Thesis (MScAgric) -- University of Stellenbosch, 2003. / ENGLISH ABSTRACT: Cowpeas are produced under low and irregular rainfall in most of arid and semi-arid
areas of sub-Saharan Africa. Growth and yield are therefore reduced due to the occurrence
of water stress during the growing season. Knowledge of the responses and adaptive
mechanisms of cowpeas to water stress may help to improve the management practices for
these areas. Therefore, three glasshouse experiments were conducted at Welgevallen
Experimental Farm of the University of Stellenbosch to test the responses of two cowpea
cultivars to water stress. In the first experiment, physiological responses were used to
identify those physiological parameters, which can be used to distinguish between drought
tolerant and susceptible cowpea cultivars. In the second experiment, some of the identified
physiological parameters together with some morphological growth responses, yield and
grain protein content of the same two cowpea cultivars were used to identify which is the
more tolerant cultivar. Tn the third experiment, the hypothesis that increased phosphorus
supply may improve the tolerance of cowpea plants to water stress and their ability of
recover from the stress was tested. The results showed that water stress affected water
relations, morphological growth parameters, yield and grain protein content, but
increasing P supply reduced the effect of water stress and promoted more rapid recovery
after re-watering. Water relations were affected by water stress because it reduced relative
water content, which resulted in reduced water potential and increased leaf diffusive
resistance and proline accumulation. Morphological growth responses and yields were
affected because water stress reduced the leaf area, which resulted in reduced biomass
production and seed yield. Lower leaf area under water stress was the result of the reduced
number of leaves and leaf expansion rate, but the number of leaves was the most
important parameter. Reduced seed yield was due to reduced number of pods. The
responses of the two cultivars tested were different. AB Wit, which performed better
under well-watered conditions was more affected by water stress due to its larger leaf area
that resulted in excessive water loss by transpiration. ACH14 was more drought tolerant
than AB Wit due to a combination of a more rapid stomatal closure and proline
accumulation, which induced osmotic adjustment, and which in tum helped to maintain
higher water potentials. The increased P supply reduced the effect of the water stress.
High-P level plants showed higher root growth, which resulted in more water uptake and
larger leaf area during the water stress period, and after re-watering these plants recovered more rapidly. The more rapid recovery from stress was the result of enhanced root growth
and leaf expansion rate and most probably due to increased water uptake. High-P level
plants also showed more rapid leaf appearance and plant growth at earlier stages compared
to the low-P level plants. / AFRIKAANSE OPSOMMING: Akkerbone word onder toestande van lae en wisselvallige reenval in baie ariede en
semi-ariede gebiede van Afrika verbou. In hierdie gebiede word groei en produksie
dikwels beperk deur water tekorte gedurende die groei seisoen. Kennis van reaksies en
aanpassingsmeganismes van akkerbone teenoor water tekorte mag dus help om
produksietegnieke in bogenoemde gebiede te verbeter. Om hierdie rede is drie
glashuiseksperimente onder gekontroleerde toestande op die Welgevallen Proefplaas van
die Universiteit van Stellenbosch uitgevoer. In die eerste eksperiment is fisiologiese
reaksies van twee cultivars gebruik om eienskappe te identifiseer wat gebruik kan word
om tussen droogteweerstandbiedende en droogte gevoelige cultivars te onderskei. In die
tweede eksperiment is sommige van die geidentifiseerde eienskappe asook morfologiese
groei, opbrengs en kwaliteitsreaksies van dieselfde twee cultivars gebruik om die meer
droogte weerstandbiedende cultivar te identifiseer. In die derde eksperiment is die
hipotese dat P-bemesting die droogteweerstandbiedendheid teen en herstelvermoe na
droogte kan verbeter, getoets. Die resultate toon dat water tekorte beide plantwaterverhoudings,
morfologiese eienskappe asook opbrengs en proteieninhoud beinvloed,
maar dat hoe P-peile die invloed van water tekorte verminder en herstelverrnoe na die
droogte verbeter. Plant-waterverhoudings is bemvloed omdat water tekorte relatiewe
waterinhoud van plante verlaag wat aanleiding gee tot verlaagde plantwaterpotensiale,
verhoogde huidmondjie weerstand en 'n toename in prolien inhoud.
Morfologiese eienskappe en opbrengs is benadeel weens 'n veri aging in
blaaroppervlakte wat fotosintetiese vermoe en gevolglik ook biomassaproduksie en saad
opbrengs benadeel. Verlaagde blaaroppervlakte tydens water tekorte was hoofsaaklik die
gevolg van 'n vermindering in aantal blare, terwyl verlaagde saadopbrengs grootliks die
resultaat van 'n vermindering in aantal peule was.
Die cultivar AB Wit wat die hoogste opbrengs onder gunstige groeitoestande gelewer
het, is die meeste bemvloed deur water tekorte omdat die welige blaargroei van hierdie
cultivar, luukse waterverbruik en groter transpirasie verliese veroorsaak het. Die cultivar
ACH 14 daarteenoor het waterverliese beperk deurdat die huidmondjies vinniger gesluit
het en verhoogde prolien-inhoude, osmotiese aanpassings veroorsaak het. Dit het gehelp om waterpotensiale instand te hou. Hierdie cultivar was gevolglik meer droogte
weerstandbiedend as AB Wit.
Hoe vlakke van P-bemesting het die effek van water tekorte verminder weens
verbeterde wortelgroei. Dit het wateropname gedurende en na die peri ode van water
stremming verbeter sodat plante vinniger herstel het na die droe periode. Plante wat by
hoe P-peile gegroei is het ook 'n verhoogde blaarverskyningstempo en 'n toename in groei
tydens die vroee ontwikkelingstadiums getoon.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/49770 |
| Date | 12 1900 |
| Creators | Chiulele, Rogerio Marcos |
| Contributors | Agenbag, G. A., Stellenbosch University. Faculty of AgriSciences. Dept. of Agronomy. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
| Format | 1 v. (various foliations) : ill. |
| Rights | Stellenbosch University |
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