Zea mays

Family : Poaceae

Text © Prof. Giorgio Venturini

 

English translation by Mario Beltramini

 

The Maize (Zea mays L., 1753) is a monocotyledon of the family of the Poaceae (Graminaceae).

The name of the genus Zea comes from the Greek “zea” (ζέα or ζειά) indicating the spelt, the wheat or other gramineous plants having edible seeds, whilst the specific name “mais” comes from the word “mahiz”  in the Taino language, or Arawakan” of the islands, that in the past was spoken by many Caribbean populations.

We must highlight that the maize does not exist in the wild but originates from a wise work of selection and of cross breeding done by the pre-Columbian populations of Central America starting from wild ancestors whose identity is still now subject of discussion.

In the genus Zea are included six species whilst the species Zea mays includes four subspecies (Zea mays huehuetenangensis, Zea mays mexicana, Zea mays parviglumis and Zea mays mays).

Zea mays mays is the domesticated form, cultivated in all the world, whilst the other three subspecies, morphologically and physiologically quite different from the latter, are considered as standing among the possible ancestors of the cultivated maize. In Italy, depending on the regions, besides the name of maize are utilized various names such as, for instance, granoturco, frumentone, formentone, sorgoturco, granone, granturco, meliga, grano d’India, formentazzo.

With more than one billion tons it is presently, as annual world production, the first of the cereals, followed by the rice and wheat even if, unlike the other two cereals, an important part is intended for different uses from that of human feeding. First producers in the world are the USA, followed by China, but practically this plant is grown all over the world and represents a food and economical resource of primary importance.

Morphology and reproduction

The Maize is a big herbaceous plant measuring commonly 1,5-3 m of height. Some very early varieties however can be only 90 cm tall and some pop-corn maizes do not exceed 30-50 cm, whilst in sub-tropical and tropical regions, some varieties may reach the height of 6-7 m or even 10 m.

The plant of maize presents many characters common to the other Poaceae: like in bamboo or the cane the stalk, called also stem, or culm, is divided in nodes and internodes; each node supports one single leaf and the leaves are alternate, distributed on the culm in two opposite rows. The basal nodes tend to form ramifications or tillering culms (basal shoots) and develop adventitious roots.

The stem, in the varieties cultivated in Europe, has usually a diametre of 3-4 cm and has 8 to 21 internodes.  The internodes are close together and their diametre is bigger at the base of the plant, whilst are more spaced in the upper part. The number of the leaves stands between 8 and 48, considering also those of the possible suckers, but usually varies between 12 and 18. The length of the leaves is 30 to 150 cm and the breadth may reach 15 cm. Each leaf presents three distinct parts: the sheath, that embraces almost completely the internode towering over the origin node and is well visible starting from the sixth leaf stage when begins the culm elongation; the hem or lamina, that represents the real leaf, of lanceolate shape with parallel longitudinal veins of which the median is the biggest; the ligula, a laminar expansion resembling a colourless and pellucid membrane, placed between sheath and hem, wrapping strictly the stem obstructing the entry of water or possible parasites and allowing the more or less horizontal position of the lamina.

The maize is a monoecious diclinous plant: this means that the plant bears male flowers as well as female ones, however contained in separate inflorescences. As the two sexes flowers of the same plant ripe in slightly different moments, usually a self-fertilization does not occur except in a very few cases (estimated less than 5%).

The common denominations of the female inflorescence and that of the male are wrong from a botanical terminology point of view: the female inflorescence commonly defined panicle, is actually a ear, or, better, a spadix, whilst the male inflorescence, apical, usually called spike or pennache, is to be defined a ear or panicle.

Adapting to the custom here we shall use the common name of ear for the female inflorescence, for instance that carrying the caryopses, that is the edible grains (seeds or granella).

The male inflorescence is located at the apex of the plant and has more or less numerous ramifications that bear many spikelets.

On each spikelet stand two flowers, each with three stamens, formed of a filament and an anther that when ripe protrudes from the flower. Often one of the two flowers is abortive. In total, the male inflorescence bears 1000 to 2000 flowers. Since each stamen produces thousands of grains of pollen, each plant can produce many millions of them.

The female inflorescence, commonly called panicle, is placed at the axil of a leaf, with a short peduncle with very close nodes. From each node of the peduncle stands out a bract, that is a modified leaf that wraps the inflorescence. The whole of these bracts form the cornet.

The number of ears produced by each plant is very variable depending on the variety  and on the cultivation conditions. In most cases only one panicle is produced, or two, but particular cultivars can produce even 10 or even more panicles, like in the case of the baby corn used for salads or appetizers.

Usually, along the stem can grow summary and incomplete forms of female inflorescences at the foliar axil of each of the nodes up to the twelfth or fourteenth of them, but normally only that (or those) on top will completely develop to form a panicle. Nowadays, with respect to the past, it is easy to find even more ears as, on this character, very much has affected genetic improvement.

The ear has a central axis roughly cylindrical usually 10-30 cm long (in some instances up to more than 40 cm), called corncob, carrying the spikelets, inserted in a more or less regular way in vertical rows. The rows are in variable numbers depending on the cultivar, usually 8 to 24. The total number of spikelets, and consequently of potential caryopses, varies from some few hundred to 1000 about. Each spikelet contains an ovary and from each ovary gets out a very long style, the beard or silk, ending in the stigma, about 1 cm long. The silk is provided with very thin hairs (trichomes), particularly abundant on the stigma, having the function of holding the pollen.

