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Oryza sativa L. Rice - Cereals - Herbaceous crops

Oryza sativa L. Rice - Cereals - Herbaceous crops


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Class: Monocotyledones
Order: Glumiflorae
Family: Graminaceae (Gramineae or Poaceae)
Tribe: Orizeae
Species: Oryza sativa L.
The species O. glaberrima Steud. Belongs to the genus Oryza, cultivated only in small areas of western tropical Africa.

French: riz; English: rice; Spanish: arroz; German: Reis.

Rice is a very ancient cultivation plant, originally from Southeast Asia with a tropical and subtropical climate.
Rice is one of humanity's main food resources: more than half of it bases its diet on rice. Worldwide, over 550 million tons of rice are produced annually on over 150 million hectares, mainly in the hot and very humid regions of the tropics and subtropics, where other cereals do not thrive.
In Italy, rice growing is spread over about 220,000 hectares and is located almost entirely in the Po Valley and especially in areas where large quantities of low-cost water are available for irrigation. In Italy, the annual per capita consumption of rice is around 5.5 kg (in Laos it reaches 170 kg per capita / year).
The most rice-growing provinces are those of Vercelli, Pavia, Novara, Milan, which alone group together just under 90% of the total area invested in rice; other rice-growing provinces are Mantua, Verona, Rovigo and Ferrara. Sporadic traces of rice cultivation are also found in central (Siena, Grosseto) and insular (Sardinia) Italy; which means that rice can be grown anywhere, as long as there is plenty of water and low price.

Rice - Oryza sativa L. (photo CS Kuoh www.efloras.org)

Botanical characters

Almost all the rice grown in the world belongs to the Oryza sativa species, a grass of the Oryzeae tribe. Only in Africa did a species of its own origin originate and is still limitedly cultivated: the Oryza glaberrima.
LOryza sativa is very rich in forms; according to the most recent and affirmed classification the cultivated forms can be ascribed to two subspecies:
- Oryza sativa subsp. indicate
- O.s. subsp. japonica.
The rice of the indica type is very sensitive to the photoperiod (they are short-day) and therefore suitable for tropical climates (they are widespread between 0 ° and 25 ° latitude), have a long cycle, are rustic but subject to lodging and the grain is long (length ratio / width greater than 3), narrow, flattened, resistant to cooking and non-gluing.
Japonica type rice is widespread in temperate areas, being not very sensitive to the photoperiod; they have lower thermal needs than indica rice, but higher nutritional needs; straw is rather short and robust, high productivity; the grain is short (length / width ratio less than 3) and stocky, not very resistant to cooking and tending to stick.
Rice has a root system consisting of embryonic and adventitious roots.
Also in rice there is a phase of tillering and emission of the adventitious roots, more vigorous than the embryonic ones; then a rising phase which corresponds to the length development of the stems.
In the adventitious roots no longer young, "aeriferous vessels" appear, which ensure the aeration of the roots also in the submerged environment in which the rice lives.
The culm has hollow internodes and full knots and develops similarly to wheat. The leaves, in different numbers according to the variety, but usually 5-7 per stalk, are made up of a sheath and a sheet, rough due to the presence of short and hard hair. The ligula is long and the hairy auricles.
Linfiorescence is a branched terminal panicle bearing uniflore spikelets, formed of glumes much smaller than the glumellae, the latter being highly developed, superimposed on the margins, flattened and enclosing the caryopsis like a case.
The lower giumella can be mutated or briefly aristata. Glumelle can also present at the maturation of the typical pigmentations in many varieties.
The flower is hermaphrodite and includes a uniovular gynaeceum, with bifid stylus and plumed stigma and an androceum of six stamens.
The fruit is an always dressed caryopsis (which constitutes the "paddy rice"), compressed on the sides, oblong, with a white or pigmented pericarp, constituted in a similar way to the wheat caryopsis. Fertilization is strictly autogamous. The weight of 1,000 kernels dressed varies from 25 to 45 grams.
Of course, the size of the stalk, the bearing of the foliage, the size of the panicles, their shape and their bearing, the size of the spikelets and therefore of the kernels, their appearance (glassy or opaque) as well as the yield to the processing are very variable. productivity, earliness, organoleptic characteristics. The average height is 1-1.2m tends to be lowered under the meter with the selection.

