2/20/2009

1

Foods from leaves, stems and roots

Stems and leaves are part of the shoot system, usuallythey have different functions:

leaves conduct photosynthesisstems support leaves and flowers

conduct water and nutrients to leavesconduct photosynthetic sugars to the rootscontain the meristem tissuecontain the meristem tissue

Arrangement of the vascular tissues is different in monocots and dicotsIn monocots bundles of xylem and phloem tissue are

scattered throughout the stemIn dicots the bundles of xylem and phloem are arranged in

a ring at the periphery of the stem

Shoots, roots and leaves

Monocot stem

Dicot stem

Meristem tissue

Plant growth (cell division) can only occur in certain undifferentiated tissues called meristems. As plant cells mature and differentiate they become surrounded by a rigid cell wall that prevents cell division.

Meristems are classified as primary, secondary, apical and lateral. Primary meristems are the apical meristems of shoots and roots, where cell division occurs throughout the lifetime of the plantcell division occurs throughout the lifetime of the plant.

Secondary meristems are derived from cells produced by primary meristems. Vascular cambium and cork cambium are examples of secondary meristems.

Apical meristems occur at the ends of shoots, roots, branches and by dividing increase the length of the shoot.

Lateral meristems occur in rings or bands of tissue parallel to the long axis of the stem. By division of lateral meristem plants grow in diameter.

Apical and lateral meristems

lateral meristem

Roots

Not all the below-ground tissues of plants are roots and not all roots occur below ground. Various kinds of modified stems and even some fruits, such as peanuts, develop underground.

How can you distinguish a true root from a modified stem?

Roots do not have leaf scars or axillary budsRoots do not have leaf scars or axillary budsRoots do not have nodes and internodes where bundles of vascular

tissue diverge into leaves and branchesRoots do not have stomates, stomates are present on stemsRoot apical meristem is located behind the root cap, covering layer is

absent on stemsRoots have root hairs, not present on stemsVascular tissue in a central core of the root, surrounded by endodermis

vs. multiple bundles in stemsBranch roots develop from within the pericyle, branched stems are

derived from apical meristem

2/20/2009

2

Edible leaf, shoot and roots

Leaf crops Root crops Stem cropsBrussel sprouts Beets ArtichokeCabbage Carrots AsparagusBroccoli ParsnipsCauliflower Radish BulbsCelery Turnips LeekE di R t b O iEndive Rutabaga OnionLettuceSpinach Rhizomes/tubers

ManiocSweet potatoWhite potatoTaroYam

Brassica

One of the most diverse food plant families, Brassicaceae (mustard family)

A variety of modified shoots and roots used for food, as well as products derived from seeds, mustard and canola oil.

The pungent taste of mustard is due to a class of chemical compounds called glucosinolates or mustard oil glycosidescompounds called glucosinolates, or mustard oil glycosides.Glucosinolates are sulfur containing compounds attached to a glucose molecule. When the glucose molecule is cleaved off by the enzyme thioglycosidase (myrosinase) the unstable intermediate spontaneously converts to an isothiocyanate.

Isothiocyanates are highly toxic, and are the active chemical in some fumigants.

The myrosinase is kept separate from the glucosinolate in the cells, because isothiocyanate is toxic to the plant too.

Myrosinase is stored in specialized cells (myrosin cells) and also in separate organelles in normal cells. The glucosinolates are concentrated in the cell vacuoles and may also be more abundant in certain cell types.

Glucosinolates

When cells are ruptured the myrosinase and glucosinolates are released and react, releasing isothiocyanate and other toxic products. This has been called the myrosinase-glucosinolate bomb”

GlucosinolatesThe reaction products of myrosinase-glucosinolates are toxic to various fungi, insects, nematodes, and are thought to be a general defense mechanism to herbivores and pathogens.

But while glucosinolates may be an effective deterrent to generalist insect

Cabbage moths are one of the major pests of Brassica crops and they are attracted by the glucosinolates.

gherbivores, several different groups of insects have evolved mechanisms to detoxify glucosinolates and for these insects, the glucosinolates serve as attractants, feeding stimulants and oviposition stimulants.

Brassica oleracea

Modification of stems and leaves of one Brassica species has produced a variety of different food plants.

BroccoliBrussels sproutsCabbagegCauliflowerKaleKohlrabi

All are varieties of the same species, B. oleracea

Brassica oleracea

B. oleracea is native to western Europe and its varieties have been domesticated within the past 2000 years.

The first Brassica species to be cultivated was probably B. campestris(turnip), but it was first domesticated as an oil seed crop, rather than as a root crop, about 4000 years BP. The turnip form of B. campestris appeared about 3000 years later in N. Europe. Both probably first began as weeds associated with wheat and other crops.

