Thesis (MScAgric)--University of Stellenbosch, 2004. / ENGLISH ABSTRACT: Due to the extraordinary drought resistance of the grapevine, viticulture without
irrigation in the winter rainfall coastal areas of South Africa is a feasible and
commonly used practice. Wine quality is largely determined by the quality of the
grapes from which it is made. Grapevine physiology is affected both directly and
indirectly by water stress, which may vary according to soil type and prevailing
atmospheric conditions. The water status of the grapevine can affect grape
composition profoundly, either directly or indirectly, in either a positive or negative
way, depending on the degree as well as the duration of water stress. There are
three important factors involved in the development of water stress, namely the
transpiration rate, the rate of water movement from the soil to the roots, and the
relationship of soil water potential to leaf water potential. All three these factors are
affected by atmospheric and/or soil conditions.
In warm winelands such as South Africa (Western Cape), with a mediterranean
climate which is characterised by a hot, dry summer period, the most important
characteristic of soil is its ability to supply sufficient water to the grapevine during the
entire growing season. Leaf water potential (Ψl) has gained wide acceptance as a
fundamental measure of grapevine water status, and has been widely applied in
viticultural research. Shortly before dawn, Ψl approaches equilibrium with soil water
potential and reaches a maximum daily value.
The study formed an integral part of a comprehensive, multi-disciplinary research
project (ARC Infruitec-Nietvoorbij Project No. WW13/01) on the effect of soil and
climate on wine quality, which commenced in 1993 and will be completed in 2004.
This study was conducted during the 2002/03 growing season in two Sauvignon
blanc vineyards situated at Helshoogte and Papegaaiberg, both in the Stellenbosch
district, approximately nine kilometres apart. Two experiment plots, representing
contrasting soil types in terms of soil water regime, were selected in each vineyard.
At Helshoogte the two soils represented the Tukulu and Hutton forms, and the soils
at Papegaaiberg were of the Avalon and Tukulu forms.
The aim of this study was to determine the effect of atmospheric conditions and
soil water status on the level of water stress in the grapevines for each soil at each
locality, as well as the effect of grapevine water stress on yield and wine quality. This
was done by determining and comparing the soil water status, soil water holding
capacity of the soils and the evapotranspiration of the grapevines on the two different
soils, at each of the two localities differing in mesoclimate and topography. The
atmospheric conditions at the two localities during the 2002/03 season were also
determined and compared to the long-term average atmospheric conditions, and the
level of water stress of grapevines on each soil at each locality was measured. During the 2002/03 growing season, atmospheric conditions were relatively warm
and dry in comparison to the long-term averages of previous seasons. These
conditions accentuated the effects of certain soil properties that may not come
forward during wetter, normal seasons.
The usually wet Tukulu soil at Helshoogte was drier than expected during the
2002/03 season compared to the Hutton soil. Due to more vigorous growth on the
Tukulu soil, grapevines extracted more soil water early in the season, leading to a
low soil water matric potential and more water stress in the grapevines. Due to the
higher vigour, resulting in more canopy shading, and more water stress, the dominant
aroma in wines from the Tukulu soil was fresh vegetative. The Hutton soil maintained
consistency with regards to both yield and wine quality compared to previous
seasons. On the other hand the Tukulu soil supported a higher yield, but with lower
than normal wine quality.
The Avalon soil at Papegaaiberg maintained the highest soil water potential
towards the end of the season, probably due to capillary supplementation from the
sub-soil. Grapevines on the Tukulu soil at Papegaaiberg experienced much higher
water stress than ones on the other three soils, especially during the later part of the
season. This could be ascribed to a combination of factors, the most important being
the severe soil compaction at a shallow depth, seriously limiting rooting depth and
root distribution, which is detrimental to grapevine performance.
Both the soil water status and atmospheric conditions played important roles in
determining the amount of water stress that the grapevines experienced at different
stages. The air temperature and vapour pressure deficit throughout the season were
consistently lower at Helshoogte, the cooler terroir, compared to Papegaaiberg, the
warmer terroir. At flowering, Ψl was lower for grapevines at Helshoogte than at
Papegaaiberg, showing that diurnal grapevine water status was primarily controlled
by soil water content. The difference in grapevine water status between the two
terroirs gradually diminished until it was reversed during the post harvest period when
Ψl in grapevines at Papegaaiberg tended to be lower compared to those at
Helshoogte. The relatively low pre-dawn Ψl at Helshoogte indicated that the
grapevines were subjected to excessive water stress resulting from the low soil water
content. However, grapevines at Helshoogte did not suffer material water stress (i.e.