Rice - Oryza sativa L. (photo www.ubcbotanicalgarden.org)

Environmental needs

Climate

Rice is extremely demanding in terms of heat and water, but its most peculiar ecological characteristic is to tolerate the water saturation of the soil, therefore, although not an aquatic plant, it is adapted to the humid areas of the tropics and subtropics also subject to submersion.
The temperature must be high and constant as the rice suffers serious damage from temperature changes. In the equatorial regions, where the temperature is constantly high, 2-3 harvests are also made per year. In temperate climates, the only possible growing season is spring-summer and with the aid of irrigation made with systems that also perform important temperature-regulating functions. The cycle from sowing to ripening is 150-180 days.
The minimum vital temperatures are 12 ° C for germination; the rising and flowering take place optimally at 23-25 ​​° C.
The thermal requirements are less during the course of the gravel. As well as demanding in terms of temperatures, rice is very sensitive to daily temperature variations.
From the photoperiodic point of view, rice was originally short-day, but the varieties that are grown in Italy have a very attenuated sensitivity to the photoperiod, so much so that it can flower even in a regime of 15 hours of light per day.
As for water, rice can be grown without irrigation ("upland rice") only where more than 200 mm of rain per month regularly fall for at least 3-4 months. In Italy, where the climate is temperate and where rainfall is insufficient, rice is grown in submerged soil. In this way the water, in addition to meeting the very high water needs of the plant, also constitutes an irreplaceable thermal aid for the direct heat supply (when the water has a temperature higher than that of the air) and for the thermoregulatory action, giving off heat at night and on cold days accumulated in periods of intense insolation. With submersion, a daily temperature range of 10-15 ° C is reduced to just 3-4 ° C.

Ground

As far as the soil is concerned, rice adapts to any type and constitution: sandy, clayey, basic or acidic, etc. provided it is wet. In submerged rice growing, the main limitation in terms of soil lies in the hydrological characteristics of the soil itself, which must be waterproof enough to be able to maintain the necessary blade of water: approximately 0.3 m thick.
In the submerged paddy field, the soil profile is characterized by a thin oxidized layer at the soil-water interface, below which the soil is in strongly reducing conditions.
The soil must be arranged in such a way as to allow for uniform water distribution and rapid drying up in order to perform the "dry" necessary for certain cultivation operations.

Variety

Rice has been and still is subject to intense genetic improvement as regards: adaptation to the photoperiod (earliness), to rainfall, to low temperatures; resistance to lodging; disease resistance.
Classification of Italian rice varieties.
Based on the earliness, the Italian varieties are distinguished as follows:
- Early: which mature within a maximum of 150 days.
- Medium: which mature within a maximum of 155-165 days.
- Late: which mature within a maximum of 170-185 days.
In the past there were also very early (140 days) and lightning (125 days) varieties, used for the transplanted interlayer culture, which has now disappeared.
There is a linear relationship that links productivity to the length of the growing season.
On the basis of the quality, the rices are differentiated according to the size of the kernels and / or for organoleptic characteristics and cooking behavior depending on the protein content of the kernels.
Until almost the end of the 20th century, all the rice varieties grown in Italy were of the Japonica type. The intensification of international trade has recently made it necessary to take into account the needs of consumers from the most diverse countries and to adopt an internationally recognized classification of types of rice.
The appreciation that the Lungo B type, which corresponds to "indica" rice, in many markets has stimulated Italian rice growers to attempt the introduction of foreign varieties of this type and genetic improvers to create "indica" varieties adapted to the Italian climatic environment. Within a few years, "indica" rice has entered massively into Italian rice growing.

