Review Article African Orphan Crops under Abiotic Stresses

20 Pages·2017·1.41 MB·English

Hindawi
Scientifica
Volume 2018, Article ID 1451894, 19 pages
https://doi.org/10.1155/2018/1451894
Review Article
African Orphan Crops under Abiotic Stresses:
Challenges and Opportunities
ZerihunTadele 1,2,3
1InstituteofPlantSciences,UniversityofBern,Bern,Switzerland
2CenterforDevelopmentandEnvironment(CDE),UniversityofBern,Bern,Switzerland
3InstituteofBiotechnology,AddisAbabaUniversity,AddisAbaba,Ethiopia
CorrespondenceshouldbeaddressedtoZerihunTadele;[email protected]
Received 20 October 2017; Accepted 17 December 2017; Published 17 January 2018
AcademicEditor:PrateekTripathi
Copyright©2018ZerihunTadele.ThisisanopenaccessarticledistributedundertheCreativeCommonsAttributionLicense,
whichpermitsunrestricteduse,distribution,andreproductioninanymedium,providedtheoriginalworkisproperlycited.
Achangingclimate,agrowingworldpopulation,andareductioninarablelanddevotedtofoodproductionareallproblemsfacing
theworldfoodsecurity.Thedevelopmentofcropsthatcanyieldunderuncertainandextremeclimaticandsoilgrowingconditions
canplayakeyroleinmitigatingtheseproblems.Majorcropssuchasmaize,rice,andwheatareresponsibleforalargeproportionof
globalfoodproductionbutmanyunderstudiedcrops(commonlyknownas“orphancrops”)includingmillets,cassava,andcowpea
feedmillionsofpeopleinAsia,Africa,andSouthAmericaandarealreadyadaptedtothelocalenvironmentsinwhichtheyare
grown.Theapplicationofmoderngeneticandgenomictoolstothebreedingofthesecropscanprovideenormousopportunities
forensuringworldfoodsecuritybutisonlyinitsinfancy.Inthisreview,thediversityandtypesofunderstudiedcropswillbe
introduced,andthebeneficialtraitsofthesecropsaswellastheirroleinthesocioeconomicsofAfricawillbediscussed.Inaddition,
theresponseoforphancropstodiversetypesofabioticstressesisinvestigated.Areviewofthecurrenttoolsandtheirapplication
tothebreedingofenhancedorphancropswillalsobedescribed.Finally,fewexamplesofglobaleffortsontacklingmajorabiotic
constraintsinAfricaarepresented.
1.Introduction considerably affecting rice (Oryza sativa L.) cultivation on
overtwenty thousandhectaresoflanddue to highlevelsof
Every year, natural disasters such as drought, flooding, soilsalinity[2].
storms,andearthquakescauseconsiderable damage to ani- Changesintheamountandpatternoftheannualrainfall
mals,plants,andnatureingeneral.AccordingtotheUnited have been negatively affecting the productivity of crops in
NationsFoodandAgricultureOrganization,theagricultural African countries especially in Ethiopia [3, 4]. The period-
sectorabsorbs22%oftheeconomicimpactcausedbynatural ically occurring El Nin˜o weather system, formed by local
hazardsanddisastersinthedevelopingworldasagriculture warming of surface water temperatures across the Central
contributes up to 30% of the GDP of vulnerable countries and East-Central Equatorial Pacific Ocean, is the cause of
[1]. Within agriculture, the crop sector suffers the most. mostdroughtsintheHornofAfrica[5,6].TheintensityofEl
From78disastersoccurringfromtheyears2003to2013,the Nin˜ofluctuatesfromyeartoyearandisperiodicallysevere.
highest damage and losses to crops were 42% followed by Forinstance,theElNin˜oin1991/92causeddroughtinabout
those to livestock (36%) [1]. According to the same report, 350millionhaoflandwhilethatof1997/98causeddroughton
thethreemostimportanthazardstocropsarefloods(60%), only80millionha[6].SincethepeakofElNin˜odevelopment
storms (23%), and drought (15%). The 2004 Tsunami that coincides with the growing season of crops in Africa [5],
impacted several Asian countries, especially Thailand, and the effect on food security is high. As drought is usually
the2011TsunamiinJapancontributedtolargelossesofthe accompanied by famine, especially in the Least Developed
agriculturalland.TheJapaneseTsunamialonebroughthigh Countries(LDCs),theoccurrenceofasinglenaturaldisaster
levels of salt from the sea to the agricultural land, thereby does not only affect crop and livestock production but also
2 Scientifica
claims the lives of thousands or even millions of people andtef[Eragrostistef (Zucc.)Trotter]intheHornofAfrica,
depending on its intensity. A study in West Africa showed andalegumecalledbambaragroundnut[Vignasubterranean
that the frequency of dry spells in a given year could be (L.)Verdc.]insouthernandwesternAfrica[8].
predicted from the annual rainfall in the previous years; A great deal of diversity has been maintained by farm-
hence,informeddecisionscanbemadeonthetypeofcrop ers for diverse types of orphan or underutilized crops.
orcultivartobecultivatedandtheappropriatemanagement Efforts have been made by national and international insti-
practicesimplemented[7]. tutionstocollectfromrepresentativelocationsthesevaluable
Environmental stresses that cause damage to crops are germplasms of diverse crops. Collected germplasms are
broadly grouped into biotic and abiotic based on the cause conservedatfacilitiesinnationaland/orinternationalorga-
of damage. While biotic stresses refer to constraints caused nizationsincludingtheSvalbardGlobalSeedVaultinNorway
by living things such as pests, diseases, and weeds, abiotic where 860,00 samples of different crop species are avail-
stressesarethosecausedbyclimateandsoil-relatedproblems. able [68]. The Southern African Development Community
This particular review focuses on the types and extent of (SADC) Plant Genetic Resources Center based in Lusaka,
damage caused by abiotic stresses and on the tools or Zambia, preserved over 18,000 collections of diverse crops
techniquesavailabletoenhancethetoleranceofindigenous from12ofits15memberstates[69].Theselargegermplasm
cropstotheseenvironmentalcalamitiesandasaconsequence collections can be used as a reliable source for screening
to increase the productivity of crops. It gives due attention for traits of interest. The study of only 10% of the 5000 tef
to African indigenous or orphan crops since these crops collections at the Ethiopian Biodiversity Institute shows a
have not received much attention from the global scientific huge diversity in several important agronomic traits which
community.Achangingclimate,agrowingworldpopulation, includes grain yield, harvest index, and lodging percentage
andareductioninarablelanddevotedtofoodproductionare [70]. Although these and other landraces possess several
problems facing the world food security in the near future. desirable agronomic traits, only few traits in limited crop
Thedevelopmentofcropsthatcanyieldunderuncertainand typeshavesofarbeenharnessed.