Th fi t f f B l t b d ti t d b bl th l fThe first forms of B. oleracea to be domesticated probably were the leafy kales, which are mentioned by Greek writers around 2600 years BP. an early stage of domestication must have involved reduction in the amounts of the bitter tasting glucosinolates that are found in high levels in wild B. oleracea ssp. oleracea.

Other forms of B. oleracea emerged much later as humans selected for enlargement of different parts. Cabbages were grown by Romans about 1000 years BP. Broccoli, brussels sprouts and cauliflower appeared in the last 500 years.

2/20/2009

3

Brassica oleraceaBroccoli and cauliflower both are derived from predominantly floral tissue but a rgarded as different varieties (var. botrytis and var cauliflora).

Broccoli heads are composed of fertile flower buds that develop into blossoms if they are not harvested early.

Cauliflower heads are formed by a proliferation of stem tips that produce a mass of tissue called “curds”. In early-flowering “snowball” cauliflowers the flower tissue does not develop. Modern cauliflower cultivars have been selected that lack the ability to produce chlorophyll in the flowering head. White cauliflower used to be produced by tying leaves surrounding the floral head to cover itused to be produced by tying leaves surrounding the floral head to cover it, preventing the initiation of chlorophyll synthesis.

More recently purple and orange headed varieties have been developed

Brassica root crops

Brassica species grown for their roots are turnips (B. campestris) and rutabaga (B. napus). Both species are also grown as oilseed crops.

Although these are called root crops, the portion of the plant that is used for food technically is the hypocotyl, the portion of the plant where root and stem meet.

Turnips and rutabagas are very similar in appearance although they produced by different Brassica species.

B. campestris was in use at least 4000 years BP in India as an oil seed crop, but its cultivation in Europe as an oil seed crop did not occur until the 1200s. Other varieties of B. campestris are used as leaf vegetables (taatsai) in China.

Turnips were a major dietary staple in Europe during the middle ages but this may have been the high point of its popularity.

Rutabagas (“Swedish turnips”) (n=19) are hybrids between B. oleracea (n=9) and B. campestris (n=10). Rutabagas in US markets usually are yellow and larger than turnips. Canola oil is also produced from B. napus.

Radish

Radish (Rhaphanus sativus) is another Brassicaceae root vegetable. It is used in the US as a minor ingredient in tossed fresh salads, but historically and in other parts of the world it has been used more extensively.

Egyptian records from 4000 years BP suggest that radishes were an important food both for the root and the oil produced from crushing the seeds.

Daikon, a variant of R. sativus, is an important vegetable in Japanese and Korean food.

Starchy tubers, rhizomes and roots

The underground stems of plants have evolved to serve various functions, such as overwintering organs and asexual propagation.

Underground stems that store carbohydrate serve a similar function to the endosperm of seeds. The carbohydrate provides an energy and carbon source for the emerging shoot before before it can conduct photosynthesis.

Underground stems that store carbohydrates are especially important food for humans. Underground storage organs have served as human foods since prehistoric times and along with fleshy fruits were probably the main foods gathered by preagricultural societies. Fleshy tubers collected from wild plants are still an important part of the diet of many hunter-gatehrer societies.

Because most of the storage organs of plants can also function as propagules, they also lend themselves to use in plant cultivation and the beginnings of agriculture.

White potato

Wheat, rice and other grains are the major source of carbohydrate for most of the world. But potato, Solanum tuberosum, is the fourth most important staple crop worldwide. This is remarkable because white potatoes have only been grown outside the Americas for about 300 years.

S. tubersosum is native to Central and South America. It has been cultivated in South America for at least 7000 years and archaeological evidence from Chile that potatoes were being consumed, probably as wild collected tubers, 13,000 years BP.

The seasonally dry climate of the high elevation valleys of the Andes mountains enabled a early form of freeze-drying for food preservation. The night temperatures are below freezing and the late summer days are generally dry.

The process of making chuño is to lay harveted potatoes out overnight to freeze, then the next day and successive days to stomp on the tubers to squeeze out water. Eventually this produces a dry, storable source of carbohydrate. Chuño is still a traditional food in Bolivia and Peru.

Making chuño

2/20/2009

4

White potato

By the European arrival in the New World potatoes were being cultivated the length of the Andes from Columbia to Chile. Spanish explorers recognized the food value of potatoes and introduced them to Spain around 1570. The potato was introduced to the British Isles (legend has it by Sir Francis Drake) soon after ~1580. Potatoes were introduced to North America from Europe in 1621.

