Thesis (MScAgric)--University of Stellenbosch, 2002. / ENGLISH ABSTRACT: Daily daylight fertigation regimes (involvinq drip fertigation, on a daily basis,
during the daylight hours) are becoming widely used in the South African
citrus industry in the endeavor to enhance tree productivity. Such regimes
could provide sensitive nutrient and moisture management, reducing plant
stress in general as well as the response time to root-directed cultural
activities.
There is a need to evaluate the efficacy of daily daylight fertigation systems
relative to conventional irrigation systems. Standard horticultural evaluation of
orchard management practices is very time consuming. We opted for
physiological studies comparing plant stress levels, in an attempt to quantify
plant performance under each system. Citrus trees under daily daylight
fertigation and conventional micro-jet and drip-irrigated regimes were
monitored to establish plant stress levels as indicated by sap flow, xylem
water potential, stomatal conductance and chlorophyll a fluorescence. Plants
under a daily daylight fertigation regime are believed to have good soil water
conditions in their rooting volume, and therefore experience negligible
baseline levels of stress. The trees do, however, experience midday
depression in stomatal conductance, to a lesser degree, but not unlike trees
under conventional regimes. It appears as if a larger rooting volume of microjet
irrigation regimes enhances recovery from the midday depression. It is
recommended that producers optimise productivity during the moming hours,
by early irrigation, so that plants can function optimally, whilst environmental
conditions are most favourable for high physiological activity.
We also assessed the effect of withholding water from trees adapted to a daily
daylight fertigation regime to evaluate the risk involved with short-term water
deficits in trees adapted to this regime, as well as the usefulness of
physiological techniques for identifying water stress. Stomatal conductance
and xylem water potential indicated water stress sooner than the other
physiological parameters. Citrus trees seem to be relatively insensitive to
water deficit stress as measured by sap flow and chlorophyll a fluorescence. Sap flow is buffered by tree capacitance, and although mediated via stomatal
conductance, atmospheric conditions and not the soil water content primarily
determine it. As daily fertigation is applied to trees under DOF regimes, they
exhibit more optimal levels of xylem water potential and stomatal
conductance, compared to trees from which water is withheld. Although
alleviating it to a degree, daily irrigation did not mitigate the midday
depression in these values. Seen over a season, even small enhancements
of stomatal conductance (and with it photosynthesis and possibly, growth) and
xylem water potential, could incrementally produce higher yields. / AFRIKAANSE OPSOMMING: In die strewe na verhoogde boomproduktiwiteit, word daaglikse
sproeibemesting (deur 'n drupbesproeiingsstelsel toegedien tydens die
dagligure) al meer algemeen in die Suid Afrikaanse sitrusbedryf gebruik.
Hierdie praktyk verminder algemene plantstres deur baie spesifieke voedingsen
vogbeheer, en verkort ook die plant se reaksietyd op wortelgerigte
bewerkingsaktiwiteite.
Dit is nodig om die relatiewe voordeel van daaglikse sproeibemesting teenoor
konvensionele besproeiingssisteme te evalueer. Huidige tuinboukundige
evaluering van boord-bestuurspraktyke is baie tydrowend. In 'n poging om
plantreaksie onder verskillende praktyke te beskryf, het ons besluit om die
plantstresvlakke met fisiologiese metodes te vergelyk. Sitrusbome onder
daaglikse sproeibemesting, en konvensionele mikro- en drupbesproeiing, is
onderskeidelik gemonitor om die plant se stresvlakke vas te stel, soos
aangedui deur sapvloei, xileem-waterpotensiaal, stomatale geleiding en
chlorofil a f1uoresensie. Die plante onder daaglikse sproeibemesting
ondervind lae viakkke van waterstremming, waarskynlik weens hoë
grondvogtigheid in die wortelsone. Die bome ondervind wel, soos dié onder
konvensionele besproeiing, middagdepressie in stomatale geleiding, hoewel
tot 'n mindere mate. Dit blyk asof die groter wortelvolume van mikrospuit
besproeide bome die herstel na middagdepressie bespoedig. Produsente
word aangeraai om die oggendure optimaal te gebruik deur vroeg te besproei
sodat plantproduktiwiteit hoog is terwyl die omgewingsfaktore op hul
gunstigste is en wanneer die hoogste fisiologiese aktiwiteit voorkom.
Ons het ook die effek van wateronthouding gemeet op die bome wat
aangepas is vir daaglikse sproeibemesting. Sodoende is die risiko verbonde
aan 'n korttermyn watertekort op hierdie bome ge-evalueer, asook die
bruikbaarheid van fisiologiese tegnieke om waterstremming in sitrus te
identifiseer. Stomatale geleiding en xileem-waterpotensiaal het
waterstremming vroëer aangedui as die ander fisiologiese parameters.
Sitrusbome blyk redelik onsensitief te wees teenoor droogtestremming soos gemeet deur sapvloei en chlorofil a fluoresensie. Sapvloei word gebuffer deur
boom-kapasitansie, en alhoewel sapvloei gereguleer word deur stomatale
geleiding, is dit die atmosferiese toestande (hoofsaaklik dampdruk verskil) wat
dit primêr beïnvloed, en nie die grond-water inhoud nie. Omdat bome
daagliks sproeibemes word, het hulle meer optimale vlakke van xileemwaterpotensiaal
en stomatale geleiding in vergelyking met bome waarvan
water weerhou is. Alhoewel daaglikse sproeibemesting die middagdepressie
verlaag het, is dit nie daardeur opgelos nie. Oor die typerk van 'n seisoen kan
selfs minimale verhogings in stomatale geleiding (en daarmeesaam
fotosintese en moontlik groei) en xileem-waterpotensiaal, hoër opbrengste tot
gevolg hê.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/53000 |
| Date | 12 1900 |
| Creators | Schoeman, Stephanus P. (Stephanus Philippus) |
| Contributors | Rabe, E., Wand, S. J. E., Stellenbosch University. Faculty of AgriSciences. Dept. of Horticulture. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | Unknown |
| Type | Thesis |
| Format | 129 p. : ill. |
| Rights | Stellenbosch University |
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