extremeclimaticandsoilgrowingconditionscanplayakey Among the root crops grown in Africa, cassava (man-
roleinmitigatingtheseproblems.Africanindigenouscrops ioc; Manihot esculentaCrantz),yam (Dioscorea spp.), sweet
canplayakeyroleinthisquest. potato [Ipomoea batatas (L.) Lam.], and enset [Ensete ven-
tricosum (Welw.) Cheeseman] are the source of food for a
largenumberofpeople.Cassavaisastaplefoodforabout600
2.SignificanceofOrphanCropsinAfrica
millionpeopleworldwideandformorethan200millionpeo-
ple in Sub-Saharan Africa [71]. In 2012, yam was cultivated
Orphancropsarealsoknownas“neglectedandunderutilized
globallyonaboutfivemillionhectaresofwhichover95%was
species(NUS)”[60],“underutilizedcrops”[61],or“cropsfor
inAfrica[8].Bananaandplantain(Musaspp.)areamongthe
thefuture”[62].Thesecropsareimportantforfoodsecurity,
major fruit crops grown in Africa. In the year 2012, about
nutrition,andincomegenerationinmanydevelopingcoun-
16 million tons of banana and 27 million tons of plantain
tries, but they have not been significantly researched [63–
were produced in the continent [8]. Although a substantial
66].Theyareoftenwell-adaptedtolocalgrowingconditions
amount of land in Africa is devoted to indigenous crops,
andfulfillthesocialandeconomicneedsoflocalpeopleand
theproductionconstitutesaconsiderablylowpercentageof
are often tolerant to many abiotic stresses compared to the
global production. For example, 10.1% of the global land
world’s major crops like wheat (Triticum aestivum L.) and
devoted to barley (Hordeum vulgare L.) cultivation is in
maize(ZeamaysL.)[66,67].
Africa,butthecontinentcontributesonly4.5%oftheworld
barley production (Table 1). This is because of the use of
2.1. Orphan Crops Are Extensively Cultivated in Africa. nonimproved cultivars as well as lack or minimal use of
Orphan crops are staple foods for millions of people in otheragriculturalinputssuchasfertilizersandpesticides.The
the developing world, particularly in Africa. These crops production per unit area of chickpea (Cicer arietinum L.),
are annually cultivated on large tracts of land and play a however, is slightly higher in Africa than in the rest of the
key role in the livelihood of the resource-poor farmers and world.
consumers.Table1showsimportantorphancrops,including
majorcropsandorphancrops,intermsofthetotalamount 2.2. Orphan Crops Are Tolerant to Environmental Stresses.
of area on which they are cultivated in Africa and the total Table 2 shows some benefits of indigenous crops in terms
productioninAfricacomparedtotheareaandproductionin ofagronomy,nutrition,andhealth.Milletsespeciallyfinger
theentireworld[8].Inaddition,thedistributionoflandand millet(EleusinecoracanaGaertn.),pearlmillet[Pennisetum
productionbetweenmajorandorphancropisshown. glaucum(L.)R.Br.],fonio,andtef,atiny-seededstaplefood
Thesecropsbelongtothemajorcategoriesofcropssuch in Ethiopia, are dominantly cultivated in semiarid areas of
ascereals,legumes,vegetables,rootcrops,andfruitsandhave Asia and Africa due to their extreme tolerance to moisture
diversecentersoforigin.Althoughsimilartypesofcropsare deficit [72, 73]. While finger millet and pearl millet are
cultivated in the rest of the world, Africa has unique crops cultivated in both continents, fonio is exclusively cultivated
whicharecultivatedandconsumedsolelyonthiscontinent. in West Africa. African rice (Oryza glaberrima), grown in
Africa’suniquecropsincludecerealssuchasfonio[Digitaria West Africa, matures early allowing it to escape terminal
exilis(Kippist)StapfandD.iburuaStapf]inwesternAfrica, drought.Itisalsoresistanttodiseasesandpestsandtolerates
Scientifica 3
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totalpr YielGlobal 2.71.04.90.94.41.51.63.1 0.70.80.90.50.8 29.37.733.7 1.10.52.31.5 12.92.318.912.77.611.7 20.68.128.56.9 0.50.9
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10globally Totalpr󸀠(000tons 6,02258769,63616,08326,82323,3124,70024,710 1595,0696965,402632 2,8051,83317,937 901,7081,9862,223 149,47972828,16917,9997,36756,500 15,8637,1351,33026,545 3,2891,000
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ndprod Areac󸀠000ha) 4,99756633,71019,97710,53823,1693,00010,216 2327,82959310,402754 1245161,010 2503,2831,6371,790 13,6623121,8943,5061,1274,801 1,4936911344,368 6,3371,990
a (
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ultivation,totalSource:[8,9]. Centeroforigin Ethiopia o&CentralAmEthiopiaIndiaEthiopiaEthiopiaMiddleEast DiverseCentralAsiaIndiaIndia ChinaEthiopiao&CentralAm EthiopiaCentralAsiaChinaNorthAmerica NorthAmericao&CentralAmIndiaIndia IndiaIndiao&CentralAm o&CentralAmEthiopia
reaofcothers. Mexic Mexic Mexic Mexic Mexic
ad
balan
rowninAfricawiththeirglopearlmillet,andprosomillet CropBotanicalname HordeumvulgareDigitariaexilisZeamaysDiversespeciesOryzasativaSorghumbicolorEragrostistefTriticumspp. VignasubterraneaDiversespeciesCicerarietinumVignaunguiculataCajanuscajan Brassicaspp.AbelmoschusesculentusLycopersiconlycopersicum RicinuscommunisSesamumindicumGlycinemaxHelianthusannuus ManihotesculentumEnseteventricosumSolanumtuberosumIpomoeabatatasColocasiaesculentaDioscoreaspp. Musaspp.MangiferaindicaCaricapapayaMusaspp. TheobromacacaoCoffeaspp.