The silks of beards begin to develop in the ovaries closest to the base of the panicle and then gradually in the most apical, therefore the first silks emerging from the cornet are the basal ones.

The enormous length of the style, that may exceed 30 cm, must make us reflect on the fact that, at the moment of the pollination, from the pollen grain will have to get out a pollen tubule able to go through the entire style until will reach the ovule where it will take the sperm cell. The fecundated ova will be the edible caryopses of the maize.

Pollination and fecundation

The male and female blooming are staggered over time: the release of pollen made by the male inflorescence precedes by two or three days the appearance of the female styles (it’s a proterandrous species). The time lag between the ripening of the male inflorescence and the female one renders very rare the self-fecundation, that however may be realized artificially. The release of the pollen usually lasts one week, more rarely up to two weeks, with a peak of releasing around the third day from the beginning. The diffusion of the pollen granules, emitted as already mentioned in a huge number, occurs thanks to the wind and to the gravity (anemophilous pollination). Because of their relatively high weight, the pollen grains usually do not move for distances of more than 10-20 metres from the plant that has released them. Only a very few grains, in case of strong wind, may be carried away for 100-200 m.

Usually, the silks emerge from the cornet in 2-3 days from their birth from the ovary, but, in case of very long panicles, the time needed may be greater and for this reason the apical part of the panicle may result scarcely pollinated. The silks elongates of 2-3 cm per day and go on growing until when they are pollinated or till when they get old. Normally a single silk remains vital for about 10 days, but since the silks do not emerge simultaneously, in each panicle we shall be able to find pollinable silks for about 14 days. The delayed appearance of the apical or basal silks entails the fact that often in the ear we shall find more frequently fertilized the basal ova and less the apical ones.

The silks are receptive as soon as they emerge from the bracts. When the pollen adheres to the humid surface of a silk, it is captured by the trichomes, thin hairs  present in the stigma, and begins to germinate forming the pollen tube that penetrates the style and goes down in the the vascular tissue towards the ovary it usually reaches within 24 hours. Often the grains are transported by the wind on the leaves but later on roll away until when intercepted by the silks. The grains of pollen reaching the style adhere thanks to a mechanism of chemical species-specific recognition. After that at least one grain of pollen has anchored on the silk are still necessary various important steps for reaching the fecundation. The pollen grain contains two cells, one of which generates the pollen tube, whilst the other divides producing two sperm cells that, moving through the tubule, reach the ovary. Here one of the two sperm cells will fecundate the ovule producing the embryo, whilst the other will join the two female polar nuclei to produce a triploid cell that will form the endosperm of the caryopsis, that is the accumulation of food reserves (practically what we nourish of when we eat the kernels). This phenomenon is called double fecundation and is typical to the angiosperms.

Even if the pollen grains may germinate on any silk, only one pollen tubule manages to fertilize the ovule. In one or two days the pollinated silks dry up and turn brown. The pollination is a continuous process and interests the silks as soon as they emerge. The environmental conditions affect reproduction. The duration of the emission of the pollen as well as its quantity are affected by the climate conditions. Usually the emission begins in the morning, with the increase of the temperature, and lasts for about 8 days in each inflorescence.

Due to the individual variability between plants in a same cultivated field, the overall emission may get abbreviated, in fact due to their enormous length and thinness the silks are very exposed to the loss of water, which may compromise their functionality in the fertilization. Moreover, not always fertilization leads to the production of a kernel. If in the weeks following the fecundation, due to not favourable environmental conditions the photosynthetic activity avers reduced, the fecundated ova may abort. This occurs more frequently in the youngest ova, placed at the apex of the panicle. It is possible to distinguish the abortive kernels from the non fecundated ova from the presence in the first ones of an accumulation of starch that, conversely, is absent in the second ones.

Origins of maize

After archaeological studies the history of the modern maize dates back to the beginnings of agriculture, about 9.000-10.000 year ago. Probably were the first farmers of a region of the present Mexico, the valley of the Rio Balsas, who started a process of improvement of a spontaneous cereal or of a hybrid. These farmers most likely did select for the sowing the kernels of the plants with the most desirable characteristics, gradually getting plants with bigger ears, with more seeds and more easily cultivable, until modern maize was generated. From the origin zones the maize has rapidly expanded northwards, in the southern regions of the present USA, as well as southwards, crossing the Isthmus of Panama, probably about 7500 years ago. The maize was  amply diffused in the northern part of South America around the 6000 years.

Archaeological data suggest that the main changes that originated the modern maize had already occurred at least 6500 years ago.

The diffusion of the maize from Rio. Balsas regions northwards as well as southwards has been possible due to its capacity to adapt to very diversified environmental conditions. Its adaptability is testified by the fact that now it grows in much more ample range than any other cultivated plant.

The identity of the ancestor of the maize has remained unknown for centuries, since in the wild we do not know any plant really looking like the modern maize, contrary to what happens for the rice or the wheat.

Thanks to studies done starting from the 30s of the twentieth century, corroborated by genetic studies, has been identified the possible ancestor in the Teosinte, a plant or, better, a group of species, present in the wild in America. The word teosinte, “teocintli” in the language Nahuatl of the Aztecs meaning “holy panicle” (teotl = holy, sacred, cintli = dry panicle).