Ψl < -1.20 MPa) during the warmest part of the day, suggesting that partial stomatal
closure prevented the development of excessive water stress in the grapevines.
This suggests that low pre-dawn Ψl values do not necessarily imply that
grapevines will experience more water stress over the warmer part of the day, or visa
versa. This does not rule out the possibility that side-effects of partial stomatal closure, such as reduced photosynthesis, can have negative effects on grapevine
functioning in general. These results also suggest that measurement of diurnal Ψl
cycles at various phenological stages is required to understand and quantify terroir
effects on grapevine water status. / AFRIKAANSE OPSOMMING: Danksy die droogteweerstand van die wingerdstok is die verbouing van wingerde
sonder besproeiing ‘n praktiese en algemene verskynsel in die winterreënval-areas
van Suid-Afrika. Wynkwaliteit word grootliks bepaal deur die kwaliteit van die druiwe
waarvan dit gemaak word. Wingerdfisiologie word direk en indirek beïnvloed deur
waterstres, wat kan varieer volgens die grondtipe en die heersende atmosferiese
toestande. Die waterstatus van die wingerdstok beïnvloed druifsamestelling, direk of
indirek, en positief of negatief, afhangend van die graad en tydsduur van die
waterstres. Daar is drie belangrike faktore betrokke by die ontwikkeling van
waterstres, naamlik die transpirasietempo, die tempo van waterbeweging vanaf die
grond na die wortels, en die verhouding tussen die grondwatermatrikspotensiaal tot
blaarwaterpotensiaal. Al drie die faktore word beïnvloed deur die atmosferiese en/of
grondtoestande.
In warm wynboulande soos Suid-Afrika (Weskaap), met ‘n meditereense klimaat
wat gekarakteriseer word deur ‘n warm, droë somerperiode, is die belangrikste
eienskap van grond die vermoë om voldoende water aan die wingerdstok te verskaf
gedurende die hele seisoen. Blaarwaterpotensiaal (Ψl) het wye aanvaarding bekom
as die fundamentele meting van wingerstokwaterstatus, en word wyd toegepas in
wingerdkundige navorsing. Kort voor sonsopkoms, nader Ψl ‘n ewewig met die
grondwatermatrikspotensiaal en bereik ‘n maksimum daaglikse waarde.
Die studie vorm ‘n integrale deel van ‘n omvattende, multi-dissiplinêre
navorsingsprojek (ARC Infruitec-Nietvoorbij Projek No. WW13/01) op die effek van
grond en klimaat op wynkwaliteit, wat in 1993 in aanvang geneem het en in 2004
afgehandel sal word. Hierdie studie is uitgevoer gedurende die 2002/03 seisoen in
twee Sauvignon blanc wingerde geleë by Helshoogte en Papegaaiberg, beide in die
Stellenbosch distrik, ongeveer nege kilometer van mekaar. Twee eksperimentele
persele, elkeen verteenwoordigend van kontrasterende grondtipes in terme van
grondwaterregime, is geselekteer in elke wingerd. By Helshoogte word die twee
gronde verteenwoordig deur die Tukulu en Hutton grondvorms, en die gronde by
Papegaaiberg is van die Avalon en Tukulu vorms.
Die doel van die studie was om die effek van atmosferiese toestande en
grondwaterstatus op die wingerdstok se waterstatus vir elke grond by die twee
lokaliteite te bepaal, sowel as die effek van die wingerdstok se waterstatus op die
opbrengs en wynkwaliteit. Dit is gedoen deur die grondwaterstatus, die
grondwaterhouvermoë, sowel as die evapotranspirasie van die wingerdstokke op die
twee verskillende gronde by elk van die twee lokaliteite, wat verskil in mesoklimaat
en topografie, te bepaal en vergelyk. Die atmosferiese toestande by die twee
lokaliteite gedurende die 2002/03 seisoen is ook bepaal en vergelyk met die langtermyn gemiddelde atmosferiese toestande. Die vlakke van waterstres in
wingerdstokke op elke grond by elke lokaliteit is ook gemeet.