Table1:Majorcropsgreferstofingermillet, CommonnameCereals∗BarleyFonio∗Maize∗∗Millets∗RiceSorghumTef∗WheatFoodlegumes(pulses)BambarabeansBeans,dryChickpeaCowpeaPigeonpeaVegetablesCabbagesOkra∗TomatoOilseedsCastorseedSesame∗SoybeanSunflowerRoots&tubers∗CassavaEnset∗Potato∗SweetpotatoTaroYamFruits∗BananaMangoPapayaPlantainOthersCocoaCoffee
4 Scientifica
Table2:DiversetypesofbenefitsrelatedtoorphanAfricancrops.
Beneficialtrait Crops Reference
Agronomy-related
Africanrice,bambaragroundnut,cassava,cowpea,enset,Ethiopian
Drought-tolerant [10–16]
mustard,grasspea,pearlmillet,tef,yam
Waterloggingtolerant Tef [17]
Salinity-tolerant Sorghum(moderatelytolerant) [18]
Heat-tolerant Cowpea,pearlmillet [19]
Adaptationtopoorsoil Africanrice,cassava,cowpea [12,14,19]
Earlymaturing(droughtescape) Africanrice,amaranth,fonio [12,13]
Erosioncontrol(fastgroundcover) Cowpea [20]
Diseasetolerance Africanrice,Ethiopianmustard,tef [12,16,17]
Pesttolerance Africanrice,Ethiopianmustard,tef [12,16,17]
Nutrition-related
Completefood(protein,
bambaragroundnut [13]
carbohydrate,fate)
Amaranth,bambaragroundnut,cowpea,fingermillet,fonio,grass
Richinessentialaminoacids [10,12,13,21]
pea
Richiniron Banana,fonio,tef [12,22]
HighvitaminAcontent Banana [23]
Highoilcontent Noug[Guizotiaabyssinica(L.f.)Cass.] [24]
Health-related
Gluten-free Tef,millets [25,26]
Lowglycemicindex Fingermillet [27]
Anticancerproperty Millets [28]
fluctuations in growing conditions [45]. Tef is moderately 2.3.OrphanCropsProvideNutritiousandHealthyFood
toleranttomoisturescarcityand,inaddition,itresistspoorly
drainedsoils,conditionsthatcropssuchasmaizeandwheat 2.3.1.Nutrition-RelatedBenefits. Nutritionalbenefitsofsome
cannot withstand [74]. Noug (Guizotia abyssinica Cass.), orphan crops are indicated in Table 2. Orphan crops often
an oilseed crop related to sunflower, grows best on poorly providemorenutrientsthanthefoodsthatdominateglobal
drained,heavyclaysoils[75]. production.Africancereals,particularlymillets,containhigh
Legumessuchascowpea[Vignaunguiculata(L.)Walp.], amountsofvitamins,calcium,iron,potassium,magnesium,
bambara groundnut, and grass pea (Lathyrus sativus L.) and zinc [77]. The seeds of finger millet contain valuable
tolerate extremely low soil moisture[10, 76]. Cowpea is the amino acids especially methionine [12], which is lacking
most widely cultivated food legume in Africa. In the year in the diets of hundreds of millions of Africans who live
2012, it was cultivated globally on 11.2 million ha of land of on starchy staples such as cassava. The seeds of fonio are
which98%wasinAfrica,particularlyinthreecountries(i.e., nutritious especially in amino acids such as methionine
Niger;Nigeria;andBurkinaFaso),whichaccountfor82%of and cysteine [78] which are essential for human health but
thetotallanddevotedtothiscrop[8].Cowpeaistolerantto deficient in major cereals such as wheat, rice, and maize
drought,heat,andsoilswithlowlevelsoforganicmatterand [21].Amaranth(Amaranthusspp.)whichislargelyconsumed
phosphorus[19].Grasspeaisconsideredasaninsurancecrop in Togo, Liberia, Guinea, Benin, and Sierra Leone, has a
inEthiopiaasitproducesreliableyieldswhenallothercrops relatively high protein content and a balanced composition
fail. ofessentialaminoacidsandminerals[79].
Among the root crops grown in Africa, cassava, yam, Whilethegrainisthemainsourceoffoodforhumans,
sweet potato, and enset are the source of food for millions crop residues, particularly the straw from cereals, are an
of people. Cassava is tolerant to drought and also performs invaluablesourceoflivestockfeed.Thestrawfromtefismore
betterthanothercropsonpoorsoils.Enset,whichisrelated palatableandnutritiousthanthoseofwheatandbarleyand,
tobananabuthasanundergroundcormastheediblepart,is hence,itfetcheshigherprices[80].
amajorfoodsourceforover10millionpeopleinthedensely Legumes such as cow pea, bambara groundnut, and
populatedregionsofEthiopiaduetoitsextremetoleranceto grass pea are the major source of protein for consumers.
droughtandadaptationtodiversesoiltypes[11]. Duetotheirabilitytofixatmosphericnitrogenandconvert
Scientifica 5
it into a form usable for plants, legumes also contribute shortshelf-lifeandthushavelowacceptancebyconsumers.
towardsimprovingsoilfertility.Bambaragroundnutisgrown Due to these desirable properties, the grain of tef fetches
for human consumption and is the third most important higherpricethanthatofothercrops.
grainlegumeinAfricaaftercowpeaandgroundnut(Arachis Similarly,indigenouscropsarecompatibletotheagroe-
hypogaea L.) [76]. The seeds of bambara groundnut are cology of the region in which they are cultivated. The
considered to be a complete food because they contain studyinNorthwesternEthiopiashowedthatwhenawidely
adequate quantities of protein (19%), carbohydrate (63%), cultivatedindigenoustefcropwasreplacedbyexoticmaize,
andfat(6.5%)[13]. theincidenceofmalariaincreasedbyabout10-fold[84–86].