Many scholars use to indicate as Teosinte all species and subspecies of wild Zea growing in Mexico and in other Central American countries, that is Zea diploperennis, Zea perennis, Zea luxurians and the three wild subspecies of Zea mays, considered as the real Teosinte: Zea mays huehuetenangensis, Zea mays mexicana, Zea mays parviglumis. Zea mays mays  is the cultivated maize.

The Teosinte are morphologically quite different from the cultivated maize: in fact the differences are as such that in the past the taxonomists had attributed the two plants to different genera. The female inflorescence of the Teosinte produces only 5-12 caryopses, covered by a very hard tegument (pericarp) rich in silica and in lignin and that spread spontaneously when ripe, whilst modern maize has several hundreds of them, without hard involucre and strongly adhering to the corncob in such a way that they cannot disperse. For this reason the maize is strictly dependent on man for the dissemination and for the protection from the predators. From a genetic point of view the two species are however surprisingly similar, have the same number of chromosomes, with analogous placings of the genes and the DNA sequences are very similar. Furthermore, maize and teosinte can hybridize producing fertile hybrids.

Even if presently maize is the most cultivated plant in the world and therefore amply studied, biologists have not yet been able to explain how a wild grass with a small spike, with few very hard seeds that disperse spontaneously, could transform in the modern maize, that produces hundreds of big chewable kernels and that remain adherent to the corncob. With the domestication, the maize has lost the capacity of self-propagation and consequently has become dependent on man for survival. Archaeological data do not help in solving the problem, as the oldest findings of maize already have most of the fundamental characters of the domestic maize. A hypothesis foresees that the maize has developed starting from the teosinte Zea mays parviglumis by selection of a series of cumulative mutations affecting a few genes. It is also possible that other species of teosinte have contributed to the genome of the modern Maize with the production of hybrids that should have been selected by the old farmers (some varieties of maize contain numerous genes derived from Zea mais mexicana). Still nowadays in Mexico many farmers sow some teosinte along the borders of the fields of maize as they are convinced that the pollen of these plants may fecundate the maize and improve its resistance to the pathogens.

An alternative hypothesis, advanced some decades ago and recently taken back into account, proposes that the maize was born after the selection done by the man of spontaneous hybrids between the teosinte and another perennial Poaceae related to it that grows spontaneous in a vast range between North and South America, the gamagrass (Tripsacum spp.) Several species of  Tripsacum do exist and are utilized as fodder. The seeds have a high protein contents and during the last years they are developing the cultivation of these plants for the production of high quality flours and of oil destined for human feeding.

From the crossing Tripsacum dactyloides and one teosinte (Zea diploperennis), are obtained some fully fertile hybrids endowed with morphological characters very similar to the oldest archaeological finds of domestic maize. These hybrids may in turn hybridize with maize, what allows a gene stream between Tripsacum and maize that could generate important improvements in the cultivation of the maize, in terms of a major resistance to dryness or to the diseases and of a better protein content of the caryopses. Also molecular analyses that prove the presence in the maize of DNA sequences typical to the teosinte and of others typical of the Tripsacum favour this hypothesis.

What have been the main changes that have transformed the teosinte in maize?

The teosinte disperses freely the pollen and the caryopses: these fell on on the ground where they may take root and also, if swallowed by some animal, thanks to the hard and not digestible skin (pericarp), will be dispersed with the faeces.

With the domestication the teosinte has maintained the capacity of dispersing the pollen in the air, but has modified other characters that rendered it more suitable for human use. The skin of the grain, rich in silica and in lignin, has become softer and therefore chewable, the diametre of the corn cob has increased carrying in such way a greater number of rows of caryopses and these are not dispersed any more, but keep stuck to the corn cob requiring the human intervention to be detached. The bracts, that is the cornet, are more robust and avoid that birds and other predators may nourish of the kernels. Whilst the teosinte presents numerous ramifications each one ending in a male inflorescence, the maize lacks totally of ramifications and has only one inflorescence.

The teosinte is very sensitive to the duration of the day and blooms only when the days are short. The acquisition of plants capable of blooming independently  from the duration of the day has represented an important progress for the domestication of the maize, which has allowed its diffusion in the various latitudes.

Genetics studies suggest that changes borne by a small number of genes are responsible for the transformation of the teosinte in a primitive domestic maize. It is thought that in the history of the domestication of the maize the most important transformations have appeared early, generating a common ancestor of the various modern varieties, that later on has differentiated only for details concerning especially number and dimensions of the caryopses and adapatatiins to different climates and soils.

Among the main genes involved we remember tb1 (teosinte branched1), so called because the mutant maizes present a great number of ramifications that culminate with male inflorescences, like in the teosinte.

A second important gene is ba1 (barren stalk1), involved in regulating the ramifications, and that most probably has allowed the bushy teosinte to be transformed in the modern maize with long stem, probably interacting with tb1. In the teosinte are present various variants of this gene whilst in all varieties of modern maize is present only one form of it: after the scholars this suggests that the Mesoamerican farmers probably have used this characteristic, in combination with few others, to transform the teosinte in maize, one of the key events of modern farming.

