harvested early and kept well nourished directly b y the use of n i t r a t e a n d phosphate a n d lime u n d e r a so-called " l e y - f a r m i n g " system. T h u s the m a x i m u m use m a y n o w be m a d e of well-nourished p a s t u r e w h i c h is grown especially for its high content of leaf protein. W o r k is also u n d e r w a y to m a k e , from t h e harvested foliage, a nutritionally effective source of leaf protein even for h u m a n s which, if necessary, m a y b e supplemented b y the critically limiting a m i n o acids such as m e t h i o n i n e a n d lysine. Although this is a still somewhat visionary possibility of solving the food prob-

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lems of large populations, nevertheless t h e inefficient conversion of leaf protein to a n i m a l protein for h u m a n n u t r i t i o n m i g h t eventually be cir- cumvented in this w a y . T h u s w e can see t h a t t h e food chain links the fertility factors t h a t d e t e r m i n e t h e growth of plants, as regulated b y supplies of inorganic n u t r i e n t s , to t h e state of balance or imbalance between agricultural a n d industrial production a n d to t h e nutritional status of W e s t e r n u r b a n communities w i t h their h i g h protein require- ments.

Certain regions of southwest E n g l a n d — f o r long t h o u g h t to be u n - suited to cattle—are n o w k n o w n to produce pasture w h i c h is toxically rich i n m o l y b d e n u m , a condition w h i c h is paradoxically aggravated b y

" i m p r o v i n g " t h e pasture w i t h clover b u t w h i c h m a y be alleviated by the use of a m m o n i u m sulfate to discourage t h e clover a n d to foster the growth of grasses. Also, large areas of Australian pasture, hitherto deficient i n traces of m o l y b d e n u m , h a v e been brought into m o r e efficient production b y supplying this essential n u t r i e n t . T h u s , t h e late discovery of t h e role of m i n u t e a m o u n t s of m o l y b d e n u m in p l a n t nutrition, w h i c h m a y seem academically remote from the considerations t h a t determine the complex balance between a n industrial population a n d its food supply, nevertheless plays a p a r t in t h e over-all dependence of m a n a n d his society on t h e nutrition a n d growth of plants.

T h u s science has come a long w a y from J o h n W o o d w a r d ' s (1699) insistence t h a t some sort of terrestrial m a t t e r determined the growth of m i n t sprigs! But as m a n embarks u p o n t h e space age, his nutritional problems are once again being posed in u n f a m i l i a r t e r m s ; these prob- lems m a y be left to t h e future to solve. However, for a n y k i n d of con- tinuously balanced system of m e n in missiles, or on space platforms, the inorganic nutrition of plants in all its ramifications will be needed to harness light energy to m a k e carbohydrates a n d t h e n c e to convert inorganic n i t r a t e into protein.

Some Modern Concepts and Future Trends

I n the n i n e t e e n t h c e n t u r y the cell doctrine a n d the study of cells a n d organisms—with t h e i m p e n d i n g rise of genetics—produced unifying concepts t h a t permeated the whole of biology. Some n o w familiar aphorisms gave expression to essential truths, as it was seen t h a t all cells came from preceding cells, all nuclei from preceding nuclei, etc.;

a n d t h a t self-duplication is a n i n h e r e n t characteristic of the w a y cells grow a n d divide. W h i l e cell biology in general profited greatly from these broad generalizations, the students of p l a n t nutrition, for a while, seemed to become bogged down in a search for a fastidiously prescribed

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10 F . C . S T E W A R D

"best" n u t r i e n t solution for this or t h a t plant. Indeed, long before the full r a n g e of variables a n d p a r a m e t e r s was properly realized, there was a somewhat sterile a t t e m p t to control the osmotic pressure of n u t r i e n t solutions a n d to v a r y only the relative proportions of those three k n o w n m a i n constituents of culture solutions, n a m e l y the salts calcium nitrate, potassium dihydrogen phosphate, a n d m a g n e s i u m sulfate, to w h i c h a small a m o u n t of a n iron salt was added. I n retrospect this approach monopolized far too m u c h t i m e a n d effort, u n t i l b y the greater use of statistical methods it was shown about 1921 t h a t m a n y of t h e supposed differences between the growth in the different solutions w e r e often not statistically significant.