Banana, especially the orange-pulped type with high This was mainly due to a higher survival rate of mosquito
carotenoid and iron content, could reduce Iron Deficiency larvaefedonmaizepollenthanofthosefedontefpollen.
Anemia (IDA) by over 50% and also Vitamin A Deficiency
(VAD)inEastAfrica,wherebothIDAandVADaffectalarge
3.ChallengesofAbioticStressesto
numberofpeople[22].Theseedoilcontentofnougranges
OrphanCrops
from39.8%to46.9%oil[75].Thefattyacidcontentoftheoil
issimilartotheoilsofthesunflowerfamilywithlinoleicacid
Although indigenous crops possess a variety of desirable
beingthedominantoil.
traits, they are under continuous pressure to detect and
adapt to the environmental constraints prevalent on the
2.3.2. Health-Related Benefits. The benefits of some orphan continent [65]. Orphan crops are normally cultivated in
crops as health-food are shown in Table 2. Finger millet is marginal environments in terms of climate and soil. For
a popular food among diabetic patients because of its low instance, millets are crops of dry and hot climate while tef
glycemic index and slow digestion [27]. Finger and pearl iscultivatedonpoorlydrainedVertisolsinthehighlandsof
milletswereshowntohaveanticancerpropertiesandmight Ethiopia where other crops fail to survive. Hence, harvests
have potential in the prevention of cancer initiation [28]. fromthesecropsaremostlysuboptimal.Theseconstraintsare
Thisantiproliferativepropertyisassociatedwiththepresence broadlygroupedintobioticandabioticstresses.Bioticfactors
and content of phenolic extracts. Tef, sorghum, and millets such as weeds, diseases, and insect pests cause significant
areconsideredtobeahealthyfoodsincethegraindoesnot crop damage in Africa due to environments conducive to
containgluten[25,81,82],thecauseofceliacdisease. these crop nuisances. The current review, however, focuses
on abiotic stresses, which cause huge losses to the biomass
ofthesecrops,especiallyinrelationtothetypesandextent
2.4. Orphan Crops Are Compatible with the Socioeco-
ofdamagetheycauseandthetoolsdevelopedtoenhancethe
nomicConditions. Agricultureprovidesmorethanjustfood
toleranceofcropsagainstthisdamage.
production—it affects nonfood products, environmental
Yield loss estimates have been documented for major
andlandmanagement,economicdevelopment,employment
crops.Accordingtoawebsitetabulatinglossestothesecrops,
opportunities, social stability, and maintenance of cultural
yieldlossesduetoabioticstressesrangefrom50%forsugar
tradition and identity. Recognition of the many social and
beetto82%forwheat[87].However,informationonorphan
economic functionsof agriculture has resulted in increased
cropsislimited.
efforts of developing land use practices that support the
functionsinanintegratedmanner[83].
In southern Senegal, African rice is thought to have 3.1. Drought. A considerable amount of water is used in
beendomesticated2000–3000yearsago.TheJolapeopleof bothrain-fedandirrigatedagriculture.Irrigatedagriculture
Senegal still cultivate this rice and choose the varieties to accountsforabout60percentofproductioninthedeveloping
beplantedbasedonmaturationtime(fastorslowmaturing countries.Althoughtheglobalwaterwithdrawalfromrivers,
or both to stagger the harvest time), plant height (a tall lakes, and aquifers for agriculture is estimated at 2,000 to
3 3
or short variety based on harvest considerations), and soil 2,500km peryear,only900km ofthisisusedbyfoodcrops
type (the soil composition and the amount of irrigation whiletheremainingislosttoevaporation,deepinfiltration,
whichisparticulartoeachfield)[45].Likewise,somefonio orthegrowthofweeds[88].Countrieswithanextensivearea
varieties produce grain in only 6 to 8 weeks after planting. underirrigationutilizelargeamountofwater.Forinstance,
Such varieties extend the period of time for which food from the total water consumption in Egypt 85.9% is for
is available and can bridge the gap between the last and irrigation,with2.6%forindustrialandtheremaining11.5%
the next harvest. Planting crops with varying maturation formunicipaluses[89].SincedataforthemajorityofAfrican
timesincreasesthechancesofharvestingunderchallenging countriesisnotavailable,itisdifficulttoknowthecountries
environmentalconditions[12]. whichutilizesmallproportionoftheirwaterforagriculture.
In additiontoadaptationtolocalenvironmentalcondi- Although agriculture requires a large amount of water, the
tions, orphan or indigenous crops are preferred because of scarcity of moisture as it frequently occurs in most African
theiruseinpreparinglocalrecipes.Forinstance,tef,whichis countriesiscriticaltoincreasingproductionandproductivity
thestaplefoodforover60millionpeopleinEthiopia,isthe ofcrops.
mostpreferredgrainsinceitmakesthebestqualityspongy SemiaridandaridregionsinAfricaarefragiletosustain
breadcalledinjera.Comparedtoinjeramadefromtef,those theecosystemmainlyduetothescarcityofmoisture.About
made from sorghum and maize have poor consistency and 40%ofthepopulationintheSub-SaharanAfricaliveinarid
6 Scientifica
3+
todryenvironmentandthepopulationisexpectedtodouble solubilizedintoAl whichistoxicforplantroots,inhibiting
inthedryareasby2050[90].Accordingtotheseauthors,crop theirfunctionanddevelopment[102].Ageneralruleisthat
failureinAfricaismostlynotduetoscarcityofprecipitation aluminumconcentrationsbetween2and5ppmaretoxicto
but due to dry spells which occur at critical growth stages aluminum-sensitiveplantswhileconcentrationsabove5ppm
particularlyduringflowering. aretoxictotolerantspecies.Toxiclevelsofaluminumaffect
Astudyonprosomillet(Panicummiliaceum),littlemillet root growth dramatically and stunted growth, small grain
(Panicum sumatrense), foxtail millet (Setaria italica), and size,andpooryieldaretheresultsoftheconsequentlackof
wild millet (Setaria glauca) showed that drought treatment nutrition[103].TheeffectsofAl-toxicityoncerealcropswere
beforefloweringcontributedtoasignificantyieldreduction recentlyreviewed[104].