Cultivation technique

Turnover

In swampy or too shallow ground, where rice is the only feasible crop, rice occurs indefinitely (permanent paddy field) since it is a species that tolerates repeated cultivation even if parasitic problems tend to occur.
In most Italian rice-growing areas, the paddy field alternates, albeit with some difficulty, with other crops.
The difficulties lie in the fact that the particular arrangement of the land for the paddy field does not allow the easy hydraulic remediation required by the other crops and that the expensive work of arranging the paddy field is depreciated only with several consecutive rice crops.
A very popular rotation is that in which rice occupies 50% of the sown area, with the rice taking place for 3-6 years and then being followed by a succession of equal duration of wheat, meadows and / or crops from renewal. Between one rice and another, where possible, the inter-cultivation of a green manure plant (e.g. embodied clover) is useful.
The paddy field, although giving rise to a reduced state and to a considerable washing away of the soil, exerts a very favorable action of planting out from the terrestrial weeds, for which it constitutes a good precession for the wheat and is to be considered an improvement plant for this cereal. Instead the renewal crops find unfavorable conditions immediately after the paddy field.

Land arrangement

Having to practice irrigation by submersion, the basis of the arrangement is the perfect leveling of the land and the delimitation of this with embankments. In Italy there are two systematic models of rice fields, linked to pedological, topographical and land structure differences: one is widespread in the western Po Valley (Piedmont and Lombardy), the other in the eastern Po Valley (Mantua and the Emilian and Venetian provinces).
In the first case, the farms are not very large and the slope of the land is not negligible, so the surface of the cultivation units, called chambers, is modest (2-3 hectares or less); the rooms are delimited by temporary embankments redone every year. Mostly the various chambers are sloping and the water enters the discharge outlet located at a higher altitude and then passes into the lower altitude chambers (irrigation with dependent chambers). In the case, frequent in this area, that the water of the alpine rivers is cold, it is necessary to heat it; for this purpose the caldana is prepared: a portion of the upper chamber is arranged with opposing comb embankments that force the water to make a long tortuous path, during which the water warms up before being introduced into the actual rice field.
In Venetian and Emilian rice growing, the soils are already naturally almost perfectly flat, so the leveling is not very demanding and allows you to make cultivation units, called basins, of large extension (10-12 hectares) delimited by large banks functioning as headlands.

Soil preparation

In the north-western rice areas, the tillage is preceded by the remaking or restoration (through the carryover and compaction of earth) of the arginelli. The tools used are the squeegee and the roller for small stacks. In rice growing in the eastern provinces, on the other hand, there are no small banks, but only the large permanent banks.
The preparation of the soil for rice consists of a complex of works which are mostly carried out in winter-spring. These jobs include: plowing, refining, leveling (embankment), slotting, leveling, compaction (or clogging).
Laratura with complete overturning of the slice is useful to restore the structure and, in the case of stable paddy or rice after rice, to ensure the loss of soil layers that prolonged submersion leads to a reduced state. To establish the depth of plowing, the permeability of the subsoil must be examined: if this is high, the plowing must be superficial to avoid excessive dispersion of water by percolation; if the subsoil is tenacious and not very permeable, the furrow can be deepened without fear but always keeping to a modest depth, never exceeding 0.30-0.35 m.
Normally only one plowing is done, in autumn in clayey and sortumous or damp soils, in late winter in peaty or loose ones.
The rotary tiller is a valid alternative to winging, but it can predispose to infestations of species propagated by vegetative means such as Scirpus maritimus, Butomus umbellatus, etc.
The spading machine is also used by some for the main process. Alaratura follows refinement, equalization and possible clogging.
Refinement is done with harrows of various types.
The balancing, which has the purpose of ensuring the perfect leveling of the chamber, is carried out by introducing water into the rice paddy which, acting as a level, allows to identify ridges and basins, and intervening with passages of a wide surface, with a smooth surface or equipped with teeth or hoes. . In the past, the leveling was distinct in the two operations of the hand slotting of the outcropping ridges and of the subsequent smoothing with a wide edge.
The clogging of the active layer is an operation necessary only in excessively permeable soils, to reduce losses by percolation. It is a question of causing turbidity in the flooded paddy field which, by sedimentation, reduces the bibacity of the soil. Special clogging tools are also needed for this purpose or, very well, repeated rapid passages of tractors equipped with cage wheels. In countries with non-mechanized agriculture, clogging is obtained by prolonged trampling on buffalo.