Moreover, we have to cite the gene tga1 (teosinte glume architecture1)  that in the present form in the maize produces soft caryopses, whilst the form present in the teosinte originates hardened caryopses. tga1 has been recently cloned and the form present in the maize appears to be the outcome of a mutation of that of the teosinte. The release of the caryopses from the hard protecting involucre present in the teosinte, rendering them accessible for human consumption has been probably the most critical passage in the domestication of the maize.

The changes affecting the first two genes have therefore produced a plant not ramified and with numerous panicles, whilst those of the second  have produced relatively morbid kernels and consequently edible of the maize.

An important rôle is probably done by the gene ramosa1 that regulates the arrangement of the rows of kernels in the ear and its ramification: the first farmers should have selected plants with versions of the gene ramosa1 that suppressed the ramification of the panicle, thus obtaining straight rows of kernels and compact panicles like those of the modern plants.

A last but not least important difference between the teosinte and the modern maize stands in the friability of the spine, or corncob. In the wild cereals the rachis is friable and when the spike is ripe, it falls apart starting from the summit. This allows a good dispersion of the seeds and hence the reproduction of the plant. Mutant forms with non-friable rachis do not disperse the seeds with great benefit for the grower. However the plant gets dependent from man for the reproduction. This character is ruled by the genus already described tga1, that regulates the hardness of the caryopsis and consequently its adhering to the corncob, as well as from the genes btr1 and btr2 (brittle rachis) regulating the fragility of the corncob.

Besides those previously mentioned, other genes may have been interested by the selection during the process of domestication but, despite the big morphological and functional differences between maize and teosinte the two species share most of the sequences of DNA. The DNA sequence shows in fact that only 3% of the genome displays signs of having been modified during the domestication.

As proof of this observation is the fact that maize and teosinte are interfertile.

The possibility of a flow of pollen between the domestic maize and other wild teosintes has further contributed to its diversification and still continues, the origin regions, to increase its genetic diversity.

Actually, the forms typical to the maize of the cited genes are present also in the teosinte, albeit with modest frequency, consequently it can be understood how the omd Mesoamerican growers through selections and crosses have been able to get plants presenting with high frequency the desired characteristics.

Maize in Europe

The Europeans met for the first time the maize in Cuba, during the first voyage of Columbus, already one month after the discovery of America. It is not sure if Colombo brought the mize in Europe in 1493, when back from his first voyage, or in 1496 with his second one. From America, where it was widely cultivated and in many regions formed the main food of the population, it was introduced in Spain since the beginning of the ‘500 and then rapidly diffused in France and in Italy, before in the gardens as a curiosity and then in the orchards for the feeding of the peasants or as fodder. The diffusion was also favoured by the fact that the new product, unknown to the bureaucracies, was not subject to taxes.

Whilst in Spain the cultivation did not develop until recently, it seems that the first great diffusion of the production on a large scale has occurred in the Balkans where, thanks also to the favourable climate conditions, could be obtained double yields in respect to other cereals. The worldwide diffusion was rapd and, during the second half of ‘500 the maize was cultivated in the Philippines and in the East Indies and had reached China.

In Italy the large scale use began in the north-eastern regions, closer to the Balkans, and then all over the Po Valley. The popular name of granturco (Turkish wheat)could come rightly from its Balkan provenance , but most probably can be explained with the fact that as exotic plant was attributed not to America, place not yet entered in the imagination of the people, but to the exotic place par excellence, that isTurkey (think also to the turkey, of American origin, that was rightly christened in English “turkey”, that is “Turkey” and also the Italian name “tacchino” is a deformation of turkey.

Conversely, after other Authors the diffusion on a large scale in northern Italy was encouraged by the Spanish occupants.

A stimulus to the cultivation of the maize came also from the landowners who pushed the peasants to produce it for the auto consumption, reserving to the owner the most precious wheat.

The maize was cultivated with the technique of the “three fields”: one cultivated with maize and two with wheat. In this way the peasants obtained the products necessary for paying the taxes, that is the wheat, as well as what was necessary for feeding, that is the maize transformed into polenta.

The flour of maize took the place of the other flours, of spelt, barley or rye, used in the preparation of one of the most popular foods since the old remote times, that “poltos” (πολτοσ) of the Greeks, the “puls” of the Romans (“puls”,  genitive “pultis”, from which comes polenta and also mush).

About this food in 1568 the humanist and doctor Pietro Andrea Mattioli wrote that “the peasants living at the boundaries separating Italy and Germany make with the flour the polenta, that after having cooked in a mass, is cut with a thread in wide and thin slices and arranged on a plate with cascio or with butter and eat it quite greedily”.

In 1580 the maize is already sown in the open field especially in the sites of the lower Verona area, Polesine and Ferrara, where gradually replaces the cultivation of spelt and wheat; by the end of the century it extends to several areas of the Republic of Venice, to Piedmont, Romagna and the Marches. Presently the Italian regions with the greatest production of maize are Veneto, Friuli, Lombardy and Piedmont.

Towards the middle of the 17th century in northern Italy the cost of the wheat is of about the double than that of the maize and quite soon, with the diffusion of the already mentioned practice of the “three fields” (two with wheat to pay the rent and one with maize for feeding the peasant family), the flour of maize, becomes the fundamental (and often unique) food of the rural populations of central-northern Italy.

The owners of course favour this practice as the abundance of maize, increasing the food availability, allows the farm a greater marketing of wheat and rice. This diet based on polenta, lacking some essential nutrients, causes the diffusion of the “polenta disease”, the “sour skin” or pellagra.