inthesefourmillets[91].Asimilarstudyontwofingermillet
landracesindicatedthatdroughtoccurringfourweeksafter 3.4. Soil Salinity. Soil salinity, characterized by a high con-
sowingresultedinacompleteyieldloss[92]. centrationofsolublesalts,affectscropproductivityalthough
Inpearlmillet,terminaldroughtoccurringbetweenthe some indigenous crops have some level of tolerance to this
floweringandthecropmaturitystagesisthemostcriticalas abioticstress.Soilsalinityisabroadtermwhichreferstothree
itresultedin60%yieldloss[93].Yieldlossesduetodrought typesofagriculturallyproblematicsoils.Theseare(i)saline
wereestimatedtobe40%intef[94],51%inpearlmillet,and soil:withelectricalconductivity,ECe> 4dS/m;exchangeable
57%inbambaragroundnut[95]at50%moisturetreatment. sodium percentage, ESP < 15; and pH < 8.5; (ii) sodic soil:
MostorphancerealcropsinAfricasuchasmilletandtef ECe<4dS/m,ESP>15,andpH>8.5;and(iii)sodic-saline
are C4 plants which efficiently utilize moisture; hence, they soil: ECe > 4dS/m, ESP > 15, and pH < 8.5. Chloride and
generally outperform C3 plants in hot and dry climates. A sulphate salts of sodium, magnesium, and calcium are the
study in the Sahel region where the annual precipitation is mostcommonsolublesalts.
aslowas200mmshowsthatsorghumandmillethadsimilar It is estimated that approximately 4.8% of the land in
wateruseandbiomassproduction,hencesimilarbiomassper Africa is considered to be affected by salinity or sodicity
unitofevapotranspiration[96]. [105].IrrigatedlandsintheMediterraneanregionhavebeen
Althoughreportsarenotavailableforindigenouscrops, converted into extensive salt accumulation where 60% of
thestudyonwheatindicatedthatmoisturescarcityaffectsnot thefarmareainEgyptisaffectedbysalineandsodic-saline
only crop productivity in terms of grain yield but also the conditions [106]. Although salinity occurs due to diverse
characteristicsofgrainincludingproteinquality,glutenlevel, causes, the main cause of the elevated salinity level in the
anddoughstrength[97,98]. Mediterraneanareaistheuseofhighlysalineirrigationwater
inthehotanddryenvironmentwhereevapotranspirationis
3.2. Waterlogging. Waterlogging is among the major con- high[106].
straintstoincreasingcropproductivityparticularlyinareas Irrigationinaridandsemiaridclimatesisassociatedwith
which receive high precipitation and have agriculturally salinesoilsasalmostallwatercontainssomesaltsand,after
problematic soils. Since soil pores during waterlogging are high evapotranspiration, the salt remains on the surface of
filledwithwater,thediffusionofgasesishamperedresulting thesoiland/orrootzoneofcrops.Theaccumulationofsalts
inanaerobicconditionswhichmightalsobeaccompaniedby dependsonthequalityoftheirrigationwater,theirrigation
theaccumulationoftoxiccompounds.Asaresult,thenormal management, and the drainage of the soil. Often salinity
functioningofstomata,photosynthesis,androotsareseverely problemsareassociatedwithpoorlydrainedandwaterlogged
affected[99]. soils as excess water allows the salt to rise to the root zone
WaterloggingespeciallyonVertisols,theblackclaysoils via capillary action [107]. Once salt has accumulated, only
withhighwater-holdingcapacity,restrictsthecultivationof additionalwatercanleachthesaltsawayfromtherootzone.
mostmajorcropssuchasmaizeandwheat.About43million Thestudyintefshowedthatsalinesoilcausesupto93%yield
haofVertisolsexistsin28countriesinAfrica[100].Astudy lossintef[108].
on millets indicated that waterlogging treatment from two
weeksaftersowingtocropmaturityreducedthegrainyieldof 3.5. Heat (High Temperature). Heat causes a multitude of
prosomilletandwildmillet(Setariaglauca)by16%and18%, molecular, cellular, and physiological changes to plants.
respectively[101]. Among these, effects on respiration and photosynthesis are
Inadditiontohinderingtheaerationoftherootsystems themajoronesaffectingcropproductivity[109].Extraordi-
due to poor drainage, Vertisols are less suitable for the naryheat waves have alreadycaused significantyield losses
cultivationofrootcropssuchasgroundnutsincepullingof acrosstheglobeandposeadirethreattoglobalfoodsecurity.
thepodsfromthesoilduringharvestingishinderedbythe Climatologicalextremesingeneralhaveanegativeeffecton
hardynatureofthesoilupondrying. plantgrowthandcanleadtocatastrophiclossesincropyields.
Studiesonmajorcropsshowedthat,withheat,yieldsincrease
∘ ∘
3.3. Soil Acidity. An estimate of the total amount of land until a crop-dependent threshold (29 C for maize, 30 C for
∘
with acidic soil in the world is about 30% with about soybeans,and32 Cforcotton)buttemperaturesabovethese
50% of the arable land being acidic [102]. In acidic soils, thresholdsbecomeveryharmful[110].Oncethethresholdis
aluminumtoxicityisthemostimportantfactoraffectingcrop passed,thepotentialforadaptationwithinthecropislimited.