Sowing

Choice of varieties. In the cultivation of rice, the choice of varieties to be sown is subject to climatic conditions, the temperature of the water and the cultivation precession. The choice of the varieties to sow usually sorts on two or three types of rice which differ from each other for the duration of the development cycle and for the commercial characteristics of the product: it is not convenient to grow many breeds. Early varieties are preferable when the soil must be cleared early for the subsequent sowing of the wheat, when the work needs to be distributed over time and when the water is cold.
We remind you that rice seed must meet the requirements of purity, germination and health. Germinability must not be less than 85%, but good seeds must have more than 90% germination and high germination energy.
Also for rice there are three categories of seed based on the varietal purity: basic seed, certified for 1st reproduction, certified for 2nd reproduction. The tolerances concern the varietal purity and the number of red grains, i.e. grains with red pigmented pericarp (which must not be more than 2 in 500 grams of basic paddy rice or 5 in 500 g of the other categories).
The sowing season of rice varies according to the temperature of the water, the previous crop, the earliness of the variety, etc. Typically it is between mid-April and mid-May, but with very early varieties you can go up to the end of May for organizational reasons.
To have a satisfactory emergency, the temperature must reach 12-14 ° C.
The quantity of paddy rice commonly used ranges from 150 to 220 kg per ha and sometimes even more; the goal is to create a population of 250-300 plants per m2.
The sowing is preceded by two preparatory operations of the seed; soaking and disinfection.
Soaking consists in immersing the sacks containing the seed in water for several hours, in order to facilitate the sinking of the kernels at the moment of sowing and also to anticipate the germination and the birth of the rice.
Traditionally, sowing is done on flooded ground. The sowing immediately follows the passage of the field, so that the seed is covered by the depositing of the turbid raised by it. The most commonly used system is machine-spreader; common centrifugal fertilizer spreaders carried by the tractor or centrifugal seed drills equipped with an auxiliary motor are used; universal seeders can also be used: without distributors they ensure very regular seeding.
Sow in dry rice paddy. A sowing technique that tends to extend, because it is suitable to simplify the control of weeds, is the sowing of rice on dry soil followed by the submersion after 20-35 days with rice already born and with 2-3 leaves.

Transplant

The rice, as well as directly in place, can be sown in a seedbed and then transplanted.
The transplant in Italy has completely disappeared; it was practiced extensively in the past to gain time and to be able to make rice as an interlayer crop after wheat or after the first cut of a lawn. This system is still very popular in the tropics because it saves time, allowing up to three crops per year, and saves seeds.

Water governance

The conduct of irrigation in the paddy field is of great importance to ensure the crop in its various phases the best conditions of temperature, availability of nutrients, control of weeds or certain pests, and consequently requires great expertise in those who must regulate the supply and discharge flows of the rooms or basins.
There is no precise general rule.
The water consumption in the paddy field is enormous: according to the lower or higher permeability of the soil, continuous flow rates of 1 to 5 (average 2.5) liters per second per hectare are required (and sometimes more, in very permeable soils). Considering a 5-month season, this leads to seasonal watering volumes ranging from 13,000 to 65,000 m3 / ha and more.