In favour of the diffusion of the maize cultivation stands also its very high productivity (the yield in Italy nowadays may exceed 20 t/ha (tons per hectare), usually 9-12 t/ha, to compare with 25-90 q/ha (quintal per hectare) for the tender wheat and even less for the durum wheat. The plant of maize, apart from its extraordinary productivity in terms of granella, utilized in various forms in the human and animal feeding, produces about as many tons of stems, leaves and cornets destined to be interred as fertilizer or as fodder, litter for animals or fuel.

Also in the traditional agriculture the maize is utilized  in all its parts, not only for the previously mentioned applications but also as roof covering for the canopies, as padding for mattresses (with the bracts, commonly called cornets: maybe someone not so young remembers the very noisy but comfortable mattresses padded with maize cornets; in the bedroom there was also a biforked wood that was inserted in the padding for plumping it up). The corncobs besides as fuel are utilized for fabricating the pipes.

The granella is utilized for human or cattle feeding, fresh or dried, entire, broken or grinded, for the extraction of edible oil or for producing starch, eventually hydrolyzed to produce glucose syrup. The granella is also used for the production of alcohol-free drinks (in South America the chicha morada from black maize), or fermented for the production of alcohol or alcoholic drinks (in North America the bourbon, in South America the chicha de jora).

We must still mention the industrial utilizations such as, for instance, the production of plastic materials and of tissues starting from the starch of maize and especially the production of ethanol, as fuel for petrol engines, and of biodiesel. These last utilizations absorb an important part of the world production and are indeed responsible for its recent strong increase. This problem arouses concerns and controversies due to its influence on the cost of the maize for feeding use and for the subtraction of lands that are dedicated to the biofuels instead to food, with obvious consequences on the feeding of the most disadvantaged populations.

Varieties of maize

Very numerous varieties of maize are known, differently spread around the world (more than 50 varieties are cultivated only in Peru), often united in subspecies by some scholars.

Following a classification generally accepted, the subspecies are as follows:

Zea mays tunicata ( “pod corn”)

Zea mays everta ( “pop corn”)

Zea mays indurata ( “flint corn”)

Zea mays indentata (“dent corn”)

Zea mays amylacea ( “soft corn”)

Zea mays saccharata (“sweet corn”)

Zea mays ceratina (mais waxy)

It is interesting to discuss about the variety or subspecies Zea mays tunicata called pod corn (literally maize with pod): the kernels are not “naked” but present covered by long membranous sheaths, called glumes, that confer the panicle a very showy look that has intrigued naturalists for centuries.

Also the male flowers are surrounded by long sheaths. The rest of the plant has a normal look.

Several populations of American natives attributed to the pod corn some ritual meanings and consequently this variety has amply diffused in the American continent.

In the past some scholars have thought that the pod corn might be a wild ancestor of the modern cultivated varieties. Actually, during the recent years studies of genetics have refuted this hypothesis, demonstrating instead that this variety of maize comes from a mutation after which a gene responsible for the production of the leaves is active also in the inflorescences. The visible sheaths in the male flowers as well as in the female ones are nothing else than ectopic leaves that popped up in an anomalous site.

Maize as food for humans

The maize has an average contents of 9.4 g/100 g in proteins, 4,7 g/100 g fats, 74.3 g/100 g in carbohydrates, with a caloric intake of 353 kcal/100 g. Its feeding value is however inadequate due to the absence of some essential amino acids and of some vitamins. A feeding based uniquely on the maize or where the maize is predominant causes consequently severe states of malnutrition and pathologies like the pellagra.

Essential amino acids

Our body, in order to synthesize its own proteins, needs 20 different amino acids, some of which can be produced by our own cells whilst others must be necessarily introduced with the feeding; these last are defined as essential amino acids.

Their deficiency determines serious states of malnutrition, even mortal. Whilst the animal proteins, like those of the meat or of the milk give us all types of amino acids necessary, many vegetable proteins lack some of them. This is the case of the maize whose proteins are severely deficient of two essential amino acids, the tryptophan and the lysine.

The Pellagra

The pellagra is a serious pathology due to the deficiency of vitamin B3, also called vitamin PP (Pellagra Preventing).

With the term of vitamin B3 are meant two similar molecules, the Niacin or Nicotinic acid and the Nicotinamide: these substances are necessary for the production of cofactors (NAD⁺ e NADP⁺)  indispensable, that intervene especially in the reactions of oxidoreduction of our metabolism.

The symptoms of the pellagra are: severe de-epithelialization, that is desquamation of the skin of the hands and of the neck (hence the name pellagra, rough skin), diarrhoea, loss of appetite and weight, stress, red and swollen tongue, depression and anxiety (it is called the 3D diseases: Dermatitis, Dementia, Diarrhoea). If left untreated, the pellagra leads to death within a few years.

In the maize the niacin is present but in little bioavailable form, as linked to carbohydrates under the form of niacytin or to proteins under the form of niacinogens.

Actually, niacin may also be produced by our body starting from the amino acid Tryptophan that is however very deficient in the maize.

A diet based on maize, not integrated by other foods rich in niacin or in tryptophan, leads to the appearance of the pellagra.