∘
productivity since aluminum under a soil pH below 5 is Itisestimatedthatatemperatureincreaseof3to4 Ccould
Scientifica 7
causecropyieldstofallby15–35%inAfricaandAsia[109]. stress,andROS(reactiveoxygenspecies)scavengingsystems
ModelingsuggeststhatcerealproductioninSouthernAfrica areemployedtoprotecttheplantfromdrought[31].Drought
and Southern Asia is most likely to be affected by climate toleranceisknowntobeassociatedwiththetensilestrength
changeparticularlybyglobalwarming[111]. ofthestemofEragrostisspeciesofwhichtefisamember[32].
Roots also contribute to drought tolerance, and especially
theefficiencyoftheuptakeofwaterduringreproductionand
3.6.ColdandFrost. One-thirdofthetotalgloballandisice-
freewhile42%oflandhastemperaturesbelow−20∘C[112]. grain filling is critical since moisture scarcity during these
developmental stages is detrimental to the survival of the
Inthesecoldregions,plantshavespecializedadaptationsthat
plant [118]. The expression of dehydrin in wheat has been
help them to survive freezing temperatures. The amount of
correlatedwiththeacquisitionofdroughttoleranceatearly
damagecausedbyfreezingdependsonthetemperatureand
growthstage[119].
durationofthecoldstressaswellasthedevelopmentalstage
of the plant. Damage from cold particularly from frost is
common in the highlands of the eastern Africa especially 4.2.WaterloggingTolerance. Plantsrespondtowaterlogging
whenitcoincideswiththegrowingseasonofcrops.Although withavarietyofmechanisms[120].Inwaterloggedsoil,the
climate change is warming rather than cooling the planet, diffusion of gases to and from the roots is severely limited.
warm winters can lead to early blooming of perennials and Oxygen at the root surface is severely decreased and toxic
consequentfrostdamagewhencoldconditionsoccurinlate byproductsareunabletodiffuseawayfromtheplant.Many
spring[113]. aspects of the plant response are a direct consequence of
anoxia, a complete lack of oxygen or hypoxia, and reduced
4.MechanismofCropToleranceto oxygen. One of the responses of plants to low oxygen is to
change from aerobic to anaerobic metabolism as has been
AbioticStresses
foundforfingermillet[37].Thoughmuchlessefficientthan
aerobic metabolism, the production of ATP by glycolysis
Understanding the response of plants to resist and tolerate
can provide the plant with ATP for a short period of time.
abioticstressesisimportantfordevisingbreedingstrategies
This requires an abundant supply of soluble sugars and
toensureproductivityunderallconditions.Diversetypesof
thuschangesincarbohydratemetabolismareoftenfoundin
mechanisms are involved in the abiotic stress tolerance of
tolerantspecies. Increasesinantioxidantsarealsoobserved
plants[114,115].Mechanismsemployedbysomeindigenous
intolerantplantssuchaspigeonpea[36].
crops towards adaptation and/or tolerance to diverse types
Changes in the cellular structure of the plant are also
of abiotic stresses are shown in Table 3. Some of these
a response to waterlogging. For example, aerenchyma, air
mechanismsagainstmajorabioticstresseswhichaffectcrop
spaces in spongy tissue that let gases diffuse from the
productivityinAfricaarebrieflyindicatedbelow.
stem to the roots, is often formed in plants tolerant to
waterloggingstress.Theseairspacesaregeneratedeitherwith
4.1.DroughtTolerance. Majorstrategiesand/ormechanisms
(lysigenous)orwithout(schizogenous)celldeath.Waterlogged
ofdroughttoleranceoradaptationofmilletsthataremostly
sunflowerdevelopedlysigenousaerenchymawithintwodays
cultivatedindrought-proneareasoftheworldwererecently
of the application of stress [33]. Another strategy found in
reviewed [72]. Four of these mechanisms are as follows:
waterloggingtolerantspeciesistheformationofadventitious
(i) drought escape which refers to the condition in which
roots when stressed [120]. Both sorghum [34] and finger
plantsreachmaturitybeforethedroughtoccurs;(ii)drought
millet[35]havebeenshowntoformadventitiousrootswhen
avoidancewhichreferstotheabilityoftheplanttomaintain
waterlogged.Astrategytoescapethefloodviafastgrowthhas
a favorable water balance under moisture stress in order to
beenshownformungbean[38].Inaddition,plantstolerant
avoidwaterdeficitintheplanttissue;(iii)droughttolerance
towaterloggingpossesshighamountofsolublesugars[121].
whichreferstotheabilityoftheplanttoproducesomeyield
In tef, the occurrence of nitrogen reductase activity in the
bywithstandinglowwaterpotential;and(iv)droughtrecovery
shoots rather than in the roots has been implicated in its
whichreferstoaconditioninwhichplantsrecoverfromthe
waterloggingtolerance.
adverse effects of drought in order to provide some yield
and/orbiomass.
Most African orphan crops are cultivated in a marginal 4.3. Soil Acidity Tolerance. Two general mechanisms of
environmentalwheremoistureislimited.However,theyare aluminum (Al) tolerance exist in plants. These are (i) Al
adapted to efficiently utilize the scarce moisture in order exclusion,whichreferstothepreventionofAlfromentering
to sustain their growth and produce at least some yield for the root apex, and (ii) Al tolerance, in which Al is allowed
farmersdesperatelywaitingforthem.Theseneglectedcrops to enter the plant but detoxified [122]. The Al exclusion
guarantee future food security either directly as alternative mechanism is also described by the exudation of organic
crops in drought-prone areas or indirectly as germplasm acidsorthereleaseofphenoliccompoundsaswellasrelated
3+
resourcesforcropimprovement[116]. products which have the ability to chelate Al ions in the
Several morphological and physiological traits are rhizosphere[122].