There are still a number of different species that are grown in South America by native South Americans that display different ploidy levels. Hybridization has apparently occurred multiple times between different Solanum species leading to diploid, triploid, tetraploid and hexaploid species.

Solanum tuberosum var. tuberosum, the species now cultivated worldwide is a tetraploid. Because of the history of hybridization of the genus, the exact parentage is unclear, but likely involves at least four different wild species.

White potato

After its introduction in the British Isles, the white potato was widely accepted, particularly in Ireland, where the climate was favorable for its growth. A potato crop could be produced on a small parcel of land sufficient to support a family and also provide for the payment of taxes.

As a result the Irish population grew rapidly between 1750 and 1840. In 1844 an epidemic of potato late blight, caused by the pathogen Phytophthora infestans caused a

Two subspecies of S. tuberosum are recognized, ssp. tuberosum and ssp. andigena.

The first potatotes to be introduced to Europe were all the ssp. andigena type. This subspecies was grown exclusively in northern Europe prior to 1840. Since the 1840s the more disease resistant ssp andigena has been used to produce potato cultivars, but late blight is still a very significant problem.

y pmassive failure of the potato crop in Ireland during the next 5 years, resulting in a calamatous famine and mass emigration from Ireland.

Yams and Sweet Potatoes

Yams, Dioscorea spp., are the second most important tuber in terms of world production. Various species are cultivated and are native to Africa, Asia, and South America, and are believed to have been domesticated independently on the different continents.

D. trifida, “cush cush” is native to South America and is grown in tropical areas including Brazil and Jamaica, D. rotundata (white yam) and D. cayenensis (yellow yam) are growm mainly in Africa, although D. rotundata is the main species grwon worldwide, D. alata (water yam) and D. esculenta (asian yam) are produced in Asia. D. alata (“uhi”) was brought to Hawai’i with early Polynesian colonists.

Yams have a major cultural significance in manyof the regions where they are grown. Annual yam festivals are celebrated in Nigeria and Ghana andin Papua New Guinea.

Yams contain poisonous oxalic acid crystals justbelow the skin of the tubers which is removed by peeling and boiling.

Yams and Sweet Potatoes

Sweet potato, Ipomaea batatas, has a tuber that resembles that of the yams, but the two are not closely related. Ipomaea belongs to the morning glory family (Convolvulaceae).

The origins of sweet potato are not completely clear. There is archeaological evidence of sweet potato from Peru from 10,000 to 8000 year BP, but could be the result of gathering. Other ancient remains have been found in Hawai’i, New Zealand (the Maori call it kumara and it is still a major staple) and across the Polynesian islands. It is believed that the center of origin is Central America, but it is not clear how Ipomaeawas spread throughout Polynesia from the New World prior to European arrival.

Columbus was the first European to encounter the sweet potato in the West Indies in 1493 and introduced it to Europe. It was then spread to Africa, India and and Brazil by Portugeuse traders, and to the Philippines by the Spanish.

TaroTaro, or dasheen, Colocasia esculenta, belongs to the family Araceae and is one of the principle strach crops of millions of people in the tropics of the Pacific, Caribbean and west Africa.

The starchy storage organ is an underground corm (stem tissue). Taro is grown on swampy land that usually will not support other crops. Wild C. esculenta is found in south and central Asia. Taro cultivation began between 7000 – 4000 years ago in the Indo Malaya and spread across Oceania by Polynesian migrations. Traders and explorers introduced taro to west Africa about 2000 years ago, and from there it was brought to the Caribbean islands with African slaves.

Like the other tuber crops, taro is propagated vegetatively. Taro is used by a variety of different cultures but is probably best known from the traditional Hawai’ian dish poi.

It is also an important staple food in India, Nepal and Africa.

Cassava, Manioc

Cassava or manioc, Manihot esculenta, is a staple food in tropical parts of South America, Africa, Asia and Oceania. Like taro, manioc is grown mainly in wet lowlands, but can be grown in more arid environments and higher elevations up to 2000 m.

Cassava is considered the third most important starch staple in terms of carbohydrate consumption. The edible part is a starchy root.

Cassava is not an annual like potatoes and yams but actually a perennial shrub. The tubers are higher in carbohydrate than rice or maize, but are poor sources of protein. However the leaves are relatively high in protein and are also used as a pot-herb in Africa.

Cassava is native to South America and archeologicalevidence of cultivation in Peru and Chile for at least 4000 years.

Cassava was used as a ship’s staple provision by slave traders and by this means was introduced to west Africa in the 1700sand was introduced to India around 1800.

2/20/2009

5

Cassava, Manioc

Cassava contains cyanogenic glycosides (like peaches) that are toxic. Bitter and sweet forms of cassava are distinguished, based on the relative amount of the glycosides. Soaking and cooking are generally required to remove the bitter (and toxic) glycosides.