For t h e next great w a v e of development the science of p l a n t n u t r i - tion was to be enriched b y t h e stimulus of enzymology a n d b y concur- r e n t developments in genetics. T h e gene-enzyme hypothesis of Beadle and T a t u m ; the accumulated knowledge of proteins as e n z y m e s a n d of their regulatory role i n metabolism; t h e purification a n d crystallization of e n z y m e proteins, all consolidated the view that certain metals, k n o w n to be essential in trace quantities for the growth of plants, could owe their essentiality to their role in metalloproteins w h i c h also function as enzymes concerned w i t h some reaction w h i c h is essential for growth or metabolism—so m u c h so that a n e w metal, found to be essential for growth, n o w leads almost inevitably to the first presumption that it m a y function b y virtue of its relation to a n e n z y m e . Nevertheless, despite the stimulus of this m o d e r n approach, there are still trace ele- m e n t s whose essential role is not yet adequately explained—for ex- ample, boron.

But some developments t h a t m a y well determine m u c h of the future trend of research w e r e slow, a n d still are slow, to come about. For a long time the inorganic nutrition of plants seemed to require r a t h e r fixed n u t r i e n t s in fixed amounts, at least above some ill-defined m i n i - m u m . T h e idea t h a t there is no universally applicable n u t r i e n t require- m e n t to cover all e n v i r o n m e n t a l conditions a n d all phases of plant development was seemingly slow to emerge. Also, the need to see the importance of the n u t r i e n t elements not m e r e l y in terms of their indi- vidual a n d separate actions, b u t also in terms of their interactions with each other a n d w i t h climatic a n d e n v i r o n m e n t a l conditions, is a still emerging b u t potentially v e r y i m p o r t a n t concept. Interactions a m o n g potassium, nitrogen, a n d light w e r e p r o m i n e n t l y noted years ago ( 1 9 3 5 ) ; a n d interactions a m o n g nitrogen, phosphorus, a n d respiration were also seen in the same general period. For one reason or another, such pairs of factors as calcium a n d boron; copper a n d m o l y b d e n u m ; iron a n d m a n g a n e s e ; zinc a n d insolation; need to be considered to-

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gether, because t h e y h a v e interacting effects w h i c h suggest t h a t t h e y impinge u l t i m a t e l y u p o n the same site of metabolic action.

But w h y w a s it ever supposed t h a t t h e inorganic n u t r i e n t require- ments of plants a n d of their constituent cells are fixed irrespective of t h e conditions t h a t affect their growth a n d development? N u t r i e n t r e q u i r e m e n t s a r e c o m m o n l y held to begin with t h e seed, b u t does not this neglect t h e all-important development of the zygote in the ovule a n d its consequential dependence on its p a r e n t sporophyte? W h y should all cells of the p l a n t body, despite their variety of form a n d function, be assumed to r e q u i r e t h e same essential n u t r i e n t s as t h e whole plant?

Do such morphologically distinct plants as a long- or a short-day plant, as a high- or a low-night t e m p e r a t u r e plant, require the same n u t r i e n t s in t h e same concentrations? W h y indeed should n u t r i t i o n h a v e ever been regarded as a r e q u i r e m e n t w h i c h is fixed t h r o u g h o u t develop- m e n t ? T o t h e extent t h a t these problems become obtrusive, questions of t h e mobilization of specific n u t r i e n t s in the different regions a n d organs of t h e p l a n t body also arise; this also involves those problems of uptake a n d accumulation of particular ions b y cells, as well as t h e m e c h a n i s m of their transport, w h i c h are dealt w i t h in V o l u m e I I . T h u s there is still m u c h room for n e w w o r k a n d n e w discovery, b u t w o r k in this field poses some especially difficult logistic problems.