involved in drought tolerance of plants [117]. In little millet The MATE family of proteins are transporters of
(Panicum sumatrense Roth), osmotic adjustment, which metabolic and xenobiotic organic cations. Members of this
referstothemaintenanceofwaterpotentialduringmoisture family responsible for malate and citrate efflux have been
8 Scientifica
e
Referenc[29][30][31][31][32] [29] [31][33][34,35][36][37][38][39,40][41][42][43]
CropPearlmilletPearlmilletLittlemilletLittlemilletTef Pearlmillet LittlemilletSunflowerhum,fingermilletPigeonpeaFingermilletMungbeankpea,PearlmilletCowpeaFababeanSorghum
Sorg Chic
s.
e
s
s
e
r
st
c
oti
bi on
msoforphancropstolerancetodiversea Responsetostressghyieldfordrought-tolerantgenotypesAdjustphenologytorainfallpatternDecreasedunderdroughtIncreasedunderdroughtIncreasedindrought-tolerantplantsxtractionbeforeflowering;moreextractiafterfloweringDecreasedunderdroughtAerenchymaformationFormationofadventitiousrootsGenerationofantioxidantsChangetoanaerobicmetabolismFastgrowthGrowthandnutrientuptakeUnknownEarlyflowering,protractedrosetteEarlyflowering
nis Hi se
ha Les
c
e
M
3:
e
l
b
a
T
TraitSeedyieldFloweringtimeShootlengthRootlengthLeaftensilestrength Waterextraction Chlorophyllcontent Cellularstructure Metabolism PlantheightGrowthYield Growth
Abioticstress Drought Waterlogging Heat Coldandfrost
Scientifica 9
implicatedinaluminumresistanceinwheat[123],sorghum 4.6. Cold or Frost Tolerance. Many plants increase their
[124], and barley [125]. An aluminum susceptible cultivar tolerance to cold if they are allowed to acclimate to it in
of barley was made tolerant after transformation with the low, nonfreezing temperatures. For example, rye exposed
TaALMT1genefromwheat[126].Ninebarleytransformant to a period of low nonfreezing temperatures can withstand
lineswiththeTaALMT1genealsoshowedincreasedmalate temperature as low as −30∘C, while the same rye without
efflux and aluminum resistance and some transformants the acclimation period dies at −5∘C [135]. The acclimation
surpassed the original line in aluminum toxicity resistance mechanismsincludetheexpressionofgenesthatfunctionto
[127]. protectmembranes,theprimarysiteoffreezinginjury,such
as dehydrins that prevent ice nucleation [136]. In addition,
CBF/DREB1 proteins, a family of transcription factors in
4.4. Soil Salinity Tolerance. Three mechanisms of plant tol-
Arabidopsis, control a group of genes that confer freezing
erancetosoilsalinitywerereported.Theseare(i)tolerance
tolerance[135].
to osmotic stress which reduces cell expansion in root tips
+ Cold tolerance is the ability to grow and produce a
andyoungleaveswhichresultsinstomatalclosure;(ii)Na
satisfactory yield under relatively cold (but not freezing)
exclusionfromleafbladesinordertoremovethetoxiceffect
+ conditions. Several sorghum accessions from Ethiopia and
ofNa+;and(iii)tissuetolerancetoaccumulatedNa [128].
Ugandahavebeenshowntobecoldtolerantduringboththe
Management of saline soils includes the provision of
reproductiveandthevegetativestages[43].Themechanism
appropriate drainage, controlling irrigation, and the use of
forthistolerancehas,however,notbeeninvestigated.
deep-rooted plants, which maintain the water table at a
Seed yield in chickpea can be increased fourfold by
level appropriate for the land. Considerable improvements
sowing in early winter as opposed to traditional spring
in growth and yield have been obtained by either priming
sowing. However, this increases the probability of exposure
seedwithchemicalsorextremetemperaturesbeforeplanting
to cold, necessitating cold-tolerant lines. Screening of 3276
orbyapplyingexogenouschemicalstoplantsgrowingunder
chickpeagermplasmaccessionsandbreedinglinesidentified
saltstress.Effortsinbreedingsalt-tolerantcultivarsarebeing
21cold-tolerantlines[137].Whenexposedtocoldstressinthe
pursued[129].Theuseofsalinetolerantcropsbothincreases
reproductivephase,cold-tolerantchickpeasproducedviable
the amount of land available for growing and reduces the
seeds,whilecoldsusceptiblecultivarssufferedfromabortion
quality of the water that must be used for irrigation. Crops
due to pollen sterility [137]. The analysis of the bambara
describedassalttolerantincludedatepalm,asparagus,barley,
groundnut transcriptome obtained from suboptimal low
andsaltgrass,whilemoderatelytolerantspeciesincludeolive, ∘
temperature of 18 C showed that transcription factors such
beet,sorghum,wheat,andrice[18].
as MYB, NAC (NAM, ATAF, and CUC), and WRKY are
Plant microRNAs are known to be involved in diverse
involvedinresponsetothestress[138].
abiotic stresses. A list of microRNAs from model and crop
plantswithdifferentialexpressionundersalineconditionwas
recentlydocumented[130]. 5.TechniquesEnhancingAbioticStress
ToleranceinOrphanCrops
4.5.HeatTolerance. Heatstressisamajorconcernforcrop
Theapplicationofmoderngenetic,genomic,andagronomic
productionandmuchworkisbeingdonetodevelopplants
tools to the improvement of indigenous crops can provide
thatcanstillmaintainyieldunderhotconditions.Itisknown
enormous opportunities for ensuring global food security.
thatheatstressinterfereswithphotosystem(PS)II,increases
A recent review indicated that thereare large gaps between
the amount of reactive oxygen species (ROS), and reduces
farmers’averageyieldandpotentialyieldforseveralAfrican
electron transport [131]. Plants have evolved a number of
crops [66] indicating that productivity of these crops could
adaptive, avoidance, and acclimation mechanisms to cope
be boosted using improvedcultivarsand proper crop man-
withheatstress.
agement practices. Recent reviews have suggested several
Tolerance mechanisms involve heat shock proteins, ion
efficient breeding and genomics techniques with immense
transporters, osmoprotectants, antioxidants, transcription
potential in the development of crops tolerant or resistant
factors,andsignalingmolecules[132,133].Lipidmembranes
toabioticstresses[139–142].Table4summarizessometech-
aresusceptibletolipidperoxidation,andmembranethermal
niqueswithahighpotentialfortheimprovementofAfrican
stabilitycanbeaselectioncriterionforheatstresstolerance
orphancrops.
as has been used in wheat [134]. Reactive oxygen species
(ROS) are normal byproducts of cell metabolism that can
5.1. Genetic Tools. Diverse types of genetic and breeding
be toxic at high concentrations. Many plants have systems
techniques and their applications to African orphan crop
containingantioxidantstocombatROSasprotectionagainst
improvementwereearlierreviewed[65].Onlywidelyimple-
awidevarietyofabioticstresses.
mentedtechniquesarebrieflypresentedbelow.