Examples of water management in rice fields of different areas
In the Vercelli area the most common technique today involves the following maneuvers.
1) Submersion with 20-30 mm of water to make the leveling (approximately in early April).
2) Increase of the water blade to 55 mm (maximum 70-80) for sowing and its maintenance for the next 15-20 days (germination); indicative date: 10-30 April.
3) Increase the water blade to 0.1 2-0.1 3 m for 10 days, in order to submerge
completely the seedlings to do the treatment against the javones (early May).
4) Reduction of the water blade to 80-100 mm for 30-35 days (i.e. up to the middle of
June).
5) Dry for 2-3 days for weeding against cyperaceae, alismataceae and other non-graminaceous species.
6) Submersion with 80-100 mm of water for two weeks.
7) Dry for a week to make cover fertilization at the beginning of the rising (last days of June).
8) Submersion with 80-100 mm of water until the end of August, milky maturation stage.
With the suspension of irrigation, the land is left to dry up so that the rice fields are accessible for harvesting machines in September.
In the peaty lands of the Ferrara area, water management is a little different. Meanwhile, the blade of water is always much higher than in the case seen before: 0.20-0.25 m. A dry root called rooting is practiced in mid-June to encourage the development of the root system, while there is no practice of early rising, which would be counterproductive by promoting an excessive release of nitrogen from the soil which is very rich in organic substance.
Organic fertilization, in the form of manure and / or green manure, has been the main form of paddy field fertilization in the past, especially in soils poor in organic matter.
Now that the disappearance of livestock farms from the rice areas has reduced the availability of manure, fertilization is mainly based on the use of mineral fertilizers and the reintegration of all crop residues in the soil.
Mineral fertilization is therefore the indispensable basis for ensuring maximum yields.
For phosphatic fertilization the quantities of P2O5 commonly used are 70-80 kg / ha.
Potassium fertilization is practiced almost exclusively in the rice area of ​​the western pre-Alpine area where the soils are washed out and acid, in the amount of 100-150 kg / ha of K2O; little or nothing is used are potassium fertilizers in the Po Valley where the soils are alluvial.
Phospho-potassium fertilizers must be given before plowing, so that they are in the layers explored by the roots.
For nitrogen fertilization, the same principle as for wheat is valid, namely to give as much nitrogen as the resistance to the variety's lodging allows, also bearing in mind that an excess of nitrogen predisposes the cultivation of rice also to the attack of brusone.
Obviously, the doses of nitrogen vary widely, as well as according to the variety, according to the nature of the soil, previous crop, etc.
With the current varieties the doses of nitrogen ordinarily supplied are 100-150 kg / ha.
To ensure the nitrogenous nutrition of rice during the tillering, rising and flowering phases, it has been seen that the best technique is to make nitrogen fertilization in three times: the first giving 20-25% of the expected seeding rate, under urea form; the second nitrogen must be made on the roof towards the end of the collection, the third at the beginning of the rising.
The shape of the nitrogen fertilizer for paddy is very important. Nitrogen in the nitric or nitro-ammonia form is to be categorically excluded because it is too soluble, washable and subject to denitrification in the subsurface layers of the soil which are in a reduced state.
Lurea is the ideal nitrogen fertilizer for rice paddy.

Weed control

The submerged paddy field is a very particular agro-ecosystem in which the weed vegetation that settles there has equally particular characteristics, for example including algae as well as higher plants adapted to the particular habitat of the rice field.
The fight against rice paddy weeds has always been indispensable, even if extremely demanding, tiring and expensive.
The infesting flora of the rice fields is characteristic for having marshy or water-saturated environments as habitat, so it includes species different from those found in the mainland agro-ecosystems: algae, real aquatic plants (Potamogeton, eterantera); marsh plants (cyperaceae, butomacee, alismataceae); plants tolerant of humid environments (between grasses, javones and wild rice).
In past rice growing, weed control was done with the copper salts dissolved in the water of the rice fields to control the algae, and the "monda", that is, the hand scerbing, done with two passages 15 days apart in June , which required a commitment of 45 days per hectare. Economic reasons have long made the scerbatura no longer available, toxicological reasons have limited the use of copper for the disposal.
Therefore, chemical control means have been developed which have had a very wide diffusion in all rice areas thanks to their irreplaceable efficacy.