Just think that among the peasants of some North Italy regions between late XIX century and the XX century beginning, feeding was formed by three kg of polenta per day, practically without integration of other foods.

To describe this situation it is related that in poor families the diners, in order to flavour their slice of polenta, did rub it in turn on a smoked herring that hung, attached to a string,on the centre of the table.

To this food shortage added the deleterious effects of quite diffused alcoholism, that causes an alteration of the intestinal absorption of vitamins.

The pellagra had a strong diffusion in North America and in Europe and especially in northern Italy between the XVIII and XX centuries. In 1878 in Italy there were about 100.000 cases.

After having understood that the pellagra was due to deficiency of vitamin B3 consequence to a feeding based only on mize, they wondered why this disease was practically unknown in South America, where the maize represented the base of feeding.

The answer essentially stands in the treatments to which the natives submitted the maize before consuming it: after various procedures the granella was heat treated with water and lime or water and ash, treatments that renders the niacin available detaching it from the molecules that render it not bioavailable.

This process, called nixtamalization (“nixtamal” in the Nahuatl language, spoken by the Aztecs, is a food made from maize treated with water and lime, “nextli”(ash or powder of lime) and “tamalli” (maize dough).

With the name of “tamales”, in many Latin-American countries, are indicated some maize flour rolls, variously seasoned, wrapped in a leaf of banana or of maize and steamed or grilled. Still today the procedure of nixtamalization is applied for the preparation of the dough of maize, for instance when preparing tortillas, as well as for the whole caryossids that are then consumed in various ways under the name of  “mote”. In many recipes after nixtamalization the maize is made to ferment, which further increases the bioavailability of the vitamin.

The conquistadores, in their blind arrogance, took the maize but despised the traditional practices of the natives.

Besides the practice of nixtamalization we have however to keep in mind that the South-American natives, though having a feeding based on maize, integrated it with other foods, among which especially beans, rich in niacin and in tryptophan. As proof of this variety of food, we must consider the traditional cultivation practices, that foresaw to plant together maize, beans and pumpkins (the three sisters): the maize with its robust stems gave support to the climbing plants of the beans, that, in turn, with their tendrils strengthened it, whilst the pumpkins, with their wide leaves protected the soil from the excessive evaporation.

Besides the beans, in the feeding of the American natives, even if very poor in meat and without dairy products, entered two pseudo-cereals very rich in essential amino acids, the quinoa (Chenopodium quinoa,  relative to the spinach) and the kiwicha (Amarantus caudatus).

Improvement of maize 

Since the twenties of the XX century are known varieties of maize containing proteins of high food value and in 1963 has been selected a mutant, called “opaque-2” rich in lysine and tryptophan, the two amino acids missing in the maize.

Unfortunately, however, this variety of maize had a poor productivity, was susceptible to various diseases; its caryossids were too soft and chalky and especially the taste and the consistency renders it unpleasant to the consumers. Consequently, its production was abandoned.

Around 1970, some researchers working in Mexico at the CIMMYT (International Maize and Wheat Improvement Center) resumed the study of this variety and in the mid 1980s were able to produce a variety called QPM (Quality Protein Mais) with excellent characteristics and high contents in lysine and tryptophan.

Starting from the 1990s the QPM is successfully cultivated in many countries, in Africa as well as in Asia and in America, with remarkable benefit for the nutritional status of the populations. We want to emphasize that it is not a matter of an OGM product but of the result of the selection  and cross of spontaneous mutants.

Hybrids of maize

Since the self-fertilization in the maize occurs only exceptionally, the cultivations of maize are formed by genetically heterogeneous individuals. Consequently the attempts to select better strains choosing for the sowing only the caryopses coming from the best ears have not given appreciable results, being not verifiable the male parent.

A notable success has been obtained with the so-called first generation hybrids.

Subjecting repeatedly to forced self-fertilization of the plants are obtained pure lines, from which, by selection, are eliminated the not desired recessive characters.

These pure lines, practically homozygote, lose vigour and productivity (inbreeding depression) but the cross of two different pure lines often originates hybrids that are all the same, very vigorous and highly productive, equipped with the required characteristics (heterosis or luxuriance of the hybrids). Already Darwin, in 1876, had described in the maize the inbreeding depression, that is the decrease in size and in productivity due to self-pollination and the vigour of the hybrids that is the increase in size and vigour after cross pollination. Nowadays in the industrial agriculture are utilized mainly the hybrid maizes, whilst the races with free pollination are mainly limited to the small scale cultivation.

As thes hybrid maizes are the outcome of the first generation of the cross of two pure lines, the farmer must buy back the seed each year, with the consequences of high costs. Recently, they have been able to significantly lower the cost, allowing  good diffusion of this type of hybrids.

Transgenic maizes

Because of its enormous economical importance this species is subject to an intense work with techniques of genetic engineering that are leading to realize several types of transgenic maize. The main engineered characters concern the resistance to pathogens, such as the Flour moth, one of the most important parasites, and the resistance to herbicides, such as the Glyphosate and the resistance to drought. Plants of maize modified in these directions are presently cultivated in many countries of the world.

The Maize and the creation of man after the Popol Vuh Maya mythology

The gods created humanity several times, initially with mud, but the men produced in this way melted with the rain and had no intelligence; then they made them with the wood, but these men had no soul and did not recognize their creators and so the gods destroyed them with the Great Flood. Only a few specimens survived, the monkeys. Finally the gods used the ground and kneaked maize thus obtaining satisfactory results. These men made by maize were the first ancestors of mankind and were perfect because they had the same wisdom and intelligence as the gods and could see and understand the whole world.