Heattolerantaccessionshavebeenfoundforpearlmillet
[40], chickpea [39], and cowpea [41]. A study in chickpea
identified 18 SNPs from five genes (ERECTA, ASR, DREB, 5.1.1. Hybridization. Hybridization refers to crossing two
CAP2 promoter, and AMDH) that were significantly asso- plantsofthesameorrelatedspeciesinordertocreategenetic
ciated with different traits under heat and drought stress variability which is utilized to improve trait(s) of choice.
[39]. Successes in hybridization resulted in semidwarf cultivars
10 Scientifica
Table4:Majortechniquesappliedtodevelopabioticstress-tolerantAfricanorphancrops.
Technique Improvedtrait Crop Reference
Breeding&genomics
Selection/screening Soilacidity Tef [44]
Hybridization(intra-&interspecific) Droughttolerance NERICArice [45]
Mutationbreeding Diversetraits Tef [46]
Marker-assistedmethod Droughttolerance Pearlmillet [47]
TILLING(TargetingInducedLocalLesions
Severaltraits Chickpea,millets,tef [48,49]
INGenomes)
Agronomy&physiology
Conservationtillage Droughttolerance Sorghumandchickpea [50]
Soilaciditytolerance;
Biocharapplication Tef&othercrops [51,52]
improvedsoilproperties
BBF(broad-beds&furrows) Improveddrainage Fababeanandtef [53]
Fingermillet,tef,
SCI(SystemofCropIntensification) Several [54]
mustard
Planthormones Salt&coldtolerance Severalcrops [55,56]
of wheat and rice, which boosted the productivity of both number of planting materials can be generated from small
crops during and after the famous Green Revolution [143]. area; (ii) planting materials are maintained free of diseases
The major breakthrough in hybridization in Africa was the particularly from viruses; and (iii) the regeneration process
interspecific crossing between the Asian rice (Oryza sativa is made all year round irrespective of climatic conditions
L.)andtheAfricanrice(OryzaglaberrimaSteudel),whichis sinceregeneratesaregrowningreenhousesundercontrolled
widelycultivatedinWestAfrica.Thisledtothedevelopment environmentalconditions.Anefficientregenerationmethod
of the popular NERICA (New Rice for Africa) varieties. resulting in plantlets has been developed for wild banana
NERICA harbors desirable properties from its two parents: [146].
high grain yield and protein content from the Asian rice
and tolerance to drought and poor soil fertility as well as 5.1.4. Marker-Assisted Selection (MAS). This refers to the
early maturity from the African rice [45]. Due to its early utilization of molecular markers in breeding for traits of
maturing property, African rice escapes terminal drought, interest. When markers are found that are linked to genes
which normally occurs after the flowering stage. Hence, controlling traits, the marker (not the gene) can be used
it is the source of food during food shortage particularly to identify and select breeding stock. Markers are selected
just before other crops are harvested. NERICA is currently that are easily identifiable with common laboratory tests.
cultivatedinseveralAfricancountries[144]. Commonly utilized markers in crop breeding are SSRs
(Simple Sequence Repeats, or microsatellites) and SNPs
5.1.2. Mutation Breeding. This refers to the application of (Single Nucleotide Polymorphisms). SSR refers to a repeat
eitherphysical(such asgamma-rayandX-ray)orchemical of two to six nucleotides in the DNA sequences while SNP
(e.g., ethyl methane sulfonate or EMS) mutagens to alter refers to a single nucleotide polymorphism. Other recently
the composition or arrangement of DNA of an organism developedSNP-basedmethodsincludeGBS(genotyping-by-
so that mutants harboring traits of choice are selected and sequencing) [147] and GWAS (Genome-Wide Association
bredtopopularcultivar(s).EMS,whichmainlycreatespoint Studies)[148].ThepotentialapplicationofGBSwasrecently
mutations, is the most desirable mutagen as it results in a investigatedinseveralcropswithdifferentgenomesizesand
singlenucleotidelesioninthegeneofinterest.Mostmutation breedingsystems[149].Inbreedingforabioticstress-tolerant
breedingprogramsaimatalteringtraitssuchasplantheight crops,GWASwasefficientlyappliedinidentifyingaluminum
anddiseaseresistanceinwell-adaptedplantvarietiesofrice, resistancewheatcultivars[150].
barley, and wheat. Mutation breeding is a cornerstone for
releasingover3000globallyknowncropvarietiesincluding 5.1.5. Transgenesis and Genome Editing. The transgenic
somevarietiesfromAfricanindigenouscrops[145].Mutation methodisthefastestadoptedagriculturaltechnologydueto
breeding and molecular breeding techniques have been its high acceptance by farmers. Since the release of the first
successfullyimplementedtodevelopsalt-tolerantcrops[129]. commercial transgenic crops, the annual cultivated area by
transgenic crops has increased 106-fold and the production
5.1.3.PlantCellandTissueCulture. Tissueculturetechniques volume has increased from just 1.7 million tons in 1996 to
orinvitroregenerationhavebeenwidelyutilizedforseveral 181 million ha in 2014 [151]. The development of cisgenesis
indigenous crops especially in tree crops due to a number involvesasysteminwhichplant-specificpromotersareused
of benefits. Some of these benefits are as follows: (i) a large to drive the gene of interest, instead of foreign promoters