- Replacement flora
Unfortunately, the generalized use of herbicides on rice crops that repeat on the same ground even for several years, has given rise to a progressive and profound change in the infesting flora of the rice fields because species that in the past had secondary importance have become dominant because they are resistant to herbicides. widespread ("substitution flora"). This has made the search for new chemical weapons against emerging weeds incessant and, consequently, the technique of their control not easy, which must be done in a diversified way according to the specific flora and with weight, taking into account that the products are put into the water. of the paddy field that can transport them to the subsoil or to the surface water bodies.
The multiple problems that arise in the control of paddy weeds can be schematically indicated as follows:
- algae
- giavoni
- cyperaceae, butomacee, alismataceae
- etherantera
- wild rice.

- Algae control («disalgo»)
Algae infestations are harmful especially during the first development of rice due to the felt that they form by weaving their filaments on the bottom of the rice paddy or on the surface.
The algae prevalent in the rice fields are green and blue; in the past the former were prevalent but easily controlled, while the latter are currently increasing.
The green algae form a floating felt that hinders the emergence from the water of the rice seedlings, whose leaves remain entangled in the algal felt, finding it difficult to come out into the light. The second ones form their felt first on the bottom, where the rice seedlings are making their first development, and then they rise and become floating: in this way the rice seedlings are uprooted and brought to the surface where, among other things, the risk of being driven by the wind and the swell, ending up piled up in the leeward part of the field.

- Control of the javones
Giavoni include several graminaceous species of the genus Echinochloa and are the most frequent and invasive rice weeds, against which it is almost always necessary to intervene because very few plants per meter2 (6-7) are enough to cause serious production losses. For weeding we have products that can be used in pre-sowing or in early post-emergency (classic product: the molinate) or in post-emergency, even late, therefore as a possible supplement (typical product is propanane). Weeding against javons allows very interesting combinations to extend control to other species.

- Control of Ciperaceae, Butomacee, Alismatacee
The control is done with post-emergence herbicides mainly based on bensulfuron-methyl or hormonal compounds, generally combined with javonicides. Ethereter control (Heteranthera limosa, H. reniformis).
It is a new pest, introduced by Central America a few years ago and which now infests almost all Italian rice fields. The only effective treatments are the preventive ones made before sowing on dry paddy. The specific and effective active substance is loxadiazon.

- Control of perennial grasses and «crodo rice»
A group of species typically infesting paddy fields is made up of grasses among which the most fearsome are Leersia oryzoides and especially crodo rice. Crodo rice is wild rice which has the characteristic of disseminating the grain («crodatura») very early, already after the milky maturation, determining in the soil an infesting load of wild rice plants uncontrollably in the middle of the cultivated rice. For both of these species the struggle is difficult and is based on the tactic of making a «fake sowing» and delaying the sowing of the rice to allow the weeds to emerge and be controlled.

Collection and use

Collection

The rice sown in April reaches physiological maturation in different periods according to the earliness of the variety: for example the early ones reach ripening in September, while the late ones reach it at the end of October.
The harvest is preceded by the definitive dryness that takes place a couple of weeks before ripening to speed up this and make the soil practicable. Harvesting must be done promptly because a delay increases the losses by cropping (or "crumbling") and the share of kernels that do not "whiten" during the paddy processing.
The harvest can be done, as for wheat, with the system of harvest or combine harvesting. The second system completely supplanted the first.
The threshing of the sheaves was then done with fixed threshers, sulia.
Rice harvesting can present some difficulties due to the problematic practicability of the paddy field by the heavy combine harvester. To overcome this drawback, rice combine harvesters are generally semi-tracked. These are self-propelled machines with cutting bars from 3 to 4.5 m in length and a working capacity of about 1 ha then.
The product obtained from threshing is paddy rice or dressed rice.