The glycosides are linamarin and lotaustralin and are acted on by the enzyme linamarase to release hydrogen cyanide. The two components are separated from one another in the cells (because HCN is toxic to the plant too) but if the cells are ruptured the enzyme and substrate are brought into contact. This is a very similar system to the glucosinolate-myrosinase system in Brassicas. Perhaps because of this defense system, cassava is not affected by many insect pests or diseases.

I S th A i t i i h i ll d t th t iIn South American countries cassava is mechanically processed to remove the toxic glycosides by first shredding the roots and allowing them to sit for 24 hr to allow the HCN to be volatilize. Cooking the mashed paste further helps drive off the volatile HCN gas.

The cassava past is used to make large “pancakes”

In Africa, where mainly sweet forms are grown the roots are prepared by peeling, boiling and drying. The dried roots are then made into a flour.

Tapioca, a thickening agent, is made from cassava starch by heating to cause formation of gelatinized pellets.

Onions, leeks, garlic, shallots

Members of the genus Allium have have edible bulbs that have been collected and eaten by humans since prehistoric times.

Onions and shallots, A. cepa, and garlic, A. sativum, originated in eastern Asia. Leeks, A. ampeloprasum originated in the Near East. All were being cultivated in Egypt by 5000 years BP. Onions and leeks are grown for their single bulb, garlic for its cluster of bulbs (cloves). The bulbs of leeks are very slightly differentiated from the leaf bases. “Elephant garlic” is actually a variety of leek.

All of these are used as flavoring agents and not as main staple foods. Onion o ese a e used as a o g age s a d o as a s ap e oods O obulbs contain more sugars than the other Allium species.

Sugar cropsSugar cane, Saccharum officianarum, and sugar beets, Beta vulgaris, have to be discussed because of the great importance of sucrose as a plant product.

Sucrose is a disaccharide, a molecule of glucose and a molecule of fructose.

Sugar cane is a perennial grass. It was probably first domesticated in or near New Guinea about 10,000 years BP, and subsequently spread north to Asia where it hybridized with other Saccharum species to yield more cold tolerant hybrids.

Extraction of sugar from sugar cane was being practiced about 5000 years BP in India, and was reported by Alexander the Great as a honey extracted from reeds. But because it required tropical and subtropical environments, it was not until the discovery of the New World in the 15th century that suitable land for its cultivation was available to Europeans.

Columbus introduced sugar cane to the West Indies in 1493 where it became a successful crop and soon became a major export and contributed to the developing economy and population of the new continent. The “sugar triangle” was a trade system during the 1600s-1700s that involved sugar and molasses produced in the West Indies used to produce rum in the northern colonies, which was then used to purchase slaves in west Africa, who were then brought to the West Indies to work in sugarcane fields.

The burning of the British vessel Gaspee, which was enforcing a tax on sugar in the colonies predated the Boston Tea Party as an early act of American rebellion against Britain.

Sugar cane

On average Americans now directly consume about 20 kg of sugar, corn sweeteners, etc per year and another 30 kg. of sugar and 39 kg of corn sweetener consumed indirectly, as various food additives.

Sugar cane as a biofuel

Brazil is the world’s largest producer of sugarcane at around 515 million metric tonnes per year

An increasingly large proportion of this production is being used to produce ethanol for use as an alternative to gasoline. Brazil is also the world’s largest exporter of ethanol, producing more than 40% of the ethanol used for fuel worldwide.

Increases in productivity and efficiency are projected to result in yields of 9000Increases in productivity and efficiency are projected to result in yields of 9000 liters of ethanol produced annually per hectare.

In the US, ethanol production is based on corn sugar (starch) which is a less efficient substrate for generating ethanol than sucrose. Corn starch must first be hydrolyzed to glucose for fermentation to ethanol.

Sugarcane based bioethanol plants are currently under development in Hawaii and Louisiana.

Sugar Beets

Beets, Beta vulgaris (Chenopodiaceae) are also a source of sucrose. At about the same time that sugar cane plantations and the sugar triangle were fueling slavery and the economy of the New World, the French under the direction of Napoleon, began operating a beet sugar extraction factory (1802).

Breeding has increased the sucrose content of beet roots from about 6% in the 18th century to about 20% today. Beets obviously have the advantage of being a crop that can be grown in temperate climates.

T d b t th j f f E t i b tToday sugar beets are the major source of sucrose for European countries, but the inexpensive cane sugar available from Puerto Rico, Hawai’i and Gulf States has kept demand for expansion of sugar beet production low.