E v e n after t h e problem of interacting effects is recognized a n d it is also granted t h a t t h e criteria of n u t r i e n t action should be extended to include the full r a n g e of developmental a n d metabolic processes t h a t m a y be affected b y n u t r i t i o n (even w h e n visible symptoms of abnor- m a l i t y are not a p p a r e n t ) , t h e r e is still a real dilemma. H o w should one design the experiments, collect all the necessary data, a n d t h e n inter- pret t h e m in such a w a y t h a t d u e weight is given to all the p a r a m e t e r s of this complex system a n d to t h e factors w h i c h interact w i t h each other? T h e use of statistics a n d the design of experiments w h i c h will permit subsequent statistical analysis of the data are n o w conspicuous features of t h e c u r r e n t scene. T h e s e w e r e largely stimulated, initially, b y R. A. Fisher a n d b y those in p l a n t nutrition, notably b y F . G. Greg- ory a n d his school, w h o seized u p o n the significance of Fisher's m o n u m e n t a l work. But, nevertheless, the full complexity of the task t h a t faces those w h o would m a k e even further contributions to p l a n t nutritional knowledge m a y , even yet, not be widely or fully appreci- ated. As growth-controlling installations a n d climate-controlling devices come into general use in p l a n t physiology, the problems of the complex design in experiments w h i c h r e q u i r e a t e a m approach to t h e problems of nutrition will need to be faced and, no doubt, m o d e r n computing machines will also be needed to a n a l y z e a n d formulate w h a t all the

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12 F . C . S T E W A R D

data m e a n . Indeed, if the science of p l a n t n u t r i t i o n w e r e ever to be complete, would it not t h e n be feasible, in advance, to prescribe all t h e requirements a n d the responses of a given fertilized egg, or of a spore, throughout its subsequent g r o w t h u n d e r all conditions?

A n astonishing a m o u n t of c u r r e n t p l a n t nutritional knowledge derives from b u t a few economically i m p o r t a n t plants. I n fact, the essentiality of trace elements has been largely demonstrated for crop plants which a r e often g r o w n in habitats w h i c h a r e v e r y different from those to w h i c h t h e plants w e r e first adapted. T h u s , crop plants will often show field symptoms of nutritional disorders w h e n t h e adjacent native plants, or even t h e trees, show n o such signs. T h i s observation leads to the following considerations.

T h e inorganic nutrition of plants is essentially a function of the e n v i r o n m e n t d u r i n g their growth, even as it is of the plant in question.

It is also beginning to a p p e a r as a function of the genetic constitution of t h e plants involved. As recently as 1953 P o p e a n d M u n g e r found in- organic nutrition to be governed b y a single gene which regulated the r e q u i r e m e n t of celery plants for boron, w h i l e another gene determined the r e q u i r e m e n t for m a g n e s i u m . Such genetically determined m i n e r a l requirements a n d genetically determined nutritional levels m e a n t h a t constant watch should n o w be kept u p o n inorganic nutrition from this point of view. By mutation, or b y the work of plant breeders, n e w nutritional disorders t h a t can be corrected only b y the intervention of specific chemical elements m a y even be created. It would be interest- ing, for example, to re-examine the required trace element nutrition of a wild, still uncultivated species compared w i t h t h e derived varieties and strains t h a t have been bred from it to fit t h e m for practical use.

Inorganic plant nutrition, therefore, n o w impinges u p o n all other branches of p l a n t science, and the book of p l a n t nutrition that seemed about to be closed at t h e t u r n of this c e n t u r y n o w presents as m u c h challenge to the investigator, or r a t h e r to the t e a m of investigators, as at a n y period in the history of plant science.

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