Thesis (MScAgric)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: Irrigation scheduling decisions are based either on the direct measurement of soil water status
(SWS) or on physiological measurements like plant water status (PWS). Soil based measurements
are quick and easily automated, but the plant response for a particular quantity of soil moisture
varies as a complex function of evaporative demand.
A plant-based approach measures the plant stress response directly, but is an integration of
environmental effects as well. In contrary to soil-based methods, plant based measurements can
indicate when to irrigate, but not the quantity.
Pre-dawn leaf water potential (ΨPD) is determined mostly by the soil moisture level, and can serve
as a measure of static water stress in plants and an index of bulk soil water availability or even as
an estimate of soil water potential at the root surface. Therefore it should be possible to establish a
link between SWS and PWS, but it is largely unknown how stable the link in a heterogeneous
vineyard would be, and how the grapevine vegetative and reproductive response relates to this
link.
Plant water status plays a large role in determining vigour and yield of the plant. The levels of PWS
are influenced by irrigation, but it was mostly affected by the season and vine location in the
vineyard. More negative plant water potentials reduced vigour, but had a less pronounced effect on
yield, while also reducing overall wine quality.
Vigour variability in the vineyard was largely attributed to soil heterogeneity, which seemed to have
a strong effect on SWS. SWS measurements were calibrated according to the observed variability,
increasing the accuracy of measurements significantly. Soil water content values were used to
establish a link between SWS and PWS. This link was determined over time using nine plots,
consisting of rain-fed and irrigated regimes, in variable vigour areas. A non-linear relationship was
found between ΨPD and percentage extraction of plant available water for rain-fed plots. When
irrigation was applied, no correlation could be found. In this study, for Merlot in the Stellenbosch region, PWS differences affected vigour, and to a
lesser extent yield, as well as wine quality. More negative plant water potentials reduced vigour
more in high vigour areas than in lower vigour areas, which in turn led to unbalanced vegetative:
reproductive ratios. This disturbed vine balance may have had a bigger impact on wine quality than
PWS levels. Therefore a well-managed and balanced vine is able to withstand more stress, with
less detrimental effects. This study also highlights the danger of limiting the assessment of soil and
plant water status conditions to point measurements in vineyards with high levels of vigour
variability. / AFRIKAANSE OPSOMMING: Besluite rakende die skedulering van besproeiing word gewoonlik gebaseer op die direkte meting
van grondwaterstatus (GWS), of op fisiologiese metings soos byvoorbeeld plantwaterstatus
(PWS). Grond gebaseerde metings is relatief vinnig en maklik om te outomatiseer, maar die
plantrespons vir ʼn spesifieke grondwaterinhoud varieer as ʼn komplekse funksie van
dampdruktekorte.
‘n Plantgebaseerde benadering meet die plantstresreaksie direk, maar is ʼn integrasie van
omgewingstoestande. In teenstelling met grondgebaseerde metodes, kan plantgebaseerde
metodes aandui wanneer om te besproei, maar nie die hoeveelhede wat besproei moet word nie.
Voorsonsopkoms blaarwaterpotensiaal (ΨPD) word grootliks deur die grondwaterinhoud bepaal, en
kan as ʼn maatstaf van statiese waterspanning in plante en as ʼn indeks van bulk
grondwaterbeskikbaarheid dien, of selfs as ʼn benadering van die grondwaterpotensiaal by die
worteloppervlak. Dit behoort dus moontlik te wees om ʼn verwantskap te bepaal tussen GWS en
PWS, maar dit is grootliks onbekend hoe stabiel hierdie verwantskap sal wees in ʼn heterogene
wingerd, asook hoe die wingerdstok se vegetatiewe en reproduktiewe reaksie die verwantskap kan
beïnvloed.
Plantwaterstatus speel ʼn groot rol in die bepaling van groeikrag en opbrengs in die wingerdstok.
Die vlakke van plantwaterstatus word deur besproeiing beïnvloed, maar word skynbaar meesal
deur die seisoen en wingerdstok se ligging in die wingerd bepaal. Meer negatiewe
plantwaterpotensiaalvlakke het gelei tot laer groeikrag, maar het ʼn minder uitgesproke effek gehad
op opbrengs, terwyl dit in die algemeen wynkwaliteit verswak het.
Groeikrag variasie in die wingerd kon grootliks aan grond heterogeniteit toegeskryf word, wat
skynbaar ʼn sterk invloed op grondwaterstatus gehad het. Grondwaterstatus metings is gekalibreer
volgens die variasie wat waargeneem is, wat die akkuraatheid van metings beduidend verhoog
het. Grondwaterinhoud waardes is gebruik om ʼn verwantskap aan te dui tussen SWS en PWS.
Hierdie verwantskap is oor tyd bepaal vir nege persele, wat bestaan het uit droëland asook
besproeide persele, in areas waarvan die groeikrag verskil het. ʼn Nie-liniêre verband is gevind
tussen ΨPD en die persentasie onttrekking van plantbeskikbare water vir die droëland persele.
Waar besproei was, kon geen verband gevind word nie. In hierdie studie, vir Merlot in die Stellenbosch area, het PWS vlakke groeikrag en tot ʼn mindere
mate opbrengs en wynkwaliteit beïnvloed. Meer negatiewe plantwaterpotensiaal vlakke het
groeikrag meer beïnvloed in hoër groeikrag areas as in die laer groeikrag areas, wat ook gelei het
tot ongebalanseerde vegetatiewe:reproduktiewe verhoudings. Hierdie versteurde balans in die
wingerdstokke kon dalk ʼn groter impak op wynkwaliteit gehad het as PWS vlakke. Daar moet dus
gepoog word om goed bestuurde en gebalanseerde wingerdstokke te hê, sodat strestoestande
beter weerstaan kan word met minder nadelige gevolge. Die studie beklemtoon ook die gevaar
verbonde daaraan om die bepaling van grond- en plantwaterstatus te beperk tot puntmetings in
wingerde met groot variasie in groeikrag.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/96088 |
| Date | 12 1900 |
| Creators | Van Zyl, Albertus J. |
| Contributors | Strever, Albert E., Stellenbosch University. Faculty of AgriSciences. Dept. of Viticulture and Oenology. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | Unknown |
| Type | Thesis |
| Format | 84 p. : ill. |
| Rights | Stellenbosch University |
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