Gedurende die 2002/03 groeiseisoen, was die atmosferiese toestande relatief
warm en droog in vergelyking met die langtermyn gemiddeldes van vorige seisoene.
Hierdie kondisies aksentueer die effek van sekere grondeienskappe wat nie
noodwendig na vore kom gedurende normale, natter seisoene nie.
Die gewoonlike nat Tukulu grond by Helshoogte was droër as verwag gedurende
2002/03 in vergelyking met die Hutton grond. As gevolg van sterker groekrag op die
Tukulu grond, het wingerdstokke meer grondwater onttrek vroeg in die seisoen, wat
gelei het tot ‘n lae grondwatermatrikspotensiaal en meer waterstres in die
wingerdstokke. Die sterker groeikrag het meer beskaduwing van die lower asook
meer waterstres veroorsaak, wat gelei het daartoe dat die dominante aroma in wyne
vanaf druiwe op die Tukulu grond vars vegetatief was. Die Hutton grond het
bestendig gebly in terme van opbrengs en wynkwaliteit in vergelyking met vorige
seisoene. Daarteenoor het die Tukulu grond ‘n hoër opbrengs gelewer, maar met laer
as gewoonlike wynkwaliteit.
Die Avalon grond by Papegaaiberg het die hoogste grondwatermatrikspotensiaal
behou tot die einde van die seisoen, heelwaarskynlik a.g.v. kapillêre aanvulling
vanuit die ondergrond. Wingerdstokke op die Tukulu grond by Papegaaiberg het
heelwat meer waterstres ondervind as op die ander drie gronde, veral later in die
seisoen. Dit kan toegeskryf word aan ‘n kombinasie van faktore, die belangrikse
daarvan die erge grondkompaksie vlak in die grond, wat worteldiepte en
-verspreiding ernstig beperk het, wat op sy beurt nadelig is vir wingerdprestasie.
Beide die grondwaterstatus en atmosferiese toestande het ‘n belangrike rol
gespeel in die bepaling van die hoeveelheid waterstres wat die wingerdstok op
verskillende stadiums ondervind het. Die lugtemperatuur en waterdampdruktekort
was regdeur die seisoen laer by Helshoogte, die koeler terroir, as by Papegaaiberg,
die warmer terroir. Gedurende blom was die Ψl laer vir wingerdstokke by Helshoogte
as by Papegaaiberg, wat daarop wys dat daaglikse wingerdstok waterstatus
hoofsaaklik deur die grondwaterinhoud bepaal was. Die verskil in wingerdstok
waterstatus tussen die twee terroirs het geleidelik verminder totdat dit omgekeer was
gedurende die na-oes periode toe Ψl in wingerdstokke by Papegaaiberg geneig het
om laer te wees in vergelyking met die by Helshoogte. Die relatiewe lae voorsonop Ψl
by Helshoogte het daarop gedui dat die wingerdstokke aan oormatige waterstres
onderwerp was. Die wingerdstokke by Helshoogte het egter nie materiële waterstres
(i.e. Ψl < -1.20 MPa) gedurende die warmste gedeelte van die dag ondervind nie, wat
aandui dat gedeeltelike huidmondjiesluiting plaasgevind het om die ontwikkeling van
oormatige waterstres te verhoed.Dit dui aan dat lae voorsonop Ψl waardes nie noodwendig impliseer dat
wingerdstokke meer waterstres gedurende die warmste gedeelte van die dag sal
ondervind nie, of visa versa. Dit sluit nie die moontlikheid uit dat negatiewe neweeffekte
van gedeeltelike huidmondjiesluiting, soos ‘n vermindering in fotosintese, ‘n
negatiewe effek kan hê op die wingerdstok se funksionering in die algemeen nie.
Hierdie resultate stel voor dat die meting van daaglikse Ψl siklusse gedurende
verskeie fenologiese stadia benodig word om die effek van terroir op die wingerdstok
se waterstatus te verstaan en te kwantifiseer.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/16376 |
| Date | 12 1900 |
| Creators | Laker, Mareli S. (Mareli Susan) |
| Contributors | Archer, E., Myburgh, P.A., University of Stellenbosch. Faculty of Agrisciences. Dept. of Viticulture and Oenology. |
| Publisher | Stellenbosch : University of Stellenbosch |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
| Format | iii, 107 leaves : ill. |
| Rights | University of Stellenbosch |
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