The gods thought that this situation was not acceptable, because in this way there should not be any difference between the creators and their creatures. Therefore, the gods breathing on men, decreased their intelligence and caused their gaze to cloud over and the result are the present men. It is interesting the analogy with the expulsion from the biblical earth paradise of Adam and Eve who, eating the fruit of the forbidden tree had acquired the knowledge of good and evil: And the LORD God said: «Behold, the man has become as one of us, to know good and evil”. Genesis, 3, 22

The mythical origin of the maize after the Aztecs

The god Centeotl (in the Maya language, Centli is the panicle and Teotl means deity) gave the man the maize, and also the cotton, the batata and the chenopodium as well as a drug, perhaps the pulque, carrying them back from the underworld.

The Maize in the science

The maize occupies an important place in the history of genetics thanks to the studies done by Barbara McClintock on the variegated colour of the caryopses.

These studies proved for the first time the possibility that the genes or parts of them could move inside the chromosomes (transposable genes or transposons).

The importance of this discovery, with the very important implications for biology as well as for medicine, has been recognized with the attribution to the American geneticist of the Nobel Prize in 1983.

The maize in art

The frequency with which, during the Renaissance, the ear of maize enters the artistic representations is an indication of the now wide diffusion of the cereal and of the fascinatio that this big golden ear with its regular disposition of the caryopses exercised on the artists.

As an example, we can remind the works “Vertumnus” and “l’Estate” (the summer) by Arcimboldo but especially the frescoes by the student of Raphael, Giovanni da Udine, in the loggia di Psiche in the Villa Farnesina of Rome, containing probably the first representation of the Maize in Europe.

It is noteworthy that these frescoes were realized between 1515 and 1517, hence after very few years from the arrival of the maize in Europe.

The traditional cultivation of Maize

Nowadays, in the developed countries the cultivation of the maize on a large scale is completely mechanized and operations like the harvest, the shucking and the hulling are done on the field by one unique machinery. In many developing countries, and also in Europe for what the small production for family use is concerned, however are still followed traditional methods mainly manual, similar to those of generalized use still used in the first half of the XX century.

In autumn, the soil was prepared and fertilized, waiting for the sowing that took place in spring: with the hoe were dug at regular distance some holes where the seeds were dropped to be then covered by earth. After the birth of the seedlings these were thinned out leaving only one plant per hole.

At the beginning of summer, when the maize plants are well grown, the earth was replenished and fertilized with manure. Then followed the process of shearing that consisted in the pruning of the highest nodes of the plant that bear the male inflorescence, with the aim of reinvigorating the development of the cobs. Naturally, the material removed with the topping was utilized for cattle. When the ears have reached the ripening, usually in September in central-northern Italy, was the moment of harvesting, done manually detaching the ears from the stem and putting them in a wicker pannier carried on the shoulder.

By the end of harvesting the ears were put to dry under a porch, or in the farmyard depending on the climate, until the moment of shucking and of the hulling, operations that, usually, involved the whole family unit. Meanwhile, the plants remaining on the field were mowed and utilized as food or as litter for the cattle.

The shucking consisted in the elimination of the cornet (bracts) handmade, possibly with the help of  a large nail the peasants kept hanging on the wrist with a string.

The outermost leaves of the cornet, more coriaceous, were used as litter for the cattle, whilst the innermost and more morbid ones were used for stuffing the mattresses. All the bract were not always eliminated, as some, folded back, formed a sort of a handle used to tie the cobs together and form bunches that were then hung to dry.

For the shelling they could help themselves with the same nail used for husking, whose tip was made to flow between the rows of the kernels in order to take them off from the corncob. In some countries, for the shelling they helped themselves with the jaws of an animal like the sheep whose teeth served as a sort of rasp. For shelling they could also use a crank sheller, equipped with a metallic disk rotating while the panicles were hand pushed or by a spring against its toothed surface. Operations like husking and shelling that required a lot of time and involved the whole family unit, that in past ages represented important moments of socialization during which the grannies told stories and circulated news and gossip in an environment not marred by tv.

After shelling the kernels were stretched out to dry, periodically stirred with a rake. At the end of drying, transferred in sacks, the kernels were transported to the mill for the production of the polenta flour or conserved.

Maize diseases

The maize can be the target of various pathogens, of animal, mycotic or viral nature that are able to alter its production as quantity as well as quality. Globally, it is estimated that the maize pathologies cause the loss of about 9% of the production, with strong differences between the various geographic areas.

The most important maize pathogens are microscopic fungi and oomycetes (the oomycetes, classified in the past as fungi are now ascribed to the kingdom of the Chromista ) that, in addition to attacking the plant, may act also on the ears or on the granella even after harvest during the conservation. An important problem concerning some of the fungal pathologies is given by the production by the fungus of mycotoxins, potentially dangerous to man or to the animals eating the infected maize. The rot of the seeds and of the seedlings is caused by oomycetes of the genus Pythium or by ascomycetes of the genus Helmintosporium. The pathogens can be present in the seed or in the soil, their development is favoured by humid and poorly drained soils. The infection can cause the death of the seedling.