Drying

The paddy rice that comes out of the threshing machine always has such a high humidity (25% on average) that it is not possible to store or process it; therefore it must be dried. In the past, only solar heat was used by spreading rice on the bush; but the often unfavorable seasonal course hindered the operation and sometimes caused even serious damage to the quality of the product. Therefore, the practice of artificial drying in special hot air dryers at moderate temperatures (35-40 ° C) has met with great favor; this operation must be done immediately after harvesting, however no later than 15-20 hours from this, under penalty of fermentation.
Il riso uscito dallessiccatoio subisce una pulitura per ventilazione e vagliatura onde liberarlo dalle impurità inerti, dai semi di malerbe e dalla gra­nella vuota, immatura, ecc. Poi si immagazzina in attesa di essere ceduto allin­dustria che lo lavorerà.
Produzione e lavorazione del prodotto.
Buone sono da considerare per lItalia rese di 7-8 t/ha di risone; sono segna­late punte non eccezionali di 10 t/ha. Nella maggior parte delle plaghe risicole del mondo le rese sono molto più basse (2-2,5 t/ha).

Lavorazione del risone

Come sè detto, il prodotto della trebbiatura è composto dalle cariossidi di riso ancora avvolte nelle glumelle. Scopo della lavorazione è quello di staccare le giumelle ed anche parte del pericarpo della cariosside insieme a parti proprie del seme, mediante una serie di operazioni che descriveremo qui appresso nelle loro fasi principali: pulitura (separazione delle impurezze dal risone), rot­tura delle reste (nel caso di risi aristati), sbramatura (distacco e separazione dalla cariosside delle glumelle, che vanno a costituire la lolla), sbiancatura o raffinatura (si allontanano gli strati esterni del granello e lembrione, o gemma, con ripetuti passaggi alle macchine sbiancatrici).
Il riso raffinato, che è bianco e conservabile, può subire un altro tratta­mento, la oleatura o la brillatura.
Quindi nel corso della lavorazione si ottengono successivamente questi prodotti.
- Riso greggio (o r. «cargo»), privato della lolla, che conserva ancora pericarpo ed embrione.
- Riso sbramato speciale, cioè semigreggio, che ha subito una lavorazione incompleta alla sbiancatrice.
- Riso mercantile, riso adatto al consumo, però non lavorato a fondo (due pas­saggi di sbiancatrice) e di non grande serbevolezza.
- Riso raffinato, riso bianco passato 3-4 volte alla sbiancatrice che ha privato completamente la cariosside del pericarpo, e che perciò ha subito una lavo­razione completa. Lungamente serbevole, adatto ad essere esportato, è il riso che si adopera per sottoporlo a lavorazioni speciali.
- Riso camolino od oleato, che si ottiene oleando leggermente la superficie del riso raffinato con olio inodoro di lino o di vasellina.
- Riso brillato, che si prepara da quello raffinato rendendolo brillante a seguito di lavorazione con talco e glucosio.

Il riso raffinato, oleato, brillato si usa nellalimentazione umana; quello sbramato trova impiego nella fabbricazione della birra.
In epoche recenti si sono ideati dei sistemi speciali di lavorazione che migliorano le caratteristiche organolettiche e il valore alimentare del riso e lo rendono più resistente alla cottura («parboiling»). Con il «parboiling» il risone viene ammollato, cotto a vapore ed essiccato; in tal modo lamido viene gelati­nizzato e i sali, grassi e proteine si diffondono allinterno dellendosperma. Ne risulta una riduzione delle rotture durante la successiva lavorazione e un aumento della digeribilità e della resistenza alla cottura e allo spappolamento.
La resa alla lavorazione, cioè il riso lavorato che si ricava da 100 kg di risone, si aggira su tra i 60 e i 66 kg. La lavorazione modifica notevolmente la composizione del riso. Infatti, oltre che della lolla, la cariosside viene privata del pericarpo, del germe e dello strato aleuronico, perdendo quindi una notevole quantità di cellulosa, di sostanze minerali, di grassi e di sostanze proteiche.
La paglia del riso trova usi analoghi a quella del frumento sebbene sia meno assorbente di questa, e viene anche destinata alla fabbricazione di cellu­losa da carta.