Fungi of the genus Helmintosporium  may also infect adult plants and the infection causes desiccation of the leaves, weakening of the plant and  poor productivity. A fungal infection affecting maize and other gramineous plants is the downy mildew caused by the mycete Sclerospora macrospora.

The downy mildew, that may manifest in all the organs of the plant, is facilitated by the water stagnation and consequently by the submersion of the sown ground and of the seedlings.

Morphologically, the most conspicuous alterations are borne by the male inflorescence, hence the common name of crazy cyme for this disease, that is of modest interest in Europe but is more significant in the USA.

An important pathology is represented by the fusarium head blight, that causes softening and disintegration of the stem, even until the plant dies, and is caused by the mycete Fusarium graminearum  (also called Gibberella zeae) or other species of the same genus. This fungus also hits other gramineous plants like wheat, barley, or rice and is diffused all over the world.

The corn smut is one of the most known and common fungal diseases that may attack the maize and is caused by the basidiomycete Ustilago maydis. The smut causes the formation of outgrowths, called galls or tumors, on the ear or also on other parts of the plant. The galls are initially whitish and fleshy and are covered by a thin film, but later on these formations darken producing a powdery mass of blackish spores that are then freed due to the breaking of the film covering them. The released spores fell on the ground, where they remain vital for several years  and in spring they germinate producing new spores that infect other plants. It is important to highlight that the galls, that have variable size and can reach the diametre of 15-20 centimetres, are edible and have been used since ancient times by the populations of Central America, where they are known as truffles of the maize or huitlacoche. In recent times the food use of the maize truffles has diffused in other countries, especially in the USA, where they are traded fresh as well as conserved in pots.

The remarkable commercial success of the galls has stimulated studies aimed at selecting particularly aggressive strains of the fungus, able to produce more numerous and bigger galls and also for obtaining hybrids of maize particularly sensitive to the Ustilago (paradoxical situation if we think that most studies in this regard aim at finding resistant hybrids and to eliminate the infections).

Among the pathologies due to mycetes being able to produce mycotoxins potentially toxic for man or for livestock we remind the Fusaria verticilloides, which causes rot in various parts of the plant and Aspergillus flavus that even if usually being a saprophyte, can also attack the live plants.

Fusaria verticilloides produces dangerous toxins known as fumonisins. Though the damages to the production done by this fungus are not huge, the attention to this affection is high as it is thought that the toxins produced may cause serious damages in the man as well as in the cattle. It must be noted that the toxins accumulate in the caryopses and that their production does not occur only on the field but also after harvesting.

Similar situation is the one concerning the Aspergillus flavus that can cause rot and the appearance of green molds on the caryopsis. This fungus produces aflatoxins whose dange for man and for the animals is well known. Also for this fungus the production of toxins continues after harvest, during storage.

Other diseases that usually do not develop in severe form are the “rusts” (Puccinia maydis, Puccinia purpurea, etc.) that preferably attack the leaves and the bracts.

Animal parasites

The most dangerous insects for the maize are lepidopterans such as Ostrinia nubilalis (synonym

Pyrausta nubilalis), those of the genus Sesamia and, particularly, the Chrysomelid coleopter Diabrotica

virgifera. This last parasite is native to North America, where it is considered as the most important pathogen agent of the maize. In the early ’90s the coleopter was reported in Serbia, and from there in a few years has diffused in a good part of Europe. The damages to the plants of maize are caused especially by the underground larvae that seriously damage the root system.

It is interesting to note that the old varieties of maize, cultivated in America and also in Europe, were capable to defend themselves from parasites such as the Diabrotica virgifera  producing and freeing in the ground substances like the caryophyllene that attract small nematodes of the genus Heterorhabditis that eat the larvae of the parasites. The more modern varieties of the maize, selected for obtaining a greater productivity, have instead lost this capacity.

Through genetic engineering have been obtained plants of maize that have got back the capacity of producing the caryophyllene, but the results have been lower than expected as the Diabrotica virgifera  in turn has organized defensive strategies. In fact, the maize, like other plants, produces antiparasitic substances, the benzoxazines, endowed with insecticidal and antibacterial activities.

The Diabrotica virgifera is able to absorb these substances and then to release them after having modified them in such a way to render them toxic for the nematodes. In this way the parasite defends itself from the toxic substances released by the maize as well as from the nematodes of the soil.

Also other insects like the cockchafers, the elaterids, the mole crickets, may cause damage to the maize.

Among the viruses the remind the virus of the maize rough dwarf virus (MRDV) transmitted by the true bug

Laodelphax (hemipter Delphacidae), the maize dwarf mosaic virus MDMV, transmitted by aphids, and the barley yellow dwarf virus, also this transmitted by aphids.

Besides the traditional methods, the defense against the pathogens of maize is based, for what concerns the mycetes, on the treatment of the seeds with fungicides that allows to eliminate the pathogens present on the seed and may act also against spores present in the soil and especially on the choice of resistant hybrids, already available for various diseases.

It must be remembered above all that the onset of mycotic diseases is facilitated by environmental conditions favourable to the parasite and therefore the prevention based on an appropriate choice and treatment of the soil can be very effective.

For the insects it is aimed at insecticides with low environmental impact based on Bacillus thuringensis. For the viral diseases, finally, the strategy consists mainly in the struggle against the vector insects.

 

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