Adversity and pests

Numerose sono le avversità che il riso incontra.
Avversità meteoriche
Il vento è dannoso quando, dando origine a moti ondosi, compromette il radicamento delle piantine. Più avanti nello sviluppo il vento può essere causa di allettamento o sgranatura.
Avversità di origine fisiologica
La colatura apicale consiste nellatrofia di una parte del panicolo, solitamente quella distale, che può portare alla sterilità fin del 50% delle spighette. Oltre alla predisposizione varietale, cause predisponenti sono le basse temperature nel periodo tra il viraggio e la spigatura.
Una qualche importanza occasionalmente riveste anche il gentiluomo (straighthead: testa alta, in inglese) che si manifesta con colorazione verde cupo della pianta, foglie erette e panicolo che resta completamente sterile e, per questo, assume portamento eretto. II fatto che si manifesti in risaie in successione a prati vecchi, fa ritenere che ne siano causa fenomeni putrefattivi a carico della sostanza organica.
Analoga come eziologia è una malattia di natura fisiologica sporadica in Italia ma molto comune in Giappone, lakiochi (o declino autunnale), che sarebbe conseguenza della presenza nel terreno di acido solfidrico formatosi nellambiente anaerobico del terreno a risaia.
Parassiti animali
Particolare importanza assumono i nemici che attaccano le piante nel periodo tra la germinazione e lemergenza dallacqua.
Tra i ditteri danni notevoli al germinello provocano le larve del lecca­riso (Cricotopus spp.), erodendo i germinelli e le foglie sommerse o adagiate sullacqua, e la Hidrellia griseola, le cui larve provocano diradamenti minando il tessuto verde delle foglie delle giovani piante appena emerse dallacqua. In genere questi ditteri si combattono indirettamente con asciutte. Il crostaceo Triops cancriformis (coppetta) può provocare fallanze per mancata germina­zione o per sradicamento con i suoi movimenti che sollevano la terra del fondo e intorbidano lacqua. Unasciutta è un trattamento agronomico che può limi­tare il danno.
Parassiti vegetali
Il più dannoso parassita del riso è un fungo (Piricu­laria oryzae) responsabile di una sindrome molto variata che prende nome di brusone quando colpisce precocemente le foglie (provocando un danno limi­tato) e di mal del nodo e di mal del colletto quando colpisce la pianta ai nodi o allultimo internodo, con danni ben più gravi dato che ne consegue il dissecca­mento dellintero panicolo. La diffusione della malattia è favorita da elevata umidità dellaria durante o subito dopo la spigatura, da eccesso di azoto, da semine fitte, da abbassamenti bruschi di temperatura. Limpiego di varietà resistenti è il mezzo di prevenzione più efficace.
Anche lelmintosporiosi (Helminthosporium oryzae) arreca danni gravis­simi, soprattutto fuori dItalia, colpendo tutte le parti aeree della pianta. Attual­mente sta destando crescente preoccupazione anche in Italia. Linfezione si tra­smette con il seme che quindi deve essere scrupolosamente trattato.
Il mal del piede del riso (Sclerotium oryzae) si manifesta durante la maturazione con il disseccamento e il conseguente allettamento delle piante. Lattacco, visibile come lesioni nerastre, comincia sulle guaine delle foglie basali e poi passa sugli internodi. Anche in questo caso il rimedio migliore è ladozione di varietà resistenti.
Una virosi, il giallume, sta destando qualche preoccupazione nellam­biente risicolo italiano; essa è diffusa da un afide, il Rhopalosiphon padi.

a cura di Elena Nelli e Francesco Sodi


Video: Sneak peek: secret found to increasing crop yield (May 2022).